Chapter 10: The Search for Greater Mobility in Ground Warfare
Factors Determining Vehicular Development
The keynote of US Army operations in World War II was sounded by the roar of the internal-combustion engine. Two decades of American automotive research and development had relegated animal power, the major tactical prime mover of 1917–18, to the category of military curiosa. Billions of mechanical horsepower in more than two million combat and transport vehicles supplied by the Ordnance Department lent American armies unprecedented mobility and maneuverability, two of the primary requisites for attaining the ultimate objective of military operations—the destruction of hostile military forces in battle.1
Destruction itself is the result of fire power, but fire power minus ability to maneuver is ineffective both in offense and in defense.2 World War I demonstrated that offensive fire power lacking a high degree of battlefield mobility cannot, even though quantitatively vastly superior, force a decision over an equally static, resolute defender suitably armed for his role. A swath of machine gun fire against unprotected assault infantry produced the same result as the murderous hail of artillery fire: it forced the opponent into the bowels of the earth, into safety, instead of annihilating him. Warfare then deteriorated into a meaningless contest of stamina in which ephemeral victor’s laurels went to the captor of a few acres of shell-pocked soil. Insatiable, the Moloch of attrition was also impartial, demanding ruinous sacrifices from victor and vanquished alike. With neither side able to break the stalemate with the means at hand, both searched frantically for ways in which to regain freedom of maneuver. The British were the first to come up with a workable solution: mechanical transport with fire power and crew protection in a vehicle capable of traversing almost any kind of terrain over which foot troops would have to advance. The cover name given the contrivance in its development stage hung on; the track-laying armored combat vehicle became known as the tank.
Advantageous as it proved in lending the attacker once more the ability to move on the field of battle, the tank of 1916–18 was far from a panacea for the ills of position warfare. To begin with, the tank itself was a hulking, lumbering affair that traveled more slowly cross country than man could
walk. Quite apart from its thin coat of armor—crew protection against only small arms fire—its speed, or lack of speed, spelled extreme vulnerability to hostile defensive weapons. Secondly, friendly artillery, as well as supply, was drawn by horse, or at best, slow-speed tractor, and as such was incapable of rolling fast enough cross country to support a sustained advance in the face of organized resistance. Trucks, though used for transporting infantry, were road bound. Their solid rubber tires, primitive springing, and, above all, lack of adequate motive power, precluded their use save on improved traffic routes. The division, the basic tactical troop unit, could move as a whole only on foot or by rail. True mobility of ground forces in combat was not to be achieved until technology perfected mechanical transport to the point where its inherent characteristics—speed, great tractive power, and economy of operation—could be employed in front and rear echelons alike, and an entire army could fight on the move over most types of terrain. By 1939 that point had been reached.
World War II soon dispelled whatever doubts existed about the merits of mere fire power, however concentrated, versus a lesser degree of fire power coupled with mobility. The fall of France dramatically proved that an army unable or unwilling to maneuver was doomed when confronted by an adversary resorting to highly mobile conduct of operations. Intricate fortifications bristling with heavy artillery—the embodiment of memories of 1914–18—proved worse than useless when the enemy chose to bypass rather than breach them. Tanks employed as pillboxes instead of mobile weapons to carry the fight to the enemy were deathtraps pure and simple. Throughout the war the same lessons were repeated over and over again. Mobile attack invariably carried the day over immobile defense, whether its name was Maginot Line, Atlantic Wall, or Siegfried Line. In 1940 the German tide in little over a month engulfed the same blood-drenched territory between the German frontier and Paris that in World War I had been the scene of four years of struggle. In 1944 the relentless sweep of American mechanized armies covered the same ground in less than twenty days.
Remarkable enough in itself, the complete motorization of US ground forces, the basis for their unrivaled striking power, becomes even more extraordinary in the light of the swiftness with which it was accomplished. Beginning with only a handful of completely developed military motor vehicles at the outbreak of war in Europe, the Ordnance Department eventually furnished to the Allies some forty major types of combat vehicles and sixty-odd major types of transport vehicles.3 This achievement became possible only through closest cooperation with industry, a long-standing tradition in Ordnance automotive research and development. During the interwar years of lean funds and public apathy toward armaments, only assistance such as that of the Ordnance Advisory Committee, sponsored by the American Society of Automotive Engineers, had enabled the Ordnance Department to keep step with developments abroad.4 With the advent of war challenging America to outproduce the Axis in equipment capable of superior performance in all four corners of the earth, the Industry-Ordnance team proved one of the most potent weapons in the arsenal of democracy. Automotive, metallurgical, electrical, and
rubber engineers from industry, serving on numerous specialized advisory boards and committees, helped solve a million and one perplexing, sometimes seemingly insuperable, problems arising over the design of ordnance vehicles. The military and civilian engineers of the Ordnance Department in turn familiarized their colleagues with the many particular requirements of military motor transport foreign to private industry.
Paramount in the design of any military motor vehicle stood reliability. Since American equipment saw action thousands of ocean miles away from its factories, distance alone ruled out shuttling to the United States for major overhaul. Not even the huge industrial plant of America would have sufficed to equip US and Allied troops if part of industry were devoted to the repair rather than the production of weapons. Finally, shipping space was so limited throughout the war that each cubic foot diverted from the build-up of Allied strength overseas postponed the prospect of victory. Once overseas, motor vehicles had to be capable of traveling under their own power the oft-times considerable distances from dockside to battlefield before embarking on their intended missions. Mechanical failure in action was intolerable. Each deadlined tank and truck impaired the striking power of Allied ground forces, put even greater strain on already overburdened supply lines, and added to the workload of rear area maintenance and repair facilities. However well suited commercially produced vehicles were for civilian use, they were unable to withstand the rigors of military employment. Though some classes, notably wheeled transport vehicles, were largely adapted from standard commercial design, these too required numerous modifications emphasizing cross-country mobility, ruggedness, dustproofing, waterproofing, corrosion-proofing, minimum bulk, and minimum weight. Appearance of vehicles and components had to yield to the purpose they were meant to serve. Ease of operation, maintenance, repair, and replacement were prerequisite to efficient field service.5
Perhaps the broadest and most basic question to be answered was whether large or small-size transport best answered military needs. Honest differences of opinion existed, with each proponent mustering almost equally cogent arguments. Those favoring large vehicles set forth the economies of reduced over-all requirements in material, labor, and fuel, and in operating and maintenance personnel; in rebuttal, the other camp pointed out the greater maneuverability of small vehicles and their greater ease of operation and maintenance, which required less highly skilled manpower. Less bulky to ship, moreover, light trucks could be sent overseas in greater numbers than heavy types, and the more trucks an army had in the field, the less vulnerable it was to immobilization for lack of transportation. The ultimate decision in favor of the small vehicle gave little cause for regret:–
The greatest advantage in equipment the United States has enjoyed on the ground in the fighting so far [wrote General Marshall in the summer of 1945], has been in our multiple-drive motor equipment, principally the jeep and the 2½-ton truck. These are the instruments which have moved the United States troops in battle while the German Army, despite the fearful reputation of its ‘panzer armies’ early in the war still depended heavily on animal transport for its
regular infantry divisions. The United States, profiting from the mass-production achievements of its automotive industry, made all its forces truck-drawn and had enough trucks left over to supply the British armies with large numbers of motor vehicles and send tremendous quantities to the Red Army.6
Not so unqualified was the praise accorded Ordnance track-laying equipment, especially when it came to that best known of combat vehicles, the tank. From the landing of US troops in North Africa until V-E Day, tanks drew increasingly severe criticism. In January 1945 Hanson Baldwin wrote in The New York Times:–
Why at this late stage in the war are American tanks inferior to the enemy’s? That they are inferior the fighting in Normandy showed, and the recent battles in the Ardennes have again emphatically demonstrated. This has been denied, explained away and hushed up, but the men who are fighting our tanks against much heavier, better armored and more powerfully armed German monsters know the truth. It is high time that Congress got to the bottom of a situation that does no credit to the War Department. This does not mean that our tanks are bad. They are not; they are good. They are the best tanks in the world—next to the Germans’,7
And on 22 March The Washington Post took up the cudgel with the statement:
A Bronx cheer comes out of Germany to greet the news that the Pershing tank has gone into mass production. It is the opinion of the men at the front, apparently, that they will get the new tank in numbers when it is no longer needed, i.e., when the war is over ... an investigation is thoroughly in order. It should take up the reasons for the long delay in getting the Pershing into production. It should likewise find out why our tanks are inferior to the enemy’s.
No investigation ever materialized. The facts were clear. From the very beginning of the tank program, the Army had staked its fortunes on the medium tank as the fighting tank of its armored divisions and, for better or for worse, remained unshaken in its choice until 24 January 1945 when, after extensive testing, the Armored Board finally recommended that the Pershing or, as it was then known, the heavy tank T26E3, be considered battleworthy after incorporation of minor modifications, and be standardized and shipped to troops.8 Up to then no recommendations of the Ordnance Department had been able to persuade the using arms to adopt a heavier vehicle than the Sherman. A heavy tank, the M6, had been developed, standardized, and put into production in 1942, but a letter from the commanding general of the Armored Force to the commanding general of the Army Ground Forces on 7 December of that year stated that because of its sixty-ton weight and limited tactical use no requirement for it existed.9 The same laconic “no requirement” was the standard reply to any proposed vehicle violating the weight limits of Army Regulations 850-15, which prescribed that no tank weigh more than 30 tons or exceed 103 inches in width, though as one Ordnance tank specialist observed, Hitler’s tanks violated this American rule.10
That, tank for tank, neither the American Grant nor its successor, the Sherman, was a match for the more heavily armored and armed German Tiger, US troops
Heavy Tank M6
Medium Tank M3
learned in the early days of the fighting in North Africa. Nor were the troops’ chances any better when, in Italy and France, they came up against the Panther. The only unquestioned advantages of the American vehicles were their reliability and their somewhat greater radius of action. For the rest, they had to depend on superiority in numbers to surround their adversaries and knock them out in flank attacks. “But,” as General of the Army Omar N. Bradley observed, “this willingness to expend Shermans offered little comfort to the crews who were forced to expend themselves as well.”11 Well known though they were to the men at the front, the inadequacies of the Sherman failed to sway the using arms in their determination that this was the very tank with which to defeat Germany. As early as August 1943 the Ordnance Department pointed out that the Sherman was becoming more obsolescent each month and urged the standardization of two types of the newly developed T20-series tanks, the T23E3 and T20E3, in order to set up production facilities for these better gunned and better armored vehicles, “Attention is invited to the fact that unless action along this line is taken at an early date it will not be possible to supply field units with any quantity of the T20 series tanks during the calendar year 1944 …” wrote General Barnes. The request was denied.12 When Lt. Gen. Jacob L. Devers, Commanding General, ETO, in November 1943 recommended that highest priority be given the development of the T26, armed with a 90-mm. gun, in order to counter the increased armor protection and fire power of German vehicles,13 the Army Ground Forces voiced its misgivings about the trend toward heavy tanks as inconsistent with American combat doctrine. A memorandum from Brig. Gen. William F. Dean, the chief of the Requirements Division, AGF, to General McNair remarked:–
... [the radiogram from General Devers] intensifies the pressure upon Army Ground Forces to immediately commit ourselves to the early production of a thick-skinned tank carrying the 90-mm. Gun. The British and the Ordnance have been convinced for some time that we should initiate such procurement without further delay. , . . Action recommended: a. That the Army Ground Forces go on record as not favorably considering procurement of T26 at this time. b. That any further procurement be deferred pending full service test of pilot models.
General McNair, in reply, approved those recommendations, adding, “I see no reason to alter our previous stand in reply to a communication from the Armored Command—essentially that we should defeat Germany by use of the M-4 series of medium tanks. There has been no factual developments overseas, so far as I know, to challenge the superiority of the M-4.14 Once again the verdict read that no requirement existed at that time for a medium or heavy tank of the T26 type.15
By D Day the status of American armor was as precarious as that of the panzer divisions in 1941–42, at which time Germany had lost qualitative superiority on the battlefield to the Soviet Union. Only
the causes underlying these crises differed. The American situation was one of choice, while that of the Germans had been one of necessity. Before beginning its Eastern Campaign Germany, counting on subjugating the Red Army in the customary few months of blitzkrieg, had let armor development lag, while a badly informed intelligence apparatus fed the belief that existing panzer types far outstripped anything the Russians had been able to build. As a result, German tanks that crossed the Soviet frontier during the morning hours of 21 June 1941 were identical with those that the year before had terrorized the world by their exploits in France. They all were there: the machine-gun-toting Panzer I which lasted exactly thirteen days before being recommended for retirement as a burden on the troops; the Panzer II with its 20-mm. cannon, so ineffective that production of the series stopped the month after the invasion;16 the Panzer III with its face lifted by the addition of armor and a 50-mm, gun replacing the former 37-mm, primary armament; and finally the Panzer IV, unchanged from the 1940 version save for similarly strengthened armor protection. Initial successes of the German armies bade fair to substantiate the estimates of Soviet tanks. Knifing their way through unorganized resistance, the panzers took a murderous toll of antiquated Russian machines. But even within the first two weeks of the campaign, an ominous note was sounded in the East: reports of Soviet vehicles topping anything the Germans had in the way of armament and armor.17 Though first data turned out as exaggerated as is usual in the case of surprise encounters of new weapons in combat, the truth was formidable enough. The panzer divisions had stumbled on the first-line tanks of the Russians: the 32-ton T34, and the 52-ton KV1 which outgunned, outarmored, and outmaneuvered every other tank then on the battlefield.18 At that time began the race of gun power against armor protection which, for the rest of the war, was to become the biggest problem of both Allied and Axis designers.
A large part of Germany’s tremendous losses during the first six months of campaigning in the East were, to be sure, due to factors other than enemy action.19 The greatest foe of mechanized equipment, for example, turned out to be the muddy seasons and, axiomatically, the Russian winter. In 1941, as in the years following, these natural phenomena wreaked more havoc with German fighting strength than Allied ground and air efforts combined.20 Lacking the ground clearance and flotation system of their Soviet counterparts, German vehicles helplessly floundered in the bottomless quagmires of autumn and spring mud. Attempts to plow forcefully ahead only compounded disaster. Engines and bearings burned out, gears stripped, and, once winter frost or summer sun made possible the resumption of movement, the countryside was littered with
unsalvageable wrecks. The trials of winter proved equally severe since neither German soldiers nor their weapons were equipped to fight in the bitter cold. The Russians, adapted to the climate and terrain of their homeland, held the upper hand.21 Decimated and nearly stripped of arms, the Wehrmacht emerging from its first winter in the East was no longer the fighting machine of the blitz years. Its cadre of battle-tempered veterans had been shockingly thinned and in another year, after Stalingrad, would be only a memory. Replacements, trained in the short period that wartime permits for this purpose to friend and foe alike, were at best only substitutes. No superweapon, however powerful, could ever fill that void.
Chastened by their encounter with Soviet armor and antitank defenses, the Germans proceeded to overmatch them. No other course was open. In 1941 production had fallen to some 500 vehicles short of six months’ battle losses,22 and even with all-out industrial mobilization Germany lacked the plant facilities to compromise quality to get quantity. With new vehicle designs still in the development stage but more powerful guns ready to be installed, the first step was improvisation. Existing tanks were up-gunned and up-armored, albeit with little permanent success since the USSR invariably was prompt in countering with similar measures. Moreover, each modification entailed additional weight and, in the absence of equal increases in power and flotation, contributed nothing toward bridging the gap between Russian and German mobility and maneuverability. On the contrary, the added strain on already overloaded engines and gear trains made German tanks less and less reliable.23 But in German eyes the mission of the tank—the same, incidentally, as in American combat doctrine, break-through and exploitation—demanded that fire power and armor keep step with the evolution of defensive weapons.
Any conclusion that the German trend toward heavier vehicles denoted a departure from the tried and proven concepts of armored warfare would be erroneous. German service schools in 1943, for example, reiterated the maxims that had been the key to the earlier successes of the panzer divisions. “The mission of the tank unit,” students were taught, “consists of opening the way for other elements [of the armored division] into and through the enemy. All missions (combat missions) are executed by means of the concerted unit attack, in which antitank weapons and artillery are to be annihilated and hostile armored formations counteracted.24 In other words, the tank had to be capable of overcoming all types of hostile weapons, which is a long way from saying that these were its primary objectives, American authorities determining the characteristics of US armor held different views. “There can be no basis for the T26 [90-mm. gun] tank,” Army Ground Forces officially replied to the suggestion of introducing the better armed and armored, and consequently heavier, vehicle than the Sherman, “other
than the conception of a tank versus tank duel—which is believed unsound and unnecessary. Both British and American battle experience has demonstrated that the antitank gun in suitable numbers and disposed properly is the master of the tank. Antitank guns either must be put out by armored infantry or equivalent means, or avoided by tanks. The primary mission of tanks is the destruction of those hostile elements which are vulnerable to them—not antitank guns.25
Underlying the Ordnance Department’s insistence on introducing a basically new tank was its awareness of the inherent limitations of the Sherman, or for that matter, of any tank, for despite its far-from-fragile appearance, a tank represents a mechanism as finely balanced as a watch. Its merits lie in the fact that it combines four essential military characteristics: fire power, maneuverability, speed, and crew protection. Each of these is, in the final analysis, a function of weight. Cross-country mobility, for example, requires low unit ground pressure, which means either a light hull—thin armor, light armament, light power train—or else a wider, and therefore heavier, track with a correspondingly larger and heavier engine and transmission. During World War II no tank of practical size could simultaneously feature maximum armor, fire power, speed, and maneuverability. Every vehicle was a compromise, with qualities deemed more desirable by its users accentuated at the expense of those they considered of lesser importance. But once a satisfactory compromise had been devised, further modifications of major import would inevitably upset that balance and punish the tank by limiting its effectiveness and reliability. It was precisely this danger that loomed in the case of the Sherman.
Its early participation in the fighting in North Africa had shown the Sherman to be in every respect superior to the Axis tanks then on the battlefield. It had contributed a large share to the British victory at El Alamein, its baptism of fire, and had played a prominent role in the westward pursuit of the Italo-German forces. The British forces had the highest praise for the one tank that finally ended a long reign of German qualitative superiority. German reports, in turn, gloomily forecast the doom of Rommel’s troops unless equipment capable of dealing with the new American vehicles was sent promptly and in force.26 But only a few months later, during encounters with a token contingent of German Tiger tanks in Tunisia, the Sherman proved to be outgunned and out-armored—a state of affairs that became even more pronounced with the advent of the Panther tank in Italy.
True, the Sherman had qualities not even remotely duplicated in any German vehicle. Time and again, for example, in both Africa and Italy it took enemy strongholds in mountainous terrain that no German tank could hope to traverse.27 In point of reliability, it similarly outshone both the notoriously undependable Tiger and the Panther. But it was small
comfort to the tanker to know that he could count on reaching the scene of action, if at the same time he was equally certain of adverse odds upward of two to one of ever leaving that scene alive.
Placing American armored forces once more on a par with their opponents meant either up-gunning and up-armoring the Sherman or supplanting it with an entirely new vehicle. The first solution, Ordnance designers knew, would prove at best only a stopgap because the balance between fire power, speed, maneuverability, and crew protection that distinguished the tank in the days of El Alamein would be lost. General Barnes opposed such a policy of improvisation on principle, though for a long time without success. The inevitable result was that during the fighting in France, Belgium, and Germany the now badly overloaded Sherman not only was still out-gunned and out-armored but on too many occasions, particularly in mud, snow, and ice, outmaneuvered as well by the Panther. The second solution, one that might have been adopted as early as August 1942 when the first of the T20-series tanks was released for production, came to pass only after General Barnes almost single-handedly overcame the determined opposition that for more than two years had prevented the introduction of a vehicle radically departing from the tried and true pattern of the Sherman. But while accolades for the Pershing tank from the ETO, proved, if nothing else, that the Ordnance Department’s labors had not been in vain, one poignant question remained unanswered: Did the intervening advances in development warrant the delay in getting the new weapon onto the battlefield?28
From almost the first day of World War II, sharply differing points of view prevailed on the acceptability of new ground weapons. Two issues were involved: the development of items for which no formal requirement had been established, and the battle testing of new equipment. As to the first, Army Ground Forces, for example, vigorously opposed development of weapons that it considered not absolutely essential, regardless of how much they might be desired by men in the field. The Ordnance Department, on the other hand, believed in maintaining a strong lead over the using services in the development of new items. For one thing, Ordnance technicians spent their entire service careers in the study of ordnance, so that their knowledge of the capabilities and inherent limitations of weapons exceeded that of line officers, whose careers were concerned with the tactical use of equipment. As Generalmajor Heinz Guderian, the father of the German panzer forces, once put it when reminded that all technicians were strangers to the truth: “Certainly there is a lot of lying, but one to two years as a rule uncover that fact, when the ideas of the technicians turn out to be unworkable. The tacticians also lie; but in that instance the truth comes out only after the next lost war, and then it is too late.”29
In the case of tanks, the Ordnance view hardly proved incorrect. The tank capable of holding its own against enemy armor, in other words, the heavy tank advocated by the Department since 1942, proved indispensable in large-scale ground operations. The Ordnance Department, which
Medium Tank M4, the Sherman
Heavy Tank M26, the Pershing
as late as 1943 was criticized for proposing such a vehicle, was barely a year thereafter criticized for its absence from the battlefield, “... our tanks when forced to engage in tank vs. tank action,” wrote the Army Ground Forces in January 1945, “have had to close to short ranges in order to destroy the opposing tanks. The destruction of the enemy has been accomplished at great cost in tank matériel and personnel and is reflected in the current critical shortage of tanks.”30
The issue of battle testing was more complex. Army Ground Forces insisted that no new weapons, however promising they looked, be sent overseas until a small number had been tested by the prospective users and all corrections deemed necessary had been incorporated. Ordnance engineers for several reasons deplored the seemingly interminable delay in getting a new tank into action.31
Had the Germans been equally insistent on mechanical perfection and as reluctant to battle test new tanks, they would hardly have been able to regain the lead in fire power and armor once they lost it in 1942. As it was, they scored telling successes by rushing virtually the pilot models of their new heavy vehicles to the battlefield. Thus the Tiger, which to the very end of hostilities remained ridden with glaring mechanical weaknesses, was a formidable enough foe in action to become almost synonymous with German prowess in weapons design. Similarly the Panther, with its high-velocity gun and sloping frontal armor, found no match in American tanks until the advent of the General Pershing in 1945. That the Panther in 1942 had been rushed from the drawing board to production line in a scant nine months and consequently was so full of the proverbial bugs that another year and a half passed before it was pronounced really fit for combat, detracted little from its killing power. In the ETO US troops, whose Shermans mechanically outlasted their German adversaries as much as five to one, “were reaching a point where they were becoming afraid to fight in the M4 [Sherman] due to lack of fire power.”32
Perhaps the most vital clue to the American tank problem during World War II could be found in that indefinable standard of tactical utility, reliability, and durability called “battle worthiness” which, in effect, meant all things to all men. Time and again an alleged lack of that quality resulted in a delay in getting a heavier tank than the Sherman into action. More often than not the sole reason was the limited durability of mechanical components. Yet no measuring stick, statistical or otherwise, was ever devised to ascertain the life expectancy of combat vehicles on the battlefield. The Russian view, though founded on a much less complicated communications problem, offered an interesting parallel in that respect. On the assumption that a tank was almost certain to be knocked out after a brief period of fighting, the Russians considered a lifetime of fourteen hours for its mechanical components to be excellent. American tanks, by comparison, were required to last for a minimum of forty hours. Arbitrary or not, this emphasis on durability rather than
reliability for the useful life of a tank deprived American troops of weapons that might, with telling effect, have contributed toward shortening the war.33
The mobility of each combat and transport vehicle depended above all on the performance of its components. Power plant, gear trains, and the like had to function reliably and had to be properly attuned. Maximum output at minimum bulk and weight was particularly important because light-weight, heavy-duty vehicles required less power for just their own propulsion and furnished more for the job they were meant to do. Component development therefore comprised one of the principal phases of Ordnance automotive research and development during World War II.
The magnitude of the work accomplished precludes any comprehensive treatment save in scores of volumes. A tank had more than a dozen major components, each consisting of several subassemblies that in turn were made up of perhaps hundreds of parts. Over 25,000 separate parts in all went into a single tank, and each might require complete reworking to permit the construction of an improved vehicle. Space limitations alone dictate an account of only some high lights from the record. Others, intrinsically significant, are of too technical a nature to discuss here. So notable an innovation as the cross-drive transmission, for example, was as complex as it was promising. Hence, discussion in the following pages will deal only with the development of two vital but more readily described features—engines and flotation devices.
Engines
The basic factor determining military mobility was the internal-combustion engine. Two of the obvious advantages of the internal-combustion engine over pack and draft animals were its greater power output per unit of weight and its ability to propel heavy loads at high speeds. From a military point of view, particularly that of an army with supply lines as long as those of US forces in overseas theatres, its other characteristics were even more valuable. Fuels and lubricants took up less cargo space than forage and, unit for unit of delivered energy, were less expensive. Motor vehicles were more easily transported by rail and water than animals and required fewer men with less training for their operation. In service, the gasoline or diesel engine did not eat when it was not working, was not subject to fatigue, and was less vulnerable to injuries than the horse and mule.
To be suitable for military purposes, engines had to pass tests far more stringent than for commercial purposes. They had to function with equal certainty in tropical heat and arctic cold, in desert sandstorms and jungle moisture. They had to be capable of long periods of trouble-free performance with a minimum of care and maintenance. Above all, they had to furnish sufficient power to permit sustained high speeds over all kinds of terrain.
In some types of military motor transport the engine problem could be solved with relative ease. Wheeled cargo and personnel carriers, for example, had much the same power requirements as civilian trucks. Designed and developed by America’s automotive industry in cooperation
with the Quartermaster Corps, these vehicles presented relatively few difficulties in point of motive power when the Ordnance Department assumed responsibility for their design, development, and production in the summer of 1942. But all the more exasperating were the obstacles that had to be surmounted in powering motorized equipment such as tracked vehicles in general and medium tanks in particular. An Ordnance expert intimately acquainted with wartime automotive research and development summed up the situation as follows:–
Our World War II difficulties in obtaining an engine of approximately 500 horsepower for the medium tank is an excellent example of our military engine problem and the awful confusion, loss of time, inefficient utilization of management, manpower, facilities, and material occurring at a critical time. In order to power the medium tank we had to employ six improvised engines, build two new plants, completely tool four plants (one of them twice), and partially tool two plants. These engines came with 5,165 spare parts, 6 sets of tools, 6 sets of maintenance literature, and a constant flow of engineering changes and mass tests to make the improvisations suitable for tank use.
The effect of this situation on training, supply, and maintenance is apparent. The fact that our tanks dominated all battlefields of this war is a tribute to those of the military and of American industry who had the responsibility of getting tanks into the hands of fighting soldiers. The confusion and waste is chargeable directly to the fact that our lack of vision as a nation resulted in insufficient appropriations to have an engine for military use developed, tested, and ready for the emergency.34
All told, at least five factors combined to make the medium tank engine one of the thorniest problems of Ordnance research and development during World War II. First and foremost stood the peculiar power requirements of tanks in general.
The tank, unlike trucks or other wheeled vehicles, had to operate primarily away from improved roads and highways and consequently needed a much bigger supply of power to insure maximum speed and maneuverability. (See Chart 9) In addition, the space limitations of the tank called for a power plant and installation of unusual compactness with no sacrifice of accessibility for quick adjustment, repair, or re-placement—the larger the engine, the larger and heavier the hull had to be, and the more time required for routine maintenance and repairs, the longer the tank would be out of action. This need for compactness in turn endangered technical difficulties with cooling systems, air intake and engine exhaust arrangements, air filters, and the like, all of which had a direct bearing on the net horsepower available for the primary job of propelling the tank. Since no analogous power or installation requirements confronted the designers of commercial automotive vehicles, the Ordnance Department during the interwar years had had to begin virtually from scratch in arriving at any semblance of a solution to the tank engine problem. Here entered the shortage of funds, the second factor responsible for the difficulties in procuring a satisfactory engine for the medium tank during World War II.
During the 1930s money for the design and development of a power plant specifically adapted to the unique requirements of full-tracked combat vehicles had simply not been available. Private industry understandably had been little interested in developing a specialized engine in view of the limited orders that the Army was able
Chart 9: Power for Tanks
Comparative Power conditions for modern vehicles
| Gross weights | Tons |
| Passenger car | 2 |
| Medium tank | 34 |
| Heavy duty truck | 13 |
Source: James R. Custer, “Power for Tanks,” Automotive and Aviation Industries, Vol. 89, No. 5 (1 September 1943), p. 17.
to place.35 Consequently, the Ordnance Department had had to compromise on adapting to tank use whatever existing type of engine would most closely live up to the desired standard, and the modern tank engine had from its very beginning been an improvisation.
The Wright-Continental R-975
The engine chosen by the Ordnance Department for its prewar tanks was a radial, air-cooled aircraft model, the Continental R-670, later redesignated W-670. Practical experience had indicated that an engine of minimum length and with a cross section fitting into a square was the ideal answer to the space problem. Cooling by air similarly made for minimum bulk as well as weight, and eliminated the plumbing intricacies of water-cooled power plants. The 250 horsepower eventually delivered by the Continental W-670 after several years of modification provided a high degree of mobility for the peacetime vehicles, which up to 1938 weighed a maximum of 15 tons. Average engine life mounted to better than 500 hours before a general overhaul became necessary. In short, over-all performance was eminently satisfactory.36 But, needless to say, for the 30-plus-ton General Grants and General Shermans of World War II an output of 250 horsepower was woefully inadequate. This introduces the third element of the medium tank engine problem—weight.
On the basis of their favorable experiences with air-cooled radials in light vehicles, Ordnance designers in 1938 had looked for a more powerful engine of similar type for the new 20-ton medium tank. They finally chose the 9-cylinder, 400-horsepower Wright Whirlwind, a power plant widely used in training planes and other light aircraft. After several engineering changes, the first installation in the medium tank T5, Phase III, proved a marked success.37 The R-975, as the Whirlwind was known officially, was adopted for the recently standardized medium tank M2. But by mid-1940, proof tests of America’s newest and heaviest tanks had barely begun when events in Europe necessitated the development of an even heavier vehicle, the medium tank M3 or General Grant. With the 30-ton M3 the engine problem became acute as tests of the pilot tank at Aberdeen Proving Ground uncovered serious deficiencies of the R-975. The drawbacks of improvisation became painfully apparent. Available space was insufficient for the engine itself, for proper cooling, and for ready access to accessories. Excessive oil consumption, carburetor air temperatures, and the like, substantially lowered power output and resulted in poor performance. “The engine as presently installed,” reported Aberdeen, “is definitely underpowered. Improvements to this installation have increased the horsepower available but the H.P./Wt. ratio is still too low to give completely satisfactory performance.”38 Reports from maneuvers in the southeastern United States similarly indicated that the engine was unsatisfactory as to performance and life.39 Officers of the Proof
Department at Aberdeen recommended “that additional consideration be given to other power plants with a view to increasing the H.P./Wt. ratio as well as improving the accessibility.”40
Now, if ever, the moment had come for tackling the tank engine problem at its very roots. Funds for research and development were plentiful. But the hour was late, too late to await the completion of a basically new engine. By 1941 US and Allied troops desperately needed every vehicle that could possibly be produced. As a result, the Ordnance Department had to retain the R-975 despite its shortcomings.
Following the adoption of the General Grant and the subsequent rapid expansion of medium tank production, manufacture of the Wright engine was substantially increased. Continental Motors Corporation, the manufacturer of the W-670 engine for the light tank, began to turn out the R-975 as well. But even the increased production was insufficient to satisfy the huge demand. In fact, the President’s directive of 25 September 1941 that a production of 1,000 medium tanks per month be reached by April 1, 1942 and that this figure be subsequently further increased, perhaps even doubled, made it clear that the facilities of no single engine producer would suffice. Moreover, the Ordnance Department soon found itself competing for radial engines with the similarly mushrooming aircraft production program of the Air Forces, a fact that underscored the necessity for not only enlisting additional manufacturing facilities but also for including different types of power plants in the tank program.41 Thanks to the initiative of Ordnance engineers and the wholehearted cooperation extended them by America’s automotive industry, development of alternate engines had meanwhile progressed far enough to forestall a crisis.
The General Motors 6046 (Twin 6-71) Diesel
The first type to be ready was the General Motors 2-cycle diesel engine 6046 Twin 6-71, a modification of a commercial 6-cylinder design with which the Ordnance Department had originally experimented in its light tanks.42 The medium tank installation consisted of two of these engines joined at their fan ends by a heavy junction plate and at the flywheel ends by a double clutch housing and by a transfer unit that transmitted power to a single propeller shaft. The two clutches were operated by a single pedal, with adjustable linkage providing uniform engagement. Engine synchronization—the two units were geared together and consequently had to operate at uniform speeds—was originally accomplished by two separate hand throttles, an arrangement that later was changed to a linkage system terminating at a single throttle lever. Clutch lockout cables leading to the instrument panel permitted the driver to disengage either
engine in event of its failure, so that one might operate without the drag of the other. The margin of reserve power was generous. As the result of an innovation in fuel injection, the rated horsepower of each of the two component engines had been raised from 165 to 210. At the governed engine speed of 2,100 revolutions per minute, a medium tank on hard-surfaced roads could travel at 30 miles an hour with both, and at 20 miles an hour with only one engine in operation.
Installation of the engine in the Grant or Sherman, both of which had been designed around the shorter Continental radial, necessitated some slight modifications of the tank interior. The bulkhead of the engine compartment, for example, was cut away to allow the transfer gear case to protrude slightly into the fighting compartment. This solution avoided the weight increase that would have accompanied a lengthening of the hull. Since the heavier engine added about one ton to the weight of the tank, such a saving was highly desirable.
The pilot tank equipped with the 6046 unit arrived at Aberdeen Proving Ground on 30 December 1941, and tests continued until April 1942, when a total of 4,201 miles had been accumulated. A number of difficulties such as the failure of an injector came to light during that period, though none was regarded as serious. A special report on the then existing power plant for the medium tank stated:–
The power plant has very good performance. Mileage and cruising range are better than that of the vehicles with gasoline engines. The cooling and starting characteristics of these engines are equal to or better than those of the standard production M3 tanks. When the engine failures encountered in this new installation have been corrected, the dual 6-71 engines should make a very dependable powerplant. Most of the engine repairs can be done without removing the engines from the vehicle.43
But tests on other pilot models as well as experience with production Grants and Shermans powered by the 6046 units revealed a variety of more far-reaching defects. Most of them were attributable to faulty manufacturing, inspection, and assembly caused by the rapid expansion of production. Some, on the other hand, were due to inherent weaknesses of either the unit itself or the installation dictated by the limited space of the tank. The dual clutches, for example, were a prime source of trouble. Unless they were perfectly synchronized, one carried a heavier workload than the other and quickly failed. Since synchronization demanded almost day-to-day adjustments and troops in battle were either too busy fighting or prone to neglect this bothersome maintenance task, clutch failures were common.
Another trouble source was the air cleaners which, because of space limitations, were too small to keep dust and grit from reaching working parts and shortening engine life. There was no similar difficulty in connection with air cleaners on other tank power plants because they did not have two characteristics of the 6046 unit: diesel engines, and 2-cycle engines.
Unlike their gasoline counterparts, diesel engines consume the same volume of air at high or low speeds, and for this reason increase was comparatively simple from 165 to 210 net horsepower in the component engines of the 6046 unit. An increase in the output of the fuel injectors accompanied by a modification of the cylinder liners sufficed to obtain the higher power output. In a gasoline engine no similar
increase would have been possible even with supercharging. Though advantageous in this instance, the unvarying air consumption tended to make for a correspondingly large volume of air intake and necessitated larger air cleaners. As a 2-cycle engine, moreover, the 6046 used much more air than the other tank engines, which were of the 4-cycle type. In a 4-cycle engine the burned gases are expelled positively by a separate stroke of the piston, while in a 2-cycle engine they are blown out by a blast of scavenging air nearly 30 percent in excess of the volume displaced by the piston. A 2-cycle engine running at the same speed as a 4-cycle engine takes in air twice as often and therefore needs at least twice as much air.
Considering these circumstances, the air-cleaner problem in diesel tanks becomes readily understandable. Given adequate space, cleaners large enough to prevent engine failures due to dust could readily have been installed. Dust was no problem for the General Motors diesel when buses were driven over hard-surfaced highways, but became a serious one when that same engine powered tanks driving in convoy over dirt roads and cross country, or when tank crews failed to service the air cleaners at the required intervals. Furthermore, the fact that servicing required breaking the air duct between the cleaners and the engine created a hazard that added another, perhaps unnecessary, cause for breakdowns. For since the air cleaners were mounted directly on the engine, this operation opened a large hole into which nuts or even wrenches disappeared all too easily. Starting the engine then wreaked havoc with the blower, and the vehicle was deadlined.44
Despite its mechanical imperfections, the 6046 proved its mettle in combat. No doubt much of its popularity originally stemmed from a belief that diesel tanks were less apt to burn than their gasoline cousins upon being hit by enemy projectiles. Ordnance and Armored Force tests conclusively proved this assumption wrong by establishing that the large majority of tank fires were started not by the ignition of fuel but by the explosion of ammunition unprotected in bins. Other reasons for its preference were fully warranted. Like all diesels, the 6046 developed greater torque at low engine speeds than gasoline power plants and therefore required fewer gear changes. For plain lugging power it had no equal. In the summer of 1943, for example, the US military observer in Cairo reported that gasoline-powered tanks frequently were overspeeded and overworked in an effort to keep up with the diesel vehicles.45 A later report from the same source stated that in the opinion of drivers, repairmen, and officers of the British Eighth Army the diesel-powered Sherman was the best.46
The Chrysler A-57 Multi-Bank
Of all the wartime engine developments, the Chrysler multi-bank A-57 was the most striking example of the resourcefulness of America’s Industry-Ordnance team. Frankly an expedient for averting the threatened shortage of air-cooled radials in 1941, the A-57 came into existence within the spectacularly short time of four months.47 A hurry-up call from Ordnance
for medium tank engines reached Chrysler in early July 1941, and on 15 November the first of the new engines was completed and installed in a Grant. Test results were impressive enough to warrant its prompt adoption.48
The speed with which the A-57 came into being was made possible by the use of standard commercial parts and accessories. Rather than design an entirely new power plant requiring many months to reach the production stage, Chrysler resorted to a multiple installation of its tried and proven “Royal” six-cylinder in-line engine. Original plans called for four of these to be spaced at 90 degrees around a common crankcase, but when Ordnance insisted on more power, a unit of five engines was chosen. The output of each was transmitted to a common bull gear that drove the propeller shaft. Production models of the A-57 delivered 370 horsepower at a governed speed of 2,600 revolutions per minute.
Despite complex appearance, the Chrysler engine was rugged and, if removed from the tank, easy to maintain. No failures of the gear box were reported. The radiator, one of the most vulnerable parts of liquid-cooled tank engines, was well protected by the power unit placed between it and the fan. Accessories such as carburetors and water pumps were the same as those used in automobiles. Nonstandard parts such as ignition wire harnesses and distributors were generally made in accordance with automobile specifications. Because engine, radiator, and fan were mounted together, the power unit could be removed from the tank in one simple operation. Removal involved lifting the engine cover, disconnecting the propeller shaft, two air-cleaner ducts, two exhaust pipes, the gasoline and oil lines, a single plug for all wires, the choke, throttle, and tachometer, and finally unscrewing three mounting bolts. A crew of four required only one hour for the entire process. Placed on a dolly, the unit could be repaired, tuned and tested, and then put back into the tank.
Since the A-57 was longer and heavier than the Continental radial, some structural changes in the tank were necessary. The hull had to be lengthened by some 11 inches; the rear bogie and idler of the track suspension had to be moved toward the rear to correct the imbalance resulting from the addition of roughly three tons of armor plate and excessive engine weight. So modified, however, the Chrysler-powered tank turned out to have slightly lower ground unit pressure than other versions. Many tests proved that the engine provided greater drawbar horsepower than competing gasoline types. The usual experience was that the A-57 could pull its vehicle in one gear higher.
Aberdeen Proving Ground began testing the multi-bank Grant in February 1942. General operation of the engine was satisfactory and the installation furnished ample power for the vehicle. But when the engine was disassembled following the test, inspection revealed that overheating had caused two piston rings to stick and had damaged the connecting rod bearings and inserts. Installed in the tank, moreover, the component engines and accessories were difficult to get at because the compartment, originally intended for the 45-inch-wide Continental radial, was badly crowded by the 55-inch-wide Chrysler unit. That the light automobile accessories
and materials failed to stand up under the extreme heat and heavy vibration in the tank aggravated the problem considerably.49 Subsequent tests of a Sherman tank with the Chrysler engine showed similar results, and the conclusions and recommendations of the proof officers read:
Conclusions
1. On the basis of this test it may be concluded that:–
a. The engine furnishes adequate power for the Medium Tank M4A4 but continual failures of a minor nature make maintenance of this vehicle extremely difficult.
b. The maintenance of the Multi-Engine power plant is difficult due to the inaccessibility of the parts requiring constant attention. This condition is caused by an extremely crowded engine compartment.
c. The water pump and generator drive belts are subject to overload and resultant failure which may result in serious damage to the engine due to faulty cooling.
d. The ignition system is especially complicated and troublesome.
e. The engine cooling radiator is easily clogged with dirt and foreign particles which seriously interfere with cooling of the engine. …
f. The operation of the vehicle in general is satisfactory. Due to the added power of the engine, the driving effort for this vehicle is less than that required for Medium Tanks powered with Wright radial engines.
Recommendations
1. On the basis of the tests conducted on this vehicle it is recommended that:–
a. Considerable redesign be initiated in order to make the parts requiring constant attention and minor adjustments more accessible so it will be possible to work on these parts in a minimum of time with the engine installed in the vehicle.
b. The five individual water pumps should be replaced by a single, large, shaft driven pump. This change would eliminate the necessity of having five water pump drive belts as well as clean up the engine in general.
c. The electrical system be made more reliable by installing heavier, dust proof, connections in all wiring. Particular attention should be paid to the distributor caps and similar parts whose successful operation depends on porcelain or plastic dielectrics. These parts should be made heavier so the rough treatment they receive in combat vehicles will not cause them to crack or break.
d. The engine radiator be made more accessible to facilitate easier cleaning of this unit. To accomplish this it is believed that a redesign of the fan shroud could be made whereby the entire radiator could be easily reached by an air or steam hose.
e. The cylinder heads on #3 and #4 engines be held in place by means of cap screws instead of studs in order that the cylinder head gaskets could be replaced without the necessity of removing the whole power plant as is now the case.
f Extensive redesign should be made in an effort to reduce the weight of the multi-engine power plant. It is believed that the concentration of weight in the rear of the vehicle shortens the life of various suspension parts.50
Except for the weight problem, none of the reported difficulties proved insurmountable. And had aluminum been used as extensively on the A-57 as it was on the other tank engines, that objection, too, could have been overcome. The belt-drive troubles were corrected by using a single gear-driven water pump and placing the generator in a cool spot on the floor, where it was driven by a belt from the propeller shaft. Accessibility was improved by raising the five carburetors to a line above the engine and moving the thermostats from the cylinder heads to fittings in the top
tank of the radiator. All modifications completed, the distributors, thermostats, fuel pump, water pump, and oil filters were readily serviced through the rear doors of the engine compartment. The spark plugs on the bottom banks were accessible through openings in the floor. But before all these improvements had taken place, experiments on other engines had been started.
In October 1942, five Chrysler-powered Grants and an equal number of Shermans underwent tests by the Desert Warfare Board. The board was greatly impressed by the pulling power of the engine, but found that breakdowns of accessories made it unreliable. Neither tank was found suitable for combat. Doubtlessly influenced by this verdict, a conference of representatives of the Allies in early November decided that the M4A4 was not acceptable. Other influences responsible for that decision were probably the facts that the trend of operations in the European and African theatres had allowed a considerable reduction in the medium tank program for 1943 and that the passing of the engine crisis had made it more feasible for the Ordnance Department to limit the number of engines. Nevertheless, 7,500 Chrysler-powered Shermans were built, and after several improvements the engine ultimately gave a remarkable account of itself.
Gasoline versus Diesel
While before 1942 the Armored Force had favored all-out dieselization, it completely reversed its stand during the early months of that year. The turning point came on the occasion of a conference on 26 February for the purpose of reviewing the fuel problem and its impact on the standardization of engines. A thorough discussion brought out a number of maintenance and supply difficulties likely to be encountered on the battlefield if diesel as well as gasoline-powered equipment were used. Adequate quantities of diesel fuel would be available in the United States but not in most theatres of operations. The majority of vehicles were gasoline-powered, and the use of diesels would necessitate a duplicate supply system. General Barnes and Colonel Christmas, the representatives of the Ordnance Department, stated that the time for deciding on any particular type of power plant had not yet arrived, that by the end of another year the Ordnance Department could, if necessary, supply an entire line of either diesel or gasoline engines, but that in the meantime it would have to resort to the maximum number procurable of both in order to assure the success of the tank program. General Devers, representing the Armored Force, declared that the Armored Force would not push the requirement for diesels and would not require diesel engines for various combat vehicles. “This represents,” noted General Barnes, “a complete ‘about-face’ for the Armored Force which up to this time has been demanding that the Ordnance Department use diesel engines 100% for all combat vehicles and tanks.”51
Following the conference General Somervell, Assistant Chief of Staff, G-4, advocated a stringent curtailment in the use of diesels by US forces. He listed his reasons for such a policy as follows:
1. The Chief of the Armored Force has recommended that diesel engines be used in
all equipment of the Armored Force. This, of course, will greatly complicate the fuel supply problem ... the maximum production of diesel fuel required will interfere to some extent with production of aviation gasoline, toluene for explosives, and butadiene for synthetic rubber. It will also result in a large surplus of gasoline with no outlet.52
A directive from the War Department issued a week after General Somervell’s memorandum adopted his recommendations virtually verbatim. All wheeled and half-track vehicles were to be gasoline powered. For tanks, development of gasoline engines should be pushed in order to supplant all diesel-powered tanks as quickly as possible. The latter were not to be shipped overseas but held for service in the United States and for training. Use of the Guiberson diesel engine was to be discontinued as soon as other types were available.53
This final decision marked a particularly sharp reversal of previous trends. The Guiberson engine, a radial, air-cooled diesel, had been in use in American light tanks since 1935.54 A more powerful model developing 370 horsepower had been under development for medium tanks since 1938.55 After testing this engine, the Armored Force Board in September 1941 had recommended that it be considered more suitable for use in medium tanks than the Continental radial and that as many medium tanks as possible be equipped with it. On the strength of that recommendation an entire new plant had been built at Garland, Texas. But during the conference of February 1942 General Devers announced that he wanted no further orders placed for Guibersons because they had been found unreliable. In April he wrote to General Somervell, now Commanding General, Services of Supply, stating:
In view of our past experience, and the present world situation as to the supply of Diesel fuel, the Armored Force does not desire any type Guiberson Radial Diesel engines for use in Light or Medium Tanks. It is requested that action be taken at once to discontinue the production of Guiberson Radial Diesel engines, and that existing facilities be utilized to increase the production of other standard types of tank engines.56
General Barnes vigorously dissented, outlining the position of the Ordnance Department in the gasoline versus diesel controversy:–
... [General Devers] makes reference to the present world situation as to the supply of diesel fuel as a reason for discontinuing the manufacture of Guiberson diesel engines. Reliable information available to the Ordnance Department does not indicate any difficulty in obtaining diesel fuel in any theaters of operation where gasoline may be obtained, and diesel fuel has a number of obvious advantages over gasoline. In fact, the Ordnance Department considers the diesel engine the proper ultimate engine for tanks and believes that every effort should be made to expedite the development of adequate diesel engines for all tanks. It should be noted in passing that our Army as well as the armies of other United Nations employ large numbers of track laying tractors of commercial origin which tractors are equipped with diesel engines; these tractors are being procured for the Corps of Engineers, the Air Corps, Field Artillery and other branches of the Army. Gasoline propelled commercial tractors are to
all practical purposes unobtainable. Further, both the British and the Russians are using and are obtaining from the United States tanks and other vehicles employing diesel engines.
With respect to the Guiberson Model T-1400 Radial Diesel Engine for the medium tank, this is a new development type engine of which 670 only are on order. The cancellation of this project would therefore have but slight effect on the 1942 production of medium tanks, however, since the War Department has already invested a considerable sum of money in this project, it is recommended that this project be allowed to carry on for several months until a reasonable quantity of these engines have been installed in tanks and given an extended test in the hands of troops. This is considered to be in the best interest of the Government and should have no appreciable effect on our tank program. It will be recalled that it was in the summer of 1940 that the Chief of the Armored Force requested that all tanks for the Armored Force be equipped with Guiberson diesel engines, at which time the Ordnance Department took the position that this engine was not well enough developed to warrant such a decision and the decision was accepted to continue with the production of the Guiberson diesel engine in a limited way to further its development. This is the policy which has been followed since that time.
In any case, the development of new tank engines such as the Guiberson diesel tank engine is a matter of years and since the Ordnance Department considers that a diesel engine for tanks should be developed, we believe these projects should be allowed to continue in their present status until further data are available.57
Nevertheless, in the summer of 1942 production of Guiberson diesels was canceled and the plant at Garland turned over to the Continental Motors Corporation for the manufacture of R-975 gasoline engines. Production of the General Motors diesel continued, in order to fill British and Russian requirements which at the time amounted to two thirds of America’s tank output. While development work on diesels for medium tanks proceeded apace, none of the projects—an 18-cylinder Guiberson radial and a 9-cylinder radial were to be put into production, since in the meantime a satisfactory gasoline engine, the Ford GAA, had made its appearance and development of other gasoline power plants had shown sufficient promise to warrant abandonment of diesels.
The Ford GAA
The Ford GAA tank engine had its origins in a development by the Ford Motor Company for the Air Force. Realizing the need for an aircraft engine with nominal displacement but higher specific output than existing designs. Ford in 1939 initiated development of an upright 60-degree, V-12 liquid-cooled power plant with a displacement of 1650 cubic inches.58
Design studies began in the summer of 1940, and by November of that same year a 2-cylinder model was fully assembled and ready for testing. There followed extensive investigations of combustion-chamber design, bearing materials, and engine timing in an effort to obtain peak performance. Within three months the output was raised from 115 to roughly 150 horsepower, much better progress than could have been made with a multi-cylinder model. August of 1941 saw a full-scale 12-cylinder engine mounted on the test stand, ready for a trial run.
Knowing of the good results obtained with the 2-cylinder model, and urgently needing a high-output engine for its 30-ton
medium tanks, the Ordnance Department approached Ford with a proposal to develop a power plant to take the place of the underpowered R-975 radial. After a survey of the medium tank’s engine compartment, Ford engineers suggested that shortening their engine from twelve to eight cylinders would permit the early production of an adequate power plant necessitating a minimum of alterations to the existing hull. Design and layout of the proposed engine commenced in September 1941, and by January of 1942 the first model was completed.
The new tank engine consisted of five major assemblies: the cylinder block and crankshaft, including pistons and connecting rods; the cylinder heads; the accessory drives; the end cover; and the oil pan assemblies. Each of the assemblies constituted an integral unit and was arranged for easy handling and replacement by soldier mechanics. Extensive use of aluminum accounted for light weight. The cylinder block and crankcase, for example, the bulkiest and therefore heaviest components of every engine, were made of one single aluminum casting. Aside from the weight saved and the ease of handling, this one-unit treatment also resulted in structural rigidity in absorbing the high loads and stresses of the engine without undue distortion. Pistons, cylinder heads, the end cover, the oil pan, and the camshaft cover likewise were aluminum castings. The exhaust manifolds, ordinarily made of cast iron, were made of two stainless steel stampings welded together—another weight-saving feature. Installed, the engine weighed only 1,575 pounds, 2,825 pounds less than the hitherto lightest-weight liquid-cooled power plant for medium tanks—the General Motors diesel—and only slightly over 200 pounds more than the air-cooled Continental radial. The Ford GAA’s output of 450 net horsepower was the highest developed by any of the medium tank engines mass produced during World War II. The Continental radial developed 400, the General Motors diesel 375, and the Chrysler multi-bank 370.59 Because of the high horsepower-to-weight ratio, high output, and compactness of the Ford engine, the Ordnance Technical Committee in January 1942 authorized its use as an alternate power plant for the Sherman tank.60
The first three vehicles of the new series were completed in June and immediately
underwent tests at the General Motors Proving Ground. Proposing minor modifications and further testing, the Proving Ground recommended that the Ordnance Department accept the vehicle. Equally favorable reports resulted from subsequent trials. Beginning in November 1942 the Armored Force conducted 24-hour-a-day operations of seventeen tanks and upon their conclusion stated that the engine was sufficiently satisfactory to warrant further development. At the same time a series of endurance tests was conducted at Aberdeen Proving Ground and, under the supervision of Ordnance engineers, by Chrysler and General Motors. Nothing more strikingly illustrates the cooperation of the Industry-Ordnance team than the fact that here two competitors were actively engaged in furthering the cause of a third.
Although first results revealed that engine life was unsatisfactory, continuous improvements eventually remedied the most objectionable faults. Structural
weaknesses in the crankcase, for example, were overcome by increasing wall thickness and ribbing sections and by changing the design of the main-bearing caps. When the rigid crankcase caused fatiguing and breaking of the crankshaft, a development program was instituted for evolving a sturdier crankshaft by combining various formulas of steel-making and heat treatment. In July 1943 the proving center of Aberdeen Proving Ground, reporting that the Ford GAA engine was very satisfactory for medium tanks and, because more accessible, more easily maintained and serviced than other medium tank power plants, recommended that:
a. The Ford model GAA engine be approved for production as a power plant in the medium tank in such quantities as are deemed necessary to supply demands of the present and immediate future.
b. The necessary corrective modifications be placed on the production units as soon as possible.
c. Further development work be carried out on this engine with a view to increasing its mechanical reliability.61
By the summer of 1943 the Ford engine was well on the way to becoming the standard power plant for medium tanks. Following a comparative endurance test of Sherman tanks powered by the several engines in current use, the Armored Force Board had in May of that year found the Ford to be the best and had recommended its adoption for all medium tanks. Although the outcome of an endurance run at Aberdeen Proving Ground between October 1943 and February 1944 was not wholly so favorable, the Ford engine performed more satisfactorily than the Continental radial, its only serious rival. The General Motors diesel was out of the picture for two reasons: first, because of the ban on the use of diesels overseas, and, second, because its endurance qualities were unpredictable and its reliability the lowest of all engines tested. Employment of the Chrysler multi-bank unit had long since been vetoed, although, interestingly enough, it was found to exceed all others in reliability as well as in economy of oil consumption and in maintenance. Endurance of the Continental was unsatisfactory, particularly under full-load operation over hilly terrain. Overspeeding the engine to brake the vehicle while descending hills was responsible for four out of five major breakdowns.
While the endurance qualities of the Ford GAA were not considered satisfactory because none of the test engines met the 400-hour requirement, its performance before failure was excellent and all failures were similar. Correction of two basic defects—burning out of exhaust valves and breakdown of cylinder-head gaskets offered prospects of an immediate increase in engine life. Accessibility was equal or superior to other engines tested; fuel consumption was lowest of all gasoline-powered plants; maintenance requirements were lowest and rose only after the valves and head gaskets began to give trouble. All told, the Ford GAA presented better possibilities of immediate improvement than any other engine tested.62
The service record of the Ford engine fully justified the expectations of its superior performance. Continuous modifications of design weaknesses ultimately resulted in a power plant that was by far the most popular with the men on the battle fronts. Reports from Europe, for example,
noted that the engine “served well and reliably during combat operations. Using personnel preferred this engine to the air cooled radial type engine because of its higher horsepower and torque outputs.”63 Had production capacity been great enough, the Ford engine would unquestionably have been adopted as the one and only power plant for American medium tanks.
Flotation for Tracked Vehicles
In the early 1930s when Ordnance engineers faced the problem of increasing the cross-country mobility of combat vehicles, they turned more and more to the use of tracks instead of wheels. Tracks not only enabled tanks and gun motor carriages to cross ditches and pass over other obstacles that stopped wheeled vehicles but also, because of greater ground contact area, provided more support in mud and sand. The support, or flotation, provided by tracks came to be of particular importance as the weight of tank armor and armament increased beyond the capacity of wheels to support it on soft ground.
Design of tracks for the light-weight, slow-moving tanks of World War I had been a comparatively simple matter, but during World War II, as more powerful engines and improved suspensions increased the speed and cruising range of track-laying vehicles, development of suitable tracks became extremely difficult. When designers thought in terms of producing the ideal all-purpose track, they found themselves confronted with a number of irreconcilable requirements. Smooth operation on the highway, for example, could be achieved only at the cost of drastically reduced traction in mud and on rough ground. Wide, cleated tracks, which provided the most flotation and traction for off-highway operations, had many features that made them undesirable for general use: they caused more noise and vibration and added to steering difficulties; they increased the wear on the suspension mechanism; and they offered more resistance to movement, thus adding to the already great burden on the tank engine. Ordnance engineers constantly strove to develop a light-weight track, but they could not save on weight at the cost of durability, for the track had to be strong enough to support the vehicle, take the severe pounding of cross-country travel, and withstand the impact of gun fire and mine explosions. Throughout World War II four aspects of track development were of primary importance—the demand for tracks wide enough to carry heavy loads under all circumstances, the need of better traction, the conflicting requirements for steel and rubber tracks, and the search for adequate, inexpensive track pins.64
Wide Tracks and Extension Devices
The theoretical solution to the problem of providing adequate flotation for tanks and other track-laying vehicles operating on soft ground was the essence of simplicity: make the tracks wider. Widened tracks distributed the weight of the vehicle over a larger area and thus lessened the pressure exerted on each square inch of
ground. But the use of wider tracks raised a host of thorny problems of design. They made steering more difficult, required the use of dual bogie wheels, and made necessary the adoption of altogether different suspension systems. Further, as the Ordnance Department reported in the fall of 1944, tracks wide enough to bring the ground pressure down to seven pounds per square inch, as requested by the Army Ground Forces, “will result in an overall vehicle width in excess of the 124-inch maximum shipping width now permitted under AR 850-15.65 Nevertheless, during the winter of 1942–43 when observers in North Africa reported that American tanks had run well in sand but had bogged down in the mud of Tunisia because their tracks were too narrow, Ordnance engineers promptly made the development of wider tracks a high-priority project.66 At that time the tracks of the Sherman tanks were 16.5 inches wide, with a ground pressure of 14 pounds per square inch. Their immediate replacement by wider tracks was impossible, not primarily because of any difficulty in designing a wider track, although that presented its problems, but because a wider track would not fit existing suspensions. Because of the problems involved in redesigning the whole suspension system and making the production changeover, tanks with wide tracks did not become available until late in 1944.67
While tanks with new suspensions were under development, Ordnance engineers produced, as temporary expedients, extension devices known formally as extended end-connectors, informally as duck bills. These were short metal plates that could be bolted to the outside end of each track block to widen the track of the Sherman from 16,5 to approximately 20 inches. They increased flotation by 21 percent and brought the ground pressure for vehicles weighing 35 tons down to less than 12 pounds per square inch.68 They could be attached to the tracks of any medium tank in the field without undue difficulty and with a total weight increase of only 350 pounds. By the spring of 1944 these extension devices, packed in kits, were available for shipment to overseas theatres to be installed for specific operations at the discretion of the theatre commanders. In October 1944 the Army Service Forces directed that all narrow-track medium tanks shipped overseas be equipped with extended end-connectors, and at the same time theatre commanders were notified that similar extension devices were available for the M5 light tank, increasing its track width from 11.5 to 15 inches.69
To develop extended end-connectors for attachment to the outside of tank tracks was not a particularly difficult task, but attaching them to the inside of the track was a different story because there was no room for them between the track and the tank hull. Pending development of tanks with narrower suspensions, Ordnance engineers devised so-called outboard spacers to hold the suspension of the Sherman several inches out from the hull to make
Track extensions being installed by the crew of a Sherman tank during a lull in combat operations at Baesweiler, Germany
room for attachment of extended end-connectors.70 When the extension devices were attached to both sides of the track they gave it a width of approximately 23.5 inches and a unit ground pressure of only 10 pounds per square inch, as compared with its original 16-inch width and 12-to-14-pound pressure. The speed with which they could be attached and the length of time required to get new tanks with wide tracks into production led the Ordnance Committee to approve installation of extended end-connectors on both sides of tracks in so far as castings and other critical components were available. Although the Ordnance Department considered the installation of the spacers and inside track extensions in the field impractical, it approved, at the request of Army Ground Forces, procurement of kits that would permit field installation.71
A further application of the extension principle was approved in January 1945. It was an extended grouser, nicknamed the platypus, that could be bolted to
extended end-connectors to improve both flotation and traction. The term grouser was used to describe either detachable cleats, which could be fastened to the track to provide more traction, or the tread design on the track block. In the latter sense it was termed an integral grouser. The 32.5-inch grousers, which extended from the inside of the track block outward beyond the extended end-connectors, required no modification of the suspension and gave the tank a ground pressure of approximately 8 pounds per square inch. Longer grousers, 37 inches in length, were also developed for attachment to Sherman tanks equipped with spaced-out suspensions and extended end-connectors on both sides of the track. The long grouser brought the ground pressure down to 7 pounds, the figure that the Army Ground Forces had earlier fixed as the maximum for effective operation. Both the long and the short grousers could be installed in the field. Both were approved for production early in 1945.72
Late in 1944 tanks with newly designed suspensions and wide tracks came off the production lines. The new Sherman, the M4A3E8, had a 23-inch track and a horizontal volute suspension. The light and heavy tanks both now had torsion bar suspensions, and tracks 16 inches and 24 inches wide, respectively. One of the major design changes necessitated by adoption of the new suspensions and wide tracks was the use of dual bogie wheels. Although this change doubled the number of bogie wheels required for each tank, it distributed the load on each wheel more evenly and resulted in longer wheel life. By the time these tanks saw service in Europe, experience had shown that a track 23 or 24 inches wide was not wide enough to keep a tank from bogging down in deep mud.73 Extended end-connectors on the old-model medium tanks had widened the track to 23.5 inches, but the use of extended grousers to provide additional width had proved necessary. To make the new wide-track tanks serviceable in deep mud the only answer was repetition of the earlier process of adding extension devices. In March 1945 the Ordnance Committee approved production of kits for field installation of 24-inch grousers on the 16-inch tracks of the light tank M24. These grousers added 3,000 pounds to the weight of the vehicle but lowered the ground pressure to about 8 pounds per square inch. By mid-July even longer grousers, 28 inches in length, had been developed for the M24, and kits to permit their installation were available but did not arrive overseas in time for combat use. V-J Day found the 39-inch extended grouser for the wide-tracked M4A3E8 and M26 tanks still in the development stage.74
Track Profiles
While developing wider tracks to increase flotation, Ordnance engineers were also concerned with designing tracks with sufficient traction to keep the vehicles going in deep mud and on icy roads. In 1940 all American tanks rode on smooth rubber-block tracks which, in addition to
being shock absorbent and long lasting, gave adequate traction on hard-surface roads under normal conditions. When greater traction was needed for muddy roads and cross-country operation, tank crews put on detachable steel grousers, much as a motorist would put chains on his car. These detachable grousers could be bolted to the track blocks and then be removed when the tank returned to the highway. As a rule, they were not put on every track block but only on every second, third, or fourth block, depending on the amount of traction needed and the number of grousers available. The combination of smooth rubber-block tracks and detachable steel grousers was highly satisfactory in most respects, but it had one major disadvantage: the installation and removal of the grousers was difficult and time consuming. In cross-country operations, vehicles frequently encountered so many different types of terrain that it was impractical to install and remove the grousers at every turn.75
In 1941 Ordnance engineers attempted to solve this problem by developing the so-called rubber chevron track with a V-shaped tread on each block to give increased traction. To a remarkable degree these tracks combined the good highway performance of the smooth-block rubber track with much of the traction of the detachable steel grouser. Although tanks equipped with this type of track also carried detachable steel grousers for use on extremely soft ground, the grousers were used so little in North Africa by tanks with either smooth or chevron rubber tracks that the Army Ground Forces in 1943 recommended that tanks no longer be required to carry them.76 The rubber chevron track came closest to meeting the needs of the armored units in North Africa, but just as its development was completed early in 1942 the rubber crisis forced Ordnance to search for a substitute.
Well before Pearl Harbor experiments were under way with steel tracks as possible alternatives to rubber tracks. Most of these early steel tracks had cleats, or integral steel grousers to provide traction in mud. In comparison with the steel tracks developed later, these early types were regarded as flat tracks because their grousers were quite shallow. Deep grousers were not provided in 1941 and 1942 for two reasons: shallow grousers afforded sufficient traction for most purposes, and deep grousers were too hard on the suspensions and caused too much resistance to movement. Toward the middle of the war when Allied armored forces had to contend with deep mud and swamps, steel tracks with deeper grousers to provide more traction were adopted in spite of their disadvantages in other respects. The designs used for the grousers on steel tracks included parallel bars, interrupted parallels, and chevrons, not unlike the tread designs on truck tires. Generally, the armored forces preferred the chevron design, but all types remained in use throughout the war because of the production difficulties in changing over to chevron only.77
When American tanks encountered deep snow and ice, crews found smooth-block rubber tracks and steel tracks of all kinds totally unsatisfactory.78 As tests by the Ordnance detachment at Camp Shilo in Manitoba, Canada, during the winter of 1942–43 had demonstrated, steel tracks were particularly unsatisfactory on icy roads. Designed for use in mud, not for operation on hard, slippery surfaces, flat steel tracks as a rule failed to cut into the ice. When sharper steel grousers with parallel-bar design were used, they offered no resistance to sideslipping and were sometimes derisively referred to as ice skates. The steel track with chevron design was more effective in preventing sideslipping, but the occasions when it cut into the ice were so infrequent that the track was of little use. The rubber chevron track gave by far the best traction on ice but, because of the loss of natural rubber imports and difficulties in producing chevron tracks of synthetic rubber, most tanks were not equipped with this type of track. In the absence of a suitable track for operation on ice, units in the field resorted to improvisation. Some welded small sections of steel bar to the running surface of the track blocks to form ice cleats; others welded sharp steel spikes to end-connector wedges; still others substituted a rubber track block for every sixth steel block and attached a steel grouser on the block mid way between. The war ended before a satisfactory track or detachable grouser for operation on ice was developed.79
Steel Versus Rubber Tracks
Throughout World War II the relative merits of steel and rubber tracks constituted one of the most controversial aspects of the track development program. From 1940 to 1945 the steel track and the rubber track competed for favor; each had its loyal supporters and each could claim superiority under certain conditions. But no track, it should be noted, was made entirely of steel or entirely of rubber. Rubber track blocks were molded on a steel framework consisting of two tubes, or binoculars as they were often called, that extended horizontally through the block to hold the track pins. In both steel and rubber tracks, rubber bushings were used on the track pins to absorb vibration and reduce wear, and were credited with giving American tracks a much longer life than enemy tracks.80 Early in 1944 steel tracks with rubber backs to cushion the shock on bogie wheels came into use, for experience had shown that all-steel blocks materially shortened the life of the running gear.81
Before the United States entered the war, particularly during 1940 and 1941, the smooth rubber track dominated the field. In those years all US Army tracked vehicles rode on rubber because it had smooth riding qualities, did not damage highways, provided adequate traction on hard-surface roads, and, by cushioning shocks, added many miles to the life of bogie wheels, support rollers, and other suspension components. Steel tracks gave
better results in mud than did smooth rubber tracks, but caused more vibration, added to steering difficulties, and increased resistance to movement. Steel tracks were less desirable in terms of weight, always an important consideration in Ordnance design, for they weighed about 25 percent more than rubber tracks. As far as traction in mud was concerned, development of rubber chevron tracks in 1941 partly evened the score; when detachable steel grousers were used, a vehicle equipped with rubber tracks could plow through mud as well as one equipped with steel tracks.82
The loss of rubber imports after Pearl Harbor forced a revision of plans for production. As synthetic rubber was not yet available in quantity, nor expected to reach mass production for another year, the only solution was adoption of steel tracks. In January 1942 the Under Secretary of War directed the Ordnance Department to discontinue the use of rubber for tracks at the earliest possible date, and in June the Chief of Ordnance reported that development work on steel tracks had progressed sufficiently to permit changing over from rubber to steel before the end of the year.83 But by March 1943 the changeover had not yet been made, and General Campbell reported that large quantities of natural rubber would still be required for tank tracks during the last quarter of 1943. He described some of the difficulties of converting to steel tracks as follows:
The Ordnance Department has been intensively working on the development of steel tracks for light and medium tanks to replace the present rubber block track as a conservation matter. ... Success has been attained in building steel tracks which will have satisfactory life, but the principal difficulty has been the destructive effect of the steel track upon the running gear of the tank. Our tanks are noted abroad for their sturdiness, reliability and their ability to keep going under adverse conditions. An important factor in this result has been the use of rubber tracks. It is believed that the tank mechanism, especially the suspension system, in time can be changed to withstand the beating which it receives from the steel track. The time required to make the tank equipped with steel tracks the equivalent in reliability to the present tank equipped with rubber tracks, however, cannot be accurately predicted.84
In view of the excellent performance of rubber tracks in North Africa and the difficulties encountered in developing satisfactory steel tracks, the Ordnance Department recommended in the early part of 1943 that steel tracks be abandoned altogether and that all tanks be equipped with rubber tracks. After thorough study of the Ordnance proposal, General Minton, director of the ASF Resources and Production Division, agreed that henceforth all tanks destined for shipment overseas should be equipped with rubber tracks. “This opinion is concurred in by everyone with whom I have discussed the question.” wrote General Minton. “General Devers will back it 100%. …” At the same time, because of the shortage of natural rubber, ASF directed Ordnance to push its synthetic rubber track program to a conclusion as soon as possible and to put steel tracks on tanks to be used in the United States for training.85
No sooner had these policies been established than demand arose for their reversal. First came a strong appeal from the Armored Force for the elimination of steel tracks on training vehicles because the steel threatened to destroy the paved roads over which they ran. ASF, promptly acceding to this request, directed Ordnance to supply rubber tracks for all tanks, whether for service overseas or in the United States, and to cancel all contracts for steel tracks.86 This policy had not been in effect more than a few months when strong criticism of rubber tracks, and an appeal for steel tracks, came from troops fighting in Sicily and Italy. Armored forces in Italy reported that, during operations on rocky mountain roads, holes were frequently gouged in rubber tracks, reducing their average life to less than 500 miles.87 Under these circumstances, field commanders requested that rubber tracks be replaced with the more durable steel tracks in spite of the disadvantages of more difficult steering, greater wear on bogie wheels, and reduction of speed. In view of this situation, ASF authorized Ordnance to ship steel tracks from existing stocks when requested by overseas commanders, and to resume steel-track production.88 In the meantime, the rubber industry had developed a rubber-backed steel track that overcame many of the drawbacks of the all-steel track. New steel-track production in 1944 consisted largely of this improved type.89
During the early part of 1943 the rubber industry had succeeded in producing a satisfactory smooth-block track of synthetic rubber but had not been able to make an acceptable synthetic chevron track. In August ASF directed Ordnance to limit its production of synthetic rubber tracks to the smooth-block type but to continue its efforts, in cooperation with industry, to develop a suitable synthetic chevron track. Although the national stockpile of natural rubber was running low, Ordnance requested permission to continue production of chevron tracks of natural rubber since combat forces had found them superior to all other tracks. It was while this request was under consideration that troops overseas reported difficulties in using rubber tracks on rocky terrain and asked for steel tracks instead. In December 1943 ASF headquarters, recognizing the futility of over-all directives prescribing the type of track to be issued, granted the Chief of Ordnance authority to manufacture both steel and rubber tracks and to issue whatever type he and the using arms jointly determined to be most suitable for specific operations.90
Track Pins
Still another important factor in track design was the type of connections used to assemble tracks.91 In World War II American tanks were the only combat vehicles that rode on tracks assembled with rubber-bushed track pins. These pins had been
developed during the early 1930s as a replacement for plain steel, or “dry,” pins, which performed adequately on low-speed tractors but wore out rapidly on higher-speed combat vehicles. “Doughnut” rubber-bushed pins, rubber rings vulcanized to steel track pins and then inserted into track blocks under pressure, gave rubber-bushed tracks several advantages over those with plain steel pins. Tractive resistance was materially less, especially at speeds over 20 miles an hour, and under heavy loads power loss was proportionally smaller. Track life was longer because the bushing itself absorbed vibration and so prevented wear, and, finally, noise was greatly reduced. The diameter of the track pins and the thickness of the bushing were varied to give the strength required for each vehicle.
The high manufacturing cost of doughnut rubber-bushed pins prompted further research. In the summer of 1941 Ordnance engineers carried on experiments with sleeve or Harris bushings that were nearly as long as the pins.92 This design was expected to be cheaper to manufacture, provide more uniform stress in the rubber, and have higher strength under load and deflection. But, because the rubber filled the tube completely, there was insufficient room for compression, and continuing deflection caused the bushing to disintegrate. Consequently, American tracks throughout the war used only the doughnut rubber bushing. The Germans and Japanese employed all-metal pins but encountered problems of enormous proportions in keeping their vehicles supplied with tracks. Life of enemy tracks was approximately 600 miles, in contrast to 3,000 miles for many American tracks. At the end of the war Ordnance designers were continuing unabated the search for a dry steel track pin as efficient as the expensive rubber-bushed pin.93
The double-pin method of track block construction was used exclusively until late in the war when single-pin steel tracks were introduced on the M18 gun motor carriage, the M24 light tank, and certain wide suspension tanks of the M4 series. While lighter in weight and cheaper to manufacture, single-pin tracks were difficult to disassemble in the field, made more frequent adjustment of track tension necessary, and had shorter life than the double pins since the angular movement that had to be taken up by the track-pin bushing was approximately twice that required if two pins were used. The single-pin tracks, tried on the wide suspension M4A3E8 tank when first used in the ETO early in 1945, gave such inferior service that they were quickly replaced by the double pin.94 But single-pin tracks for the M18 gun motor carriage and the M24 light tank gave less trouble and remained in use.95
By the end of 1943 one fact had become clear: no single track could meet all
combat requirements, Best results could be achieved only by employing several types, each designed for a particular purpose. Smooth rubber tracks were best for fast travel over good roads; steel or rubber tracks with detachable steel grousers were best in mud; rubber chevron tracks gave the best traction on ice; on rocky terrain steel tracks lasted longer than rubber. Development could not be limited to pursuit of the ideal all-purpose track but had to be spread out over a wide field, including tracks made of steel, natural rubber, and synthetic rubber, and embodying various designs to improve traction.96
Flotation for Wheeled Vehicles
The major problem of World War II in providing flotation for wheeled vehicles lay in developing suitable synthetic rubber tires to replace natural rubber. This task became imperative when crude rubber imports from the Far East were cut off by the Japanese after Pearl Harbor. The ensuing shortage became so critical that only rigid conservation and rapid development of synthetic tires could stave off collapse of both civilian and military wheeled transport.97 The vast majority of tires for military wheeled vehicles were standard commercial tires of the highest quality obtainable—whether of synthetic or natural rubber, or a mixture—and most had a modified tread to give increased traction for off-road travel. But for two particular purposes tires had to be specially designed. First, to enable combat vehicles to travel some distance after tire deflation by puncture or gun fire, tires had to have extra strong shoulders and sidewalls. And second, low-pressure, high-flotation tires had to be developed for traversing soft ground.
Combat Tires
Long before World War II the Ordnance Department had begun the search for tires proof against gun fire. Experiments in the 1930s with sponge rubber fillers inserted in standard casings and with bullet-sealing (self-sealing) tubes proved the former generally unsatisfactory. The sponge fillers, while puncture proof and impervious to small arms fire, were difficult to mount, apt to develop flat spots when a vehicle remained idle for a few days, added considerable weight, and were subject to blowouts resulting from the heat generated by the semisolid fillers.98 Bullet-sealing tubes, on the other hand, added negligible weight and rode as well as ordinary pneumatic tires but had unreliable bullet-sealing qualities. This type of tube could seal a hole two inches long without loss of more than 60 percent of initial air pressure in the tube, but could not seal the larger holes caused by projectiles 20-mm. or larger or by small caliber bullets tearing through the tube longitudinally. Furthermore, the plastic coating inside the tube hardened at extremely low temperatures, losing its effectiveness, and under ordinary conditions the heat generated within the tire sometimes caused the plastic to flow to such an extent that the wheels became unbalanced. Despite these limitations, the Ordnance Committee in the spring of 1940 approved bullet-sealing tubes for pneumatic-tired combat vehicles. A year later changes in tube composition gave somewhat improved performance.99 In addition to work on bullet-sealing tubes and
modified solid rubber tires,100 engineers in 1940 began experimenting with standard commercial pneumatic tires equipped with beadlocks, devices designed to prevent deflated tires from creeping on the rim. To use beadlocks, the tires had to be mounted on special divided rims. Since tests showed that standard tires lacked the strength to support the vehicle for more than a short distance after a road hazard or gun fire had deflated the tube, attention centered on developing tires with strengthened sidewalls and shoulders. The stronger tires with beadlock devices, called combat tires, had many more plies than a standard tire of the same size and also interliners of highest quality rubber, which added greatly to the stiffness of the tire casing. Thickness notwithstanding, combat tires when inflated had riding qualities similar to standard tires. By the spring of 1941 combat tires that could run seventy-five miles deflated had been developed. The Ordnance Committee approved limited procurement of these in May 1941 and the following October standardized combat tires with commercial heavy duty innertubes and divided rims for all combat vehicles,101 and later for scout cars, half-tracks, and a few transport vehicles.
The American combat tire was patterned after a somewhat heavier “run-flat” tire developed by the British for their armored cars.102 In contrast to the British rubber beadlocks, American metal bead-locks, by permitting the tube to carry more air, resulted in a lower operating temperature within the tire. Trouble with segmented beadlocks at first used on American combat tires led to design of a more rugged hinged type standardized in October 1942.103 The run-flat advantage of the combat tire was largely offset both by the extra quantity of rubber needed in its construction and by the complexity of manufacture. During 1942 and 1943 the Ordnance Department, besides carrying out a development program for synthetic combat tires, conducted unsuccessful experiments with a tubeless combat tire and with steel restrictor rings for standard tires.104 As a conservation measure, the Ordnance Committee in March 1943 approved construction of lower quality natural rubber combat tires and a reduction of approximately 50 percent in the original 75 mile run-flat requirement. Approval of a 40-mile run-flat requirement for all combat tires came in November 1943.105 Tests had shown that the high operating temperature generated by synthetic sidewalls thick enough to support the vehicle over greater distances when no air was in the tube seriously restricted inflated mileage.
In the meantime, as a further rubber
conservation measure, use of combat tires on certain antiaircraft artillery carriages was discontinued because, the Army Service Forces stated, “These guns are generally used outside the actual combat zone, they are usually in a semi-permanent emplacement, the prime mover is not equipped with combat tires.106 Indeed, by late 1943 the shortage of military truck tires had become so acute that sharply curtailed use of combat tires for all vehicles was considered. Two to four truck tires could be produced for every combat tire, and experience had shown that the latter, though highly desirable, was not essential.107 Fortunately, drastic curtailment proved unnecessary, and throughout World War II large quantities of combat tires continued to roll off production lines.
High Flotation Tires
Operations in North Africa in 1942 and early 1943 proved that regular tires did not provide sufficient flotation in desert sand. Concurrently, need arose for greater flotation in muddy cross-country terrain and on beaches. Tires developed to fill these dual requirements were originally called “desert” tires. But, in as much as desert warfare ended while they were still under development, they came to be used primarily for cross-country operations, and the name gradually became “cross-country” tire.
Initial development work centered on tires for the two most important transport trucks, the 2½-ton 6x6, and the 4-ton 6x6. By January 1943 a satisfactory tire for the 2½-ton transport and amphibian DUKW had been developed, and by May one for the larger truck.108 The design of both tires was a compromise. The mud and snow tread, which was thinner than the standard because of the larger ground contact area, gave traction in mud. The large cross section and the greater flexibility from thin sidewalls provided flotation and cooler running in hot sand. The extra plies afforded protection against rocks. These tires were operated at low pressure on sand and in mud and were reinflated by air compressors when they ran on hard roads. Because of their greater size, a single high-flotation tire was mounted on each wheel instead of dual tires on rear wheels.
High-flotation tires, like combat tires, had to be mounted on wheels with divided rims and beadlocks. The beadlocks clamped the casings to the rims to prevent creeping at low pressures. These larger tires in some cases also required vehicular modifications such as the alteration of brake drums or limitation of the spring action to prevent tire interference with cargo bodies.109 For installation on transport
vehicles with regular wheel equipment, Ordnance engineers had to devise complicated kits containing the new tires, chains, air compressors, proper tubes, rims, bead-locks, and other equipment. Since the Ordnance Department was concerned with over-all vehicle performance, not tires alone, these so-called desert kits also contained equipment for improving engine-cooling characteristics. Unfortunately, most of the kits were still in the development stage when the Germans were defeated in North Africa, and only a few for the 2½-ton truck were available for service in that campaign.110 Although the series of desert kits was later standardized, no widespread use was ever made of them.111
Interest in the tires themselves remained because, in addition to excellent flotation in sand, they offered some advantage in mud. When the end of the North African campaign shifted emphasis from tires for desert warfare to problems of mud flotation, Ordnance engineers developed a slightly different high-flotation tire for the 2½-ton truck and modified the tire for the larger truck. Both tires had a full depth tread that gave better traction and greater wear. The desert tire for the 2½-ton truck was used mainly on the DUKW.112 For ordinary highway use the high-flotation tire had certain disadvantages. It made trucks more difficult to drive and maintain and reduced their ability to climb grades. Moreover, it required more crude rubber than a regular tire and, even if pressure were carefully regulated to match changing terrain, was less durable.
Although recognizing the drawbacks of high-flotation tires, the Ordnance Department, in response to requests from the using arms and services,113 developed another series of kits called cross-country kits, which enabled men in the field to install these tires on transport trucks, trailers, and tractors, and .to substitute larger, regular-size tires for smaller tires on certain vehicles in order to improve mud flotation.114 Development work continued on kits for a limited number of additional vehicles.115
Auxiliary Flotation Devices for Artillery Carriages
The flotation difficulties encountered by wheeled artillery in European mud were intensified in jungle terrain. The Borden mission sent to the Pacific in 1943 reported that to increase the maneuverability of artillery pieces, “track-laying vehicles of low unit ground pressure and excellent grouser action must be employed as prime movers, and greater flotation must be given to the towed load by use of skid plates or other suitable means.” Of field experiments with B-25 aircraft tires on the 105-mm. howitzer carriage, the mission commented:
... greater ground clearance was provided, the tires did not hang up on stumps but bounced off and the stability of the carriage in firing was not impaired. However, these desirable features were measured against the casualties which would result from the use of
these tires which are more susceptible to deflation when hit by fragments than are combat tires, and as a result the theater did not favor introduction of these larger airplane tires.116
The first auxiliary flotation devices developed by the Ordnance Department were shoe plates, or skid pans, for the 105-mm, and the 155-mm. howitzer carriages.117 These steel plates, fitted under the axle and trails of the carriages, supported the load when the wheels sank in the mud. However, the ditch-like ruts that the wheel dug in the mud often hindered prime movers that followed. To overcome this, in 1944 wooden mud sleds that fitted under the wheels were designed for the 105-mm. howitzer, lighter artillery, and certain cargo and ammunition trailers. As wooden sleds could not support the weight of the 155-mm. howitzer,118 early in 1945 teams were rushed to the theatres to introduce steel sleds, combining less weight and more durability with sufficient strength to support greater loads.119
Self-Propelled Artillery
Self-propelled artillery was one of the most controversial weapons of the war. In the 1930s, when the Ordnance Department had urged the advantages of motorizing guns and howitzers, the Field Artillery had contended that towed artillery was more maneuverable, less conspicuous, less likely to be deadlined for repairs, and less expensive. In the case of the self-propelled field gun, these arguments persisted down to the summer of 1944. On the other hand, the self-propelled antitank gun or “tank destroyer” advocated by the Armored Force was accepted early in World War II. It differed from the tank in having thinner armor and an open, rather than enclosed, turret. It was therefore lighter and faster but, while giving the crew greater visibility, also gave them less protection from enemy fire. A more vulnerable vehicle than the tank, the self-propelled antitank gun was designed for hit-and-run tactics rather than for slugging it out with the enemy. Since combat demonstrated that it was valuable not only against tanks but also in support of infantry and armor, the term “tank destroyer” came to be a misnomer. The self-propelled field gun bore little resemblance to either tank or tank destroyer. The big gun dwarfed its carriage, a tank chassis without turret or inner compartment for crew. But the carriage enabled the gun to move out of action before the enemy could get the range and to get closer to the target than had hitherto been possible for heavy artillery. In the end, the using services were converted to gun motor carriages for field guns as well as for antitank guns.120
Gun motor carriage development for World War II began in June 1940 when the Secretary of War, on the recommendation of the Board of Officers on the Development of Equipment for Armored Divisions, directed the Ordnance Department to develop a mount for the 75-mm. antitank gun and suggested the chassis of the light tank. The newly established Armored Force appended its own recommendation for the medium tank chassis, because of its capacity to carry more ammunition and a larger crew, and asked that the 105-mm. howitzer be considered as the weapon. Substitution of the howitzer—inherently support rather than antitank artillery—was comparatively easy, but the mount presented problems. The first requisite, high speed, meant light weight, but light weight meant either less carrying capacity or thinner armor. The Ordnance Department was inclined to favor a new commercial high-speed tractor and proposed using the more powerful 3-inch gun as the weapon.121
Offsetting the difficulty of adapting the tank chassis to this new purpose was the advantage of expediency. Eventually a faster motor carriage designed especially for self-propelled artillery was developed; in the meantime, after experimentation with wheeled carriages and half-tracks, attention centered on the medium tank chassis. By dispensing with a closed turret and reducing the armor, engineers could give the medium tank M3 or M4 the mobility and speed of a light tank, together with adequate power and room for ammunition and crew.122 Artillery so mounted soon proved itself on the battlefield. The first to see action was the 105-mm, howitzer mounted on the medium M3 tank, rushed to the British early in 1942. Designated the M7, but called “The Priest” because of its pulpit-like machine gun platform, it helped to defeat Rommel at El Alamein. Observers were impressed by its effectiveness in getting the enemy off balance.123 Another early weapon developed for antitank use was the 3-inch gun mounted on the medium tank M4, the motor carriage designated the M10. It was popular in North Africa and Italy.124 To combat the thicker armor encountered in Europe, the new 90-mm, gun was mounted on the same M4 chassis. When a new light tank, the M24, became available, the 105-mm. howitzer was mounted on its chassis. For antiaircraft work, two 40-mm. guns on the M24 chassis served.125
The one motor carriage designed especially for self-propelled artillery was an outgrowth of tank destroyer development initiated by the War Department G-3 in
1941. The objective was a very fast, lightly armored, cross-country tracked vehicle with a low silhouette; it was to be equipped with the Christie suspension. Though G-3 proposed using the 37-mm., in April 1942, following a conference between General Moore of the Army Ground Forces’ Requirements Division and General Barnes of Ordnance, a 57-mm. gun was substituted. A few months later when the Tank Destroyer Center asked for the 75-mm. gun, a carriage designated the T67, mounting the larger gun, was built.126 After tests at Aberdeen comparing this model with other types of tank destroyers, the newly established Special Armored Vehicle Board in the fall of 1942 selected the T67 as the most satisfactory. Further development brought about the substitution of one Wright radial engine for two Buick engines and the more powerful 76-mm, gun for the 75-mm. In its final form the carriage, now the T70, had the new torsion bar suspension. It was faster than any track-laying vehicle ever before produced; on level ground it could do better than 50 miles an hour. So promising was the design and so great was the demand for an effective antitank weapon, that the Army Service Forces in January 1943 ordered 1,000 T70’s manufactured without extensive service tests. Testing was carried on throughout 1943 concurrently with procurement.127
Improvements provided better slope climbing and better performance in low gear. The T70 was standardized in April 1944 as the M18. Despite early forebodings about the thinness of its armor, it gave excellent service, especially when the 76-mm, gun was fired with tungsten carbide cored ammunition. Like the self-propelled 3-inch and 90-mm. guns, the 76-mm. often functioned as an assault gun in support of infantry and armor. Much of the credit for getting it to the battlefield belonged to Brig. Gen. Andrew D. Bruce, commander of the Tank Destroyer Center, but Ordnance engineers were justifiably proud of the M18 as one of the major artillery developments of the war.128
Self-propelled field guns might not have got overseas at all had the Ordnance Department not early developed a motor carriage for the 155-mm. gun and contrived its acceptance by the using services. In the spring of 1941 the Ordnance Technical Staff began development of self-propelled mounts for field guns in calibers up through 155-mm. Experiments showed that tank chassis could be successfully adapted by adding a spade to keep the vehicle steady when the gun recoiled. Ammunition and crew could be carried in an accompanying vehicle.129 In the face of AGF opposition, it was only by persuading General Somervell to go to the Secretary of War that General Barnes got authority to manufacture a model, the M12, mounting a 1918-type 155-mm. gun. The Army Ground Forces turned down the 4.5-inch self-propelled gun, authorized only a pilot model for the 155-mm, howitzer, and anticipated no requirement during 1943 and 1944 for self-propelled field guns beyond a hundred
M12’s. Standardized in 1942, the M12’s did not get overseas until 1944.130 Nevertheless, successful tests of medium self-propelled field guns at Fort Knox and reports from overseas, especially of the Russians’ tactics in bringing big guns out of protected positions to fire point-blank on German dugouts, impelled the Armored Board in December 1943 to recommend immediate production of gun motor carriages for medium and heavy artillery. The Army Ground Forces disapproved the recommendation pertaining to the heavy guns, but the Ordnance Department went ahead with pilot models even heavier than the M12. Thus a new 155-mm. gun, the “Long Tom,” and the 8-inch howitzer mounted on M4 tank chassis were ready when demands from both European and Pacific theatres brought about authorization for procurement in July 1944.131
In the advance across France and through the Siegfried Line, the M12’s lived up to General Barnes’ expectations. General Hodges considered them invaluable. They could be brought up to within a few hundred yards of strong fortifications and blow them to pieces. Even more effective was the self-propelled Long Tom, the first gun to fire on Cologne. Gun motor carriages, by making possible the employment of heavy cannon for direct fire, introduced a tactical innovation and thus showed how technicians at times could influence tactical doctrine. By V-E Day the foresight that inspired the development of these powerful weapons and the drive that got them into combat was vindicated. By V-J Day the 8-inch gun and the 240-mm, howitzer mounted on heavy tank M26 chassis were ready for shipment to the Pacific.132
Airborne Equipment
When in April 1940 German paratroopers dropped from the skies into Norway, and in May landed behind Allied lines at Fort Eben Emael in Belgium, military men realized that the Russian parachute demonstrations in prewar maneuvers had inspired a new application of mobility, perhaps even a new form of warfare. Yet since soldiers minus effective weapons could have scant value in the enemy rear save as intelligence agents, in the United States search for matériel suited to parachute and glider delivery necessarily went hand-in-hand with training men and designing aircraft to employ the new technique. Paratroopers could land carrying shoulder and side arms with them, and machine guns and ammunition chests could be separately parachuted without great difficulty. Splitting the load into several parcels might even permit dropping light-weight artillery. Heavier weapons, if made to fit into aircraft, could be flown in later. But if infiltrating units could be supplied with greater mobility than their legs
would furnish, their fighting potential would clearly be enormously enhanced.
Airborne Tanks
With this thought in mind, the Ordnance Department in February 1941 held a conference with representatives of G-4 of the General Staff, the Armored Force, and the Air Corps to consider the possibilities of developing a special light-weight tank and an aircraft to transport and land it. Plans for Ordnance to develop the vehicle and for the Air Corps to develop the carrier took shape quickly.133 As the Pressed Steel Car Company of Pittsburgh had already informed the British Purchasing Commission that an airplane could be obtained without major change in design to transport a 7,5-ton tank, the list of tentative military characteristics for the proposed tank included a weight limitation set at that tonnage.134 Design studies began at once.
Of the several designs submitted, that of the Marmon-Herrington Company was considered most satisfactory. Manufacture of one pilot model, designated the T9, was approved in the fall of 1941 and followed in January 1942 by a contract for two additional pilot models.135 In the two later pilot models changes in the original specifications led to assigning a separate designation, T9E1. In as much as experiments with the first pilot had demonstrated that an increase in weight to 7.9 tons was necessary if the tank were to retain many of the important features required, both the Army Air Forces and the British, who were also interested in airborne tanks, agreed that new models might run to that weight.136 The first T9E1 pilot was completed in November 1942 and was sent to Aberdeen Proving Ground for various road and firing tests. The second pilot, completed shortly thereafter, was shipped to England for test.137
Meanwhile, ASF in April 1942 had approved quantity production of T9E1 tanks even before development and standardization were completed. Consequently, the first production models came off the line in December 1942. Extensive tests in 1943 and 1944 by Ordnance and by the Armored Board, together with flight tests in C-54’s, initiated several essential changes both in new vehicles and, by field modifications, in those already produced.138 In August 1944, after production had ceased, the T9E1 airborne tank was adopted as a limited standard vehicle and redesignated the M22. Altogether 830 had been built. Although several hundred were shipped overseas to both US and Allied forces, none was used in combat. In mid-1945, with no future need contemplated, the M22 tanks were declared obsolete.139
This checkered career was the consequence of discovery that while the M22 tank could be satisfactorily transported in C-54’s, the tank itself possessed many limitations. It had insufficient armor to withstand .50-caliber armor-piercing ammunition; its engines developed very low horsepower; its meagre gas capacity gave
it a limited range of operation; it carried only the light 37-mm. gun; it had too little space for cargo and crew; and it had poor over-all mechanical reliability. AGF exhibited little interest in this airborne tank not only because of these shortcomings but also because of the time consumed in getting it into action. Landing, reassembling, and driving it from the nearest airfield capable of handling C-54’s to the scene of combat, perhaps as much as 100 to 200 miles, took so long that the enemy could get to the battlefield tanks with much greater fire power and armor. The airborne tanks would probably then be outnumbered.140
The Airborne Center in the summer of 1944 submitted to the ASF plans for an improved airborne tank to be carried inside larger planes then under development and suggested investigation of the possibility of transporting the tank suspended beneath the plane so that launching from a low-flying carrier could be made near the scene of battle. The AAF commented that the C-82 was the only plane under development probably capable of carrying the proposed tank internally. Flight range of the C-82 carrying such a tank would be limited by the reduced fuel load. Moreover, previous research had established the impracticability of launching a tank from a plane in flight, since the average loaded cargo plane had a minimum flying speed of well over 100 miles an hour and the speed the tank could attain was only 40 to 50 miles an hour. Attempts to attach a tank beneath the fuselage of a plane and then lower the tank to the ground after the plane had landed also proved unsuccessful. The Ordnance Department pointed out that a tank with the larger engine, more powerful gun, and heavier armor desired could not be built within the essential weight limitations imposed by aircraft and gliders under development. Improvement of an airborne tank would have to depend upon improving components such as infinitely variable transmission, torsion bar suspension, center guide tracks, and a new turret.141
Since rapid development of such an airborne tank would entail establishment of priorities that possibly would interfere with other high-priority projects, the entire matter was restudied. Since it was known that a tank could not be launched from a plane in flight, that the M22 was insufficiently armored to be employed properly in a tank role, that the largest planes under development would be unable to carry a sufficiently heavy model, and, finally, that aircraft could be used more effectively in airborne operations to transport other weapons and additional troops rather than an inadequate combat vehicle, the Airborne Center in December 1944 declared that no need existed for a special airborne tank.142
Nevertheless, the decision to forego development of a special airborne tank did not lessen the long-standing AGF desire for air transport of standard tanks. In the latter part of 1944 Ordnance engineers began studying the possibilities of carrying the M24 light tank in the C-82 plane, which,
though still in the development stage, was designed to carry 10 tons of cargo. The solution was to use two planes. disassembling the tank so that its total weight could be distributed into equal loads of less than 10 tons, loads that the dimensions of the C-82 cargo space could accommodate. Preliminary studies showed that this could be achieved by carrying the tank hull in one load and the turret, suspension, tracks and ammunition in a second. In February 1945 the Ordnance Committee approved a complete, detailed study of means of partially disassembling and transporting the M24 and T24E1 light tanks and the self-propelled 76-mm. tank destroyer M 18 in both the C-82 and the XCG—10A glider. The glider, recommended by the AGF for standardization and procurement, had cargo capacity comparable to that of the C-82. This study was to include design of equipment for dismantling and reassembling vehicles, for handling the various components, and for fastening them securely in the plane during flight.143
Of the two development projects set up at the tank center in Detroit, one concerned with C-82 transport and the other with glider transport of the light tank M24, the latter was the subject of marked differences of opinion within the AAF. The plan worked out by May 1945 during conference between Ordnance and AAF representatives met with AM’ disapproval in June. The project for transporting the M24 in the C-82 met with more success. By June 1945 the equipment required to assemble the tank in the field had been designed and trial loadings and unloadings in a C-82 fuselage were under way. Tests showed that a five-man crew using one set of tools could unload and reassemble the tank in four hours arid forty minutes or, using two sets of tools, in three and a half hours or less.144
Airborne Artillery
The development of light-weight, compact artillery had long been an Ordnance goal. To this aspiration the introduction of airborne operations gave new impetus, and development of artillery designed especially for air transport accordingly began in the fall of 1941. As larger cargo planes and gliders appeared, an ever-increasing variety of weapons could be flown to the battlefields. In 1943 the Ordnance Department began investigating the practicability of transporting by air all items of carps and division artillery. Tests conducted in 1943 and early in 1944 showed that the equipment for the 105-mm. howitzer, 155-mm. howitzer, and 4.5-inch gun battalions could be successfully carried in the largest available planes, the C-47, B-17, and B-24, though disassembly of some items before loading was necessary. The 155-mm. gun, 8-inch howitzer, 8-inch gun. and 240-mm. howitzer all proved too large.145 Experimental use of aluminum and magnesium in artillery carriage components. such as wheels and trails, began in 1944 and continued after the war. The only artillery items designed specifically for air transport during World War II were the 105-mm. howitzer M3 and its carriage M3A1, the multiple 50-caliber machine gun mount M55, and the 40-mm. antiaircraft gun carriage M5.
In the fall of 1941 need was felt for a 105-mm. howitzer transportable by air. Difficulty in getting the standard weapon, without disassembly, through the doorway of the cargo planes necessitated considerable change in both howitzer and carriage, a problem solved by shortening the barrel 27 inches and using the smaller 75-mm. field howitzer carriage. Howitzer and carriage were standardized in March 1943 as the M3 and M3A1, respectively. The M3 fired the same 33-pound shell as the original M2 105-mm. model, but maximum range was reduced from 12,000 yards to slightly over 7,000 yards. Yet its light weight, only 2,500 pounds for both the howitzer and its rubber-tired, high-speed carriage, made the weapon valuable not only in airborne operations but also in amphibious operations, mountain warfare, and for use over soft jungle terrain. Two completely assembled M3 105-mm. howitzers and carriages could be transported in a C-47 in contrast to only one M2 model and carriage disassembled into five major units.146
Equally, or perhaps even more successful was the development and production of a multiple .50-caliber gun mount and trailer for airborne operations. When paratroopers captured an enemy airfield it was of the utmost importance to set up an adequate defense immediately in order to hold the field. Against dive bombers and strafing planes, the new trailer mount M55 was invaluable. Its four heavy barreled .50-caliber machine guns in a power-operated turret were mounted on a two-wheeled trailer and could be carried in either a CG-4A glider or in a C-47 plane. The trailer was equipped with removable pneumatic-tired wheels and mechanical jacks for emplacing the mount and leveling it in firing position when the wheels were removed.147
To meet the need for a larger caliber automatic gun and mount that could be transported by air for use as either antitank or an antiaircraft weapon, Ordnance engineers redesigned the 40-mm. antiaircraft gun carriage M2A1. The modified carriage, standardized as the M5 in September 1943, weighed considerably less than its ground counterpart; its width was decreased to permit passage through the doorways of the C-46, C-46A, C-47, and C-54 and it rode on two pneumatic tires. The chassis consisted of a center base with one permanently attached outrigger and three removable outriggers which, along with the gun barrel, had to be removed before loading the carriage into the plane. Three men could emplace the carriage in approximately five minutes and raise it from firing to traveling position in about eight.148
Paracrates
For equipment to be dropped from airplanes to paratroopers or other ground forces, reducing the weight and bulk of each item solved only part of the problem. There still remained the matter of protecting the items from being damaged when they hit the ground, and to the development of protective containers for this purpose Air Forces and Ordnance engineers devoted considerable attention. They not only developed containers for dropping
weapons from planes but also added several new words to the English language, chief among them being “paracrate,” the official name for containers that could be floated to earth by parachute, “parachest,” a trunk-like container for ammunition, and “paracaisson,” a small, collapsible, hand-drawn ammunition cart.149
The first and most important paracrates were for the 75-mm. pack howitzer, a weapon developed in the late 1920s for use in mountainous territory and later equipped with pneumatic tires for airborne use. The relatively light units into which this weapon could be broken for transport by mule pack were equally well suited to transport by airplane and drop by parachute. To protect the parts from the shock of landing, during the early months of 1942 engineers designed a series of plywood paracrates to fit each of the seven major parts of the weapon, plus a parachest to hold ten rounds of ammunition and a para-caisson to transport the ammunition after it hit the ground. The nine loads varied somewhat in weight but averaged about 300 pounds; they were floated to the ground by parachutes of different colors to aid in the identification of the loads. When tested in the fall of 1942 in actual air drops at Fort Bragg, these containers gave full protection to the weapon and suffered little damage themselves in the process. Rock Island Arsenal immediately produced fifty sets for further test by the using arms, and in June 1943, with the approval of the Field Artillery Board and the Airborne Command, the entire series was standardized.150 When combat troops later reported difficulties in finding and reassembling all the parts of the howitzer because of their wide dispersal when dropped, a harness was designed to hold the nine packages together.
While these containers were being tested and approved, Ordnance engineers turned attention to reducing their weight and bulk, making them of some material other than plywood and redesigning them so they could be readily disassembled for shipment overseas. In September 1943 experimental paracrates of corrugated steel went to Camp Mackall in North Carolina for test. When dropped from an altitude of about 1,000 feet upon hard-baked soil they gave good protection to the weapon and withstood the shock of landing better than the plywood crates. Further, as the steel containers were held together by nuts and bolts, they could easily be shipped unassembled at a great saving of shipping space. Because of their many advantages, the steel crates eventually replaced those made of plywood. During 1945 when the scarcity of light-weight metals eased somewhat, consideration was given aluminum and magnesium paracrates, but the war ended before these lighter containers went into production.
The 75-mm. pack howitzer was by far the most important weapon for which paracrates were used, but Ordnance engineers also worked on the development of containers for a variety of other weapons from 37-mm. guns and mounts and flame throwers to 60-mm, and 81-mm, mortars and 57-mm. and 75-mm. recoilless rifles. During 1945 work continued on paracrates for larger experimental matériel and the heavy barreled ,50-caliber machine gun.151
Trucks
Pressure of time, coupled with acute shortage of light-weight metals, precluded
undertaking an elaborate program of designing special airborne trucks and focused attention on means of disassembling standard vehicles to permit their passage through the plane doorways into the narrow cargo space. The ¼-ton jeep could be easily rolled into a plane without disassembly, but the larger trucks required modifications ranging from the relatively simple removal of fenders, bumpers, and other exterior parts of the ¾-ton truck to cutting the frame of the 2½-ton truck back of the cab, splitting the vehicle into two separate units. After this operation, performed at the factory, the two units were bolted together by steel plates and the truck was then classified as an airborne vehicle. When it was to be transported by air, the steel plates were removed and the two parts of the vehicle were loaded on two C-47’s. The accompanying airborne preparation kit contained a two-wheel dolly that served to support the front half of the truck while it was being loaded on the plane.152
If the vastly greater mobility in ground warfare that airborne operations promised was not fully realized in World War II, the potentialities of the technique for the future were nevertheless well understood. Advances in metallurgy before V-J Day were still too slight to enable engineers to design and build many types of vehicles and weapons at once sufficiently compact, light weight, and sturdy to be suited to air transport. But the research and development program mapped out embodied the hope that in the postwar period developments in airborne ordnance would keep pace with the training of troops in landing and using it.