Armour-piercing fin-stabilized discarding-sabot
Armour-piercing fin-stabilized discarding-sabot (APFSDS) is a type of kinetic energy penetrator ammunition used to attack modern vehicle armour. As an armament for main battle tanks it succeeds armour-piercing discarding sabot (APDS) ammunition, which is still used in small or medium calibre weapon systems.
Armour-piercing discarding sabot (APDS) was initially the main design of the kinetic energy (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. Adding fins like the fletching of an arrow to the base gives the round stability. 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 China, Israel, France, South Korea, 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.
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.
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). 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.
- "Adiabatic Shear Banding in Axisymmetric Impact and Penetration Problems". J. B. Stevens and R. C. Batra.
- "120mm Tank Gun KE Ammunition". Defense Update. 2006-11-22. Retrieved 2007-09-03.
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