Kinetic energy penetrator

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 is commonly used for the most common modern type of armour piercing weapon, the armour-piercing fin-stabilized discarding sabot (APFSDS), a type of long-rod penetrator (LRP) and not for small arms bullets.

The opposite "technique" to KE-penetrators are chemical energy penetrators. There are two 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 of 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. The modern KE weapon maximises KE by

being fired with a very high muzzle velocity; they trade mass for velocity as the kinetic energy increases with the square of the velocity of the projectile.
concentrating the force in a small impact area while still retaining a large mass

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

History

The develpoment of the KE penetrator is that of two elements of artillery design. Firstly the highest speed of shot and secondly the concentration of the force. The first can be achieved by having a low mass and large base area in the gun barrel. This can be done by firing a small size projectile wrapped in a lightweight outer shell from a larger gun. Once the shell was clear of the barrel the sabot was no longer needed and would fall off in pieces, leaving the shell travelling at high speed and with a smaller cross-sectional area, which reduces aerodynamic drag during the flight to the target (see also external ballistics and terminal ballistics). The Germans developed sabots under the name "Treibspiegel" ("Propulsion mirror") to give extra altitude to their anti-aircraft guns during the Second World War.

The concentration of the force into a smaller area was approached by replacing the single metal (usually steel) shot with a composite one using two metals. A heavy core (based on tungsten) inside a lighter metal outer. 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.

The British between 1941 and 1943 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 give just a heavy shot of small cross-sectional area.

Modern design

The APDS was initially the main design of KE weapon. 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 gives poorer accuracy. Traditionally shells have been 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 6 or 7 times its diameter, rifling becomes less effective. Adding fins like those of an arrow to the base gives the round stability hence Armour piercing Fin Stabilised discarding sabot (APFSDS). The spin from rifling however decreases the effective penetration of these finned rounds and so they are generally fired from smoothbore guns; a practice that has been taken up by Russia, Germany and the United States in their tanks. 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 (for instance Britain) because they can then fire other ammunition but the rifling wears in use and they need more maintenance.

KE-penetrators for modern tanks are commonly just 2-3 centimeters in diameter, and 50-60 centimeters long; as more modern penetrators are developed, their length tends to increase and the diameter to decrease. To maximize the amount of kinetic energy released on the target, the penetrator must be made of a dense material, such as tungsten or depleted uranium (DU) alloy. The hardness of the penetrator is of less importance, but is still important as abrasion is a major component of the penetrator defeat mechanism. As DU is of iteself not particularly hard it is alloyed with nickel and/or zinc. A minor advantage (from a military perspective) of DU is that it is pyrophoric; the heated fragments of the penetrator ignite after impact on contact with air. Additionally, DU penetrators exhibit significant adiabatic shear band formation. During impact, fractures along these bands cause the tip of the penetrator to continuously shed material. This erosion maintains the tip's conical shape. Other materials such as unjacketed tungsten tend to deform into a less effective rounded profile, an effect called "mushrooming".

Few countries use DU ammunition despite its lower cost compared to tungsten because of adverse environmental and health effects. Battle sites where DU rounds have been used typically have residual uranium dust in and around battle-damaged vehicles. This dust is only mildly radioactive but toxic, and the lingering effects of it give public health concerns. It is disputed whether dust from spent DU rounds is linked to cancer and other illnesses such as "Gulf War syndrome".

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. APFSDS rounds generally operate in the range of 1,400 to 1,850 m/s. The sabots also travel at a huge velocity and upon separation 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 US Army, but the project was abandoned.

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History

Modern design

See also