Kinetic Energy Penetrator

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French anti-tank round with its sabot

A kinetic energy penetrator (also known as a KE weapon) is a type of ammunition which, like a bullet, does not contain explosives and uses kinetic energy to penetrate the target.
The term can apply to any type of armour-piercing shot but typically refers to a modern type of armour piercing weapon, the armour-piercing fin-stabilized discarding-sabot (APFSDS), a type of long-rod penetrator (LRP), and not to small arms bullets.
The opposite technique to KE-penetrators uses chemical energy penetrators. There are two types of these shells in use: high explosive anti-tank (HEAT) and high explosive squash head (HESH). They have been widely used against armour in the past and still have a role but are less effective against modern composite armour, such as Chobham as used on main battle tanks today.
The principle of the kinetic energy penetrator is that it uses its kinetic energy, which is a function of mass and velocity, to force its way through armour. If the armor is defeated, the heat and spalling (particle spray) generated by the penetrator going through the armor, and the pressure wave that would develop, would destroy the target.^[1]
The modern KE weapon maximizes KE and minimizes the area over which it is delivered by:

• being fired with a very high muzzle velocity
• concentrating the force in a small impact area while still retaining a relatively large mass
• maximizing the mass of whatever (albeit small) volume is occupied by the projectile—that is, using the densest metals practical, which is one of the reasons depleted uranium is often used.

This has led to the current designs which resemble a long metal arrow.

History

The first cannon fired kinetic energy ammunition. First these were round balls of worked stone, then round balls of metal. From the beginning, combining high muzzle energy with projectile density and hardness have been the foremost factors in the design of such weapons. Similarly, the foremost purpose of such weapons has generally been to defeat armour or other defensive structures, whether stone castle walls, ship timbers, or modern tank armour. Chemical energy ammunition in its various forms has consistently been the choice for those weapons which due to various factors of their design could not generate the high muzzle energy needed by a kinetic energy weapon.
The development of the modern KE penetrator combines two aspects of artillery design; high muzzle velocity and concentrated force. High muzzle velocity is achieved by using a projectile with a low mass and large base area in the gun barrel. Firing a small size projectile wrapped in a lightweight outer shell, called a sabot, raises the muzzle velocity. Once the shell clears the barrel, the sabot is no longer needed and falls off in pieces. This leaves the projectile traveling at high velocity with a smaller cross-sectional area and reduced aerodynamic drag during the flight to the target (see external ballistics and terminal ballistics). Germany developed modern sabots under the name "Treibspiegel" ("thrust shield") to give extra altitude to their anti-aircraft guns during the Second World War. Before this, primitive wooden sabots had been used for centuries in the form of a wooden plug attached to or breech loaded before cannon balls in the barrel, placed between the propellant charge and the projectile. The name "sabot" is the French word for clog (a wooden shoe traditionally worn in some European countries). According to one theory, the word "sabotage" is derived from this specific meaning of "sabot".
Concentration of force into a smaller area was attained by replacing the single metal (usually steel) shot with a composite shot using two metals, a heavy core (based on tungsten) inside a lighter metal outer shell. These designs were known as Armour Piercing Composite Rigid (APCR). On impact, the core had a much more concentrated effect than plain metal shot of the same weight and size. However, the air resistance and other effects were the same as for the shell of identical size.
Between 1941 and 1943, the British combined the two techniques in the armour-piercing discarding sabot (APDS) round. The sabot replaced the outer metal shell of the APCR. While in the gun the shot had a large base area to get maximum acceleration from the propelling charge but once outside, the sabot fell away to reveal a heavy shot with a small cross-sectional area. High Velocity Armor Piercing (HVAP) rounds were also introduced by the United States Army, and were primarily used by tank destroyers.

Modern design

APFSDS at point of separation of sabot.

The APDS was initially the main design of KE penetrator. The logical progression was to make the shot longer and thinner to concentrate the kinetic energy in a smaller area. However a long, thin rod is aerodynamically unstable; it tends to tumble in flight and is less accurate. Traditionally, shells were given stability in flight from the rifling of the gun barrel, which imparts a spin to the round. Up to a certain limit this is effective, but once the projectile's length is more than six or seven times its diameter, rifling becomes less effective.^{[citation needed]} Adding fins like the fletching of an arrow to the base gives the round stability, hence Armour-Piercing Fin-Stabilized Discarding Sabot (APFSDS). The spin from rifling decreases the effective penetration of these rounds (rifling diverts some of the linear kinetic energy to rotational kinetic energy, thus decreasing the round's velocity and impact energy) and so they are generally fired from smoothbore guns; a practice that has been taken up by Israel — a major supplier of "arrow" rounds^{[citation needed]} — China, France, Pakistan, Germany, Russia, and the United States in their tanks. Another reason for the use of smoothbore guns is that shaped charge HEAT munitions lose much of their effect to rotation. APFSDS can still be fired from rifled guns but the sabot is of a modified design incorporating bearings to isolate the spin of the sabot in the barrel from the round itself, so far as practicable. Rifled guns have been kept in use by some nations (the UK and India, for example) because they are able to fire other ammunition such as HESH rounds with greater accuracy. However, the rifling wears down under regular APFSDS use and requires more maintenance. For these reasons a British Challenger 2 tank was modified under the Challenger Lethality Improvement Programme to mount a Rheinmetall 120mm smoothbore gun. However this programme has now been discontinued because of the impossibility of stowing an adequate quantity of the new fixed (one-piece) ammunition without replacing the powerpack to create additional hull volume.

Modern 120 mm tank gun shells

KE penetrators for modern tanks are commonly 2–3 cm in diameter, and 50–60 cm long; as more modern penetrators are developed, their length tends to increase and the diameter to decrease. However the development of heavy forms of reactive armour such as the Soviet, later Russian, Kontakt-5 which were designed to shear long rod penetrators, has prompted the reversal of this trend in the newest U.S. rounds. To maximize the amount of kinetic energy released on the target, the penetrator must be made of a dense material, such as tungsten carbide or depleted uranium (DU) alloy (Staballoy). The hardness of the penetrator is of less importance, but is still a factor as abrasion is a major component of the penetrator defeat mechanism. As DU is itself not particularly hard, it is alloyed with nickel, zinc, or both. DU is pyrophoric; the heated fragments of the penetrator ignite after impact on contact with air, setting fire to fuel and/or ammunition in the target vehicle, thereby compensating for the lack of an explosive warhead in the penetrator. Additionally, DU penetrators exhibit significant adiabatic shear band formation. A common misconception is that, during impact, fractures along these bands cause the tip of the penetrator to continuously shed material, maintaining the tip's conical shape, whereas other materials such as unjacketed tungsten tend to deform into a less effective rounded profile, an effect called "mushrooming". Actually, the formation of adiabatic shear bands means that the sides of the "mushroom" tend to break away earlier, leading to a smaller head on impact, though it will still be significantly "mushroomed". Tests have shown that the hole bored by a DU projectile is of a narrower diameter than for a similar tungsten projectile.^[2]
Typical velocities of APFSDS rounds vary between manufacturers and muzzle length/types. As a typical example, the American General Dynamics KEW-A1 has a muzzle velocity of 1,740 m/s (5,700 ft/s).^[3] This compares to 914 m/s (3,000 ft/s) for a typical rifle (small arms) round. APFSDS rounds generally operate in the range of 1,400 to 1,900 m/s. The sabots also travel at such a high velocity that upon separation, they may continue for many hundreds of metres at speeds that can be lethal to troops and damage light vehicles.
The counterpart of APFSDS in rifle ammunition is the saboted flechette. A rifle firing flechettes, the Special Purpose Individual Weapon, was under development for the U.S. Army, but the project was abandoned.