Logo from Scott Cunningham.
Logo from Scott Cunningham.
riginally developed in Britain during the latter part of World War II, it is called HESH (high explosive squash head) in Britain and HEP (high explosive plastic) in the USA. It was used in World War II for the destruction of concrete fortifications and it remains the most effective type of ammunition against such targets and buildings. Considered as a replacement for HEAT ammunition it has proven to be less successful as an anti-tank projectile and has since been (almost) abandoned in favour of other alternatives.7
HESH projectiles consist of a thin walled shell filled with a plastic explosive which is squashed on impact against the surface of the target and then detonated by a delayed action base fuse. The detonation of the explosive in close contact with armour generates stress waves within it which can cause fracture of the inside surface of the armour followed by the spalling of lethal metal scabs. Actual penetration of the armour does not necessarily occur. HESH projectiles may be spin stabilised without affecting lethality, allowing better accuracy compared to fin stabilised projectiles.7
The maximum thickness of homogenous steel armour which HESH projectiles can cause to scab is not much more than their calibre. This compares unfavourably with other types of projectiles as a means of attacking thick armour.7 This is not quite as bad as it seems, as HEAT projectiles, for example, need to have a residual penetration of 150mm to 200mm to ensure lethality (say one to two calibres). Even so, since a modern HEAT projectile has a potential penetration of up to 9 cone diameters, it can be seen that they have a much better performance than HESH projectiles.
HESH can be defeated by spaced armour, and has a low muzzle velocity…
HESH can be made ineffective by spaced armour, as the outer plate prevents the generation in the inner plate of stress waves sufficiently intense to cause spalling. HESH can only be fired at muzzle velocities of less than 800m/s, because of the low strength of the thin walled shells.7 This is less than that for modern HEAT projectiles and such a low muzzle velocity must result in a lower hit probability than conventional projectiles, in the same way that HEAT projectiles are affected, despite any spin stablisation.
Britain used HESH as the sole anti-tank ammunition type for Scorpion light tanks, and as a complement to APDS starting with the Conqueror heavy tank. The USA and Germany adopted HESH, or HEP as it was known, when they adopted the British 105mm L7 gun but neither country developed HESH for new tank guns.7 The only other countries to develop HESH in recent times are: India, 120mm for the locally designed Arjun; China, 105mm on the Type 80 (although this is possibly an outright copy of the British L7); and Yugoslavia, as secondary ammunition for 105mm and 122mm artillery.11
Consequently, HESH is not considered a better alternative than HEAT as an anti-tank projectile. One further reason for preferring HEAT to HESH is the better effectiveness of HEAT as an anti-personnel weapon, which is discussed next.
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