Aircraft in Warfare (1916)/Chapter 9

by Frederick William Lanchester

CHAPTER IX.

(October 30th, 1914)

GUN-FIRE BALLISTICS. THE ENERGY ACCOUNT. EXPANDING AND EXPLOSIVE BULLETS.

§ 58. Gun-Fire. The Energy Account. The kinetic energy of a projectile commonly represents from 10 to 30 per cent, of the total energy of the explosive or powder charge by which it is projected; the lower figure corresponds to the performance of a small-bore low-velocity rifle such as a rook rifle, the latter being that approached under the most favourable conditions by the military or big-game rifle. The British Service rifle with Mark VI, ammunition thus has an efficiency of approximately 28 per cent.; in the ordinary sportsman's "12-bore" the figure is about 11 per cent.

The total energy released on combustion by black powder is the equivalent in round numbers of 500 foot-tons per pound. The corresponding figure in the case of cordite is half as much again, approximately 750 foot-tons per pound; in general it may be taken that most of the explosives in common use have an energy content between 500 and 1,000 foot-tons per pound. In the case of the Service rifle with Mark VI, ammunition the weight of the powder (cordite) is 30nbgrains (0.0043 lb.), and the bullet 215 grains (0.0307 lb.), the velocity being 2,050 ft, per second. Thus the total energy of the charge is 0.0043 × 750 = 3.2 foot-tons, and the muzzle (kinetic) energy is 2,000 foot-pounds = 0.895 foot-ton; the efficiency, therefore, is 0.895/3.2 = 0.28, as already given. It is worthy of remark, en passant, that there is very close accord between the figures applying to the gun and those which obtain in the gas-engine. In all such matters as efficiency, heat lost to barrel (cylinder walls), and heat remaining in gases ; the agreement is far closer than one would have ventured to expect in view of the great disparity of the conditions.

§ 59. Energy Available and Otherwise. It has already been pointed out that under the conditions of attack on aircraft there is very little possibility of utilising the whole of the energy of the bullet on impact. Unless the motor mechanism, or the pilot or gunner, be hit, the character of the structure employed in aircraft is such that the bullet or projectile will pass through with a comparatively insignificant loss of energy and will do little or no damage. With the ordinary military bullet, and more particularly with the spitzer model,^[1] nothing less than encounter with a heavy metal part will cause it to break up. Any non-metallic structural material, such as timber, is bored cleanly through, and if initially designed with a reasonable margin of safety, the resulting injury to it is negligible. The position is similar to that which existed, before the adoption of explosive shell, in the attack on the wooden ship by the artillery or cannon of a century ago. At close quarters the cannon ball would go clean through, often with comparatively little injury. It is said that Napoleon, observing this to be the case, himself expressed the opinion that explosive shell (a then well-known expedient in siege operations) could be adopted generally in naval warfare with advantage. The situation is considerably more acute in the case of the attack on aircraft by rifle-fire, and so we are led to consider the possibilities of the ex- plosive or expanding bullet, ignoring, for the purpose of discussion, the existence of the Declaration of St. Petersburg.^[2]

§ 60. The Explosive Bullet. The simplest form of explosive bullet, and one of the most effective, is that devised by Mr. Metford about the middle of the last century; this, as applied to an Enfield bullet of the period 1860, is illustrated in Fig. 11. An explosive charge is inserted in the fore part of the bullet, and consists of equal parts of sulphur and chlorate of potash,^[3] this mixture acting both as detonator and "burster."

The hollow-ended form, or "drilled-up" end, has the incidental advantage that it alone will determine the

Fig. 11.	Fig. 12.	Fig. 13.	Fig. 14.

expansion of the bullet on impact, quite apart from the action of the explosive charge. If the Metford system were applied to the modern bullet, the section would be somewhat as shown in Fig. 12, the basis of which is the Service 0.303 Mark VI. Another good form to take as the basis of an explosive bullet is the capped bullet. Fig. 13, as used in sporting rifles, the space inside the cap being conveniently filled with mixture to Metford's specification.

It is difficult, however the cavity be arranged, to devote more than about one-eighth or one-seventh of the volume of the bullet to receive the charge, and consequently, in view of the relatively low density of the explosive (about 1.6 in the case in point), the weight of the burster cannot be more than some 2 or 3 per cent, of the total. Taking the figure for cordite as representing the energy of the burster explosive, this means, in the case of the Service rifle, about 5 or 8 grains, or 1,300 ft.-lb, energy. But the efficiency of the burster is not likely to be higher than that which we associate with the main charge—it is at some advantage, inasmuch as there is no confined barrel to the walls of which heat is lost, but it is at a serious disadvantage, in that the explosion is not with any certainty confined to its work. It is doubtful whether of the 1,300 ft.-lb, total more than 300 ft.-lb, on an average will be usefully expended. We are thus led to appreciate the attributes of the explosive bullet, and more generally the explosive shell, in true perspective. The explosive only adds to an initial energy content of 2,000nbft.-lb, as due to velocity, a matter of about 300 ft.-lb, in available explosive energy, a quantity representing an addition of only 15nbper cent. It is at once evident that the value of the explosive charge is less due to its direct action than to the fact that by its spreading or scattering effect on the projectile the kinetic energy is used to better advantage. In other words, the explosion is effective as a means of initiating or causing the expansion of the bullet rather than as acting directly by its own destructive power. In the case of large shells the proportion of burster charge to total weight can be increased, and so the direct effect is relatively more important; for armour-piercing projectiles, however, the proportion is no higher than in the example taken—i.e., about 3 per cent. It might be imagined that the employment of some higher explosive would give a capacity of greater direct bursting energy, but the high explosive is not so called by reason of any greater total energy content, but rather on the effects of its rapidity of action; in brief, its power of detonation.

It is evident that, for the purpose under contemplation, the destruction of the less substantial structural parts of aeroplanes, etc., if we are able to secure the proper and immediate expansion of the bullet on impact without the use of an explosive charge, every useful purpose will be served. The bullet energy, even reduced to about one-quarter of its initial value by 1,000 yards flight, is more than sufficient, if definitely expended in the impact, to destroy any strut or spar or other light constructional part, without any aid from an explosive charge. The question is, whether the expansion of the bullet can be induced to take place with sufficient rapidity by any less drastic device.

§ 61. The Expanding Bullet. Any bullet is considered an expanding bullet if it be so made as to spread or mushroom on impact with its objective. But it is more usual to restrict the term to bullets having some special provision artificially to assist or facilitate their expansion, and, generally speaking, the objective is assumed to be game or other living quarry of some description. Evidently, if the target be hard enough, every bullet will expand to some degree. The means usually adopted in the case of the solid-lead bullet is to drill or form a hollow in the nose, as familiar to all who have used the sporting rifle. Another well-known method is to split the nose for a short distance by two cuts at right angles. In the case of the nickel-covered bullet the drilled nose again is sometimes adopted, or the nickel sheath at the nose, or point, of the bullet, is removed, the lead core being laid bare. All these devices have been practised in connection with sporting ammunition for many years. The art of designing an expanding bullet is so to proportion things that under the average conditions the degree of expansion is that found to be most desirable; thus the depth of the hole, or the extent of the slits, or the amount of the sheath cut away may be varied to whatever extent desired. The object to be attained is that the bullet shall expend its whole energy in inflicting the maximum possible injury, but at the same time it must not go to pieces or spread to such an extent that its penetration is lacking. In stopping big game it is necessary, not only that the energy should be wholly utilised, but also that it should be expended, as far as possible, in injury to the deep-seated vital organs. More recently Messrs. Westley Richards^[4] have brought out a modified form of expanding bullet in which the sheath is kept intact, but is not wholly filled by the lead core, there being an air-space in the fore end; this type (already illustrated in Fig. 13), expands to a moderate degree only, and retains a considerable power of penetration.

§ 62. Expansion due to Centrifugal Force. One of the main factors contributing to the spreading or expansion of a bullet is the centrifugal force of the bullet itself; all that is required of the impact is so to break down the structure of the bullet as to permit it to expand. The direction of motion of the peripheral portions of the bullet make at all points an angle with the axis of flight at least equal to the angle of the rifling, which is commonly about 1 in 10 to 1 in 12. This is the state of things when the bullet is discharged, but the actual angle rapidly becomes greater owing to the reduction of velocity, the speed of rotation being comparatively little affected. Thus at 1,000 yards range the velocity is reduced by half, and the relative direction of the skin of the bullet becomes about 1 in 6 to the line of flight. If then by a sudden impact or other means the bullet be broken into a number of small fragments at any point in its path, these fragments immediately spread out after the manner of shrapnel, covering a cone whose base is approximately one-third of its height; moreover, the distribution of the fragments in space—that is, within the conical surface—will be almost uniform. Such a distribution is almost ideal from the point of view of the work in hand. A desirable solution to the problem would appear to lie in the direction of a bullet composed of pellets or shot embedded in a matrix of only just sufficient strength to hold together, so that on comparatively light impact the component pellets will be released, and each will follow its individual direction of motion.

In every case the degree of expansibility requires, finally, to be determined by experiment, though with sufficient previous experience, and a proper comprehension of the conditions, it is usually possible to hit off the right thing without much difficulty. For the destruction of wooden struts, spars, etc., it is clearly necessary to obtain the most rapid expansion possible (corresponding to instant disintegration), since the bullet has to do its work in a distance rarely exceeding some 3 in. or 4 in. Owing to the fact that the "tissue" penetrated is of very low density—about 30 lb. or 40 lb. per cubic ft.—it is probable that good results would be obtained by drilling the nose with a conical aperture of large diameter, at the entrance about half the diameter of the bullet, after the manner illustrated in Fig. 14. The author is at the present time making a series of experiments with expansive bullets on a target made up of wooden scantlings of cross-section comparable to aeroplane struts or wing members. We may form some estimate of the latent destructive power of the military Service bullet from the fact that it will penetrate some 4 ft. of deal or pine, representing 3½ cubic inches of wood displaced. Taking this as a criterion, it is clear that if we can obtain the necessary rapidity of expansion, there is ample energy in a single hit to sever any ordinary wing or similar structural member.

§ 63. The Light Weight Shell. Apart from the question of the explosive or expansible bullet, we have already seen that the 14 oz. limit of projectile weight is an irksome restriction when we are dealing with aeroplane armament. Not only is the gun required weighty and cumbersome, but the weight of the ammunition is, save for machines of exceptional size, almost prohibitive. If we are able to adopt a weight of 6 oz. or 8 oz. in place of 14 oz., it will be possible to use a gun direct-mounted without the elaboration of recoil mechanism, and weighing from 40 lb. to 45 lb. In addition to this, the cartridges would weigh less than 1 lb. apiece, and provision could be made for some 300 rounds in a machine of present-day dimensions. For attack on other aeroplanes, such a shell would be almost as effective as one of twice the weight, and, owing to the recoil limit, the rate of fire can be doubled if the lighter shell be adopted.

It is convenient to make a distinction between the explosive bullet and the shell, even if the definition be considered somewhat artificial. The former is defined as containing a single charge, impact fuse and "burster" in one; the latter (the shell) contains, as well as the burster charge, an independent cap or detonator, and a fuse or mechanism, time or impact, for determining the instant of explosion. Many writers refer to the explosive bullet equally as a shell.

It is unnecessary to discuss the probability or possibility of the abandonment of the restriction imposed by the Declaration of St. Petersburg. We know that the clause in question was framed from humanitarian motives, and it is fairly evident that any expanding bullet which, from its behaviour, is tantamount to an explosive bullet, may be looked upon as infringing the terms of the Declaration, even though it contain no actual explosive; the terms of the Hague Declaration (Article 60) are virtually an admission of this. It is equally clear that neither at St. Petersburg, in 1868, nor at the Hague, in 1899, did the matter arise that now confronts us; and so it is actually a question to what extent either document will be considered binding under the conditions which have arisen. In any case it behoves us to ascertain everything there is to know on the subject, and to be prepared for all eventualities.

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1. See: Spitzer (bullet). (Wikisource contributor note)
2. See: Saint Petersburg Declaration of 1868. (Wikisource contributor note)
3. This mixture being liable to detonate by friction, the ingredients require to be separately ground and mixed with due care.
4. See: Westley Richards . (Wikisource contributor note)