At first glance, the answer to the question, "What is the ultimate caliber of the guns that aircraft will carry?" is another one, "How large will aircraft become?"
To carry cannon of 75-mm (3-inch) or larger in airplanes, it is obvious that the planes must be of medium bomber size or larger. The North American Mitchell bombers now mounting specially designed 3-inch guns in a modified nose carry the gun to fire in a fixed forward position only. The gun is aimed with the plane and fired by the pilot. The total weight of the gun and mounting with a supply of ammunition is around 2000 pounds. The hydro-spring recoil mechanism designed for this gun has been stated to reduce the recoil to a gentle "push." However, newsreel pictures showing a B-25 anchored to the ground with the gun firing definitely show plenty of kickback. It is true, however, that in the air this recoil force would be considerably reduced.
Aircraft of Flying Fortress or Liberator types could actually carry guns up to 155-mm if necessary, but these guns, too, would have to be redesigned for minimum weight and recoil, and minimum size shell. Furthermore they also would fire only in a line parallel to the line of flight of the plane itself. Mountings would have to be rigidly reinforced and all possibility of carrying smaller guns around a 155-mm gun position would be nullified.
It is best first to review the reasons for mounting artillery type cannon on airplanes at all. For either defensive or offensive action against other aircraft, guns having a high firing rate are necessary. These can be either heavy caliber machine guns or 20- to 37-mm cannon. The 20-mm is the preferred class among the shell firing cannon because it almost attains the firing rate of a machine gun, about 500-600 rounds per minute, compared to 125 per minute for the 37-mm. However, the larger the shell, the more damage done to the enemy craft, but conversely, the lesser chance one has of hitting the target due to low firing rate. The 3-inch guns mounted on our bombers are not for the purpose of engaging opposing aircraft, unless the latter are unfortunate enough to get into the line of sight of the bomber nose. They are installed for use against naval or ground targets which would normally be invulnerable to small caliber cannon fire and which are either too small or too mobile to be bombed. To bomb a target properly, a definite bombing run lasting perhaps ten seconds must be taken, which is a most dangerous period for the bomber in question, as the enemy will attempt to hold it under constant antiaircraft fire.
The bomber with its 3-inch gun could aim at a target several miles distant, be reasonably sure of hitting it with a few 3-inch shells, yet be relieved from taking a timed and dangerous bombing run. During bombing runs, the enemy AA fire directors can more accurately predict the position of the plane, and devastating AA fire may result. Aiming and firing a fixed gun at a target requires no more than a second or two.
Aircraft mounting rifled guns of artillery caliber will probably be multi-engined special purpose types, will probably be to the Air Forces what cruisers are to the Fleet. The Germans are reported to have taken some of their big bombers and converted them into flying arsenals. These lumbering fighters, for such they are, carry almost every caliber weapon that has ever been mounted on aircraft. Without a bomb load, such airplanes can carry twice as many 50-caliber machine guns as our present Fortresses, and in addition a number of flexible 20- to 37-mm cannon in turrets. The Germans in their news releases have reported the American Air Forces to be using similar bomber escorts. If this is true, the ten ton bomb load of the Fortresses can be replaced with perhaps thirty heavy machine guns, half a dozen 20-mm turret cannon, and a couple of 75-mm guns, with possibly some rocket tubes added for projecting 5- to 6-inch rocket shells. Such an array of varying fire power, protected by suitable armor plating, would enable these super fighters to perform brilliantly on almost any mission, whether escorting bombers, destroying small heavily armored stationary or mobile targets, naval vessels or warding off enemy small fighter opposition with whatever weapons are indicated by the speed, size, number and range of the enemy aircraft.
The use of rocket guns and shells is beyond the stage of infancy. The advantages that the rocket tube projector has over its rifled tube competitor are really extraordinary.
First, there is no recoil whatsoever in the rocket tube. Second, the lack of recoil precludes the necessity for building up a heavy gun with accompanying breech-block, and a rigid mounting. A rocket gun tube could be made of plastic material and weigh perhaps no more than fifteen pounds, yet would project a 6-inch rocket shell loaded with the equivalent explosive content of a conventional 6-inch shell. Compare the insignificant weight of the rocket tube to the tonnage figures of a rifled 6-inch cannon!
Third, the saving of weight results in a greater ammunition carrying capacity. With a 3-inch shell weighing around twenty pounds, and a 5-inch one around fifty pounds, the rocket gun equipped plane could carry scores of heavy shells in addition to such other lower caliber machine guns necessary for action against fast or numerically superior enemy fighters. The larger shells, from 3-inch upwards, whether rocket or rifled gun type, have in common the possibility of carrying time fuses which can be variably set during firing. If such fused shells are sent towards enemy aircraft groups, a near direct hit with the shell bursting in the immediate vicinity of the target may either damage the enemy plane or throw it out of control temporarily.
On the debit side of the rocket guns, at this writing, must be listed their lower initial velocity, somewhat around 1800 feet per second, as compared to 2300-3000 for the rifled shells. However, rocket shell initial velocity may be maintained during the entire flight of the rocket projectile, and, in some cases even accelerated. If maintained, the low speed during the projectile flight trajectory makes accurate firing difficult. If accelerated during flight, hits will be difficult to predict, as the firing rate will only be one every few seconds, as compared to many hundreds for machine gun or 20-mm cannon fire. The rapid-fire weapons have tracers interspersed every five rounds, so that fire can be checked by the pilot or gunner. This is impossible with low speed, low firing rate rocket guns. A rifled shell has a known trajectory which, while certain errors may be introduced through differences in the atmosphere, wind or ballistic density, nevertheless allows the aircraft gunner a reasonable opportunity of hitting another plane. For fire to be effective from airplane rocket guns, the range must either be very short, fire be directed in a straight line into the tail of the target, or timed near hit bursting of the shells to be considered as satisfactory.
The redesign of an artillery weapon for aircraft gun use always takes time. A couple of years must have gone into the design of the 75-mm now used by our Air Forces. The gun saw extensive service before news about it was released. In the meantime, experiments with rocket guns have been going on feverishly. The success of our "Bazooka" on land and the known use of rocket guns on aircraft by the Russians and Germans indicates future policy pretty clearly.
This writer predicts, notwithstanding the opinions of some authorities on the subject, that in lieu of increased caliber of rifled guns on warplanes, rocket guns will eventually and gradually supplant all present aircraft weapons. It may take until the next war to see this fully accomplished, but come it will. With jet propelled high speed planes in the offing, the larger we can build our aircraft, the greater the protection we can give to the turbines producing the intense propelling jet. There will come a time when planes will be so cleverly protected that machine gun bullets and perhaps even 20-mm shells w ill be rendered harmless against them. By that time, rocket shells with controllable acceleration will have been designed, and air combat will be a battle of science, a battle of machines and instruments, of radio and electronically-controlled devices manned by imperturbable air gunners pressing tiny buttons and adjusting graduated dials according to fixed charts and directions given them by automatic aircraft range and position computers.
The weights and lengths of all artillery type ammunition are based upon certain formulae which cannot be exceeded much beyond the limits given below. Variations which occur are caused by special design considerations, governed by military purposes, which directly affect the velocity, range, and penetration of the projectile.
Generally speaking, the weight of a projectile is roughly (caliber in ins)2/2, while the length is from 4 to 6 calibers. The length of a 3-inch shell may be anywhere from twelve to eighteen inches, and the weight about thirteen pounds.
The 90-mm gun for both land and anti-aircraft use seems the logical follow-up for the 75 in aircraft. The 105-mm is the next in line, while the 155-mm is the extreme possibility, and will doubtless be outclassed by rocket weapons.
The ranges of all of these large caliber shells far exceed those of the rapid fire automatic weapons, and average 10,000 yards and more. These ranges are necessary for effective air to ground gunfire.
This article was originally published in the March, 1944, issue of Air Tech magazine, vol 4, no 3, pp 33-35, 80.
Photos credited to North American Aviation, Inc, European Picture Service, Air News.
The PDF of this article includes two photos of cannon-armed B-25s (-G or -H), two photos of what appears to be a Nebelwerfer 41 rocket launcher, a photo spread showing relative sizes of .22, .30 (apparently .30-06), .50, 20-mm and 37-mm rounds, and a diagram showing internal construction of some kinds of rounds.