Read Ebook: The Development of Armor-piercing Shells (With Suggestions for Their Improvement) by De Zafra Carlos

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THE DEVELOPMENT OF

Armor-piercing Shells

CARLOS DE ZAFRA, M.E.

Faculty Lecturer, New York University

Historical

The manufacture of projectiles to meet the requirements of the modern science of warfare has been brought to its present high stage of development through a long series of experiments based, at first, more upon theory than perhaps any other branch of engineering.

In the days of the all-wood vessel the guns were of the smooth-bore class divided into various types with nomenclature according to the size or weight of the shot, very much as they are today, i.e., 3-pounder, 6-pounder, 4-inch, 10-inch, etc.

A general review of the gradual development of projectiles will be found beneficial and helpful to a more complete understanding of the complexities involved in overcoming the present day difficulties.

In the smooth-bore gun spherical shot was used. This was by no means a tight fitting device. Upon firing the gun considerable powder pressure was lost through the rapid escape of the gases past the shot between it and the bore of the gun. This would most naturally be expected since at best the surface of contact between the shot and the bore would be only a circular line quickly eliminated or worn away through friction under the high temperature of the burning gases behind the shot. The most obvious way to eliminate that wearing away of the bearing surface was to increase it, in doing which the escape of gas past the projectile would be greatly checked, and the gas pressure behind the projectile increased and imparting to the projectile greater velocity, increased momentum, and consequent increased penetration. But an increase in the bearing surface of the shot necessitated an alteration in its shape introducing difficulties affecting the accuracy of its passage through the air.

It was not an appreciation of any ineffectiveness in the early shot that first brought about a realization of the importance of obtaining the highest possible results from the material at hand, for no difficulty was experienced in penetrating the early wooden barriers. But with the introduction of rail-road and boiler iron and anchor chains along the sides of the vessels of war as a protection it was demonstrated that the old round shot previously most effective at the same range was now of little consequence. Armored vessels, though crude as was their armor, could with impunity run up along side a wooden enemy and demand immediate surrender with immediate destruction as the penalty for non-compliance. It is only necessary to refer to the Naval History of the Civil War of the United States for the most convincing proof that this was so.

Thus began one of the greatest industrial wars of the World--the Battle of Guns and Armor, which has been constantly waged through years of international peace and prosperity, and is destined to continue indefinitely or until the Utopian days of Universal Disarmament and everlasting peace arrive.

Early Developments

With the change from the spherical to the longitudinal projectile, difficulties in securing accuracy of flight arose not previously existing. It was found that the elongated projectile would tumble or revolve about its transverse axis during its flight, also wobble or describe a cork screw or spiral trajectory--capital defects requiring immediate attention.

The principle of the gyroscope to the effect that a body would maintain any desired position while revolving at a high rate about the proper axis was known and it was found desirable to adopt this principle in some practical manner to the development and improvement of the projectile. It was believed that were it possible to give a high rotative speed to the projectile about its major axis the desired object of keeping that axis co-incident with the vertical plane of the trajectory would be accomplished.

Among the first steps towards the development of the modern rifled artillery and elongated projectile we find certain improvements to have originated in the small arms pieces. In his "Report on the Art of War in Europe in 1854, 1855, and 1856" Colonel R. Delafield, U.S.A., gives the first reference of immediate importance to the subject in question. The small arms bullet was of lead which readily adapted itself to such configuration as was desired. Great contrariety of opinion existed as to the best form of ball and principle, even, by which it was caused to partake of the rifle twist of the gun barrel. The following are some of the first forms and methods adopted and are worthy of consideration:

Among the French and some others the "tige principle" was employed. It consisted in forcing the base of the ball open so "as to fit the bore and rifle grooves by driving it on a projecting spike in the bottom of the gun attached to the breech, and rising through the charge of powder," as in Fig. 1. For this purpose a countersunk rammer head to fit over the head of the ball had to be used.

In the English Enfield rifle a form of ball was used consisting of a hollow cup or cone in the bored-out base of the ball the action of the powder driving this cup into the ball causing it to expand and take the rifling. Iron cups were used in the Crimea but because of occasionally cutting off and leaving in the bore a ring of lead were discarded for solid wood or papier mach? cups . Figs. 5 and 6 show forms of hollow base balls used by the French and Russians, in which the direct action of the powder on the base caused the sought for expansion into the rifling.

The Russians at Sebastopol employed also a fourth principle consisting of two short projections or lugs on the cylindrical part of the solid ball to engage in two grooves cut in the bore of the gun. Its proportions are illustrated by Fig. 7.

The modifications of the preceding forms, shown in Figs. 8 and 9 were used in the Crimea by the Sardinian army which also used a smoothbore musket with solid ball as per Fig. 10. The French army Zouaves used a solid cylindro-conical grooved ball, as in Fig. 11, in a tige rifle.

The 1856 Austrian rifle used a solid cylindro-conical ball, "with two deep grooves cut in the cylindrical part such that the parts between the grooves are forced together and outwards, or upset by the explosion of the powder, to fill the bore and the rifle grooves," as in Fig. 12. Fig. 13 illustrates the same principle as used by the Saxon army.

Other forms used at the time by the various Powers are illustrated in Figs. 14, 15, 16, 17, and 18. But it was an open question as to which was the best form, no Power being fully satisfied.

It may be noted here that as the breech-loading rifle had not up to this time been sufficiently perfected, all the above bullets were for muzzle loading rifles. Breech-loading arms had been known for over two centuries but were as yet unreliable, clumsy, and generally imperfect.

The early methods adopted in the construction of cannon to impart to the projectile the desired rotary motion are as interesting as the early methods adopted in the construction of the projectile. Heavy rifled artillery was introduced in 1856 against Cronstadt. The English artillery at Sebastopol used the Lancaster Gun, illustrated in Fig. 19. The form of the bore section of this gun was that of an ellipse of 8" and 8-5/8" diameters, the bore being generated by the section of such an elipse making a revolution of about one-quarter turn in the length of the bore, the center of the section always co-incident with the longitudinal axis of the gun, forming thereby a continuous elliptical cylinder, the greater axis at the muzzle lying in the vertical plane and gradually becoming horizontal at the breech section, or in other words, the whole length and section of the bore was a rifle twist of one quarter of a turn in its length.

The projectile was a wrought iron shell of the form and size indicated in Fig. 20, as ascertained by measurement of one found in the trenches at Sebastopol.

The use of these guns in the siege was by no means satisfactory, giving neither precision of fire nor extraordinary range, while the gun more often failed by bursting than other types. The principle, however, met with favor and was studied and improved upon.

Another method of applying the rifle principle to heavy guns consisted in casting a segment of a sphere on the side of the cylinder part of the shot with corresponding grooves in the bore of the gun, making about one turn in twenty feet. It is somewhat like the principle of the solid musket ball, Fig. 7 with a difference in the shape of the projections, as shown in the annexed Fig. 21, giving the form and size of the shot.

Guns of this pattern were adopted for many of the gunboats fitted out by France for operations in the Baltic in 1856, some with four and others with two guns each.

The bore of the gun had a circular section of 6-1/2" diameter with two grooves cut in it, as shown in Fig. 22, which in the length of the bore had a twist equal to one turn in six meters.

Figures 23 and 24 represent cast iron shot "of very peculiar shape, intended apparently, as a substitute for the rifle groove. They were cylinders of about four inches diameter, with a flattened spherical head from which three spiral openings communicate with the open interior of the cylinder. The cylindrical part was formed with grooves...."

The Modern Type of Gun

From these earliest examples the development of artillery has been gradual until the present day of the built-up gun with an energy and range undreamed of in the earlier days. The built-up gun of today has attained to a calibre of 16 inches, a length of nearly 50 feet, a weight of 124 tons, and an extreme range at 42? elevation of 20.9 miles with a maximum height of trajectory of over 5-3/4 miles. The projectile, too, has increased in size from a few pounds to the one ton or 2,240-pound mass used in the above gun. The energy imparted to it at the muzzle amounts to 6,408-foot tons assuring a penetration at the muzzle of 33.8 inches of steel, or at 3,500 yards of 27.5 inches, the muzzle velocity being 1,975-foot-seconds and powder charge 640 pounds of smokeless. The maximum pressure in the powder chamber allowed is 37,000 pounds per square inch.

Briefly the modern gun is a built-up piece, constructed by fitting or shrinking super-imposed hoops or cylinders one over the other in size and number as diagramatically explained in Fig. 25, sufficient to re-inforce the bore to withstand the varied pressures.

The twist or rotary motion is imparted to the projectile by means of the "rifling" in the bore. Fig. 26 shows the cross-section of an 8-inch gun with the dimensions of the rifling, which is composed of two elements, the "groove" or spiral cut made in the bore and the "land" or space between two adjacent grooves. To take these grooves "rotating bands" of soft metal, generally copper, are fitted to the projectile as will be explained under "Manufacture of Projectiles."

Classification of Projectiles

Projectiles are classified according to their calibre, type of gun for which they are intended, material of which they are made, etc., as per the following scheme used in the U.S. Army for marking cases of projectiles:

Fuze Point }

Manufacture of Projectiles

While a high state of development has been attained in the manufacture of armor-piercing shells attention will be confined to their manufacture in so much as the methods for improvement hereinafter suggested are intended to affect the physical and not the chemical properties of the material, and are, therefore, applicable to all projectiles in which the stresses to be resisted exceed the resisting powers of the projectiles as at present manufactured.

The function to be performed by an armor-piercing shell is that of fully penetrating, without disruption to itself, an armor plate in thickness equal to, at least, the calibre of the shell in question, and then be in condition for effective bursting.

The following extracts from the Army and Navy specifications pertain to:

MATERIAL AND WORKMANSHIP

The base plugs must be of forged steel, annealed after forging or tempering, free from seams, cracks, and other defects, and have the following physical properties:

Elastic limit 50,000 to 60,000 pounds Tensile strength 90,000 to 100,000 " Elongation 18 per cent. Contraction 25 " "

The projectiles shall be machine-finished, before treatment, as close to the prescribed dimensions as may be consistent with that operation, and must, if necessary, be finally finished to the prescribed dimensions within the allowed variations.

Cylindrical tensile-test specimens with diameter of stem of 0.505 inch will be used in all cases when the piece is sufficiently thick to finish the stem to that dimension; when not, the inspectors will determine the exact form or diameter of the specimens to be used, the largest practicable being used, considering the piece under test. A length of stem between gauge marks of 2 inches will be used in all cases where the elongation is to be taken.

CAPPING

All steel projectiles shall be fitted, when required, with a cap of soft steel placed upon the point, the caps to be of the dimensions shown on the approved drawings and secured in a manner satisfactory to the Chief of Ordnance by means of a groove, to be turned on the head of the projectile prior to tempering.

See Frontispiece.

The steel for the cap must show a tensile strength not to exceed 60,000 pounds per square inch, an elongation after rupture of not less than 30 per cent, and a reduction in area of not less than 45 per cent on standard specimens, 2 inches long between measuring points and 0.505 inch in diameter. These caps will be thoroughly annealed before being placed upon the projectiles and will be free from cracks and all other defects.

TEST FOR DETECTION OF HOLES, CRACKS, ETC.

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