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High Explosive Squash Head (HESH) or High Explosive Plastic (HEP) Round

Category: Term of the day

HESH (HEP) Round

This is a type of anti-tank ammunition in which the explosive is contained in a thin-walled projectile which deforms on contact with the target, allowing the explosive to spread. A base fuse then detonates the explosive which sends shock waves through the armour. The shock waves are reflected from the internal face of the armour and when they meet the next incoming wave, the resulting wave front causes the armour to fracture. This type of ammunition is not velocity dependent as it relies on chemical energy to achieve its effect.

High Explosive Squash Head (HESH), for which the American term is High Explosive Plastic or HEP, is a type of chemical energy ammunition which is not velocity dependent. In this round the explosive is contained in a thin-walled projectile which collapses on striking the target, allowing the plastic explosive to spread. A base fuze then denotes the explosive which sends strong shock waves through the armour. Reflection from the internal armour surface causes an overmatch of the armour which then fails, causing a large 'scab' to form and fragments to fly off inside the vehicle. The success of HESH against armour depends on the explosive forming a suitably shaped 'pat' on the outside of the armour plate before it is detonated (the British term 'squash-head' describes the process very well). This poses several problems, because if the projectile hits the target at a very acute angle, the explosive will not form a cohesive mass and consequently its effectiveness will be greatly reduced. Similarly, if the projectile arrives at the target with very high residual velocity, i.e. if the target is at very close range, the explosive will be dispersed on impact, before the fuze has time to function. The latter problem is easily resolved by keeping the muzzle velocity low, but the result is a lower chance of a hit because of the higher trajectory. HESH can be defeated relatively easily by having a discontinuity (spaced armour) in the path of the shock wave, though the outer plate will almost certainly be destroyed.

The major advantage of HESH lies in its usefulness as a multipurpose round. It is approximately 90% as effective as a conventional High Explosive (HE) round against unarmoured targets and considerably better than HE against bunkers and buildings. It has a devastating effect against lightly armoured targets and even if a kill is not obtained against a more heavily armoured target, the secondary effects will damage all the optics, antennae and any equipment mounted externally.

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14:50 10.08.2006

As far as I know this type of projectile is used only by the British and the Americans. Other nations do not think that is type of projectile is useful enough.

01:53 16.08.2006

As the text mentions, it is possibly the best all round tank shell available. Cheaper than a SABOT round, although not as useful against heavy armour, it is a damn sight more useful against lighter armoured vehicles - a good story is os a bus hit around 5 times by American fired SABOT ronds in the '91 Gulf War - it drove off! If it had been hit, even once by a HESH shell, the result would be a lot different. It was also using a HESH round that the longest ranged tank on tank kill ever was recorded in this latest Gulf war - a British Challenger II scoring a confirmed kill at 2.5 miles.

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