High explosive squash head
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High explosive squash head (HESH) is a type of explosive ammunition that is effective against buildings and is also used against 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 acquired the early post-World War 2 British 105 mm Centurion tank, including Sweden, India and Israel. In the United States, it is known as HEP, for "High Explosive, Plastic".
HESH rounds are thin metal shells filled with plastic explosive and a delayed-action base fuse. On impact, the plastic explosive is "squashed" against the surface of the target, and spreads out to form a disc or "pat" of explosive. A tiny fraction of a second later, the base fuse detonates the explosive, creating a shock wave that, owing to its large surface area and direct contact with the target, conducts very effectively through the material. In the case of the metal armor of a tank, the compression shock wave conducts through the armor to the point where it reaches the metal/air interface (the hollow crew compartment), where some of the energy is reflected as a tension wave. At the point where the compression and tension waves intersect, a high stress zone is created in the metal, causing pieces of steel to be projected off the interior wall. This fragmentation by blast wave is known as spalling, with the fragments themselves known as spall. The spall travels at very high speed through the interior of the vehicle to injure or kill the crew, damage equipment, and/or ignite ammunition and fuel. Unlike high explosive anti-tank (HEAT) rounds which are shaped charge ammunition, HESH shells are not designed to actually perforate the armour of main battle tanks, relying instead on the conduction of the shock wave through the solid steel armor.
The round has good general purpose use being effective against most targets, though the round is generally used at relatively lower velocities because high velocity excessively disperses the pat of explosive. While only effective against tanks without spaced armor or spall liners, the round is still highly favored for combat demolition purposes. The flattened high-velocity explosive pat is capable of destroying a concrete edifice far faster than a HEAT round (which is designed for armor penetration), and without the dangerous fragmentation of a traditional high explosive (HE) fragmentation round.
[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 already 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 such as the T-55 and T-62. Later versions of T-55 and T-62 tanks contained a layer of spall liner, decreasing the effectiveness of the HESH round. Britain also devised anti-tank guided missiles in the 1960s with HESH warheads such as the 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 maintain HESH ammunition capability, 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.
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