Contact fuze
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A contact 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.[1]
Artillery fuzes
The British Army's first impact fuze for high-explosive shells was the Fuze No. 106 (illus.) of World War I. 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[2] 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.[3] An example of such a double-acting fuze is the British WW II Fuze, D.A. and percussion, No. 119[1] This fuze uses a nose striker, as for the original No. 106 , but is rather more sophisticated 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 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 spinnign 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.
Most artillery contact fuzes act immediately, although some may have a delay.[4] This allows a high-explosive or semi-armour-piercing shell to penetrate a wall before exploding, thus achieving the most damage inside the building. Where a shell is used against strong armour and requires all of its explosive power merely to penetrate, a delay is not appropriate. Most such delayed fuzes are thus switchable to a "superquick" or immediate mode.[4]
Timed fuzes are used for airbursts. They take their delay time (½ second or longer) from firing, not from impact. These fuzes may also offer a contact fuzed ability. [5] As this type of fuze is complex and more sensitive, they usually have a deliberate safing mechanism such as an arming wire that must be removed before use.
Air-dropped bomb fuzes
Triggered when its forward motion rapidly decreases, specifically when they physically strike an object after being dropped by a parachute restrained ordinance device. The detonation may be on impact, or delayed slightly.
Contact fuzes are the most common type of fuze used.[citation needed] The contact fuze is set off when a series of connected crush switches that are placed on the exterior nose of the ordinance device make contact with the ground. The contact with solid ground activates the interior firing circuits which leads to the detonation of the ordinance device. A cone shaped cover over the device prevents premature detonation while the device is being loaded and carried to the desired location by aircraft.
One form of contact fuze was invented by Glenn O. Folkins and filed for official patenting on December 14, 1971.[6]
See also
References
- ^ a b Fuze, D.A. and Percussion, No. 119, Fletcher, David (1983) [1944]. "Ammunition, 95mm, Tank Howitzer". Churchill Tank (Reprint of original WW2 operating manuals ed.). HMSO / Bovington Tank Museum. pp. 85–87. ISBN 0-11-290404-1.
- ^ Fuze D.A. No. 233Fletcher, Churchill Tank, pp. 87-88
- ^ Note that in British terminology, "D.A." stands for "direct action", i.e. contact, rather than double-acting. Many D.A. fuzes, such as the No. 233, are not double-acting.
- ^ a b M48 Point Detonating Fuze for 75mm main gun, Fletcher, Churchill Tank, pp. 74-76
- ^ M54 Time and Superquick Fuze for 75mm main gun, Fletcher, Churchill Tank, p. 75
- ^ http://www.freepatentsonline.com/3769911.pdf