History – WW2 HEAT and the Piezoelectric Fuze

Featured Image: American Soldiers firing a Bazooka, possibly an M9A1 model, during WW2.

First striking fear into the hearts of tankers during WW2, the principle of High Explosive Anti Tank weaponry offers a way to use stored chemical energy rather than brute kinetic energy to penetrate armor. Freed from their reliance on velocity and weight, warheads could be comparatively slow and light, making them infantry-portable. At the same time, tank cannons can retain their penetrative ability even as velocity drops away, meaning their effective range is greatly increased. It should come as no surprise then that the premier tank round until the advent of composite armor and long-rod KE penetrators was the HEAT round, and it remains useful today. However, there was a key technological step between the WW2 incarnation of HEAT weapons and their eventual success – that of the Piezoelectric Fuze.

There is some debate over who was the first to develop shaped charge warheads, however the British No. 68 AT grenade was the first to be paired with a launcher in an anti-tank role. With the invention of the American “Bazooka” family, recoilless rocketry as a delivery platform blossomed, and every major combatant used some form of HEAT weapons, from the German disposable Panzerfaust to the Soviet RPG-43 and RPG-6 hand-thrown grenades. However, while a number of low-velocity cannons used HEAT warheads, tanks as a whole soldiered on throughout the war with conventional solid shot penetrators.

To understand why, we must first look at the principle behind HEAT warheads, that of the shaped charge. Drawing upon the Munroe effect, a shaped charge consists of a hollow cavity in front of a cone, usually of copper. The back side of the cone is covered in high explosives, which, when detonated, form the cone into a molten jet which punches through the target.

A cutaway of the RPG-7’s HEAT warhead, the cavity and cone clearly visible, with explosives in yellow.

This importance of the copper cone in properly forming the penetrative jet is what relegated HEAT to low-velocity applications; quite simply, if the warhead was moving too fast, the cone would be crushed before the high explosives could be triggered by the fuze, meaning that no shaped charge would be formed. Here came into play the Piezoelectric fuze. When exposed to mechanical stresses, such as hitting armor plate at high speed, Piezoelectric materials accumulate an electric charge. Using this principle, fuzes could be created which triggered the high explosives in the microseconds before the cone was damaged, preserving the HEAT effect even in high-velocity tank cannons.

A Norwegian Leopard 1, who’s designers assumed HEAT made any amount of armor irrelevant, and is thus lightly armored and very fast.

Though largely supplanted by newer types of ammunition due to the advance of composite armor types, HEAT still remains in use in everything from infantry AT weapons, multipurpose main gun ammunition for tanks, guided missile warheads, and even submarine-killing torpedoes such as the Mark 50. With advances such as double or even triple warheads to defeat composite and explosive reactive armors, HEAT shows little sign of disappearing.

-Millett

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