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Chapter 16: Aircraft Armament: Guns and Rockets for Air-to-Ground Attack

Air-to-air fighting in World War II ordinarily took place to prevent enemy craft from reaching their ground or naval objectives, to forestall enemy reconnaissance, and to ward off attacks upon Allied bombers and strafing planes. Clearing enemy planes from the skies to give the Allies air superiority was an essential preliminary to employing Allied air power for offense. But because men live on the earth, not in the air, air offensives ultimately had to be directed at ground targets. Inevitably air combat and air-to-ground offensives were not wholly separable. Yet aircraft designed for use in strategic and tactical bombing missions or in close support of ground forces were usually equipped with ordnance somewhat different from that mounted on escort fighter planes. The machine guns employed on fighters were effective for strafing infantry, but for knocking out armored forces or for destroying fortifications, ships, and submarines, other weapons were needed.

Air Cannon

Use of aircraft cannon was naturally one method of achieving the power required for heavy strafing. Early in 1943, therefore, the Air Forces requested a high-velocity fully automatic cannon capable of firing a 1.92-pound armor-piercing projectile at 2,900 feet per second and a 1.34-pound high-explosive shell at 2,600 feet per second. Though the gun was to be usable in air combat, it was wanted primarily for strafing mechanized ground forces. Consequently, it was to function at positions of elevation from plus 35 degrees to minus 75 degrees, mounted either in normal position or on its right or left side. It must fire at temperatures as low as—52° F. On the other hand, for air-to-ground fire a cyclic rate of only 120 rounds per minute would be satisfactory, and weight of gun tube and feeding device might run to as much as 400 pounds. Trunnion reaction was not to exceed 10,000 pounds. The Oldsmobile Division of the General Motors Corporation undertook this project, basing design upon the standard 37-mm, antiaircraft gun M1A2, modified for aircraft installation. The pilot model failed to meet fully every specification—trunnion reaction was 13,000-pound maximum instead of 10,000, and the mount limited gun rotation to 45 degrees from the normal vertical position, instead of permitting 90 degrees, but operation

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was sufficiently satisfactory to make the design acceptable. Standardized in January 1943 as the M9, the gun had an average cyclic rate of 140 rounds per minute. Muzzle velocity with armor-piercing ammunition reached 3,050 feet per second, and, most important of all, a shot fired at a 500-yard range could penetrate 3.1 inches of homogeneous armor plate.1

Meanwhile, long before work on the M9 37-mm. cannon began, Ordnance engineers had been trying to develop a still more powerful strafing weapon. The 75-mm. field gun tested in plane-to-plane fire in the summer of 1940 had established the feasibility of mounting a big gun in aircraft. Though further work on that first development project had lapsed, the Air Corps had evinced some interest in a scheme to install a 75-mm. in a plane built to mount it for fire against ground targets. In mid-1941 the Douglas Aircraft Company, instead of using the B-18 first tried, undertook to adapt a new medium bomber to take a specially designed cannon and mount. Though a Douglas XA26B did mount a 75 and fire it successfully in June 1942, the bomber that eventually carried the 75-mm. into action was the B-25 made by the North American Aviation Corporation. In 1941 Ordnance engineers to whom design of the gun, mount, and fire control system was assigned had for guidance only the knowledge that the job was possible. It meant a completely fresh start, for the field gun tried as an air-to-air weapon could not readily be modified to meet the new requirements for air-to-ground fire. In one respect only was the designers’ task simplified: impact fuzes and aiming by ordinary gun sights would suffice for effective strafing of stationary or slow-moving ground targets, whereas proximity fuzes or preset fuzes and elaborate range finders would have been needed against aerial targets.2 Still the problems were peculiarly difficult.

The 44-inch recoil mechanism of the ground-mounted gun was impossible to use in the confined space of a plane. Shortened recoil would increase trunnion reaction, and mounting the conventional hydromatic recoil and counterrecoil cylinders above or below the gun tube would make the gun silhouette too large, Ordnance engineers found the answer in step-by-step modification of the 75-mm, M3 tank gun and in development of a new mount. The single-shot, hand-loaded weapon with its vertical sliding, automatically’ operated breechblock was fired electrically. An ejector mechanism spewed out the shell case after the round was fired. A hydro-spring recoil mechanism using two cylinders mounted above and below the gun barrel reduced the silhouette somewhat. The stronger construction of the newer bombers enabled Ordnance engineers to let the plane absorb part of the recoil shock and thus limit the recoil stroke to the 21-inch length of the round, a space needed in any case to load the gun. In the first models an automatically functioning muzzle cover that opened when the breech was closed and closed when the breech opened was provided to prevent fumes from pouring into the gunner’s compartment after firing and to ease ammunition loading, but this feature was found unnecessary and later dropped.

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The model accepted in the summer of 1942 was designated the 75-mm. aircraft gun M4, and its mount the M6. Gun and mount together weighed 893 pounds. Muzzle velocity with high-explosive ammunition averaged 1,974 feet per second, with armor-piercing-capped ammunition, 2,024.3 The ammunition was the same as that standard for ground guns, a considerable advantage in procurement. Moreover, on at least one occasion, this interchangeability was of importance in the field. Maj. Gen, Claire L. Chennault in his memoirs described how one of his officers saved the day for Chinese troops equipped with three old French 75-mm. field guns but no ammunition. Sacrificing some of his cherished supply of 75-mm. aircraft shell, the pilot dropped enough to the Chinese to put their guns into action. From the Pacific theatres after 1942 came testimony to the effectiveness of the gun for strafing. Its fire destroyed pillboxes and sank naval vessels. In July 1943, for example, two B-25 bombers mounting 75-mm. air cannon attacked a large Japanese destroyer off the coast of Cape Gloucester and in two runs, firing seven shots on each run, riddled the ship from stem to stern and left it sinking.4

Even before the 75-mm M4 was standardized, Ordnance engineers began work upon a light-weight, mechanically-loaded air cannon. An entirely new recoil mechanism in which the cylinder was concentric about the gun tube reduced both silhouette and weight. By using a new high-strength alloy steel having an elastic limit of 130,000 pounds per square inch, the designers lowered the weight of the gun alone to 406 pounds. When metallurgical tests at Watertown Arsenal and firing tests at Erie Proving Ground established the greater strength of hollow quenched over solid quenched gun tubes and breechblocks, the former method of manufacture was included in the specifications. An automatic fuze setter-rammer saved the space required for manual loading and permitted firing at a rate of 30 rounds per minute. Two models of this light-weight cannon varying from each other only in minor details were standardized in 1944 as the 75-mm. AN-M5A1 and the M10.5

Early in 1943 the 37-mm. M9 for strafing mechanized ground troops and the 75-mm. M4 for destroying heavily armored ground targets were serving their purpose well enough to inspire work upon a still more powerful gun. Tests at Eglin Field, Florida, comparing the effectiveness of existing rockets and air cannon pointed to the superiority of the latter. Development of a 105-mm. aircraft gun, therefore, was started in July 1943 with endeavor to adapt a 105-mm. howitzer to use for air-to-ground attack. A year later numerous changes necessitated by the excessive blast of the first models led to making a new approach. The resulting T7 105-mm. aircraft gun was test fired late in 1944 only to show that the feed mechanism required further study. Before the changes were completed the war ended and the project was canceled.6

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More than a year before any model of cannon for ground strafing was approved, the drawbacks of mounting heavy guns in aircraft were recognized. This knowledge intensified the search for other types of powerful air armament. Accurate bomb sights would increase the effectiveness of bombs, and airborne homing torpedoes better the percentage of hits in antisubmarine warfare. But neither of these was the direct responsibility of Army Ordnance, although work of the Ballistic Research Laboratory at Aberdeen in compiling a complete set of ballistic tables for use with the Norden bomb sight contributed greatly to more effective bombing. Rockets, on the other hand, were the immediate concern of the Ordnance Department, as well as of the Navy Bureau of Ordnance and the Air Corps. If accurate rockets could be launched from aircraft, the problem of strafing might be largely solved. In December 1940 the Ordnance Department requested NDRC’s assistance in developing a 4.5-inch rocket primarily for use in aircraft. The fruits of British research on rocketry, in 1940 far in advance of American and promptly put at the disposal of the United States, greatly expedited progress on adaptation of this ancient weapon to conditions of modern warfare.

British research before 1942 had been directed at developing rockets for antiaircraft and plane-to-plane fire, but in the United States the Army’s attention early centered upon other phases. The bazooka was the first Army device to use a rocket in combat. Yet an Ordnance officer had fired an experimental 4.5-inch aircraft rocket before anyone had seriously considered the application of rocket propulsion to an infantry weapon. This 4.5-inch rocket, though originally designed for either ground or plane-to-plane use, was in actuality employed in the latter capacity in China, and then only experimentally. For strafing, however, it came to be a valuable weapon, adding to the fears of “the American harassment” repeatedly expressed by German ground troops.

To produce dependable, powerful rockets involved solving innumerable new engineering and ballistic problems. Motor tubes strong enough to withstand the high pressures of the propellant powders must be as light as possible, particularly for aircraft rockets. Propellants must create pressure high enough to attain the desired range but must be safe to handle, easily manufactured, and of composition to burn readily at a wide range of temperatures. Getting suitable even-burning powder was, in fact, the biggest poser of all. Nozzles to reduce the rate of flow of propelling gases must not raise internal pressures to the point of bursting the tubes. Traps and cages, by which to suspend the propellant sticks in the motor tubes, must be so designed as to retain the powder in the tubes until the sticks were completely burned, but without permitting the traps to interfere with the even, quick burning of the powder and without adding excessive over-all weight or reducing the rockets’ pay load. A safe reliable ignition system was essential. Some means of stabilizing the rocket in flight and fuzes that would function properly with low-velocity projectiles must be devised.

Despite these obvious difficulties, development of a 4.5-inch rocket for air use initially proceeded with deceptive rapidity. Tentative military characteristics were agreed upon in the summer of 1941 after Ordnance ammunition experts had had opportunity to study samples of British

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rockets. That fall, Maj. Leslie A. Skinner, the only Ordnance officer to work on rocketry during the 1930s, successfully made a few 4,5-inch rockets, using old fire-extinguisher cylinders as casings. This caliber appeared to be the smallest that could contain a reasonable-size burster tube and warhead and enough propelling charge to give about 1,000 feet-per-second velocity. Fired at Aberdeen in December 1941 these rockets, for all their crudity, were stable in flight and performed fairly well. They weighed about 33 pounds apiece and carried 3.8 pounds of explosive. Redesign for production began at once. Before the war was over, considerable criticism was directed at the Army and Navy for developing two separate rockets for essentially the same purpose but with a difference of half an inch in diameter. The sheer chance of having old fire extinguisher tubes available had determined the size of the Army models.7

By April 1942 both the Ordnance Department and the laboratory staff at Wright Field dared hope that a usable aircraft rocket was about ready. Aberdeen firings from the ground had indicated reasonably satisfactory accuracy of the redesigned 4.5-inch model and the probability of no damage to the plane structure. Fins that unfolded after the projectile left the tube gave adequate stability in flight.8 Wright Field designed a mount for a launching tube under the wing of a P-40, while plans got under way for installation of projectors in the bomb bay of an A-20A plane to permit reloading while the plane was in flight. The chief of the Experimental Engineering Section at Wright Field, considering the rocket project vital, urged the Commanding General, Matériel Command, AAF, to inform the Chief of Ordnance of its importance. “In view,” he wrote, “of the rapid progress which has been made and the information available on its employment abroad, particularly by the Russian Air Forces, it is suggested that the military characteristics for such a weapon be reviewed.”9 The Ordnance Department needed no prodding. Confidence in the 4.5-inch model ran so high that the Ordnance Committee had already recommended standardization and limited procurement of some 3,500.10

Six weeks later belief still endured that the difficulties so far encountered could be overcome quickly “if vigorously prosecuted,”11 Better ammunition, namely propellant powder of uniform thickness or “web” having neither internal fissures nor external cracks to interfere with even burning, was a problem the Ordnance Department hoped to have answered by midsummer. Maj. Gen. Millard F. Harmon, Chief of Air Staff, on 10 June set 1 October as the goal for having rockets available for the AAF in TORCH, the invasion of North Africa. Delivery of 15,000 for testing to

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begin in August was requested. On 6 July a 4.5-inch rocket was successfully fired from a P-40E plane in flight without injury to the plane, a performance that fortified faith in the future of aircraft rockets.12 Procurement then was raised to 600,000. But there the project bogged down because of the powder bottleneck, General Barnes wrote that the first 15,000 rockets could not be shipped until further experimentation and tests established the safety of the propellant and the rocket tubes. That moment arrived only seven months later, in March 1943. Overoptimism earlier, plus premature notices of prospective availability of rockets for the combat zones, made the unavoidable delays hard to explain to men who failed to comprehend the magnitude of the task.13 Even Dr. Bush, Chairman of the Office of Scientific Research and Development, protested the slowness of progress. Maj. Gen. G. E. Stratemeyer, Chief of Air Staff, assured Dr. Bush in January 1943:

Realizing the many problems confronting the development section of the Ordnance Department, we have no criticism of that Department for not having had everything connected with this program available and in order “for a long time.” Personally, I feel that splendid progress has been made by the different groups concerned with various phases of this program, considering the complexity of the problems, dealing as it does with new propellants, new fuzes and new types of launching equipment.14

How to obtain suitable propellant was the first question, and the second was how to hold multiple small grains in the rocket motor. The Ordnance Department’s answer to these problems fortunately could be one and the same for aircraft rockets and ground-launched rockets.15 The plan to develop a single type of 4.5-inch rocket for Air Forces and Ground Forces alike permitted the Ordnance Department to dedicate far greater resources in technical talent and testing facilities, and, later, in production and inspection, than would have been possible otherwise. Most problems were common to the two applications.16 The differentiation of aircraft rockets from ground rockets was to be solely in the fuze. To develop the best kind suited to air use required extensive experimentation. For some types proximity fuzes were to be tried, for others impact fuzes. When in June 1943 impact-fuzed high-explosive rockets were test fired at Eglin Field against water targets, observers reported functioning satisfactory and the splash pattern of fragments effective. Proximity fuzes, on the contrary, when tested both then and at intervals later, gave many duds and some prematures. Consequently VT fuzes were not employed.17 Their

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usefulness would have been chiefly for air-to-air combat, and by mid-1944, with enemy air power on the wane, the Air Forces had largely discarded plans for special new air-to-air weapons. Some work on influence fuzes for rockets continued down to V-J Day, but it was aimed at a long-term development rather than at one for immediate use.18

Projectors were still another feature of aircraft rocket installation that proved to be troublesome. In March 1943, after successful ground tests at Aberdeen, the Ordnance Department dispatched two experimental launchers to Wright Field. These were admittedly too heavy but were to be followed by aluminum alloy tube models, which would be very much lighter. They were designed for mounting in bomb bays to permit reloading while in flight. Though the models of this design were never air-tested, one sample of a redesigned lighter-weight automatic launcher was eventually shipped to Burma, where firing established the soundness of its principle of operation. After the war this launcher supplied the basis of an extensive development project.19 But in World War II, after mid-1943, the “rocket gun” gradually dropped out of sight in planning, for at that point the Air Forces turned attention to jettison-able three-tube steel or plastic clusters to be mounted under the wings of aircraft.20 Such a device precluded the possibility of reloading in flight, but omission of a reloading feature permitted more rapid completion of safely usable launchers. The three-tube plastic clusters, in fact, appeared to be sufficiently satisfactory to warrant an initial Ordnance procurement order of 5,000, and in October 1943 the AAF asked for 10,000 more. Soon after delivery of the first 5,000 in December, the AAF pushed its requirements for 1944 to nearly 200,000, far more than could be manufactured with the limited supply of plastic. Since magnesium-alloy tubes met all essential requirements, they were manufactured in considerable numbers to supplement the plastic. Though the combat theatres used some of both types of launcher, thousands were stored in the United States by the spring of 1945, literally an unwanted, obsolescent commodity.

This accumulation of large stocks was due partly to the unexpectedly long life of tube launchers in service, and partly to the small number jettisoned in action. But the principal reason was the lack of proper aircraft mountings for the clusters. The Navy “zero rails,” short simple posts beneath the wings from which the rocket could be suspended and launched without using tubes at all, were proving perfectly satisfactory. The speed of the aircraft gave sufficient directional stability to fixed-fin rockets to make needless any guide rail. Easy to manufacture and install, the zero rails had the further advantage of creating less drag

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Aircraft Rocket Installations

Typical installation on 
Air Force aircraft

Typical installation on Air Force aircraft

Typical installation on 
Navy aircraft

Typical installation on Navy aircraft

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than tube launchers. By the time mounts were available for the latter, the Army Air Forces had adopted both the zero rails and the Navy 5-inch rockets. Still, because of the large quantities of 4.5-inch rockets available, commonsense dictated using them. Installation of adapters, which include a large fixed fin, a bayonet type of igniter, and lugs, permitted firing the smaller rocket from the zero-length launcher designed for the 5-inch. Used with these adapters the 4.5-inch, the Ninth Air Force reported, then compared favorably in accuracy with the 5-inch and were an acceptable substitute until the 5-inch were in larger supply.21

The AAF switch from the Army 4.5-inch rocket to the Navy 5-inch high-velocity aircraft rocket, HVAR, nicknamed “Holy Moses,” grew out of delay in Air Forces procurement of mounts, impatience over the slowness of Ordnance developments, and discovery in the summer of 1944 that some HVARs were immediately obtainable. Six months earlier Maj. Gen. Barney M. Giles, Chief of the Army Air Staff, had written the commanding general of the Army Service Forces listing the shortcomings of Army Ordnance rocket development. Stating that the 4.5-inch types were inaccurate, subject to fuze trouble, limited by extremes of temperature, and lacking in adequate velocity, Giles concluded:

The Ordnance Department personnel have repeatedly stated that they were working on these difficulties, and that before winter the Air Forces would have another improved rocket. To date no recent improvement in the rockets that are being furnished to the Army Air Forces has been noted; in fact present tests underway at the Proving Ground on a current lot of ammunition indicate less satisfactory operation than previous lots tested at that station.

7. The experience of the Navy and of our Allies establishes the rocket as a weapon of prime and possibly decisive important [sic]. …

8. It is requested, therefore, that the Ordnance Department redouble its efforts to furnish the Army Air Forces a rocket suitable for combat use.22

Up to the fall of 1943, the 4.5-inch rocket, it is true, had had a checkered career. The mass production begun the preceding spring had been halted in June when service tests showed that motor tubes and some other components failed to function properly in extreme temperatures. Reducing the propellant charge in rockets already manufactured, though shortening effective range, made them safe to use at high temperatures, while strengthening the motor tube and redesigning the warhead partly corrected the weakness of the new rockets. Later, a slight modification of the fin blade produced a model labeled the M8A3.23 But some months before General Giles aired his concern, the Ordnance Department itself had taken steps to “redouble its efforts” to speed rocket work. A separate Rocket Development Branch, created within Research and Development Service in September, expanded rapidly from a staff of 2 officers and 13 civilians to 15 officers and 31 civilians. Larger sums of money allotted to rocket projects enabled the chief of the branch, the gifted

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Table 12—Comparison of 5-inch and 4.5-inch Rockets

5” HVAR Rocket 4.5” M8 Type Rocket
Total weight 140 pounds 40 pounds
Maximum velocity 1,300 feet per second 865 feet per second
Weight of high explosive 7.8 pounds 5.1 pounds (M8A3)
4.3 pounds (T22)
Maximum accurate range 1,000 yards 800 yards
Approximate penetration of Class A armor 1.75 inches 1 inch
Good reinforced concrete 3 feet 1 foot
Operating temperature range 0° F to 120° F -10° F to 105° F (M8A3)
-20° F to 120° F (T22)
Weight of plane mounting installation 15 pounds (16 mounts) 196 pounds (with 2 cluster launchers).

Source: Memo, Lt Col J. W. Gruitch for C. W. Bunch, Office of Commitments and Requirements Div, 15 Aug 44, sub: Comparison of 4.5” Type and 5” HVAR Rockets, Hq USAF file, Office of DCofS for Development.

Col. Gervais W. Trichel, to intensify and widen the program and to establish closer ties with research groups of the Navy, NDRC, and AAF units at Wright Field and Eglin Field. As the AAF also enlarged its research and testing staff and opened Muroc and the Dover Air Bases, Army aircraft rocket developments moved more rapidly.24

Dissatisfaction with the first modifications of the M8 rockets revealed the necessity of designing motor tubes strong enough to withstand an internal pressure of 10,000 pounds per square inch. Experimentation proved that heat-treated alloy-steel seamless tubing gave the desired strength and extended the rockets’ temperature range from—20° to 120° F.25 The Ordnance Technical Committee designated this high strength rocket the T22.26 By August 1944 a comparison of these new types of 4.5-inch with the Navy HVAR 5-inch rockets showed that the latter was by no means superior in every respect, (Table 12, above.) While the HVAR thus carried about 50 percent more high explosive, had considerably greater muzzle velocity, and was equipped with both a nose and a base fuze, the 4.5-inch was so designed that it could be fired from an automatic launcher and could be launched in other directions than in the line of flight of the plane. The fact that the HVAR had an excellent underwater trajectory, which the 4.5-inch lacked, constituted no particular advantage of HVAR for Army Air Forces use inasmuch as the AAF had ceased to participate in sea search and antisubmarine warfare in July 1943, and Wright Field investigation of rocket launching devices for vertical bombing of submarines had faded out thereafter.27 Moreover, a comparison tabulated after zero rails and adapters for the 4.5-inch rocket had come into use

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might have made the lesser weight of the 40-pound rocket a more obvious asset. And finally, because the AAF employed rockets only for strafing, the 4.5-inch had an eminently desirable distinctive feature: its design enabled the pilot to fire both rockets and machine guns simultaneously by merely harmonizing with the gun sight.

Meanwhile, the existence of rather extensive facilities able to produce the thin-webbed wet-extruded powder grains used in Army 4.5-inch rockets, coupled with the relatively limited sources of supply for the thick-webbed dry-extruded powders needed for the 5-inch, pointed to the wisdom of designing a rocket at least nearly equalling HVAR in power, yet employing a solvent type of propellant powder. Early in 1944 the Ordnance Department requested NDRC to undertake the project, and by October experimental lots of the “H” 4.5-inch rocket were ready for test. Mounted on zero rails on P-47’s and B-25’s, these first “super 4.5-inch” rockets performed well. Damage to the planes was slight and easily preventable, and dispersion of fire was not excessive. Though the “H” rocket carried a 39-pound payload, HVAR a 48-pound, and though velocity of the former at long range was considerably lower than that of the 5-inch, the new rocket with its faster-burning propellant got up more speed quickly and, over short ranges, attained higher velocities. For many kinds of mission a weapon possessing these characteristics would be better than HVAR. A thousand of the “H” 4.5-inch rockets were accordingly made for further testing, but no production order followed, because as the tactical situation in the spring of 1945 altered, the probability shrank that any need would arise for this type of short-range rocket.28

Aircraft rockets played a smaller part in AAF combat than in naval air forces engagements, just as operations over Europe were different in character from those over the Pacific areas. Nevertheless, the knowledge gained in World War II about rocket design and performance was quite as valuable to the Army as to the Navy. An Army Air Forces officer prophetically summarized the importance of the Ordnance and AAF rocket developments when he wrote in 1943:

In view of the potentialities of rockets as a new aircraft munition, ... we should go after them hard, although I have never felt, nor suggested that any of our “new weapons” would very strongly influence the outcome of the present war. My own view is that new weapons of one war become of real usefulness during the war after that in which they are introduced, and that we shall have to slug out this war for the most part with the guns, bombs and other munitions which we had or had in sight when we entered it.29