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Projectile, Armor, and Methods to Increase Armor-Piercing Capability of Guns

Definition of the Armor Penetration

Today the majority of tank lovers, conducting endless arguments regarding the superiority of one tank over another, have not even considered what guides engineers in the creation of a new combat vehicle. Therefore, so that we can better understand the problem of the survivability of a tank and the penetrability of its armor by a projectile, we will examine some baseline information.

During the design of a tank, engineers select a survivable thickness of armor that will defeat the projectiles of a particular antitank gun whose use by an enemy is most probable. During this process the baseline parameters for calculation are: quality of armor, mass and construction of the projectile, caliber of the projectile, velocity of the projectile upon striking the target, and the angle at which the projectile strikes the armor. For the event of hitting the target, as norms these values are associated with the empirical formula of Krupp, which is in greatest accordance with the results of experiments with homogeneous steel plates.

B=(V*P^0.5)/(K*D^0.5)

B -is the desired thickness of homogeneous plate (in decimeters)

V -is the velocity of the projectile at the moment of impact on the armor (m/s)

P -is the mass of the projectile (kg)

D -is the caliber of the projectile (in decimeters)

K -is the weighted coefficient of the durability (AP resistance) of the armor and the construction of the projectile. Before the war K values ranged from 1900 to 2300. When case-hardened armor was present, the coefficient increased by 15-20 percent. In the event the impact angle deviated from normal (90 degrees), a correction was made in accordance with trigonometric principles and, though they in a majority of cases were relatively close in practice, the results were considered satisfactory.

In all tables of armor penetrability of guns there exist calculations of magnitude that differ from reality by a significant degree. Nonetheless, for baseline calculations artillerymen used the same Krupp formula by which they determined the PTP (Predel Tyl'noy Prochnosti - Strength of Rear Surface - when the armor had not been penetrated, but the rear surface of the armor plate had begun to show penetration) for a given gun at a given range.

But it was much more important to measure the magnitude of the PSP (Predel Skvoznogo Probitiya - Limit of Through Penetration) -the maximum possible measurement of armor thickness that a gun was able to penetrate under given conditions at a given range. To the point, namely here is a great disparity in the evaluation of PSP by various countries. For example, in the USSR it is considered that the armor is penetrated only in the case if all fragments of the armor-penetrating expended (that is, without explosives) projectile made it past the rear surface of the armor plate. The British follow the same rule, but Americans and Germans consider that this not required. During the evaluation of the PSP they consider full penetration to have occurred when more than 70-80% of the fragments of a projectile have penetrated the rear surface of the armor plate.

In addition, all artillerymen well understand that PTP and PSP are ideal numbers. In essence, a gun can penetrate armor of a thickness that lies between these numbers. In all countries, during the development of criteria of the threshold of armor penetrability, entered in the table, they selected a number greater than 50% of probable through penetration, and only the USSR selected 60% in 1931, and even further toughened the criteria to 75% of ideal through penetration in 1938.

For the sake of correctness it should be noted that the tabular entries of armor penetrability adopted in the USSR after 1938 were almost always fulfilled, at the same time that the tables that were published abroad had a probable nature, and only from 1943 was this question reexamined by USA and Britain.

The misunderstanding that occurred with the firm Byutast, through which the USSR obtained license from Germany for the production of the 37 mm antitank gun, was based on these variant readings. After the conduct of tests in the USSR, Soviet specialists submitted to the German firm that the armor penetrability of this gun, as indicated in the export certificate, did not correspond to reality. Approximately a month passed before this conflict was resolved.

Methods of Increasing the Armor-Piercing Capability of Guns

Before the war countless investigations were conducted of various methods of improving the penetration of armor. In this effort armor-piercing means were divided into high-explosive and kinetic energy. In the first case the destruction of armor plates or shields was accomplished by the actions of explosive waves upon them. Antitank grenades were one example of this type of weapon. Despite the fact that tests even with 10-15 mm sheets of case-hardened armor showed that an explosive charge with a mass of more than 2 kg, under the condition of close proximity was required for their penetration. Antitank grenades were periodically standardized as armament, with the recommendation that they be thrown on top of the target, where the armor usually was about 6-8 mm thick. In addition to grenades, another weapon of this type was the antitank mine for detonation against the bottom of the tank. Before the war they were not widely distributed and were used primarily to deprive a tank of its mobility by damaging suspension components.

Kinetic energy armor-piercing ammunition was studied in practically all countries. It was obvious to everyone that the velocity and mass of this round had to be as great as possible for maximum target effect upon striking armor plate. The first requirement in achieving this state was to attain the maximum possible muzzle velocity. Experiments were conducted with long barrels of small caliber. But light projectiles accelerated more slowly in the barrel (acceleration was determined by the time duration of the pressure forces of propellant gases on the projectile) and more rapidly lost their speed from air resistance. They required too long a barrel and too much propellant charge, and this in turn required a greater thickness of barrel walls and an increase in the mass of the gun carriage. In other words, the gun became too heavy and too long, at the same time remaining small caliber and, therefore, unsatisfactory for defeating personnel targets (infantry). Thus the more worthwhile path became to increase the mass of the projectile while other conditions remained equal.

Broad-scale experiments were conducted in 1936-1938 with projectiles of large cross section load and improved aerodynamic properties. Theoretically such "lances" were capable of defeating armor of great thickness. For example, a 20 mm penetrator projectile with a mass of 800 grams theoretically was able to defeat up to 120 mm of armor under ideal conditions! But in practice experiments with such projectiles concluded unsuccessfully. Their stability in flight was accomplished by rotation, and the nose portion of any rotating projectile describes a kind of cone (like the wandering of a spinning top) relative to the center of mass. And the longer the projectile, the greater the dimension of the cone that its nose portion describes. Thus, when it strikes the armor, such a penetrator has a great chance to come into contact with the surface of the armor at some angle and to be deflected to the point where the armor can begin to defeat it. This is precisely what occurred in the overwhelming number of experiments. As a result, experiments with penetrator projectiles were halted.

It was empirically established that the aerodynamic coefficient of an armor-piercing projectile should not exceed 7. Thus, it turned out that to increase without limit the mass for an existing caliber did not work. Just the same, experiments were conducted in the utilization of materials of great density as armor-piercing projectiles. But the use of tungsten carbide or other tungsten and molybdenum alloys made a shot "golden", was recognized as unjustifiably expensive, and not cost effective.

In addition, the actions of small-caliber ammunition after it penetrated armor were very weak. For example, a shot on a T-26 tank from a captured Polish antitank rifle of 7.92 mm calibre in 1940 showed that of 39 holes made in the side of the tank, only two were dangerous because they occurred in the area of the fuel cell. Not a single mannequin placed in a crew member position was seriously harmed (only the commander's leg was grazed by a bullet).

Thus, if Germany, Czechoslovakia, and France were conducting searches in the sphere of increasing armor-penetrating capability of small caliber (20-47mm) antitank artillery, then in 1940 the USSR adopted a sensible conception of increasing the caliber of antitank guns to 55-76mm and greater, because these guns would have dual purposes: antitank and anti-personnel. And in distinction from small-caliber cannons, they permitted combat with enemy tanks to be conducted at great ranges and with great effectiveness. For this same reason in 1940 the Soviet 45mm antitank (and tank) cannon was recognized as unpromising, and all work for its subsequent modernization was halted. Only the war upset all the plans and in 1942 forced the Soviets once again to return to this cannon.

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