Fuze - Fuze Safety/arming Mechanisms

Fuze Safety/arming Mechanisms

The multiple safety/arming features in the M734 mortar fuze are representative of the sophistication of modern electronic fuzes.

Safety/arming mechanisms can be as simple as the spring-loaded safety levers on M67 or RGD-5 grenade fuzes, which will not initiate the explosive train so long as the pin is kept in the grenade, or the safety lever is held down on a pinless grenade. Alternatively, it can be as complex as the electronic timer-countdown on an influence sea mine, which gives the vessel laying it sufficient time to move out of the blast zone before the magnetic or acoustic sensors are fully activated.

In modern artillery shells, most fuzes incorporate several safety features to prevent a fuze arming before it leaves the gun barrel. These safety features may include arming on "setback" or by centrifugal force, and often both operating together. Set-back arming uses the inertia of the accelerating artillery shell to remove a safety feature as the projectile accelerates from rest to its in-flight speed. Rotational arming requires that the artillery shell reach a certain rpm before centrifugal forces cause a safety feature to disengage or move an arming mechanism to its armed position. Artillery shells are fired through a rifled barrel, which forces them to spin during flight.

In other cases the bomb, mine or projectile has a fuze that prevents accidental initiation e.g. stopping the rotation of a small propeller (unless a lanyard pulls out a pin) so that the striker-pin cannot hit the detonator even if the weapon is dropped on the ground. These types of fuze operate with aircraft weapons, where the weapon may have to be jettisoned over friendly territory to allow a damaged aircraft to continue to fly. The crew can choose to jettison the weapons safe by dropping the devices with safety pins still attached, or drop them live by removing the safety pins as the weapons leave the aircraft.

Aerial bombs and depth charges can be nose and tail fuzed using different detonator/initiator characteristics so that the crew can choose which effect fuze will suit target conditions that may not have been known before the flight. The arming switch is set to one of safe, nose, or tail at the crew's choice.

Base fuzes are also used by artillery and tanks for shells of the 'squash head' type. Some types of armour piercing shells have also used base fuzes, as have nuclear artillery shells.

The most sophisticated fuze mechanisms of all are those fitted to nuclear weapons, and their safety/arming devices are correspondingly complex. In addition to PAL protection, the fuzing used in nuclear weapons features multiple, highly sophisticated environmental sensors e.g. sensors requiring highly specific acceleration and deceleration profiles before the warhead can be fully armed. The intensity and duration of the acceleration/deceleration must match the environmental conditions which the bomb/missile warhead would actually experience when dropped or fired. Furthermore, these events must occur in the correct order.

Note: some fuzes, e.g. those used in air-dropped bombs and landmines may contain anti-handling devices specifically designed to kill bomb disposal personnel. The technology to incorporate booby-trap mechanisms in fuzes has existed since at least 1940 e.g. the German ZUS40 anti-removal bomb fuze.