Armour-piercing fin-stabilized discarding sabot

Armor-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.

Improvements in powerful automotive propulsion and suspension systems following World War Two allowed modern main battle tanks to incorporate progressively thicker and heavier armor protection systems, while maintaining considerable maneuver speed on the battlefield. In addition, incorporating highly sloped armor surfaces, which effectively increase the line-of-sight thickness of armor plating well above its perpendicular through-thickness, provides considerable additional protection at reduced weight penalty. When sloped dramatically backward adding obliquity and incorporating spaced armor, the line-of-sight thickness of armor arrays can increase to several feet, with limited armor weight increase. As a result, achieving deep armor penetration with gun-fired ammunition required even longer anti-armor projectiles fired at even higher muzzle velocity than could be achieved with stubbier APDS projectiles.

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, rounds 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 standard rifling decreases the performance 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 very high rotation on a fin-stabilized projectile can dramatically increase aerodynamic drag, further reducing impact velocity. For these reasons, APFSDS projectiles are generally fired from smoothbore guns, a practice that has been taken up for tank guns by China, Israel, Italy, Japan, France, Germany, Turkey, Russia, and the United States. Nevertheless, in the early development of APFSDS ammunition, existing rifled barrel cannons were used, (and are still in use), such as the M68-105mm cannon mounted on the M60A3 main battle tank. To reduce the spin rate when using a rifled barrel, a "slip obturator", (slip obturation ring), is incorporated that allows the high pressure propellant gasses to seal, yet not transfer the total spin rate of the rifling into the projectile. The projectile still exits the barrel with some residual spinning, but at an acceptably low rate. In addition, some spin rate is beneficial to a fin-stabilized projectile, averaging out aerodynamic imbalances and improving accuracy. Even smooth-bore fired APFSDS projectiles incorporate fins that are slightly canted to provide some spin rate during flight; and very low twist rifled barrels have also been developed for the express purpose of firing APFSDS ammunition. Another reason for the use of smoothbore, and very low twist rate guns is that the most effective precision shaped charge designs, HEAT munitions, lose armor penetrating performance when rotating too fast. These deep penetrating shaped charges also require fin stabilization; (although less precise and less effective "spin compensated" shaped charges can be designed to function properly in a spin-stabilized projectile).

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Wikipedia