Fuzes

The triggering of incendiary compounds may not be too critical as they burn for a noticeable, albeit short, period of time. HE shells, however, have to be detonated at precisely the correct instant and for this, a fuze is needed.

Every fuze has two distinct requirements. First, it must reliably detonate the explosive filling at the precise instant required. This might be the instant it hits the target, a fraction of a second later, at a preset point of its flight, or when it detects the target is close enough (many fuzes are capable of being set to achieve more than one of these). Secondly, it must be completely safe when being handled (or mishandled) and fired. It is not easy to achieve these aims and fuzes are often intricate devices built with the precision of a Swiss watch. They therefore tend to be expensive, which is why they are most common in cannon calibres, where the cost can be justified by the effect.

Fuzes are of several distinct types. The original version, applied to mortar shells in previous centuries, consisted of a length of fuze cord (rope soaked in a flammable compound) which could either be ignited just before firing or be lit by the propellant charge. It was a time fuze, which detonated after the cord had burned through to the filling, and could be adjusted by varying the length of the cord. The same system is still used in fireworks.

The first 'modern* time fuzes, which were used in

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Lock Collet

FIRING PIN SPRING

BALL

^ SPLIT COLLET

FLAT SPIRAL SPRING

FIRING PIN

SHUTTER

DETONATOR

SECURITY RING

SAFETY LOCK

R E LAY

FIRING PIN SPRING

BALL

^ SPLIT COLLET

FLAT SPIRAL SPRING

FIRING PIN

SHUTTER

DETONATOR

SECURITY RING

SAFETY LOCK

R E LAY

Impact fuze for Rarden 30 X 170 H E-T projectile

(Courtesy: Ian Hogg)

both world wars, were a more sophisticated version of the same principle. They contained a powder train which could be adjusted in length by turning the head of the fuze to achieve the required burning time. During the Second World War, Germany made much use of mechanical fuzes which were, in effect, clock timers (the British adopted these halfway through the war). These were much more precise than powder fuzes, being accurate to 0.5% instead of 2%. This was particularly important in anti-aircraft fire, in which time fuzes were set to detonate shells at a specified altitude.

In more recent times, time fuzes, like clocks, are electronic. These fuzes are mainly reserved for larger artillery, but are an essential feature of the Oerlikon AHEAD system in which the time is determined by the fire-control computer and set electronically as the projectile leaves the muzzle. An intriguing variation is used in the 20mm OICW infantry weapon currently being developed in the USA; the fuze counts the number of projectile rotations to determine when to detonate the airburst shell.

Contact or impact fuzes may appear simpler to design - the general principle is the same as that used to fire a toy cap gun - but they have their own problems. The main one is to ensure that the shell will detonate at the instant of contact while remaining inert at all other times. Mechanical contact fuzes therefore have elaborate safety measures, which usually involve the detonating mechanism being locked in a safe position by devices which are moved out of the way by the centrifugal force applied to the shell as it is spun by the rifling. One of the earliest contact fuzes specifically designed for aircraft use was the British No. 131 fuze of the Great War, made extremely sensitive in order to achieve detonation when penetrating airship fabric. Somewhat surprisingly the Russians have a modern equivalent for their 30 X 165 ammunition, intended to destroy balloons.

The mechanism is further complicated by the usual requirement that there be a slight delay between contact and detonation. This is in order to achieve a more useful effect against vehicles as it is usually better for the shell to explode inside the target aircraft or vehicle rather than on the surface. This requirement can cause problems: for example, Israeli pilots, in various actions during the 1960s, discovered that their 30mm DEFA cannon fire was ineffective. It transpired that the shells were fitted with long-delay fuzes intended for use against bombers; against fighters, they had passed right through before exploding.

Another problem is concerned with ensuring that the fuzes detonate even when the target is struck a glancing blow. This is a particular problem in AA fire, and despite the best efforts of fuze designers it is difficult to ensure 100% reliability of detonation.

Not all contact fuzes are mechanical. Some, particularly in smaller calibres, use chemicals which detonate on impact. These are far simpler in theory but very difficult to get right in practice, because

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