Aircraft in Warfare (1916)/Chapter 8
CHAPTER VIII.
(October 23rd, 1914)
RAPIDITY OF FIRE AND ITS MEASURE. ARMOUR IN ITS RELATION TO ARMAMENT.
§ 49. Rapidity of Fire and its Measure. The measure of the rapidity of gun-fire from an aeroplane or dirigible as an index of its fighting value depends upon the nature of the objective or target. It is evident that in some cases the mere number of projectiles per minute is the most important factor, as, for example, in attacking any object in which a hit is a hit whether the projectile be large or small. In other cases, where the mischief done is in any reasonable relation to the weight of the projectile, the total weight of projectiles discharged per second (or per minute) affords a better criterion. In view of the comparatively flimsy and fragile nature of aircraft, it is doubtful whether the energy equivalent of the discharge will ever be of the importance which it is in the case of the battleship, where the destruction of the enemy depends to a very large extent upon the number of foot-tons with which he is assailed. Thus it is doubtful whether a factor representing the horse-power of the offensive armament would, as applied to the fighting aeroplane, will have any useful significance. Now it is scarcely probable that in the immediate future the fighting-machine can be furnished with complete bulletproof protection, at least such as can be considered effective at short range. Consequently we may take it that it is quite unimportant whether the bullets used in its destruction be of the usual British 215 grains or the 162 grains of the Mannlicher, or the 530 grains of the old Enfield. The advantage of size and weight only becomes important when a single hit is sufficient to carry away an important strut or structural member which would have been penetrated without great injury by a bullet of ordinary size. Thus, so long as we are dealing with ordinary rifle, pistol, or machine-gun fire, we are concerned merely with the number of bullets that can be discharged per unit time, and this number—i.e., number per minute or second—fairly and properly expresses the value of the armament. This, of course, does not mean that the weight and muzzle energy of the bullets are of no importance whatever; it is merely an expression of the fact that with such weapons as are commonly available the differences, such as they are, and important though they may be in other applications, are not appreciable in relation to attack by aircraft on aircraft.
§ 50. Measure of Fire Value in the case of Explosive Projectiles. When we pass to the consideration of weapons capable of throwing explosive projectiles, it is impossible to maintain, or even suggest, any direct basis of comparison. The effectiveness of shell fire depends entirely upon the conditions being present necessary to the correct timing of the fuse—that is to say, either the range must be known with great accuracy and the time-fuse mechanism correspondingly perfect, or the nature of the target must be such as to permit of the effective employment of an impact fuse of some description. Granted that the necessary conditions exist, the destruction wrought by any given type of explosive projectile may be taken as, in a measure, proportional to its weight. However, there are cases when a 3 lb, high explosive shell would be just as effective as one of 18 1b., as, for example, if it were to strike the motor or fuselage of an aeroplane in flight, and so, in assessing the value of shell-fire by the aggregate weight of the projectiles thrown, it is evident that we should only be making an approximation to the truth.
If we go further and endeavour to compare the relative value of armament of diverse type for aeroplanes, as, for example, in computing the relative merits of machine-guns and small artillery in any given case, we are inevitably thrown back on examining the service for which the armament is required; it is impossible to institute a direct quantitative comparison which would be generally applicable. If it be conceded that in any particular case a given weight in the form of shell is of greater effect than the same weight in the form of bullets, then we have a prima facie case for the use of artillery. If, on the other hand, it is conceded that the bullets would do the greater mischief, then a machine-gun armament is indicated. However, although the weight of ammunition is a matter of first importance, the weight saving and convenience of the machine-gun in itself are sufficient to give it a preference where the other advantages are not overwhelmingly against it.
§51. Weight thrown per minute; Machine-Gun and One-Pounder Compared. It has in the preceding paragraph been rather assumed that the capacity of the armament, as represented by its weight-rapidity factor of fire, is a constant; this is a matter that depends, firstly, upon the mechanism of the gun. The Lewis gun, which has been taken throughout these articles as representative of the machine-gun in its aeroplane usage, will fire as an ordinary maximum 600 rounds, or 18½ lb, of lead per minute. Unfortunately, there is no standard 1 pounder with which to institute a comparison. The 37-mm. gun is given by different makers as throwing in some cases a projectile 1nblb., and by others the same bore is given as throwing 1½ lb. Also the question arises whether an automatic gun of this size is, on account of its heavy mean recoil, an altogether workable proposition. The Vickers automatic 37-mm., for example, is made in two weights; one of these throws a 1 lb, shell at 1,800 foot-seconds at 300 rounds per minute, at which speed of discharge the recoil reaction would be about 300 lb. The weight of this gun is given as 3¾ cwt. This gun is quite unsuited to aeroplane service, both from the point of view of recoil and weight; there is, however, a lighter type by the same firm, of the same calibre, semi-automatic, throwing a 1½ lb, projectile with a velocity of 1,200 ft, per second, the weight of the gun being given as 110 lb. Presumably the maximum rate of fire of this gun would be about thirty rounds per minute, or the weight of metal thrown per minute, 45 lb. This is about 2½ times the weight per minute given above for the machine-gun.
§ 52. Weight per minute as limited by Recoil. Quite apart from mechanical details, however, a real limiting factor exists in the recoil reaction to the momentum per second permissible, and this limit may be taken as applying whatever the type of gun may be; consequently, since the muzzle velocity of one type and another is not widely different, the weight discharged per second or per minute will have an approximate maximum for any particular aeroplane no matter what the type of gun may be. This is the reason why it is possible and, in the author's opinion, advantageous to employ duplication, or even fit three or four barrels in the case of the machine-gun; whereas an automatic 1-pounder under like conditions could not reasonably be allowed to fire at over 60 or 80 rounds per minute.
§ 53. Present Advantage of Machine Guns. Future Possibilities discussed. At present there are very few cases in which the automatic or semi-automatic 1-pounder could compete with the machine-gun as an aeroplane arm. If a percussion or impact-fuse were available sufficiently sensitive to explode with certainty on encountering balloon-cloth, the 1-pounder would be an excellent weapon for the destruction of the airship or dirigible. Every part of a shell exploding within the envelope is effective, and the fragments of a shell leave wounds in the envelope and give rise to loss of gas of a more serious character than that due to the rifle or machine-gun bullet. Beyond this the danger to the crew (and structure in the case of a Zeppelin) is considerably greater under these conditions than under equivalent machine-gun fire. It may be some time, however, before the impact-fuse reaches the required degree of perfection.
In the author's opinion there is room for a well-designed light-weight automatic to fire 14-oz. shell at a moderate velocity, say 1,200 ft. per second, with a maximum rate of 100 rounds per minute, the weight of the gun to be kept, if possible, under 100 lb; the length should be kept as short as the requirements of the internal ballistics permit. The value of such a weapon, however, would depend almost entirely upon the development of suitable ammunition, and in particular, as already pointed out, the perfection of the impact-fuse to a point not yet within sight.
§ 54. Armour in its relation to Armament. We have already given considerable attention to the question of armour in connection with the primary function of the aeronautical Arm — the attack on and co-operation with the other Arms of the Service. It is now time to extend our study to the secondary function of the Arm, and discuss the question in relation to problems of aerial attack and defence. The first instalment of the conditions which need to be fulfilled by the aeroplane constructor arises directly from the consideration of the primary function; thus, it is already given us that the armament to be of maximum service must be capable of action to the full in a downward direction with the greatest angle of fire (both forward and aft, and laterally), that the limitations imposed by structural considerations permit. Similarly, we know that the initial need for armour is mainly to resist attack from below. These facts remain, and cannot be altered by the additional duties imposed when aeroplane attacks aeroplane. We may, and in fact shall, have to provide for a far wider range of fire; we shall need to make provision for training our gun or guns upward as well as downward; likewise we may find it expedient to provide protection against fire from above as well as beneath. But any extended scheme of armament or protection so developed takes essentially as its starting point the more elementary condition.
§ 55. Importance of Upper "Gage" or Berth. The first result of importance arising from the above facts is, in any aeronautical engagement, the importance of the upper berth. The machine which is able to attack from above is acting under the conditions for which its armour and armament were initially provided. Beyond this, the taking of the upper position at the start, or perhaps, we may say, before the start, gives the power to outmanœuvre an enemy, in spite even of inferior speed capacity in the ordinary acceptation of the term. The initial difference in altitude represents a store of potential energy which may be drawn upon when the opportunity occurs; this is, in fact, the principle utilised by the hawk, the kite, and other birds of prey. The objective of securing the upper berth, or position, or "gage," if we adopt the old-time word used by naval writers, will probably prove to be, and will remain, the key or pivot on which every scheme of aeronautical tactics will, in some way or another, be found to hang.
§ 56. On Protection against Attack from Above. The question of employing armour as a protection against attack from above, or against dropping fire, is one which requires consideration on an entirely different basis from that of attack from below. In the latter case, the employment of protection in some degree may be looked upon as essential. The steel employed may be thin and only sufficient to be effective above some prearranged altitude, but, nevertheless, it will be essential. Protection from attack by other aeroplanes, or, more broadly, aircraft, is another question; we may express the utility of armour under these conditions definitely in terms of gun-power.
To make this clear let us consider two machines in combat—an aeroplane duel, in fact—and we will take it that at their average distance apart or range the mean number of shots fired by either to score a decisive hit is found to be 600. Now if either aeronaut by the employment of armour or gun-shields, or equivalent device, can reduce the effective target offered by his machine to one-half that previously presented, it will on an average take 1,200 shots to knock him out in lieu of 600 without protection. But in order to provide for the weight of his armour he must cut down his armament; he must sacrifice either his gun weight, and with it his speed of fire, or he must carry a lesser total weight of ammunition, and risk finding himself without means of attack, this being virtually synonymous to being without means of defence. If the only alternative were the cutting down of the speed of fire—tersely, if he were to substitute, say, 30 lb, of armour for 30 lb, of gun—and if this represent half his total gun capacity, and involve a reduction in his speed of fire by nearly one-half, then the change might be considered as nearly justified, since he would receive two shots for every one he could discharge, but would at the same time be proportionately less vulnerable.
Obviously, rate of fire should be one of the last things to be sacrificed; but the alternative—a reduction in the load of ammunition—involves a curtailment of the period of activity, and, as a corollary, an increase in the number of machines required for a given combatant duty. Once admit the necessity for such additional machines, and we must estimate the sacrifice, or price paid for the armour, in terms of the loss of fighting strength due to the absence of a section of the air-fleet occupied in replenishing. This is evidently a serious matter under the best conditions—i.e., when fighting in the immediate vicinity of the base; if, however, an air-fleet be engaged far afield it becomes still more serious, and the sacrifice of rapidity of fire, rather than reserve of ammunition, might well prove to be the lesser of evils.
The foregoing illustration shows that, tangibly or intangibly, the matter is one of figures, or, at the worst, a balance of advantages not capable of ready numerical expression. It may thus not always be possible to lay it down definitely whether in theory given conditions mean the abandonment of armour or otherwise; but nevertheless the fact is determined by the sum of the conditions, and where theory is dumb the decision will require to be taken on actual experience, as in analogous problems in naval construction.
§ 57. Protection by Armour and Shield Contrasted. It is, perhaps, opportune to draw attention here to the difference between shield and armour as a means of protection. The shield is essentially mobile, it is moved round and about to give the best protection possible, according to the direction of attack. A shield commonly forms part of a gun-mounting, but this is by way of being an accidental circumstance; the gun has to be trained on the enemy, and so the shield is made part and parcel of the gun, thus automatically taking the best position for the gunner's protection. In the aeroplane, however, the pilot is of almost more importance than the gunner; hence this traditional method of handling the shield may not be the best possible arrangement; perhaps it will be found advantageous to provide the pilot with a shield separately mounted or otherwise adjustable. There is rarely any intention in the case of a shield to give full and complete protection as is done with armour. It is an error to suppose that partial protection is of no value; every square inch covered diminishes proportionally the chances of a fatal hit, and so increases the fighting value of the machine, just as would a commensurate increase in rapidity of gun-fire; on the other hand, as already insisted, either armour or shield which is insufficient in thickness is worse than useless.
It is probable that in cases in which it may not pay to fit armour, it will still be found profitable, owing to the considerations already discussed, to provide shields to give partial protection both to gunner and pilot.