Chapter 15: Aircraft Armament: Weapons for Air-to-Air Combat
The problems antiaircraft batteries had to deal with in countering the powerful attacks of Axis aircraft were matched, perhaps more than matched, by those the Army Air Forces encountered. Aircraft manufacturers in the United States during the 1930s had been building planes capable of ever-increasing speeds, but as specifications for military craft had slighted the concomitant developments—faster-firing guns, protecting armor, and self-sealing fuel tanks—these netted relatively scant consideration before 1939. Fuel tanks, as an integral part of the plane, were an Air Corps responsibility. Before 1940 the Air Corps had rejected the idea of armored planes, and only when General Henry H. Arnold insisted, after checking the reports of the Battle of Britain, was any attention given to use of protecting armor. Testing suitable materials and preparing specifications then fell to the Ordnance Department because of its experience in ballistics and knowledge of the behavior of plate on armored vehicles under fire.1 But the major task of the Ordnance Department in developing matériel for the Army Air Forces was to design guns and ammunition for attack.
The Problem of Speed
When President Roosevelt in January 1939 requested of the Congress vast sums of money to produce 50,000 aircraft, only Ordnance experts and a handful of Air Corps officers fully appreciated the need of equipping these planes with very fast-firing guns, with more guns per plane, or with both, in order to score hits on enemy planes traveling at new high speeds. General Arnold, to be sure, as early as the summer of 1937 had requested the Ordnance Department to increase the cyclic rate of the .50-caliber aircraft machine gun, and the Springfield Armory and Colt’s Patent Fire Arms Company had spent some time on study of the problem. But funds were skimpy, and progress had been proportionately slight. Furthermore, the Air Corps had submitted no list of required military characteristics.2 Whether to add more guns to a plane to increase its volume of fire was a matter for the Air Corps to decide; the quality of those guns, in keeping with the user’s specifications, was up to the Ordnance Department.3
American fighters in 1939 carried nothing heavier than .30-caliber and
.50-caliber machine guns in multiple mounts on the wings, fuselage, and nose of the plane.4 Though the standard .50-caliber Browning machine gun M2 of 1939 could fire 600 rounds a minute on a rigid test mount, in some planes that rate was reduced by about 100 rounds a minute, partly because of the aircraft mount’s resilience and partly because of the heavy ammunition-belt loads on the gun’s feed mechanism. The .30-caliber aircraft gun fired 1,200 rounds a minute, but its muzzle velocity was low and its bullet light.5 Aircraft guns had to be mounted in confined spaces, so that any redesign or modification must, if possible, retain the external dimensions of the originals in order to minimize modification of the plane to accommodate the new models. Not until late September 1939 did the Air Corps establish the military characteristics wanted in a machine gun of cyclic rate rapid enough to be effective in the shortened “on-target” time of new planes. By then it was clear that what would suffice to hit a target moving at 200 miles an hour would not serve against aircraft flying at 300 to 400 miles an hour.
Higher Cyclic Rates
Accordingly, the Air Corps’ major requirement for the new gun was a cyclic rate of at least 1,000 rounds a minute and as much more as other features would permit. The time of bullet flight was to be .7 second for 600 yards and at that range penetrations of .75-inch armor plate must be possible when armor-piercing bullets were used. The over-all length of the gun should be kept within 68 inches and weight as low as was consistent with efficient performance. Other requirements were full automatic fire, controlled by hand and by trigger motor, right-hand and left-hand ammunition feed, adaptability to mounting for either fixed or flexible use, and the least possible trunnion reaction, that is, the lowest possible strain on the shafts upon which the gun was mounted in the plane. Air-cooling the gun with an aerodynamic barrel jacket extending at least 20 inches aft of the muzzle was listed as essential. The Colt’s Patent Fire Arms Company, which owned John Browning’s patents, undertook design of a gun to meet these requirements, only to have each model tested during the next two years show such serious defects as to be wholly unsatisfactory. In 1940 engineers at the Springfield Armory, by lightening the barrel of the standard M2 machine gun and by substituting double driving springs for a single spring, pushed the cyclic rate up to 800 rounds a minute, but that was still far below the 1,000 rounds a minute the Air Corps wanted.6
Increasing the cyclic rate of a gun such as the .50-caliber M2, designed to fire at 650 to 800 rounds per minute, resulted in added stresses upon the barrel and all the moving parts. Even if other components were strengthened enough to avoid excessive breakage, the problem of barrel erosion would remain. The hot gases generated by the explosion of the powder charge softened the bore surface, the chemical composition of the powder attacked the metal, and the high temperature and
pressure tended to expand the bore in a very fast-firing gun. Inaccuracy would therefore soon develop unless barrel erosion could be lessened by cooling the barrel, by using a heavier barrel, or by improving the barrel’s metallurgy.7 In February 1942 the Ordnance Department requested NDRC to study the whole problem. NDRC let contracts to some twenty-six companies, universities, and other research institutions, each of which followed out a particular line of investigation. Two and a half years of work, notably that of the Crane Company and the Geophysics Laboratory of the Carnegie Institution, produced a liner of a special alloy which, fastened into the breech end of the barrel, greatly reduced erosion. Further experimentation showed that combining this material at the breech end with chromium plating extending to the muzzle end gave still higher erosion resistance and better general performance.8
In the interim, efforts to develop a machine gun of high cyclic rate had continued. Early in 1942 the High Standard Company of New Haven offered a promising design, though the models tested that summer at Aberdeen lacked both the strength and the reliability to be acceptable. Later models, built under a development contract with the Ordnance Department, showed marked improvements but still failed to meet all requirements. Throughout, High Standard worked on the basis of designing a high-speed gun in which changes from the M2 would be kept to a minimum. This came to be a big handicap. Consequently, in August 1943 the Ordnance Department entered into contract with the Frigidaire Division of General Motors Corporation, with the understanding that a gun be developed using the basic mechanism of the M2 air craft gun but with no restrictions upon the number of changes that might be made. In short, the Ordnance Department abandoned any plan of having parts of the new gun interchangeable with those of the M2. Frigidaire’s first model was ready for test in March 1944. It was essentially a new gun; only minor components were interchangeable with the standard M2. Numerous changes were still needed but, by adopting some features of the High Standard experimental models, Frigidaire succeeded by the fall of 1944 in producing a weapon, the T25E3, that had a cyclic rate of 1,250 or more rounds per minute and functioned well enough to warrant fabrication of a hundred for AAF and Navy service test. By the next April the guns that had been carefully service-tested at Wright Field were proving so far superior to the M2 that the Air Forces requested immediate standardization of the new model. The latter then became the .50-caliber aircraft gun M3, and the M2 was reclassified as limited standard.
Development of a machine gun with cyclic rate increased from 800 to 1,200 rounds per minute, at the cost of only a pound in weight and with no significant change from the over-all dimensions of the slower-firing M2, was so impressive an achievement as to merit some particulars. It was accomplished largely by twelve new
features. First was a bolt of improved metallurgy and design with holes drilled through to reduce weight. Second was an extractor with a reversible ejector which eased ammunition feeding. Third was the substitution of a pneumatic barrel buffer for the older oil buffer, a change that produced smooth operation regardless of extremes of temperature. Alteration of the curvature of the accelerator resulted in more effective use of the energy of the barrel and barrel extension to accelerate bolt recoil, while a Belleville spring back-plate buffer, using cupped steel washers, accelerated counterrecoil of the bolt. Rigidly mounted breech-lock depressers added to stability of the gun’s components during operation. Redesign of the back-plate and of the breech-lock cam strengthened the construction and gave smoother functioning, and an improved firing pin provided about five times as long a life as that of the M2 firing pin. The two features incorporated from the High Standard guns were a special cover assembly to increase ammunition belt-life capacity and split belt-holding pawls to improve ammunition feeding. Use of the new erosion-resistant lining for the barrel, moreover, permitted firing long bursts without loss of accuracy or marked drop in velocity.9
Unfortunately, few M3’s saw action in World War II, as only some 2,400 were completed before September 1945. Yet in the fall of 1944 Ordnance engineers perceived that some desirable proven features of the High Standard and Frigidaire highspeed models could be readily applied to the M2. Among the parts to be used were High Standard’s extractor assembly, recoil booster, wide top-cover assembly and split pawls, as well as two parts designed by Frigidaire for the still unperfected T25E3 model. With improved metallurgy, use of a lined barrel, removal of the oil from the oil buffer, and one or two lesser changes, this modified M2 gun, the T36, could fire slightly over 900 rounds a minute. In October 1944 the Air Forces requested 31,336 of these, but rapid progress on the T25E3 limited output to some 8,000.10
Encouraging though these developments were, still faster-firing aircraft guns would clearly be advantageous. The Germans, indeed, believed that extremely high cyclic rates would offer the best possible method of combating high-speed craft. Toward the end of the war observers reported on a German development program to introduce aircraft machine guns with cyclic rates well beyond those under development in the United States. The object was to lay down a dense curtain of fire from a very short range. These experimental guns fired short bursts, with firing initiated photoelectrically when the plane was in the proper position.11 From the American point of view the great drawback of the device was that it did not permit continuous or prolonged fire, a feature the AAF regarded as more important than a single, very fast burst. In 1945 the maximum rate the Ordnance Department set as its goal was 1,500 rounds a minute. Springfield Armory for a time worked on a design of a totally new mechanism suitable for highspeed operation, and Frigidaire designers continued efforts to increase the rate of the M3 to 1,500 rounds per minute, but the war ended before either type had reached the stage of extended testing.12
Metallic link belt installation in the nose of a P-38
Improvement in the metallic link belts for aircraft machine guns was also needed to make them sturdier and flexible enough to use in restricted spaces such as the plane’s tail. The complexity of the problem may be envisaged by study of the accompanying photograph of an installation. A project to improve the link began in the spring of 1940, and, while the Air Corps accepted a modified link early in 1941, search for a design still more serviceable and easier to manufacture continued throughout the war. If, in addition to getting a high degree of strength combined with great flexibility under strain, a lightweight link could be made, the advantages for aircraft guns would be important. Before V-J Day designers tried some twenty-eight variations of the standard one-piece link and several two-piece types. Not only design but also materials and heat-treating affected performance in all metallic links. As the light weight of aluminum and plastic suggested substitution of these materials for steel, considerable study of a nylon link went forward, but the effect of extremes of climate upon plastic links and the inelasticity of aluminum prevented development of anything as satisfactory as the steel types used in World War II. In these, lowering the hardness of the steel largely overcame brittleness.13 Addition of a sprocket to serve as a booster to the feed mechanism also relieved strains.
Higher Muzzle Velocities
Faster-firing guns were not the only answer to the greater speeds of enemy
aircraft. Flatter trajectory of fire obtained by higher muzzle velocity was an even surer answer and was sought simultaneously. The NDRC study of hypervelocity noted at the end of the war that, in attacking targets moving in three dimensions, heightening the muzzle velocity of guns by 50 percent would more than triple the number of hits. In any given gun, unless the barrel were lengthened, thereby increasing the weight of the gun, or unless the metallic ammunition components were changed, the only way to increase muzzle velocity without adding to pressures was to use higher-potential powder. But higher-potential powders, like increased cyclic rates, spelled the probability of excessive barrel erosion. This could be avoided only by keeping the temperature of powder combustion low. Extensive experimentation, largely at Frankford Arsenal, produced .50-caliber ammunition with muzzle velocity heightened from 2,700 to 2,880 feet per second, but even with lined barrels the danger of “keyholing” after firing relatively few rounds forbade more powerful charges. As the interior surface of the barrel wore, the projectile tended to deform so that it lost velocity and tumbled in flight, and upon impact made a keyhole-shaped mark. If the barrel became excessively hot the bullet might even break through the wall of the barrel. At the very end of the war tests at the Ordnance research and development center indicated that a newly developed lighter-weight cartridge, the armor-piercing-incendiary T49, using a single-base powder, would give muzzle velocity of over 3,400 feet per second, but the erosion properties of the T49 were still a drawback.14
The German point of view on high muzzle velocities in aircraft weapons offers a contrast to the American concept:
Investigation of German small arms development and production revealed that, as concerns automatic weapon design, the attainment of a high cyclic rate appeared to be a primary consideration. This was particularly true in the case of automatic weapons designed for aircraft use. Apparently German authorities believed that a relatively low muzzle velocity was acceptable if a high rate of fire could be obtained. High velocities required larger and heavier rounds with a consequent reduction of cyclic rate ...15
Training Devices for Bettering Marksmanship
Use of power-driven turrets developed by the Air Forces to enable the aircraft gunner to locate his quarry quickly and track him accurately was another means of dealing with the problem of hitting a target moving at high speed.16 There remained the human factor in marksmanship. Whatever the perfection of gun mechanisms, Army and Army Air Forces both knew that soldiers must be well trained in their use; few weapons were highly effective in the hands of the inept or inexperienced. Where speed of hand and quickness of eye were so vital as to the aircraft gunner, his training became of more than usual importance. Skeet shooting, firing at a towed airborne sleeve or “drone,” and shooting a camera gun were used in the early part of the war, but the shortcomings of those methods of training were so obvious that the Air Forces in September 1942 requested a conference with
the Ordnance Department to discuss ways of improving upon them. The Air Forces proposed that the Ordnance Department develop a projectile that would disintegrate upon impact without harm to the target or its crew, but a projectile with ballistics similar under training conditions to those of the service ammunition under combat conditions. The Air Forces, agreeing that a suitably armored target plane would be essential to successful use of a frangible cartridge, undertook to develop an armored plane. Two months later the Air Forces changed its mind about the armored target plane and informed the Ordnance Department that unless a bullet could be made that would shatter against an unarmored craft, the whole project must be abandoned. The Ordnance Department dropped it. Research men of NDRC later charged that “one or two willful men in the Ordnance Department nearly stopped the development altogether.” The Ordnance staff averred that the Air Forces had tied its hands.17
At this point, one or two Air officers, convinced of the value of the idea, persuaded the Air Forces to turn the problem over to NDRC and approve a research contract with Duke University. Experiments were to be confined to work upon a .30-caliber bullet because, though .30-caliber machine guns were ineffective in combat against the armor plate of World War II planes, the firing characteristics were nearly identical with those of the more powerful .50-caliber, and the smaller-caliber ammunition was cheaper. In the course of the next year men at Duke and NRDC came up with a 90-grain bullet of powdered lead bonded with bakelite that, fired from a modified .30-caliber aircraft machine gun, at a distance of 50 yards disintegrated against a quarter inch plate of dural placed normal to the line of fire, or, if the bullet perforated the plate, caused little or no damage. This achievement was encouraging, though it still did not meet the Army Air Forces specifications. The Air Forces then requested the Ordnance Department to continue the development, but Maj. Cameron Fairchild, AAF, Professor Paul Gross and associates at Duke, members of NDRC, and the Bakelite Corporation, who had been the chief proponents of the program thus far, largely saw it through.
The technical difficulties were various. Beside making a frangible bullet with the proper ballistics and adapting the machine gun to firing ammunition with reduced powder charge, a plane had to be built, armored enough to be safe and yet able to fly. A hit indicator system had to be devised and a plan worked out for vectorial scaling of bomber and fighter velocities, for the bullet muzzle velocity, and for the ring sight size. The bullet T44, finally produced in some quantities and used at several training fields toward the end of the war, was slightly heavier than the first experimental type. The machine gun was satisfactorily modified and the scaling problem solved. The Air Forces did build an armored plane upon which hits were scored automatically by electrically amplifying the vibration caused by the bullet’s impact and thus flashing a light on the nose of the plane. Yet in spite of these successful developments, only a small fraction of bullets manufactured were fired. After the war Air Forces psychologists concluded that trainees were prone to get false ideas of aiming and firing because the
bullet simulated too much. The frangible cartridge was then declared limited standard.18
Tracer Ammunition for Bettering Marksmanship
As an aid to accurate fire, tracer ammunition had long been held in high regard by the Air Corps. In 1924 an improved .30-caliber tracer cartridge was standardized as the M1, the combat characteristics of which remained unchanged until Pearl Harbor. But after the Battle of Britain in 1940 the British, though praising tracer as a medium of fire control, wanted a controlled length of trace and, in order to minimize the blinding effect upon the gunner, a delay of 150 yards before the bright-burning powder ignited. Satisfactory types were developed in the course of the next two years, but obsoletion of .30-caliber machine guns for Army aircraft resulted in limiting .30-caliber tracer to ground use and, in small quantities, to use by the naval air forces.19 Caliber .50 tracer, on the other hand, mounted in importance as the war progressed. A type standardized in 1931 as the M1 formed the basis of the .50-caliber tracer most widely used by the Air Forces during the first two years of the war, though improvements were made in the original bullet by changing tracer and igniter mixtures, by increasing velocity 330 feet per second, and by developing clad-steel bullet jackets. Frankford Arsenal modifications of the M1 produced ammunition with a controlled bright trace of 550 plus or minus 50 yards, and later a type, standardized as the M10, which not only eliminated risk of blinding the gunner and gave a sufficient glow during the first moments of flight to permit him to retain trace image but also had ample intensity to enable him in daylight firing to follow the trace for the duration of the ignition. Thus, the M10 could be used in both night and day combat. It had, moreover, the advantage of longer life with less deterioration in storage than several other types of tracer ammunition.20
Another variation of the MI tracer was developed between 1942 and 1944, a so-called headlight tracer, which gave a frontal visibility three times as bright as the Ml. Initial reports from active theatres indicated that the psychological effect upon enemy pilots gave this bright-burning tracer particular value. The Tenth Air Force in December 1944 stated: “Preliminary reports indicated that they [caliber ,50 headlight tracer cartridges] … make adjustment of fire easier. Enemy pilots seem to be less aggressive and show a tendency to break off combat at longer range than with standard ammunition ...21
And the Commanding General, Strategic Air Forces in Europe, cabled: “Brilliant tracer indicates enemy fighter to other gunners in formation, which enables our planes to spot enemy aircraft more effectively at greater distances. … [This] ammunition is an important factor in breaking up enemy fighter attacks at extreme ranges,”22 Furthermore, pilots at that time believed that this tracer used in ground strafing disturbed flak tower operations. It was therefore standardized as the caliber .50 headlight tracer M21. Only
after the war did the Air Forces conclude that enemy pilots had been less easily scared by this ammunition than first reports stated.23 The M21 was then dropped.
The Problem of Effective Striking Power
Incendiary Ammunition
Rapidity of fire, flat trajectory, and accuracy of aim might solve the problem of scoring hits against enemy planes flying at high speeds, but if shots failed to disable the plane or crew, the effect of accuracy was lost. Greater striking power could be achieved by increasing machine gun muzzle velocities, by using air cannon that fired bigger projectiles, or by employing rockets. The threat of encountering enemy armor plate capable of withstanding .50-caliber machine gun fire did not actually materialize in World War II, but bullet penetration of the plane’s armor did not necessarily knock out aircraft. Enemy use of self-sealing fuel tanks necessitated development of effective incendiary ammunition.24
A .30-caliber incendiary had been used in World War I but, because of difficulties in manufacture, was later discarded for tracer. When in 1939 and 1940 tests of incendiary characteristics of the tracer showed it to be unsatisfactory, search for suitable incendiary ammunition began again. The Chief of Cavalry and Chief of the Air Corps in July 1940 submitted requirements for incendiary .30-caliber and ,50-caliber rounds that upon striking would ignite a gasoline or oil tank or pipelines from the tank. Time of flight was to be approximately that of standard ammunition and the center of impact was to be within twelve inches of the center of impact of standard Air Corps ammunition. Fortunately the British had made some progress with the problem. During the Battle of Britain in the fall of 1940 they had employed a .303-caliber incendiary cartridge that was effective against German bombers. But, like the World War I incendiary, its design was so complex that simplification was essential for quantity manufacture, Frankford Arsenal, assigned the task of redesigning the British .303 both to adapt it to mass production and to convert it to .30-caliber, succeeded in evolving a satisfactory bullet and cartridge by September 1941. This became the .30-caliber incendiary M1 and was issued linked in the ratio of two armor-piercing-two incendiary-one tracer, until in 1943 the Army Air Forces discarded .30-caliber machine guns altogether.25
More urgently needed was an effective .50-caliber incendiary. The first acceptable design was the work of the Remington Arms Company; whose staff had already had considerable experience in work on Swiss patents for incendiary ammunition. The Remington development was based upon the British .303 B Mark VI Z and was adopted in September 1941. The bullet was a flat-base type with lead base closure and steel body and was charged with 35 grains of incendiary mixture, 50 percent magnesium alloy and 50 percent barium nitrate. A few months later Frankford produced a type of .50-caliber boat-tailed bullet that equalled Remington’s in performance and proved better adapted to mass production. The Frankford design was standardized and the Remington became the Caliber ,50 Incendiary M1 Alternate.26
By 1942 flyers had come to regard some type of incendiary as indispensable for air combat. “These pilots, who are in daily conflict with the enemy, swear by the effectiveness of the incendiary ammunition and would as soon go up without their machine guns as without this type of ammunition.27 But the M1 incendiary did not serve every purpose. In the spring of 1943 the air forces were suffering heavy losses of B-17’s in daylight bombing operations over Europe, partly because the M1 incendiary, though excellent against enemy fighters approaching from most angles, was ineffective against frontal attack.28 The protection afforded by the engine of the enemy craft served to exhaust both the incendiary and the penetration energy of the projectile before it got to the fuel tank. Ordnance small arms ammunition specialists consequently suggested use of the M8 armor-piercing-incendiary developed for antiaircraft defense. The M8, when manufactured in relatively small quantities, proved more efficient than either armor-piercing or standard incendiary rounds, but, when manufactured by mass production methods with the types of powder then available, retention of its high velocity became impossible. Inasmuch as armor-piercing-incendiary with less velocity lost most of its penetrating and its incendiary properties, the Ordnance Department recommended that until something better could be perfected the M1 incendiary continue to be used for general air combat and straight armor-piercing for ground strafing.29 The something better than either standard incendiaries or the M10 tracer emerged in the spring of 1944 in the T28 armor-piercing-incendiary tracer standardized in March 1945 as the M20.30 Air Forces theatre commanders were authorized to request such quantities as they saw fit.
Meanwhile, in the winter of 1943–44, increasing German employment of jet-propelled aircraft burning kerosene created the need for ,50-caliber ammunition capable of igniting aviation kerosene. Half a dozen different Ordnance plants worked on the problem. The Des Moines Ordnance Plant produced the most satisfactory model, a 500-grain bullet containing 90 grains of an incendiary mixture composed of 50 percent magnesium aluminum alloy, 40 percent barium nitrate and 10 percent potassium perchlorate. A single-base powder was used that was found to be superior to double-base powder for firing extended bursts. Quantities of the Des Moines cartridge, listed as the T48, were shipped to the theatres in the winter of 1944–45 and proved so effective that in May 1945 the T48 bullet was standardized as the .50-caliber M23 and the round as incendiary cartridge M23. A report of June 1945 from Headquarters, US Strategic Air Forces in Europe, was enthusiastic: “Most pilots stated that aircraft burst into flames more readily when hit with this type ammunition in contrast to armor-piercing-incendiary ammunition. Many enemy aircraft burned after having been hit only two or three times. ... One pilot destroyed 10 aircraft on a single mission by firing short bursts.”31 This testimony notwithstanding, design of
incendiary and armor-piercing-incendiary ammunition remained at the end of the war a problem requiring much additional study.32
Air-to-Air Cannon
To provide more destructive fire power, an alternative to the use of high-velocity machine guns and incendiary ammunition lay in mounting cannon in aircraft. Though the greater weight, heavier recoil, and smaller ammunition capacity were disadvantageous, the bigger guns would have longer range as well as greater striking power. The development of air cannon had had a considerable history before 1940. In World War I some 37-mm. guns had been mounted in planes, but in 1920 Ordnance Department engineers, believing it possible to design a gun better adapted to air combat, began work upon a fully automatic 37-mm, aircraft cannon. Though the project was suspended in 1925, some ten years later the question of the most effective type of air armament was reopened. During 1936 Aberdeen Proving Ground conducted a series of comparative tests of the destructive power of ,50-caliber machine guns and 20-mm., 25-mm., and 75-mm. guns firing high-explosive instantaneous-fuzed projectiles against aircraft frames so loaded as to simulate the stresses of planes in flight. The outcome of these tests was an Air Corps request for development of three types of aircraft cannon, two automatic, one semiautomatic. Design of a high-velocity automatic gun was given first priority, its essential features to include a caliber of not less than 20-mm. or as much larger as would permit full automatic fire, weight not in excess of 300 pounds, a minimum muzzle velocity of 2,850 feet per second, a maximum of 4,000 pounds trunnion reaction, a magazine carrying at least 50 rounds, and a high-explosive impact-fuzed projectile. A lighter gun with a lesser muzzle velocity, 2,000 feet per second, was given second priority. The third type desired, a gun capable of firing a time fuze-impact shell at a muzzle velocity of 1,500 to 2,000 feet per second, was not to exceed 75-mm. in caliber.33
While some work proceeded simultaneously upon all three projects, it was the second that was first concluded when in December 1939 a Colt automatic cannon was recommended for standardization as the 37-mm. automatic gun M4. It was not an ideal weapon: it would not fire at an elevation of more than 70 degrees, muzzle velocity was just 2,000 feet per second, cyclic rate was only 150 rounds per minute, and weight without the mount and accessories was 213 pounds. But the Air Corps felt that the need was acute for some aircraft weapon more powerful than the .50-caliber machine gun, and the 37-mm, M4 would function in most positions irrespective of gravity.34 Two and a half years later the Air Forces and Ordnance Department sponsored a modification of this cannon to provide a disintegrating link-belt feed, a device better suited to aircraft installation than the magazine of the M4. The resulting 37-mm. M10, fed from either the right or left side, was accepted in April 1944 chiefly for use in the nose of P-63’s.35 The weight was 18 pounds more
than the 213 pounds of the M4, but cyclic rate reached 165 rounds per minute.
But any 37-mm. cannon was necessarily too heavy, too bulky, and too slow-firing to meet all the specifications listed for aircraft armament to supplement .50-caliber machine guns. As several 20-mm. cannon of foreign design had been tested in the mid-thirties without giving satisfactory performance, in the spring of 1937 Ordnance designers began work upon a new .90-caliber gun, that is, about 22.8-mm. If a suitable weapon of American design could be developed, the Ordnance Department could avoid all the complications inherent in the purchase of a foreign model. The .90-caliber project was eventually canceled because the urgent need of the Air Corps for a light cannon precluded taking time to get all the bugs out of the one experimental model completed.36
Instead, the Ordnance Department found a foreign weapon that in essential features would meet the immediate demand. In the very month that the Ordnance Committee had established the .90-caliber project, a report had arrived from Paris describing a new Hispano-Suiza 20-mm. gun made under Birkigt patents. This so-called 404 type promised to meet all American requirements. The Ordnance Department accordingly purchased one gun and 2,000 rounds of ammunition to study. While waiting for the shipment to arrive, Aberdeen tested a Danish Madsen 23-mm, a 20-mm, Rheinmetall, a 20-mm, Swiss Oerlikon, and a 20-mm. French Hispano-Suiza of earlier design, the last two guns borrowed from the Navy. Ample comparative data were therefore available against which to check the newer Hispano-Suiza model. Tests of the latter took many months. The gun was a combination gas- operated blowback type and fired 600 rounds per minute at a muzzle velocity of 2,850 feet per second. Weight was 137 pounds. Tests established accuracy life to be 10,000 to 12,000 rounds. The gun could be mounted in aircraft wing or fuselage or fired through the propeller hub but was not designed for synchronized fire between the propeller blades. It fired electrically by remote control. Though some uncertainty about its adequacy still endured, inasmuch as Air Corps and Ordnance experts agreed that the 404 type Hispano-Suiza appeared to have more desirable features than any other intermediate caliber cannon tested, in the spring of 1939 the Ordnance Department bought thirty-three additional guns from the French and began negotiations to secure American manufacturing rights.37 A year later General Arnold, urging haste in procuring guns of this type, restated the need:
... as a result of the recent developments in leakproof gas tanks, the caliber .50 may become ineffective against this component in the near future. The 37-mm. aircraft cannon has a somewhat limited application due to its bulk and comparatively slow rate of fire. Its application to the tail gun and engine nacelle mounts on bombers and wing mounts on pursuit airplanes is very difficult without compromising to an unwarranted extent the desirable performance of the airplane involved. It has, therefore, became apparent that a gun of greater power than the caliber .50 and of less power than the 37-mm. will be required to meet certain installations where the 37-mm. could not be effectively employed.38
20-mm. aircraft guns. After cleaning the guns, British mechanics replace them in the nose of a plane.
The option on manufacturing rights already obtained was then taken up and in May 1940 the gun was approved for standardization as the 20-mm. automatic gun M1, Watervliet Arsenal prepared drawings for contractors because the French drawings not only would be delayed in arrival but also would give dimensions computed in metric measurement that would have to be transposed into feet and inches. Some nine months later, when changes in dimensions of some parts were adopted, the M1 was made substitute standard and the newer model declared standard as the AN-M2. “AN” meant that the Navy as well as the Army had adopted the gun. Three types of ammunition were available by 1942, armor-piercing with tracer, which had a muzzle velocity of 2,563 feet per second; ball with a muzzle velocity of 2,820 feet per second; and high-explosive incendiary with muzzle velocity of 2,820 feet per second but effective range of only about 200 yards.39
Though, in the interest of saving time, the design of the 20-mm. automatic cannon was purchased, Ordnance engineers had to devise a number of modifications to make it fully satisfactory for air combat. Different planes required different types of adapters to control the gun’s recoil, different kinds of firing mechanisms, and different types of loading devices or chargers. If with one type of adapter a 60-round magazine were used, a muzzle brake had to be screwed to the barrel to reduce recoil
distance. If that same adapter were used with a disintegrating link-belt feed mechanism, the muzzle brake had to be replaced by a thread protector. The Navy demanded a hydraulic charger, the Army Air Forces a manual charger. The British wanted a scar mechanism instead of an electric trigger. Altogether there were seven different types of this 20-mm. aircraft gun in use during the war. In 1943, after prolonged tests, minor changes were introduced to reduce malfunctions. The dimensions of the powder chamber were slightly reduced, a new type of extractor spring replaced the original, the firing pin was transformed into a floating firing pin, and the breechblock slide springs were strengthened. The 40,000 guns already manufactured were altered in keeping with the last three of the four changes.40 Toward the end of the war a faster-firing model, the M3, was developed. Weight of gun and feeder combined was reduced by one fifth, to 112 pounds; muzzle velocity was slightly lowered, but cyclic rate was increased to an average of 750 rounds per minute. Furthermore, use of new automatic belt-feed mechanisms notably increased pull.41 Of the 134,633 20-mm guns produced, over 26,000 were converted to the M3 models in the last fifteen months of the war.42 Study of ammunition for these cannon went forward simultaneously with investigations of smaller caliber cartridges.
The third type of cannon the Air Corps requested in 1937 was to be approximately 75-mm. in caliber. Preliminary planning for developing such a weapon began the next summer and experiments started in 1939 with mounting a 75-mm. field gun in an airplane. The difficulties were far greater than with light cannon. The original 37-mm, gun of World War I had indeed been designed for aircraft, and the 20-mm. was not very much bigger or heavier than the .50-caliber machine gun. But no one had ever before attempted aircraft installation of so large a gun as a 75; no one could prophesy how its weight and recoil would affect flight; and no one could state authoritatively what type of fire control would serve best if so heavy a weapon were to prove feasible in aircraft at all. In view of these uncertainties, before plans had gone beyond the drawing board stage, the Ordnance Department persuaded Air Corps technicians at Wright Field that the first gun, mount, and fire control equipment should be the simplest possible. Firing at a predetermined distance from the enemy plane would simplify range finding and fuze setting, while a fixed gun with maximum recoil would reduce the stresses on the plane. After study of performance of this type installation, both Ordnance and Air Corps would have data on which to base further developments.43 The Air Corps undertook to supply a B-18 Douglas plane for experimental mounting and firing of the gun, first at Aberdeen and then at Eglin Field in Florida.
In June 1940 the 75-mm. field gun with a special mount was fired from a B-18 against a towed target. An Air Corps pilot and co-pilot flew the plane, but no member of the Air Corps Board witnessed the tests. The Ordnance experts directing the test firings reported that, for a first phase
development, the gun and stereoscopic range finder performed encouragingly well. A fourth shot fired at 1,500 yards was a direct hit, and range errors were not excessive. Nevertheless, the final section of the report submitted by the Ordnance officer in charge, Capt. Horace A. Quinn, foreshadowed abandonment of the project:
(1) The Air Corps Board, so far as I was able to determine from discussion, although admitting the effectiveness and accuracy of the 75-mm. gun in an airplane was not prepared to accept it as a standard weapon. Opinion was expressed that the .30 Cal. probably could still be used against aircraft although (based on trends abroad) they were recommending that it be replaced with the .50 Cal. gun. Interest was also expressed in antiaircraft bombing to accomplish the same result as would be accomplished with the 75-mm. gun. The board was also anxious to know if a small caliber gun, 50-mm. for example, would be just as effective as the 75-mm. However, as I understood their reactions they would recommend that the development continue.44
Yet to all intents and purposes, there the 75-mm, air-to-air cannon project dropped. General Arnold himself was reportedly impressed by the possibilities of a gun capable of hits at a 2,000 yard range, but otherwise the Air Corps of this period was dominated by small arms enthusiasts who pinned their faith to machine guns. Tactical need of the powerful 75, moreover, failed to materialize for air combat, and not until long after World War II did the Air Forces request such a weapon.45 When in 1944 two successful models of 75-mm, aircraft guns were developed, they were designed for strafing.
Aircraft Rockets
A similar shift of original plans attended development of aircraft rockets. The comprehensive program, inaugurated jointly by Army, Navy, and NDRC in the summer of 1941, had stressed work upon antiaircraft and plane-to-plant rockets. Army Ordnance and Navy undertook work upon the latter. The Ordnance Department had built a 4,5-inch rocket, standardized in 1942 as the M8, for either a plane-to-plane or ground artillery fire, with the only differentiation the fuze. But, though the 4.5-inch was put to use in ground warfare and in ground strafing, it saw no service in air-to-air combat, in spite of three years of Ordnance and Air Forces experimentation with various applications.46 American aircraft rockets used in World War II turned into weapons for ground strafing.
To fire rockets against aircraft effectively, planes would have to be built big enough to carry automatic launchers and fast enough and maneuverable enough to keep fire directed at the target. The launching installation, therefore, must not be so heavy as to retard that essential speed. A system of reloading the projector while the plane was in flight would also be highly desirable. Though the Air Forces postponed designing a special type of plane, in November 1942 a contract with the United Shoe Machinery Company called for building automatic projectors designed by Ordnance engineers and for studying a magazine installation suited to various types of
planes. But the launchers when ground tested in the spring of 1943 were dubbed too heavy and too slow firing for air use and that phase of the project then lapsed.47 Another scheme grew out of an Eighth Air Force request for an upward-firing rocket launcher to protect B-17 formations from planes bombing them from above. The development, known by the code name SUNFLOWER SEED, was worked out in England, using a special British rocket, and a B-17 so equipped was flown to the United States for study. At the same time technicians at Wright Field evolved a somewhat similar vertical-firing installation using the American M8 rocket; this was tested at both Aberdeen and Eglin Field during the spring of 1944. The rockets behaved as the designers hoped, but the low velocity of the projectiles and the lack of flexibility in aiming them led to the conclusion that neither SUNFLOWER SEED nor its American variant would serve the intended purpose. And by September 1944, with the cessation of overhead bombing attacks against Allied bomber formations, tactical need for such a weapon disappeared. Results generally similar to those obtained with the vertical-firing launchers followed when test data were assembled on a rearward-firing breech-loading 4.5-inch rocket launcher mounted in a B-17 bomber. Consequently, until scientists could develop rockets of higher velocity, even rockets with proximity fuzes promised little for defensive air combat.48
For fighter craft the problem was somewhat different. In the summer of 1943 the Germans employed aircraft rockets with some success against unescorted Allied bombers, but as soon as Allied fighter range increased so that fighters could provide cover for bombers, this threat subsided. The Army Air Forces found fighters equipped with conventional armament adequate to combat enemy rocket-carrying planes because, one explanation runs, the rocket installation created a drag that slowed the enemy plane enough to make it an easy mark.49 For Allied fighters, which by then were operating far from their bases, plane-to-plane rockets, in Air Forces opinion, offered no advantage. Had these planes had to fight enemy craft over England or the United States, tactical need might well have dictated using American rockets in air-to-air combat. Toward the end of the war the Air Forces experimented somewhat with a scheme to fire an air-to-air bomb and asked the Ordnance Department to supply the explosive container and a special VT fuze, but the project was incomplete at V-J Day and canceled at the Air Forces’ request in 1946.50
Ordnance Department interest in plane-to-plane rockets received fresh
encouragement in June 1945 when American experts in Germany discovered that the Germans apparently had a fully developed powerful type ready for production, Colonel Simon, then on a special mission to investigate German research and development projects, at once dispatched to General Barnes a file of data on the German 55-mm. aircraft rocket “believed to be the hottest thing to come out of Germany.” A sample of this rocket was found at a research laboratory in Lübeck. Colonel Simon wrote General Barnes:
By actual trials, they had shown that when these rockets were fired from a jet-propelled plane, they were almost certain of at least one hit, and one hit is sufficient to effect the destruction of an airplane. The rocket has the right size, the right ballistics, a velocity of about 1700 feet per second. THIS LOOKS LIKE THE CHANCE FOR A QUICK PAY-OFF ON SHOOTING DOWN SUICIDE BOMBERS, AND YOU WELL KNOW HOW BADLY WE NEED SUCH A WEAPON NOW.”51
V-J Day arrived before study of this rocket was far advanced, and only postwar tests proved it no better than American types equipped with influence fuzes.52
As American techniques in making rocket powders improved and velocities accordingly increased, the potential advantages of rockets for air-to-air combat became more evident. Though having lower velocity and lesser accuracy than projectiles fired from rifled bores, wing-mounted rockets had great power and no recoil, and the launchers, by comparison with conventional gun mounts, were simple and light.
The Problem of Functioning at High Altitude
Because progress in aircraft design by 1940 was enabling planes to fly at 36,000 feet or higher, study of the effects of altitude upon air ordnance was also essential. Engineers feared that the thinness of the atmosphere at great heights might make fuel-air mixtures in gasoline tanks too rich to be ignited by the incendiary ammunition that was effective at lower altitudes, and might even affect the flash properties and stability in flight of all types of ammunition. At the temperatures encountered at high altitudes, perhaps as low as 50° or 60° F, below zero, gun steels might become excessively brittle and oils in buffer mechanisms and lubricants too thick to function properly. Here were contingencies with which no ordnance designer at the beginning of World War II was familiar.
Testing at high altitudes and subzero temperatures was not easy to arrange. Making a series of controlled recordings necessitated working on the ground rather than in the air, and no laboratory facilities existed in the United States at the altitude desired. In the late fall of 1943, therefore, experts from the Ordnance Research and Development Center at Aberdeen Proving Ground were sent to Mount Auconquilcha in Chile to conduct tests at some 19,000 feet above sea level. The outcome of tests of the .50-caliber incendiary cartridge M1 and of the armor-piercing-incendiary M8 showed ignition efficiency of the former unimpaired and of the latter somewhat decreased. Camera films proved both types able to destroy enemy aircraft at 36,000 feet. Study of the effects of severe cold on gun steels and oils was part of the mission of the Winter Detachment sent to Camp Shilo in Manitoba in December 1942. All air ordnance tested there performed
satisfactorily if lubricated with suitably thin oils.53
Nevertheless, inasmuch as improved oil buffer assemblies in machine guns would minimize oil leakage at high temperatures and congealing at low, during 1943 Springfield Armory experimented with use of new synthetic packings, new finishes, and design modifications, until the Frigidaire Division of the General Motors Corporation evolved the pneumatic type buffer that used no oil at all. Two helical springs absorbed the recoil and functioned so well in tests in the fall of 1944 that this mechanical buffer was incorporated as a feature of the M3 aircraft machine gun. Not only did the air-and-spring type buffer dispense with the use of oils, the spring action allowed markedly increased rates of fire. Furthermore, in the modified M2 aircraft gun, engineers found it possible to use the oil buffer without oil.54
The problem of powerful, dependable armament for air-to-air fighting thus emerged as one of balancing the gains against the drawbacks of every given type or combination of types of guns. To cope with the speed of an enemy plane. Allied aircraft could saturate the path of flight with fire by sheer number of guns. But as air drag increased with every additional gun mounted and, more important, as the weight of guns and ammunition belts and the limited space in aircraft for stowing ammunition put a ceiling upon numbers, the AAF installed not more than eight ,50-caliber machine guns per fighter and sixteen per big bomber. Considerably heavier guns had greater range than .50’s but necessarily had a lower cyclic rate, could fire fewer rounds, and suffered the handicap of pronounced recoil. The advantage of the heavy powder charges of rockets and the lack of recoil was obvious, but the weight of rockets and the impracticability of carrying more than 14 on a plane constituted equally clear disadvantages. And for air-to-air fighting, rockets were virtually still untried. Experience taught the Air Forces that the .50-caliber machine gun was an eminently reliable weapon for the combat conditions of World War II. The 1,453,829 .50-caliber aircraft guns produced testifies to their usefulness.55 Nevertheless, by V-J Day indications pointed to the probability that more powerful, bigger guns would be employed increasingly as aircraft structures became heavier and stronger and their speeds still greater.