A kinetic energy penetrator (KEP, KE weapon, long-rod penetrator or LRP) is a type of ammunition designed to penetrate vehicle armour. Like a bullet, this ammunition does not contain explosives and uses kinetic energy to penetrate the target. Modern KEP munitions are typically of the armour-piercing fin-stabilized discarding sabot (APFSDS) type.
Early cannons fired kinetic energy ammunition, initially consisting of round balls of worked stone and later of 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. Kinetic energy ammunition, in its various forms, has consistently been the choice for those weapons due to the need for high muzzle energy.
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-diameter 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 mirror") to give extra altitude to its 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 cannonballs in the barrel, placed between the propellant charge and the projectile. The name "sabot" (pronounced // SAB-oh in English usage) is the French word for clog (a wooden shoe traditionally worn in some European countries).
Concentration of force into a smaller area was initially 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 either as armour-piercing composite rigid (APCR), high-velocity armor-piercing (HVAP) and "hartkern" (hard core) (resp. by the British, US and Germans). 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.
The principle of the kinetic energy penetrator is that it uses its kinetic energy, which is a function of its mass and velocity, to force its way through armor. 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, ideally destroys the target.
The modern kinetic energy weapon maximizes the stress (kinetic energy divided by impact area) delivered to the target by:
This has led to the current designs which resemble a long metal arrow.
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. Main battle tanks usually use KE-penetrators, while HEAT is mainly found in missile systems that are shoulder-launched or vehicle-mounted, and HESH is usually favored for fortification demolition.