Chapter 12: The Search for Increased Fire Power: Ammunition
Neither smoothly functioning guns nor perfection of aim can make fire accurate if the projectile is unstable in flight, since the flight characteristics of a projectile fired from any kind of weapon, whether from rifled gun bore or smooth mortar or rocket-launcher tube, affect the projectile’s ability to reach its target. Hence, exterior ballistics are as important as interior ballistics, fire control, or the terminal ballistic elements that determine the effect upon the target when reached. Study of the motion of projectiles through the air and their behavior during flight constitutes a special, highly complicated branch of science, the problems of which are frequently baffling even to experts. Various design features of the projecting weapon, such, for example, as the dimensioning of the chamber of a gun or the twist of the barrel rifling, may have as much bearing upon accuracy of fire as does the character of the propellant or the shape of the projectile. The desired result depends upon a complex of factors. Gravity, air drag, wind forces, the weight and distribution of the weight of the projectile, the velocity at which it is launched, and the angle of projection must all be reckoned with.1 Careful observation of these phenomena and computation of exact data require elaborate measuring instruments and computers. Germany was far ahead of the United States and Britain in this field of work. As early as 1940 German scientists were using an intermittently operating supersonic wind tunnel in developing radio-controlled bombs, rockets, and flak shell, while the US Army was still trying to obtain data for bombs and rockets at subsonic velocities. After the completion of the Wright Field wind tunnel, the Air Forces and Ordnance Department were in a better situation than formerly, but not until the fall of 1944 was a supersonic tunnel ready for use at Aberdeen Proving Ground.2 Nevertheless, throughout the war the work of Ordnance ballisticians, particularly at the Ballistic Research Laboratory, was vitally useful. Though their services in providing the armed forces with the means of obtaining accurate fire were essential, the discussion here must be limited to some of the simpler phases of the intricate problems involved,
Barrel Rifling and Design of Projectiles for Conventional Weapons
In conventional artillery and small arms, the rifling of the barrel gives the
projectile a spin calculated to ensure sufficient stability in flight to carry it to its target. Stability is important solely because it is a big factor in achieving accuracy. Projectiles fired from smooth bore launchers usually had to rely upon fins for stability, though German technicians in World War II produced a curiously shaped finless mortar projectile which, shot from an unrifled bore at a low velocity, obtained stability from the air flow about it.3 American Ordnance engineers before the war was over evolved rifled tubes for a 4.2-inch chemical mortar and for the enormous 914-mm. Little David. But most American mortars, like most American rockets used in World War II, were fin stabilized, not spin stabilized.
Except for hand grenades and mortars, which fired only at relatively short ranges, the US Army had used rifled weapons almost exclusively since the 1870s. Consequently, despite incomplete knowledge of why projectiles shot from rifled bores behaved as they did, Ordnance ballisticians by trial and error over the years and by study of the findings of French scientists had arrived at sound general principles of gun bore and projectile design. They early learned that a long twist in the rifling might produce so little spin on the projectile that its stability in flight would be adversely affected, whereas a needlessly short twist not only put excessive pressure on the barrel but might actually lessen the projectile’s range. During World War II designers studied or restudied virtually every rifled weapon then in use or under development to determine the most satisfactory rifling twist. Though production problems forbade making desirable changes in many guns, in some the advantage offset the cost. For example, the 76-mm. tank gun, originally rifled with a twist of one turn in 40 calibers, was later made with one turn in 32 in order to get a faster spin on the projectile. Conversely, when experiments at Springfield Armory indicated that the twist in Garand rifle barrels was shorter than necessary and that two grooves would serve as well as four, because of the effect upon production no revision of design was authorized. Two-groove barrels were, however, made for the 1903 and 1917 rifles, arms for which demand was less.4
Shapes of standard projectiles, on the other hand, were not much changed from the elongated boat-tailed contour established as standard before World War II began. Thus, correcting inaccuracies in the 105-mm. high-explosive shell proved to be not a question of redesign of the projectile, but of improved banding, closer dimensions, smoother finish of the shell, and use of two granulation powders, that is, powders of two different cross-sectional thicknesses. When a new weapon was under development, refinements were sometimes effected in the projectile, such as the thin-ogived, long boat-tailed shape given the 120-mm. projectile to reduce air drag. But over-all restudy of this feature of ballistical elements in conventional ammunition had to await the postwar era,5 Mortars, also, though newer to the US Army than artillery and shoulder arms, underwent rather few important changes in projectile design. Fixed tail fins and the cylindrical or tear-shaped contours standard before
1940 remained the accepted design for all but the giant Little David. The Bureau of Standards, at Ordnance Department request, worked out a redesign of the 60-mm. shell in order to improve stability by shifting the center of gravity further from the center of pressure and by some reshaping of the fins, but this model was not ready in time for use in World War II. The shell for Little David was nearly cone-shaped and was pre-engraved.6
Special Projectiles To Give Hypervelocity
Some exploration of new types of ammunition for both artillery and small arms did take place, in hopes of attaining hyper-velocities without excessive barrel erosion or radical changes in design of the projecting weapon. Hypervelocity, producing a flattened trajectory in the projectile’s flight, would of course not only improve the chances of a hit by shortening the time of travel toward a moving target, but, by conserving the kinetic energy of the projectile, would heighten the destructive effect of a strike. Thus, the search for hypervelocity involved, simultaneously, considerations of interior, exterior, and terminal ballistics. Its advantages in increasing accuracy of fire were clear. Against a tank 1,500 yards distant moving at 30 miles per hour, NDRC later figured that a shot fired at a velocity of 3,550 feet per second reduced the lead needed with 2,030 feet-per-second velocity from 105 feet to 60 feet and quadrupled the allowable error in estimating range.7 But apart from lengthening the gun barrel and thereby adding undesirable weight, the means of getting velocities of more than 3,000 feet per second reduced themselves to two: an increase of the powder charge in relation to the weight of the projectile or use of higher potential propellant powders.
In the first category one possible solution lay in applying the “squeeze principle” to fire a specially designed projectile from a gun barrel narrowed by tapering the bore or by addition of a conical adapter. As early as 1932 the Ordnance Department had tested a tapered bore sporting rifle with a skirted bullet designed by an American-born German engineer, Hermann Gerlich. From a barrel of which the grooves in the middle section were diminished in depth from .75-mm. to .13-mm., a 125-grain monel metal bullet fired at a muzzle velocity of 4,406 feet per second. But as accuracy was unsatisfactory and the bullet tended to fall apart on impact, the rifle was clearly unsuitable as a military weapon,8 Not until 1942, following discovery of the German Army’s apparent success with artillery tapered bores and British interest in an application to small arms, did the Ordnance Department renew investigation of possibilities in a small-caliber weapon. Patterning its work in general on the British Littlejohn gun designed by a Czech named Janecek, Frankford Arsenal in July 1942 began development of a projectile to fire from a .45-inch .50-caliber machine gun barrel with a muzzle adapter that reduced the exit diameter to .35 inch. A projectile with a
German 28/20-mm. antitank gun
.35-inch hard core was surrounded by a soft envelope that bulged in the center so as to leave at that point a void between the core and the envelope. The projectile was rotated by the rifling in the cylindrical section of the barrel bore, but on reaching the tapered portion, the projectile’s deform-able envelope was swaged into firm contact with the core so that the bullet left the gun with a reduced diameter and with the shape of a conventional armor-piercing projectile. Despite extended research on a large number of types of bullets, the laboratory technicians found no design with suitable exterior ballistics. Bullets tended to disintegrate upon emerging from the barrel and had such ballistic instability that muzzle velocities proved greater in fire from a barrel without the conical adapter than from a barrel with it. Incendiary ammunition, moreover, could not be fired from this type of weapon. The whole project of producing caliber .50/.35 ammunition was therefore dropped in the fall of 1943.9
Meanwhile, the Ordnance Department had been trying various applications of the squeeze principle to artillery projectiles. In 1941, when the British captured from Rommel’s army, near Halfaya Pass, a light antitank gun with bore tapered from 28-mm, to 20-mm. and a few rounds of its “arrowhead” ammunition, American interest in Gerlich-type weapons revived. A report describing the distinctive features of both gun and ammunition, and British reports of tests, inspired the Ordnance Committee to request immediate design of weapons and projectiles as nearly identical to the German as possible. At the same time, because a report from the Ballistic Research Laboratory indicated that equally high velocity could be obtained without tapered bores, the decision was reached to make several cylindrical bore guns employing ammunition similar to the
hard-cored soft-sleeved German type, in order to compare performance with that of American copies of the German and with that of a captured model to be sent to Aberdeen.10
While awaiting completion of the American 28/20 matériel, Ordnance engineers experimented with affixing tapered adapters to the muzzles of standard 37-mm, and, later, 57-mm. guns, so that from the former the projectiles would emerge with a 28-mm. diameter, from the latter with 40-mm. Though capture of a German arrowhead projectile for a 41/29-mm, gun in mid-1942 showed that the Germans were then placing a good deal of faith in tapered bore weapons, careful tests convinced the Ordnance Department by late 1943 that in all artillery a cylindrical bore without an adapter but using a light hard-cored projectile served every purpose of the tapered bore. The former suffered far less wear, gave greater accuracy of fire, and over long ranges maintained higher velocities.11 Thereafter the Ordnance Department bent its efforts to developing tungsten carbide-cored rounds for conventional weapons and delegated to NDRC the pursuit of discarding sabot projectiles and other ways of attaining hypervelocity.12
Propellants for Conventional Weapons
Because getting a shot to the target depended on other factors than design of the launcher and the shape of the projectile, propellant powders and primers also had to be considered. Indeed, to get sufficient velocity to ensure accurate fire from conventional weapons, use of a more powerful propellant was the only alternative to increasing the ratio of propellant to weight of the projectile. Ordnance specialists had long known that in other weapons besides the 105-mm, howitzer the nature of the propellant powders influenced accuracy of fire. Unfortunately, during the 1920s and 1930s money for research in this realm had been so meagre that, apart from developing relatively smokeless non-hygroscopic powders, the Ordnance Department had accomplished little.13
The qualities of the ideal propellant were easy to name: a chemical composition producing neither flash nor smoke and causing little barrel erosion, having low flame temperature and low chamber pressure, yet giving very high velocity,14 a powder largely impervious to moisture and made of readily available materials easily manufactured in quantity. Achieving any one of these features was no poser. The trouble came in trying to combine all wanted qualities in one package. Low flame temperature threatened to cause loss of velocity; reduction of flash increased smoke. While improved ignition systems in artillery ammunition contributed to lessening both obscuration and muzzle flash, the chemical composition of the
powder remained the chief factor to consider. Adding potassium sulphate to propellant for antiaircraft fire, where flashlessness was all important, helped to solve one problem. But it was no answer to the demand for a wholly smokeless and flash-less propellant for field artillery where smoke would obscure gunners’ vision and muzzle flash reveal the tank or battery position.
The search for a compound at once smokeless and flashless had its beginning in the requirement established by the Westervelt Board in 1919. Ordnance chemists, following British experiments, in the early 1920s offered the using arms samples of nitroguanidine which, to a degree unobtainable in any other known propellant, had both properties. But nitro-guanidine in combustion gave off such noxious ammonial fumes that the Field Artillery vetoed its use. The Ordnance Department, with no customers in prospect, then abandoned all thought of building plants to make it. But ammunition specialists found no satisfactory substitute. Twenty years later the Navy seized upon nitroguanidine as the one feasible answer to novel conditions of combat. For the first time American ships in the Pacific were having to fight in small harbors where maneuvering was all but impossible. Flash by night betrayed the vessel’s position and smoke by day made second rounds inaccurate. Negotiations with Canada in 1943 for purchase of nitroguanidine from the one plant upon which British and Canadian forces were also depending succeeded in meeting Navy needs but left no surplus for the US Army. Until shortly before V-E Day the Army Ground Forces were unconvinced of the value of this propellant. By then, urgent demand could not create facilities to produce it in quantity, and the Ordnance Department could procure only small lots for test and experimental firing.
Whatever the advantages of nitroguanidine, neither that nor any other known composition was ideal for all purposes. Even in conventional artillery and small arms ammunition, where ballisticians understood propellant behavior better than in rockets and recoilless rifle ammunition, compromises were inescapable. The primary requisite for one weapon or one particular use tended to be different from that for every other. Propellants suitable for a 90-mm. shaped charge, where low velocity was acceptable, would not answer for a 76-mm. tungsten carbide-cored projectile, the design of which was directed at achieving very high velocity. In addition to these problems, World War II introduced the new element of extremes of temperature at which ground firing must take place when Allied troops were fighting in dry desert, damp jungle heat, and in the subzero winter weather of northern Europe. Series of propellants, therefore, were needed to cover widely varying contingencies.15 Since basic research as well as prolonged applied research was necessary, many problems remained unsolved at the end of the war. But the field was explored more thoroughly than ever before in the United States, and lines of investigation were clarified for postwar development.
In processing propellants, industry and Ordnance made considerable advances during the course of the war. One new method, developed by the Hercules Powder Company, for washing nitrocellulose
in a continuous filter instead of in large tubs by the old “settle and decant” system, washed more thoroughly and thus improved the stability of the nitrocellulose. The DuPont Company found that, in winter, use of preheated alcohol to dehydrate nitrocellulose reduced the dehydration time cycle, bettered the yield, and made a more uniform product, which in turn made better powder. The Radford Ordnance Works carried on extensive experiments to improve manufacturing and testing techniques as well as to find better chemical compositions. Yet in the spring and summer of 1945 reports of the Combined Intelligence Objectives Subcommittee, established to locate data in Europe on Axis research and manufacturing procedures, indicated that Germany had developed several processes more effective than those of the United States. The most novel German method was one of casting propellant grains by adding a paste of moist nitrocellulose and diethyleneglycol-dinitrate, DEGN, to molten TNT and pouring the mixture into steel molds to cool. Grains as large as 1,000-mm. were cast this way. After the war complete sets of the German equipment deemed most useful and novel went to Picatinny Arsenal for study.16
Design of Projectiles, or New Weapons
Meanwhile, rockets and, later, recoilless rifles, were introducing unfamiliar ballistic problems. Both types of weapon were so designed as to fire at low velocities, the bazooka as low as 260 feet per second, the 57-mm. recoilless rifle at just over 1,200, and the 75-mm. at only 1,000 feet per second. While those velocities would make no difference in the ultimate striking power of hollow-charge projectiles, the slowness of flight gave a curved trajectory that made judging range and lead more uncertain and thus lessened the chances of a hit. As rockets, carrying their propellant charges within their casings, were initially heavy but lost weight as the propellant burned during flight, maintaining stability was peculiarly difficult. In recoilless rifle ammunition the propellant, though firing the projectile minus the shell, necessarily lost some of the energy needed to impart high muzzle velocity because of the partial dissipation of the gases rearward. Yet tests proved the accuracy of the recoilless guns comparable to that of small-caliber artillery.17
For ground-launched rockets, as for mortar shell, fins affixed to the round appeared at first to be the most practical way to provide stability, though in retrospect the question arises why no one in 1940 recalled that the famous Hale rocket of 1855 was spin stabilized. Doubtless the example of the British, who at the time of the Battle of Britain had to produce rockets quickly, if at all, influenced American designers. And finned rockets promised to be simplest to design and make. But the optimum shape of the fins and the method of fastening them to a rocket, which must be shot out of a tube launcher, remained to be determined. The 3.25-inch target rocket, designed not for combat but for training antiaircraft gunners, was launched from long rails since accuracy of fire was unimportant. Consequently, in that model large flat-ended fins that gave adequate stability of flight could readily be hooked onto the rocket case and cause no interference
when the rocket was fired. Fins on tube-launched rockets had to be designed to fit the launcher. The principal features of the first American combat model, the 4.5-inch rocket, were folding fins and electrical firing. The rocket itself was long and slim. Fins, folded back into the neck of the case, opened up by inertia and were kept open by air flow after the rocket emerged from the tube. The smaller 2.36-inch bazooka rocket, on the other hand, was built with a series of fixed fins somewhat resembling the feathers of an arrow but fitted into the narrowed case neck so as to have the same diameter as the rocket motor. Fixed fins shrouded by an outer ring were also used on the 7.2-inch series developed in 1943.18
When Allied troops encountered German rockets and discovered that they were spin stabilized, the Ordnance Department, convinced that the Germans must have good reasons, undertook to develop a series of similar design. Only as the multiple-nozzled experimental types were tested did the virtues of spin stabilization become clear. Accuracy was greatly improved because the flow of gases through the eight canted vents rotated the rocket at about 10,000 revolutions per minute, thus keeping it on its course, whereas in firing finned models frequent slight misalignments of the thrust with the axis of the rockets caused veering. When the latter type was launched at low velocity across the wind, the fins tended to act like a weather cock and swing the projectile into the wind. Furthermore, a folding-fin rocket equipped with a proximity fuze might develop enough flutter in the fins as they unfolded to activate the fuze prematurely. Elimination of fins and fin rings reduced parasitic drag on the rocket and thus lengthened range even with a heavier explosive charge in the warhead. And, finally, finless rockets were easier to load quickly into multiple and automatic launchers. The first 4.5-inch spin-stabilized model, the T38, in fact proved to be as accurate when fired from a short launcher as from a long one, though postwar tests showed long launchers necessary for other rockets. Two variations of the T38 were standardized in the spring of 1945, while work on spin-stabilized 7.2-inch rockets was pushed vigorously.19 But as shaped-charge rockets would lose much of their effect if rotated in flight, all high explosive antitank (HEAT) rounds were fin stabilized.
Rocket Propellants
Difficult though the program was for improving propellants for conventional weapons, the question of developing suitable rocket propulsion was still harder to answer. From the very beginning everyone concerned with rocket research agreed that single-base powders would lack the necessary energy and that double-base, that is, nitrocellulose and nitroglycerine combined, must be used. Double-base powder made by the solvent process had been manufactured in the United States for a number of years for use in intermediate and heavy artillery. This method employed a solvent of acetone and alcohol to make a glue-like substance, or colloid, from which grains or flakes could then be formed. About 1939 the Hercules Powder Company found a way of producing a solventless double-base powder plasticized
by heat and pressure and then rolled out into sheets for mortar increments. But neither was readily adaptable to rockets, where a solid stick or grain, not sheet powder, was needed, and where the essential quality of even burning precluded using a grain with so much as a minute crack or fissure. For even a tiny crack in a burning grain of powder would create peaks of pressure at particular spots of the encasing rocket motor tube and thus either burst the tube or cause erratic flight.20 Consequently, to get safely usable rocket propellant made by the solvent process meant using grains of small cross section— in technical phraseology, thin-webbed powder—in which malformations occurring in drying would be few and inspection could be exacting. A method of manufacturing solventless double-base powder by a dry-extrusion process had been developed in the late 1930s by the British. This produced much thicker-webbed, and therefore longer-burning, grains but required enormously heavy presses to extrude or compact and force out the powder into the desired shape.21
In 1941 American scientists split into two schools of thought on the relative merits of solvent and solventless double-base powders for rockets. Dr. Charles C. Lauritsen of the California Institute of Technology, and in 1941 vice chairman of one of the NDRC divisions, had been much impressed by British rocket work and strongly recommended dry-extruded powders. Though at that time no facilities existed in the United States for producing it, the Navy, with contracts let through NDRC to the California Institute of Technology, chose to focus efforts on obtaining thick-webbed solventless types. The Ordnance Department, on the other hand, agreed with the views of Dr. Clarence N. Hickman of the Bell Telephone Laboratories, then chief of an NDRC group working on rocket developments at Indian Head, Maryland. Hickman advocated use of wet-extruded powder because of the shorter burning time and greater strength of the thin-webbed grains produced by this process. Quite apart from theoretical advantages, the urgency of quickly getting some usable type led Ordnance to center its program about solvent powder. When toward the end of the war the Army’s rocket specialists concluded that the long-burning, thick-webbed solventless powder was after all generally better for rocket propellant, Navy preemption of facilities for dry-extrusion obliged the Army to continue to rely largely upon the wet-extruded.22
Wet-extruded propellant was made by suspending the powder in a solvent that swelled the nitrocellulose to make a dough. The dough was forced through dies to form sticks, or grains, of powder from which evaporation then dried the solvent. As satisfactory drying to produce flawless grains could be obtained only with thin-webbed powder, it was clearly necessary to use a number of small grains in each rocket in order to get a sufficiently heavy
charge. That, in turn, complicated the problem of designing a trap to hold the series of powder sticks in the rocket motor, so that unburned portions of the grains would not be ejected by the high-pressure gases in front of the motor ends. Col. Leslie A. Skinner of the Ordnance Department and Dr. Hickman eventually found the answer in a stamped metal ring with scallops through which passed “cage” wires. The centrally perforated powder sticks were then hung on these wires. The wires had rivet heads to hold them in the scalloped ring.23 When experiments began on spin-stabilized rockets, the problem arose of how to keep the rotation from rattling the powder sticks around in the spinning tube, thus causing uneven burning. Development of a new powder composition made into a single thick-webbed grain with a redesigned trap to hold it in place promised to be satisfactory.24
Early in 1944, indeed, powder chemists found several new chemical compositions that offered advantages over the types originally employed. The new were slower burning, operated at lower pressures, and were therefore usable at a wider range of temperatures. Moreover, they could be produced by either the solvent or the solventless process. Before the new compositions were available, the fast-burning propellants tended to develop such high pressures within the motor tubes as to make them unsafe to use at very high temperatures. Premature explosions close to the launcher endangered life and limb of the user and friendly troops in the vicinity. At low temperatures the rocket might fail to ignite or might burn only intermittently. In fact, frequent motor failures in the 4,5-inch rocket, standardized in September 1942 as the M8, obliged the Ordnance Department to discontinue mass production in June 1943 and restudy the design. As reducing the amount of propellant presented the alternative of lowering the pay load proportionately or else of lessening the range, designers undertook to strengthen the motors without increasing their weight. Motors made of heat-treated, seamless, alloy-steel tubing proved able to withstand an internal pressure of 10,000 pounds per square inch and, coupled with a stronger head, extended service temperature limits to cover a range of 20° to 120° F.25
The Bazooka Rocket
Though the bazooka and the bazooka rocket had a preternaturally short history from the inception of the idea to the moment when combat troops first fired rockets from the new shoulder launcher, the story of the weapon’s development may serve to illustrate the uncertainties attending the evolution of rockets as modern military weapons. The birth of the bazooka merits attention for several reasons. The weapon was an innovation. It combined great fire power with great simplicity. It met quite admirably a particular need. It was designed, produced, and placed in the hands of troops in record time. And, perhaps because of its spectacular features, the tale of how it took shape has been confused by rival claimants for credit. As one of the participants in the project later wrote, “the number of ‘inventors’ of the bazooka has fallen and risen as troubles developed and
were cured, the stage having been reached in one part of its career where only those who worked on it could be found to claim any connection with it.”26
The bazooka and the bazooka rocket came about in a rather devious fashion. The shaped charge, fired by rocket propulsion, and the launcher were the result of several men’s pooled efforts. Rockets were used long before Francis Scott Key wrote of “the rockets’ red glare,”27 The principle of the shaped charge was promulgated by the physicist C. E. Munroe as early as 1880, when he discovered that shaping high explosive with a hollow cone at its forward end focused the explosive waves on one point and thus gave greater penetration per unit weight of the explosive. The innovation embodied in the bazooka lay in the combination and adaptation of these well-known principles and basic inventions, which imagination and skill converted into a practical new weapon. The design was steadily improved upon as production of the first models went forward.
Rockets, today part and parcel of the accepted equipment of national defense, were little considered in America between 1860 and 1940; they were superseded when rifled artillery offered greater accuracy of fire. Though signal rockets were widely used during World War I, only one man endeavored to revive interest in rockets as a supplement to conventional artillery fire. That man, Dr. Robert H. Goddard, Professor of Physics at Clark University, was the true father of modern rocketry. In the fall of 1918 this gifted physicist offered the Ordnance Department the fruits of his investigations: a 1-inch, a 2-inch, and a 3-inch tube launcher, each 5.5 feet long, and designed to fire, by an electric mechanism, rocket projectiles of 1.4 pounds, 8.5 pounds and 16.5 pounds, respectively. Just before the Armistice he demonstrated his “recoilless gun” or “rocket gun” at Aberdeen Proving Ground with results that Ordnance Department witnesses summarized as proving the validity of the principle he employed. Goddard had to use a wick fuze in place of the electric firing mechanism, which he had not had time to perfect, and solid sticks of powder instead of nitroglycerine sticks with a single perforation. Yet even with these crude substitutes marring the performance, the report of the proof officer admitted the possibility that these guns “could be developed to operate successfully against tanks.” But the lack of suitable powder and need of further work upon the electrical firing mechanism coupled with the Armistice led the Ordnance Department to shelve the project. Goddard died without receiving any acclaim for this pioneering work, though comparison of his rocket gun with the bazooka adopted twenty-four years later shows how closely the 1918 model approximated the later weapon. Only the circumstance that Dr. Hickman, then a young Ph. D. from Clark University, worked with Goddard in 1918 gave some continuity to the studies that produced the antitank rocket weapon of World War II.28
In 1933 the US Army created a one-man rocket unit by assigning Capt. Leslie A. Skinner to study the possible use of rockets. Skinner was handicapped by limited funds to expend on research and
by the indifference of his fellow officers. Hence, before 1940 the project made little headway. The British, on the other hand, in the mid-thirties perceived the potential usefulness of rocket barrages against aircraft, where volume and power of fire might compensate for lack of accuracy. By the time of the London blitz the British had developed rockets that took some toll of the Nazi bombers and fighters. The Navy in September 1940 requested NDRC to undertake a jet-propulsion research program, and in December the Army, urged on by the British experience, made a similar request. At the same time the Ordnance Department purchased British rockets and a projector for study. Thus, the American rocket program was born.29
The 2.36-inch bazooka rocket was not initially part of the Army rocket program at all. It grew out of the search for a way to use a shaped-charge projectile that an individual soldier could fire from the shoulder. The first shaped-charge projectile to get serious consideration from the Army was the rifle grenade designed by Henri Mohaupt.30 A grenade fitted with a special Mohaupt head and designed for fire from a spigot launcher was produced and standardized as the high-explosive antitank grenade M10. The spigot launcher resting on the ground much like a mortar had the serious drawback of dispersing the fire widely. Firing the grenade with blank cartridges from a rifle or from a .50-caliber machine gun necessitated resting the butt on the ground in order to get an elevation high enough to get sufficient range. The heavy recoil severely damaged the guns.31 Without a suitable projector, the powerful new projectile promised to be relatively useless.
At this point the chief of the Ordnance Department Patent Section, Gregory J. Kessenich, already familiar with the details of the Mohaupt shaped charge, conceived the idea that the basic faults of the antitank grenade could be remedied by converting the grenade into a rocket. Using a rocket made with a hollow charge and launched from a shoulder projector that an individual soldier could carry and fire would give the destructive effect of the grenade but would eliminate both the high-angle trajectory and the breakage of the rifle stock that made the M10 antitank grenade unsatisfactory. Early in August 1941 Kessenich, armed with sketches embodying his idea, presented his proposal to Col. Wiley T. Moore, chief of the engineering group of the Small Arms Division. Colonel Moore, who had just designed an attachment for the rifle to fire the rifle grenade, immediately saw the possibilities of Kessenich’s sketches and approved his enlisting the interest of Major Skinner of the Ordnance rocket unit. The sketches and a copy of the Westfaelisch Patent of 1911, which covered the hollow charge phases of the plan, Kessenich accordingly turned over to Major Skinner.
Experimentation with rockets had meanwhile been progressing at the Navy Firing Ground at Indian Head, Maryland, where Major Skinner and his assistant, 2nd Lt. Edward G. Uhl, were collaborating with Navy experts and a group of civilian scientists under Dr. Hickman of NDRC. Some months later Major Skinner completed a first conversion of the M10 grenade by adding a rocket element to the base of the grenade. About the same time Lieutenant Uhl completed a tube launcher
The Bazooka. The original 2.36-inch model M1 is shown at left, the improved model M9 at right.
that looked like a piece of stovepipe equipped with a trigger and handle. The handle contained dry batteries to supply energy electrically to ignite the rocket motor. In April 1942 Colonel Moore, then at Frankford Arsenal, produced a factory-made 54-inch launcher and factory-made parts for the converted grenade.32 With the further help of Dr. Hickman and the NDRC group, the Ordnance rocket unit had a few “respectable” antitank rockets ready for trial by May 1942. Captain Uhl, dressed like “the man from Mars,” fired the first rocket from his shoulder at the test ground of NDRC, and the next day demonstrated launcher and rockets at Aberdeen Proving Ground. He improvised a sight by using a piece of nail found on the ground. The new weapon was christened that day: its resemblance to the comedian Bob Burns’ bazooka led the colonel who fired some of the rockets to dub the device the “bazooka.” The name stuck.
The effectiveness of the rocket with dummy heads fired at a moving tank impressed the onlookers. A few days later a formal demonstration was held at Camp Sims, D. C., when high-ranking officials of the War and Navy Departments, Allied governments, and NDRC witnessed the real thing in action against a medium tank. British observers now opened negotiation for samples and Russian military staff members present at this trial immediately requested that they be supplied with some
of the new launchers even though development was still in progress. General Marshall at once issued verbal orders that 5,000 launchers and 25,000 antitank and 5,000 practice rockets be procured. The E. G. Budd Company made the rockets. On 30 June 1942 the Ordnance Committee formally standardized the 2.36-inch antitank rocket as the M6, and the launcher as the M1.33
The bazooka is thus an example of a cooperatively developed weapon in which the Army, Navy, and civilian agencies all played a part. It provided a powerful addition to infantry armament. The projected rockets could penetrate three inches of homogeneous steel armor plate at an angle of impact up to 30 degrees from normal, and retain full penetrative power up to their maximum range of 650 yards. Fired against masonry, girders, railroad tracks, or heavy timber, as well as against armor plate, they were highly destructive. While improvements upon both rocket and launcher were admittedly necessary, the first models were satisfactory enough to warrant obsoletion of the original antitank grenade from which the rocket had derived. Orders for 120,000 rockets were placed in June and in July for 75,000 launchers to be completed by the end of the year. Modifications of design of rocket fins and launcher sights were incorporated into the production units as these orders were filled.34
More drastic changes soon became necessary. Misfires obtained with the original type of ignition squib led first to substitution of a new type, but by May 1943 reports of serious malfunctions had become so frequent that the services were instructed to suspend use of the rocket pending investigation. The Ordnance Technical Committee recommended a new design of rocket motor body, using a different steel in the stabilizer tube and employing a new type of powder trap and fuze-base cover. Still more important as a safety measure was reduction of the propellant. Extensive tests showed that powder sticks cut from 23 inches to 20.75–21 inches in length gave sufficient propulsion but greatly lessened the danger of prematures even at extreme temperatures. With these changes approved, the standard bazooka rocket was designated the M6A1, the practice rocket the M7A1. Teams equipped with the new parts, materials, tools, and repair kits to make the modifications of both rockets and launchers were sent to the active theatres in July 1943.35 Some months later the M6A3 and M7A3 rockets were standardized with ogives reshaped to lower the angle of effective impact and with cylindrical fixed fins to increase stability in flight. Substitution of copper cones for steel resulted from discovery that copper cones obtained about 30 percent greater armor penetration than the steel cones of identical design.36 Moreover, in response to theatre requests the Ammunition Division of Industrial Service developed waterproof wire clamps to seal the fuze assembly against entry of water around the safety pin and thereby reduce malfunctions due to moisture.
Because rockets were relatively little known weapons, improvements often hinged upon fresh basic research. Hence, at the request of the Ordnance
Department, the Bureau of Standards in the summer and fall of 1944 conducted a number of wind-tunnel tests of fin assemblies, and Picatinny Arsenal investigated ignition characteristics calculated to give the most dependable functioning of the 2.36-inch rocket at all temperatures. Fuzes also required careful study. Experiments to develop a whole series of special bazooka rockets—smoke, incendiary, chemical, and others—brought about standardization of the M10, a smoke rocket, but no others.37 In every case the commercial companies having production contracts collaborated both in the designing work and in finding shortcuts in methods of manufacture. Joint efforts of the Atlas Powder, DuPont, Hercules, and American Cyanamid Companies improved rocket powders, while concerns assigned contracts to develop experimental components or assemblies, particularly fuzes, added to the knowledge of what new features were desirable or attainable by mass-production methods. Throughout the war the scientists of NDRC and the universities with which they contracted were amassing data on explosives that were invaluable in all phases of the rocket program.
The success of the 2.36-inch bazooka rocket inspired NDRC to propose development of a still more powerful type with greater range and a velocity increase from the 265 feet per second of the M6A3 to 500 feet per second. The resulting T59 unhappily proved in tests to be dangerous to the user and quickly became a bone of contention between scientists of NDRC and the Ordnance Department. The former, having let considerable advance publicity concerning their new “super-bazooka” rocket reach the theatres, suspected that Ordnance men were needlessly delaying its production and issue. But post war developments were to vindicate the Ordnance Department when more than six years’ work still failed to remove the bugs from the rockets.38 Work upon the T59 nevertheless led to investigation of the possibility of a larger antitank rocket, which, like the 2.36-inch, could be projected from a shoulder launcher.39 While development of a 3.5-inch Navy rocket for air-to-ground fire had begun in February 1944 only to be dropped in March 1945, a 3.5-inch antitank rocket was still wanted.40 The project was initiated in August 1944. The first experimental model, the T80, was charged with 1.9 pounds of cyclotol. Though it obtained longer range and higher velocity than the standard bazooka rocket, it fell short of that achieved by the T59 with its eight pounds of pentolite. Yet the cyclotol in the T80 would, research men believed, ensure penetration of 8-inch homogeneous armor plate. Static, flight, and penetration tests in March supplied data on which to base a revised design, but V-J Day arrived before this was proved.41
Once the projectile had reached its target, the final job was to do the greatest possible damage. The rate of speed at which the projectile was traveling at the moment of burst was of course important, but the ultimate result was determined by the quality of the ammunition itself. Some German ammunition was of better quality than American because it was more perfectly fabricated. But it had to have more exacting machining and more careful
heat-treating than US mass-production requirements permitted. To increase fire power by otherwise improving the effectiveness of the projectile was a continuing problem for the Ordnance Department. Experts in terminal ballistics attempted to solve it in several ways. One was to improve the fuze. Another was to use a more powerful explosive in the warhead or to use an old explosive in a new way, applying the shaped-charge principle. And with specific targets in mind, designers devised special types of projectiles having the mass, size, and mechanical strength required to defeat different kinds of enemy defenses.
Fuzes
Artillery fuzes were likened by an NDRC scientist to “the old-time Army mule, ornery but indispensable,”42 The orneriness was inherent in their delicate and complicated mechanism, which was directed toward detonation at exactly the right time, not too early and not too late. Extensive work in the 1920s and 1930s had produced several families of fuzes adapted to use in any caliber artillery shell.43 Designated point-detonating (PD) or base-detonating (BD) according to their position on the projectile, fuzes were further classified as “impact” or “time,” depending on whether or not they functioned only when striking a target. Time fuzes did not require impact but were set by turning a time ring before firing. As the shell left the muzzle, a time element, either a clockwork mechanism or a slow powder train, began to work, and the fuze functioned at the moment it had been set for. Time fuzes were valuable for fragmentation effect, for smoke or illuminating shells, and especially for antiaircraft fire. Impact fuzes, used against targets of varying degrees of resistance, were of several types: supersensitive, superquick, nondelay, delay, or a combination of these. Supersensitive action was necessary against insubstantial targets such as airplane fabric. Against more solid targets, superquick fuzes were used when penetration was not desired, nondelay for detonation at the moment of penetration, and delay—usually of .05, .15, or .25 seconds—when penetration in varying degrees was wanted.
Fuzes need not be single purpose, though many were. Selective-type fuzes could be adjusted in the field for more than one kind of action, such as superquick and delay, or time and superquick. For example, the M48 fuze, used with the 75-mm., 3-inch, and 105-mm. high-explosive shell, and the M51, its counterpart for larger calibers, could be set to function immediately on impact or with a delay of .05 or ,15 seconds. In all fuzes, mechanical safeguards restrained the firing pin until the right moment for detonation arrived. When the gun fired, the sudden and violent forces arising from acceleration removed one set of safeguarding devices. In flight the centrifugal force of the shell as it rotated about its longitudinal axis removed the last set of safeguards and the fuze was armed, that is, free to function.44
Engineers strove with some success to adapt the fuzes developed before 1940 to the tactical situations of World War II. The chief problem, determining the most effective delay times, was difficult to solve because the resistance of targets varied greatly.45 On the whole, troops in all
theatres preferred the combination to the single-purpose fuze.46 One important requirement for use in air-burst neutralization was an accurate, long-delay time fuze with a superquick functioning feature. The M67 mechanical time fuze could be set for a delay of 75 seconds, but as it depended on the turning of gears in flight and had no provision for detonating on impact it was unsatisfactory for neutralization.47 One possibility for development was the M54, a powder-train type of time fuze with the desired superquick action. But the maximum range of the M54 was only 25 seconds. The problem of how to triple that range was not fully solved when the war ended.48
The most important point-detonating fuze developed during the war was a radical departure from other types, as it had a steel nose that adapted high-explosive ammunition to concrete-piercing uses. Since 1940 designers had been trying to find a way to destroy fortifications made of concrete reinforced with steel bars, such as the Siegfried Line. Against such targets, standard high-explosive shells would throw the fuze and become duds. Armor-piercing ammunition, based-fuzed, though better, lacked enough power to remove earth or sandbags placed in front of the concrete, to blow the reinforcing bars and debris from the impact area, or to cause large enough craters to make successive hits effective. The best solution was a completely new high-explosive shell with a base-detonating fuze. But that meant a new round for each weapon, further complicating the already complicated ammunition situation, and long delay in getting ammunition to troops. A more expedient answer was a steel nose fuze that could be attached to high-explosive rounds already in the field. Col. George G. Eddy began the development at Aberdeen in the summer of 1943. Tests against prototypes of West Wall fortifications showed the most practical design to be a fuze body made of molybdenum steel, heat treated for greater strength, with the delay assembly of the M48 and M51 fuzes and a modified booster. Developed in a matter of weeks, the steel nose fuze, standardized as the M78, was rushed to General Devers in Africa and successfully tried out at Cassino. Later it helped to breach the Siegfried Line and the log-and-earth bunkers encountered in the Pacific.49
The concrete-piercing fuze and the long-delay superquick time fuze were examples of developments to meet new tactical conditions and new targets. Fuzes for rockets, on the other hand, presented still another problem. Conventional artillery fuzes were not suitable because, thanks to the comparatively long burning time of rocket propellant, the rocket reached its maximum velocity much more slowly than the artillery shell, had virtually no setback force, and attained very much lower velocities. With these differences in mind, engineers designed new fuzes for the 2.36-inch bazooka rocket and the 4.5-inch, the models chiefly employed for ground fire in World War II. The bazooka rocket,
primarily an antitank weapon with an armor-piercing head, used a base-detonating fuze. When the first fuze proved prone to function prematurely, designers found that a bore-riding pin provided greater bore safety. The pin, held in place by the wall of the launcher while the rocket was traveling through the launching tube, was released at the muzzle and the fuze was then armed. Later simplification of design reduced the number of parts from thirteen to seven. The new fuze, the M400A1, combined with the metal parts adopted for the M6A3 bazooka rocket, gave the M6A5 rocket greater plate penetration and sensitivity than that of any earlier model.50 Instead of the base-detonating fuze of the 2,36-inch, the 4.5-inch was equipped with a point-detonating fuze. Though two fuzes were at first thought to be necessary, one with superquick action, another with a short delay to permit airburst from ricocheting rockets, preliminary studies indicated that the two requirements could be combined successfully in one fuze, the setting of which could be adjusted for either superquick action or a delay of .10 second. The fuze embodying these features was standardized in the summer of 1943 as the M4. Used not only with rockets but also with mortars, it was especially valuable in the South Pacific. The 4.5-inch rockets, particularly when launched in rapid succession from multiple-tube projectors, had a high-explosive effect peculiarly useful for dispersed fire against enemy concentrations and area targets.51
But barrage fire depending for effect on saturating an area rather than on placing shots squarely on a particular target was frequently futile. With time-fuzed fire, relatively few bursts could be properly placed, and because the many shots needed to get on target destroyed the element of surprise, the enemy would have warning and could seek shelter. What was needed was a fuze that would operate not by time but by proximity to the target, for airbursts occurring at just the right distance would make foxholes useless and divest revetments and trenches of means of providing shelter from attack. Development of that kind of fuze occupied some of the best scientific brains of America and Britain and became one of the technological triumphs of the war.
The Navy had been considering the possibility of devising an influence fuze even before the National Defense Research Committee was established in June 1940. Hence, this project was the first that the Navy asked NDRC to study. In September the Tizard Mission to the United States made available to American scientists the fruits of British research upon the problem, data of great value in advancing American work. The British had been focusing attention upon fuzes for bombs and antiaircraft rockets. For shells, British physicists had regarded the difficulties as virtually insuperable. The Army request for NDRC help on proximity fuzes came in mid-January 1941, NDRC assigned the research on bomb, mortar, and rocket fuzes to a division under Dr. Alexander H. Ellett, and shell fuzes to a section headed by Dr. Merle A. Tuve.
Though sometimes popularly assumed to stand for “variable time” or “vacuum tube,” VT was merely a code name. The means of operating VT fuzes to detonate without regard to time but rather at a given distance from the target could conceivably be several. NDRC began
investigation of each method that appeared even remotely feasible—acoustic, photoelectric, radio and, after consultation with the British, radio-pulse fuzes triggered from the ground. The characteristics listed as essential by the Ordnance Committee for bomb and rocket fuzes give some idea of the complexity of the problem; in artillery and mortar shells space limitations and the violence of setback made a solution even more difficult than in bombs. The first stipulation was that the fuze must function when passing within effective fragmentation range of the target, whether in daylight or darkness, and in any kind of weather. Secondly, there must be a self-destructive feature to prevent injury of Allied troops and damage to equipment in case of missing the enemy target.52 Separate storage of the power unit, presumably batteries, was also required, so that fresh units could be attached shortly before use. And, lastly, the mechanism must have sufficient ruggedness to withstand the rotational and setback forces of conventional artillery.53
The answer NDRC finally came up with was a radio-operated fuze triggered by reflection from ground, aerial, marine, or submarine target. The underlying principle was simple enough. Anyone familiar with electronics would immediately comprehend it, for it was essentially an application of short-wave transmission and reflection from the target, with use of the so-called Doppler frequency. Contained within the fuze was a miniature transmitting and receiving oscillator, an amplifier, an electrical firing circuit, and electrical and mechanical safety devices. The radio transmitter broadcast a continuous signal that was reflected back to the fuze as it approached its target. As the oscillator received the signal back, an interference, the Doppler frequency, was set up, which, when intensified sufficiently by the amplifier, passed a signal through to set off the firing circuit and detonate the projectile. If this sequence was simple to understand, fitting the mechanisms into the confined space of a fuze was complicated in the extreme. Several nations had issued patents for proximity devices, but no patent explained how the mechanism was to be made. By the end of the war Germany had in production a radio-operated fuze for rockets and had made some progress in design of acoustic fuzes. None of these saw service, presumably because production problems had not been solved satisfactorily.54 The triumph of American research lay in successfully designing a fuze that could be manufactured by assembly-line methods.
As was true of fuzes operating on time or impact, VT fuzes were a considerable family. Each weapon had to have its own fuze, differing in particulars from that designed for every other weapon. The intensive study and elaborate, ingenious testing that produced effective, safe proximity fuzes for bombs, rockets, antiaircraft and field artillery shells, and, by the very end of the war, for 81-mm. mortars must command the utmost admiration. The technical research was chiefly the work of NDRC and its contractors. Three universities, the Department of Terrestrial Magnetism of the Carnegie Institution, the Bureau of Standards, and some twenty commercial companies all contributed. Infinite patience and wide scientific knowledge went into development of the proximity fuze, but without the experts of the Army and Navy who supplied the
VT fuze. Air burst of a 105-mm HE shell and dust rising as shell splinters strike the ground in the impact area.
essential data to make the elaborate electronic device practicable in ammunition, the laboratory achievement must have counted for little. The first fuzes were far too big and clumsy to be useful in combat. And after gradual, step-by-step corrections of that obstacle, the models hand-made at laboratory bench by skilled technicians had still to be adapted to mass production. Success in the end was the result of the full collaboration of civilian scientists, Army and Navy ordnance specialists, and experienced industrial engineers.55
The first VT fuzes used in combat were fired in January 1943 from Navy guns on the cruiser Helena to bring down a Japanese plane. For the next eighteen months use was restricted to the US and British Navies, lest the enemy salvage enough pieces to copy the design. In the summer of 1944 Army antiaircraft batteries in England fighting off the German buzz bomb attacks were issued some of the new fuzes and employed them with effect, but their widest and most deadly application was against enemy ground troops. In October 1944 the Combined Chiefs of Staff approved preparation for release of VT fuzes for ground warfare in Europe. From stockpiles that had been accumulating since the fall of 1943, shipments to Europe built up ample supplies. Teams of officers sent to instruct ETO artillerymen in the proper use of this new killing device had the scene set to put it into action on Christmas Day
1944.56 Generalfeldmarschall Gerd von Rundstedt’s sudden stealthy move to cut the supply lines of the US First Army in the Ardennes hastened the day. Though only 3,000 VT fuzes were issued to the VIII and V Corps stemming the enemy advance, and though the wooded terrain reduced the effectiveness of the fuzes, the surprise produced was considerable. Here was a truly secret weapon. After the Battle of the Bulge, Allied ground forces used VT fuzes in dozens of actions. In interdictory fire the effect was to deny the enemy use of key bridges and roads, for, as General Eisenhower’s headquarters cabled, “by the unprecedented effectiveness of unseen fire at all hours of day and night, the enemy has been severely upset ... ,”57 Introduced nearly simultaneously into the Pacific theatre, there also the “funny fuze” in rockets and bombs disrupted the enemy.58
High Explosives
Increasing the destructive effect at the target meant not only using suitable fuzes but also the most powerful explosive consonant with safety and dependability. The explosive in the warhead of an artillery shell had to be able to stand the shock of setback when the projectile was fired and the shock of impact against steel or concrete. That is another way of saying that it had to be obedient to the fuze. Trinitrotoluene, commonly called TNT, possessed this essential quality to a gratifying degree. It would not explode until the initial weak impulse from the detonator had set off a booster charge consisting of a small amount of a highly brisant explosive such as tetryl or PETN,59 Considerable power combined with insensitivity, which made for easy loading, stability, and safety in handling and transport, made TNT and amatol, a mixture of TNT and ammonium nitrate, the preferred fillings for most high-explosive artillery shells at the outset of World War II. That they remained so was largely due to their availability in large quantities. As the war progressed and ammunition in general became more complex in design and more specialized in function, demand arose for improved explosives. This demand could not be met to any extent because the explosives developed between wars did not get into large-scale production in time. Nevertheless, throughout the war the Ordnance Department sought both to find ways of using more powerful new explosives and to adapt old ones to special purposes.60
There were at hand several explosives of higher shattering effect, or brisance, than TNT. The most important were, first, cyclotrimethylene-trinitramine, which the Americans called cyclonite and the British called RDX, “Research Department Explosive”; second, pentaerythritol tetranite or PETN; and third, ethylenedinitramine or EDNA, later named haleite. RDX and PETN were far too sensitive to be used in the pure state in a shell. It was therefore necessary to mix them with oils or waxes, or with other explosives, to form usable compositions. The British had managed to desensitize RDX by mixing it with 9 percent beeswax to form Composition A, for press-loading into shells; with 39.5 percent
TNT and 1 percent beeswax to form Composition B, chiefly for bomb loading; and with 11.7 percent of a plasticizing oil to form Composition C, for demolition work.61 These formulae were brought over by the Tizard Mission in 1940. In America development of Compositions A, B, and C was undertaken by the DuPont Company under contract with NDRC. Because the Ordnance Department wanted Composition A in granular form instead of the lump form specified by the British, DuPont produced a granular mixture designated Composition A3; but it had low priority in the very tight RDX program and played little part in ground warfare.
More promising than RDX for shell loading was haleite. From the 1920s onward chemists at Picatinny had been trying to find a compound that would have the high brisance of RDX without its sensitivity to friction and impact. Research on this problem, principally by Dr. George C. Hale, chief chemist, led to the discovery of ethylenedinitramine, or EDNA, the first entirely American high explosive. More powerful than TNT, it was slightly less powerful than RDX but was less sensitive. Its stability gave it an important advantage in considerations of manufacture, loading, storage, transportation, and use in the field. This advantage was offset in prewar days by the high cost of manufacture of one of its intermediates, ethylene urea. But, by the combined efforts of NDRC and the DuPont Company, the obstacle was removed, and in the late spring of 1943 EDNA was adopted for testing purposes. Designated haleite in honor of Dr. Hale, this new explosive could be press-loaded into small shells without a desensitizing agent, and its derivative, ednatol, a mixture containing 42 percent TNT, could be melt-loaded into large shells as easily as was amatol. Delay in solving manufacturing problems, however, prevented haleite from getting into combat.62
The most sensitive of all high explosives was PETN, which was even more readily detonated by fuze-booster systems than was RDX. It was desensitized by mixing it with TNT to form a composition named pentolite, which was extensively used in detonators, bazooka rockets, rifle grenades, boosting devices, and in the shaped charges of high-explosive antitank shells.63
Much of the work of improving high-explosive compositions was directed at finding the most efficient filling for antitank shells. For armor-piercing projectiles, relatively insensitive ammonium picrate, “Explosive D,” had long been preferred. As it was not likely to detonate on impact, the shell could penetrate the tank before exploding. But experience with German heavy tanks in North Africa showed that something more was needed in the way of power and fragmentation, coupled with greater incendiary effect within the tank. Chemists at Picatinny accordingly tried several expedients. In armor-piercing shell, addition of a small amount of powdered aluminum to cyclotol—a mixture of RDX and TNT—to ednatol, or to TNT produced more brisance than Explosive D and increased sensitivity to impact. In high-explosive antitank shell fillings, conversely, the difficulty was the exact opposite. The high sensitivity of pentolite made it liable to detonation by target impact so
that the problem was to tone it down to the proper degree of insensitivity. Among several possibilities considered were the addition of wax to the pentolite, the reduction of the PETN content, and the substitution of ednatol or Composition B. None was entirely satisfactory.64
The search for an explosive composition of the greatest possible power and brisance took a new turn after analyses of foreign explosives at Picatinny during 1943. Hitherto, research had been concentrated on two-explosive compositions such as pentolite, Composition B, or ednatol. The examination of a Soviet 76-mm. high-explosive armor-piercing round suggested the possibility of employing ternary mixtures. Tests revealed that castable ternary explosive mixtures such as RDX-Tetryl-TNT and Haleite-PETN-RDX offered great promise not only for armor-piercing projectiles but as fragmentation ammunition for weapons designed to produce blast and for demolition charges. Further study showed that the haleite ternaries were unstable. The best combination seemed to be 28 percent PETN, 43.2 percent RDX, and 28.8 percent TNT, a mixture designated PTX-2 (Picatinny Ternary Explosive). More brisant than any of the binary compositions, it was more stable than 50/50 pentolite, and less sensitive to impact. Preliminary firings at Picatinny indicated that it would be particularly adapted to shaped-charge ammunition. At V-J Day, PTX-2 was still in the testing stage.65
In spite of intensive effort, chemists at Picatinny and NDRC’s laboratories failed during the war to develop any new explosive composition for shell loading that was wholly satisfactory and readily available in quantity. The obstacles were often disheartening. The characteristics of an explosive might be considerably affected by impurities existing in the raw material to begin with or admitted in manufacture; the instability of PETN, for example, was probably due to impurities in its raw material, pentaerythritol. Other variables that had to be taken into account were the different methods of testing and differences in interpreting results. Assuming that a composition had been hit upon that promised to combine greater brisance with less sensitivity, there was still the question whether it could be economically manufactured, safely handled, and made unchanging in character in temperatures ranging from arctic cold to tropical heat. In the search for explosives with special properties, much work had been done in the field of aluminized explosives, but, although aluminized TNT (tritonal) was used in bombs, much remained to be done. At the end of the war the Ordnance Department felt that deeper study of the fundamental properties of all high explosives was essential to effective development in the future.66
If no new explosive for artillery shell came into use during the war, a new way of employing explosives nevertheless did. The effect of a hollow-charge or shaped-charge projectile against armored targets was first successfully demonstrated by the bazooka and the rifle grenade. The intense forward jet of the charge, serving to focus part of the energy of the explosion in a limited area, gave to the light-weight low-velocity rocket the armor-piercing advantages hitherto possessed only by high-velocity artillery. The antitank rifle
Rifle grenades. M9A1 rifle grenade and launcher (left); rifle grenade, adapted from the 60-mm. mortar shell (right).
grenade, containing only four ounces of pentolite, would penetrate up to four inches of homogeneous armor plate at a normal angle of impact. If not so powerful as the antitank rocket, which had equal penetration at angles of impact as great as 50 degrees,67 the M9A1 rifle grenade still did excellent, albeit less publicized, work not only against tanks but against bunkers and pillboxes, where its good fragmentation characteristics were especially valuable. There were times when the infantryman preferred it to the bazooka because the grenade launcher could be fitted on the rifle he already had and did not involve carrying an extra weapon.68
Application of the shaped-charge principle to artillery naturally proceeded also. The choice of howitzers was logical be- cause their low velocity made conventional types of armor-piercing projectiles ineffective, whereas for a shaped charge low velocity was an advantage. Before Pearl Harbor in an atmosphere of great secrecy, work began upon a shaped-charge shell, the “HEAT,” for the 75-mm, howitzer. The designers, paying careful attention to the length of the ogive, the filler, and the striking velocity, came up with a 13,5-pound shell of the same length as the corresponding high-explosive round. The high-explosive, antitank shell, containing a filler of 50/50 pentolite, at a muzzle
velocity of approximately 1,000 feet per second would penetrate 3 inches of homogeneous armor plate. A similar shell for the 105-mm. howitzer appeared simultaneously.69 Standardized in late 1941, HEAT shells were produced in time to take part in the North African tank battles early in 1943. The Ordnance Department had high hopes that they would succeed in penetrating the German heavy armor plate that had defeated solid armor-piercing ammunition.70 But though sometimes successful, performance of shaped charges was not dependable. When they worked they worked like a charm, but they were ineffective disconcertingly often. In an effort to find out why, the Ordnance Department, with the help of NDRC and the Navy, intensified its research. In some cases observers in the field had blamed faulty manufacture, but investigators proceeded on the assumption that design of the round and the principles of operation needed improvement. Because of the difference in behavior of the nonrotated rocket and the rotated shell, the effect of spin was carefully studied as well as the method of fuzing. One of the most important discoveries was that increase in plate penetration was directly proportional to increase, up to approximately three calibers, in “stand-off” distance, that is, the distance from the base of the cone to the target at the moment of detonation. Yet this finding was only a beginning, and the solution to the puzzling behavior of hollow-charge projectiles was not found during World War II.71
Special Purpose Projectiles
Shifting conditions of battle in a war on many fronts and the constant demand for greater fire power against all kinds of tar gets spurred development of new projectiles and the adaptation of old ones to new purposes. In addition to changing the method of fuzing or using different fillers, the size, shape, and weight of the warhead itself, and the material of which it was made, had to be studied carefully with special targets in mind. The shell directed against troops or lightly armored defenses was necessarily different from that fired at tanks or concrete fortifications. Still another type would be required for smoke screening or signaling. And within these categories, a shell that might be effective in the European theatre might be unsuited to the jungle and cave warfare of the Pacific. Thus, the 105-mm. howitzer alone fired some thirteen different types of ammunition, delivering to the enemy projectiles that varied in persuasive power from the shaped charge to the propaganda leaflet.72
Of ammunition employed primarily against troops, perhaps the most interesting development was the adaptation of canister to modern warfare. This ancient type of projectile was the simplest imaginable, a cylindrical sheet-metal can filled with small steel shot set in a matrix of resin. But in jungle warfare canister proved surprisingly effective for stopping massed Japanese attacks and for clearing
jungle undergrowth. Containing neither fuze, ogive, nor bursting charge, it depended for operation on the shock of discharge, which ruptured the case and scattered the shot forward. The canister most used was the M2, containing 122 steel balls, which was fired from the 37-mm. antitank gun. Larger rounds containing 390 balls were developed for the 75-mm. and 105-mm. howitzers as a part of the extensive program for jungle weapons begun in the fall of 1943.73
Aside from canister, departures from prewar design were few in projectiles for use against troops and light emplacements. Developments were chiefly concerned with fuzes calculated to give greater range and more effective burst, and with providing terminal ballistic data to enable field commanders to make optimum use of the weapons. At the Ballistic Research Laboratory, in the course of intensive basic research on fragmentation, ultra-highspeed X-ray equipment was developed that enabled technicians to make radiographs showing what happened to a shell immediately after detonation. These remarkable pictures gave more insight into the nature of fragmentation than any yet attained and offered much promise for future development.74
Similar in ballistics to the high-explosive shell was the chemical shell. The filling, whether gas or smoke, was the responsibility of the Chemical Warfare Service, but the Ordnance Department supplied the case, the burster—a tube containing tetryl running down the center of the cavity—and the fuze, usually a superquick or combination time-and-superquick type. As gas was not used in combat, Ordnance concentrated on developing various types of smoke shells. Some had no burster. For example, the base- ejection type depended on an expelling charge of black powder that forced the smoke canister out of the base of the shell. It could lay down a smoke screen, or signal with colored smoke, but its construction made it useless for certain weapons. Of all smoke ammunition, the older and more versatile burster shell filled with white phosphorus turned out to be the most useful in combat. It was valued not so much for its screening ability, which was limited, as for its good qualities both as a spotting agent for signalling to aircraft observers and as a means of producing casualties and demoralization among enemy troops.75 Phosphorus caused severe burns that were slow to heal. The white phosphorus (WP) shell developed for the 60-mm. mortar in the fall of 1943 was immediately popular in the theatres and brought about a demand early in 1944 for WP shells in calibers from 75-mm up.76 A favorite with the infantryman was the WP rifle grenade, which was extremely useful in cleaning the enemy out of open trenches or smoking him out of bunkers and foxholes.77
Of necessity, Ordnance Research and Development Service focused much of its work in terminal ballistics on developing
projectiles able to penetrate the heavy tank armor, pillboxes, log bunkers, and strong concrete fortifications that opposed the American advance from 1942 on. Reports from North Africa early in 1942 of tests on captured German tanks showed that under existing conditions American armor-piercing shot usually failed to penetrate German face-hardened armor. At the ranges required in desert fighting, this was true even of high-velocity weapons such as the 90-mm. gun. When shot did penetrate, fragmentation was insufficient to wreck the tank interior. To increase the armor-piercing quality, Ordnance engineers added to all calibers of armor-piercing shell a device already developed for the less powerful 37-mm. antitank shell, a steel armor-piercing cap with a windshield affixed for improved ballistics. To increase the destructive effect after penetration, these armor-piercing-capped—APC—projectiles were provided with a small cavity containing a bursting charge of Explosive D and with a base-detonating fuze. With these improvements, the 90-mm. APC M82 projectile, for example, would effect a deeper penetration of face-hardened armor plate at a thousand yards than the 90-mm, armor-piercing.78 American 75-mm. APC ammunition, though then made without an explosive charge, was credited with saving the day in Libya, as British uncapped ammunition was ineffective against German face-hardened plate. APC projectiles with the explosive charge were late getting into production, partly because the base-detonating fuze was not available.79
Hardly had the new APC high-explosive ammunition reached the battlefields when the even more heavily armored Panther and Tiger tanks made their appearance. They could not be knocked out even by the 90-mm. M82 or the M62 APC provided for 3-inch and 76-mm. guns. This new threat called for a shell of radically different design. The Ordnance Department’s answer was “HVAP.” The principle underlying the effectiveness of HVAP—hypervelocity armor-piercing am-munition—was that the energy of a projectile is a function of the square of the velocity. Light-weight, hard-cored projectiles could attain hypervelocities, that is, from 3,000 to 4,000 or more feet per second, and thus have more than double the energy and hence penetrating power of those fired at ordinary velocities, which seldom exceeded 2,600 feet per second. Study of captured ammunition for the German tapered bore gun established the possibilities of the German design.80 It had a core of tungsten carbide, a material of such density and hardness that it would not shatter on impact at high speeds, as steel was likely to do. To adapt this projectile to standard cylindrically bored weapons, the best method appeared to be to use a lighter weight shell made with an aluminum alloy jacket. Testing of a first experimental model, the T24, designed for the 37-mm. gun, began at Aberdeen in the spring of 1942. That summer NDRC, at Ordnance request, undertook basic research on the behavior of hypervelocity projectiles at the target.81
Despite the approximately 50 percent greater penetration of this new type of projectile as compared to that of standard armor-piercing ammunition, no 37-mm. HVAP was made for service use, nor was
HVAP for bigger shells pushed until 1944. Before that time the Ordnance Department considered the existing 75-mm. and 3-inch armor-piercing projectiles capable of defeating any enemy tank so far met on the battlefield, and tungsten, imported from China, was in critically short supply.82 But after D Day US commanders in France cabled urgent appeals for weapons of high velocity to use against the German heavy tanks, more heavily armored than anticipated. The Ordnance Department then went quickly into action. Thanks to the earlier development work, the HVAP shot T4 for 76-mm. and 3-inch guns was in the field by September. This achievement was close to a record for speed. Unfortunately, the T4, although an improvement over any preceding armor-piercing ammunition, failed to solve the major problem, for it did not successfully penetrate the glacis plate of the Panther tank at practical ranges.83 Continued development produced an improved design, the T4E20, standardized early in 1945 as the M93. By then a 90-mm. HVAP round had been developed which, following combat tests, was standardized as the M304 in June. Containing an 8-pound core of tungsten carbide as opposed to the 4-pound core of the 76-mm. HVAP shot, and at the same time attaining a muzzle velocity about 400 feet per second greater, the 90-mm. shot could destroy the German Panther and Tiger tanks at ranges up to 2,000 yards. Against the King or Royal Tiger it was only partially effective.84 Foreseeing future need for penetrating even thicker armor at more difficult angles of presentation, the Ordnance Department carried an extensive HVAP development program into the postwar period.85
Breaching the heavy concrete obstacles and massive masonry walls encountered by the American forces in Europe required the utmost in explosive power, much more than most armor-piercing or semi-armor-piercing ammunition could deliver. The Engineers’ demolition blocks, and notably shaped-charge blocks, were effective when fighting was at close quarters,86 but at long ranges the problem remained of getting a high-explosive shell that would penetrate and not break up on impact. The best solution found was a special steel fuze that could be used with high-explosive ammunition of 75-mm, to 240-mm,87 A still sturdier fuze together with an explosive more brisant than TNT promised even better results. In the interim, the Ordnance Department pushed work on ammunition for the 914-mm. mortar then under development. Though the shell for the Little David with its 1,584 pounds of high explosive proved in tests to have far more destructive capacity than any projectile ever previously conceived, at the end of the war its accuracy was still unsatisfactory.88