Contact fuse

A contact fuze, impact fuze, percussion fuze or direct-action (D.A.) fuze (UK) is the fuze that is placed in the nose of a bomb or shell so that it will detonate on contact with a hard surface.

Many impacts are unpredictable: they may involve a soft surface, or an off-axis grazing impact. The pure contact fuze is often unreliable in such cases and so a more sensitive graze fuze or inertia fuze is used instead. The two types are often combined in the same mechanism.

The British Army's first useful impact fuze for high-explosive shells was the Fuze No. 106 of World War I. (illus.) This used a simple protruding plunger or striker at the nose, which was pushed back to drive a firing pin into the detonator. Its ability to burst immediately at ground level was used to clear the barbed wire entanglements of no man's land, rather than burying itself first and leaving a deep, but useless, crater. The striker was protected by a safety cap that was removed before loading, but there was no other safety mechanism.

The simplest form of artillery contact fuze is a soft metal nose to the shell, filled with a fulminating explosive such as lead azide. An example is the British World War II Fuze, Percussion, D.A., No. 233 ('direct action') The primary explosive transmits its detonation to an explosive booster within the fuze, then in turn to the main charge of the shell. As an artillery shell lands with a considerable impact, the "soft" nose may be made robust enough to be adequately safe for careful handling, without requiring any protection cap or safety mechanism. As a matter of normal practice though, fuzes and shells are transported separately and the fuze is only installed shortly prior to use. These simple contact fuzes are generally used for anti-tank shells, rather than high-explosive.

A more sophisticated fuze is the double-acting fuze, which is sensitive to both contact and grazing. An example of such a double-acting fuze is the British WW II Fuze, D.A. and percussion, No. 119 This fuze uses a nose striker, as for the original No. 106, but is rather more complex with an added inertia mechanism for grazing impacts and also three automatic safety devices. Simple contact impacts drive the striker back into the detonating pellet, as before. Graze impacts trigger the inertia mechanism, where instead the pellet in a heavy carrying plug travels forwards onto the striker. The striker is protected in storage by a nose safety cap. Normally this is removed before loading, but it may also be left in place if the target is behind cover. This reduces the sensitivity of the striker to light impacts through vegetation, but the fuze will still function through the inertia mechanism, or through a hard impact. Three safety devices are provided, one released by inertia during firing, which then unlocks a second that is released by centrifugal force of the spinning shell. These are mechanical locks that prevent the striker contacting the pellet. A third device is a centrifugal shutter that initially blocks propagation from the detonator pellet to the booster explosive.

...
Wikipedia