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. The plastic explosive is "squashed" against the surface of the target on impact and spreads out to form a disc or "pat" of explosive. The base fuze detonates the explosive milliseconds later, creating a shock wave that, owing to its large surface area and direct contact with the target, is transmitted through the material. In the case of the metal armor of a tank the compression shock wave is conducted through the armour 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 at high velocity. This fragmentation by blast wave is known as spalling, with the fragments themselves known as spall. The spall travels through the interior of the vehicle at high velocity, killing or injuring the crew, damaging equipment, and/or igniting ammunition and fuel. Unlike high explosive anti-tank (HEAT) rounds which are shaped charge ammunition, HESH shells are not specifically designed to perforate the armour of main battle tanks. HESH shells rely instead on the transmission of the shock wave through the solid steel armour.
HESH ammunition 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 concrete constructions much faster than a HEAT round (which is designed for armor penetration), and without the dangerous fragmentation of a traditional high explosive (HE) fragmentation round.
