High explosive squash head
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High explosive squash head (HESH) is a type of explosive ammunition designed to defeat tank armour. It was fielded chiefly by the British Army as the main explosive round of its main battle tanks during the Cold War. It was also used by other military forces, particularly those that had acquired the early post-war British 105 mm Centurion tank, including Sweden, India and Israel. In the US, it is known as HEP, for "High Explosive, Plastic".
HESH rounds contain a warhead filled with plastic explosive and a delayed-action base fuse. On impact the plastic explosive spreads out to form a disk on the surface of the vehicle armour. The base fuse then detonates the explosive, creating a shock wave that propagates through the armour, causing flakes of metal to spall off the armour's interior surface. These fragments injure or kill the crew, damage equipment, and/or ignite ammunition and fuel. Unlike HEAT ammunition, HESH shells are not designed to perforate the armour of main battle tanks, although performance depends on the thickness of the target's armour plating.
[edit] History
HESH was developed by Charles Dennistoun Burney in the 1940s for the British war effort, originally as an anti-fortification "wallbuster" munition for use against concrete. This also led to British developments in recoilless rifles as a means to deliver the shell.
HESH was found to be surprisingly effective against metallic armour as well, although the British had effective weapons using HEAT, such as the PIAT. HESH was for some time a competitor to the more common HEAT round, again in combination with recoilless rifles as infantry weapons and was effective against tanks from the 1950s and 1960s such as the T-55 and T-62. Britain also devised anti-tank guided missiles in the 1960s with HESH warheads (see Malkara missile), although the vast majority of subsequent designs used variants of the HEAT concept.
From the 1970s onwards, tank armour design has tended towards layered composites of hard metal and heat-resistant materials. This type of armour is a poor conductor of the shock wave, and furthermore "spall liners", made of materials such as Kevlar, are commonly fitted to the interior surface of the armour, where it acts to retain any spall that does occur. Another reason for the declining use of HESH rounds is the switch by most armies to smoothbore cannon, since a HESH shell relies upon spin for accuracy. The British Army has kept rifled tank guns longer than many partly to be able to use HESH ammunition, though the Challenger 2 is intended to be converted to a smoothbore in the interests of ammunition commonality with NATO partners. HESH rounds are still carried today by armoured engineer vehicles; they are typically intended for use against fortifications rather than armoured fighting vehicles. A 165-mm HESH round is used by the United States Army for the main gun of the M728 Combat Engineer Vehicle, an M60 tank equipped with a bulldozer blade. Similarly the British Centurion AVRE (Armoured Vehicle Royal Engineers) was equipped with a short 165-mm gun solely for a 29-kg HESH shell.
Amongst other ammunition types, the Stryker Mobile Gun System variant is to be equipped with a 105-mm HESH round for demolition and bunker-busting purposes. Argentina's TAM medium tanks (mounted with the same 105-mm gun as the Centurion) can also fire HESH rounds.
