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Armor-Piercing Shells

an answer to development of the tank

The first anti-tank shells achieved their objective by punching a hole in the tank's armor, relying on their mass and velocity for penetration. This type of attack is called the kinetic energy attack. Kinetic energy is the ability of a moving body to carry out work. This energy is proportional to the mass of the shell, but also proportional to the velocity squared. In other words, doubling the mass of the shell doubles the kinetic energy, but doubling the velocity increases the energy four times. To obtain the maximum effect the kinetic energy must be applied to the smallest possible area of the target and this means a long thin projectile with the maximum mass and minimum diameter, moving as fast as possible. The long, thin, heavy projectile is ideally suited to maintain its velocity as it passes through the air, but inside the gun the maximum muzzle velocity is achieved by having the largest possible diameter shell which will give the biggest base area for the gas pressure to act on. This means there is a contradiction in requirement -- maximum diameter inside the barrel, minimum diameter while the shell travels through the air and strikes the target.

This contradiction was resolved by the British invention of the Armor Piercing Discarding Sabot (APDS) shell. In the APDS shell there is a central core made of tungsten carbide, a strong and very heavy metal. This core is surrounded by a light magnesium alloy sleeve, sabot, which produces a large diameter when the shell is loaded. When the propellant is ignited the pressure of over 20 tons per square inch is applied to the large cross section area of the core plus the sabot, and this force gives the shell great acceleration. The force of the explosion breaks the sabot into sections, but the confinement of the barrel holds it together until it reaches the muzzle. The sabot then separates from the core, which proceeds towards the target. Thus at impact there is a large kinetic energy contained in a small diameter solid shot and this produces an extremely effective attack on armor.

armor piercing discarding sabot

The biggest problem with all kinetic energy shells is that they require a large and heavy gun in order to achieve maximum velocity and energy, something which is unacceptable in an infantry division, which must be able to move its guns with relative ease. To meet the need for a light launcher, methods of attacking armor other than kinetic energy had to be evolved. The answer was found in chemical energy produced by the detonation of a high explosive charge in a shell.

The first armor-piercing shell to employ chemical energy embodied the hollow charge, or shaped charge, principle and is known as the High Explosive Anti-Tank (HEAT) round. The front face of the shell is hollowed out to produce a cone. A liner of copper or aluminum is placed in front of the cone. When the shell hits the tank the high explosive is detonated by a base fuse and the energy produced is focused into a parallel-sided gaseous jet. The jet, with the now molten liner carried with it, has a velocity of about 18,000 feet per second. And, although it weighs only a few pounds, this velocity produces a very high kinetic energy, which allows it to penetrate to a depth of approximately three times the diameter of the cone.

high explosive anti-tank shell

The effectiveness of the HEAT round depends not only on penetrating the armor but also on the energy of the jet, liner and fragments of armor plate which pass through the hole into the interior of the tank to kill the occupants, cause fires, and destroy equipment.

An alternative method of using chemical energy is known as High Explosive Squash Head (HESH). In this type of round, a large quantity of plastic explosive is carried in a shell. When it strikes the armor plate, the plastic explosive filling spreads out on the armor face, and a base fuse detonates it. This system does not go through the plate -- the shock wave from the detonation is transmitted through it. When the shock wave reaches the far side it is reflected back, which overstresses the metal on the inside of the plate so that a large "scab," often a couple of feet across, is detached. This whirls around inside the tank at high velocity causing casualties and damage.

high explosive squash head shell

PICTURE SOURCE
The Illustrated Science and Invention Encyclopedia New York: H.S. Stuttman, 1977

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This page was last updated on 03/22/2017.