Contact fuse
A contact fuze, percussion fuze or direct-action (D.A.) fuze (UK) is the fuze that is placed in the nose of a bomb
A bomb is any of a range of explosive weapons that only rely on the exothermic reaction of an explosive material to provide an extremely sudden and violent release of energy...

 or shell
Shell (projectile)
A shell is a payload-carrying projectile, which, as opposed to shot, contains an explosive or other filling, though modern usage sometimes includes large solid projectiles properly termed shot . Solid shot may contain a pyrotechnic compound if a tracer or spotting charge is used...

 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.

Artillery fuzes

The British Army's first useful impact fuze for high-explosive shells was the Fuze No. 106
No. 106 Fuze
Number 106 Fuze was the first British instantaneous percussion artillery fuze, first tested in action in late 1916 and deployed in volume in early 1917.-Background:...

of World War I
World War I
World War I , which was predominantly called the World War or the Great War from its occurrence until 1939, and the First World War or World War I thereafter, was a major war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918...

. (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
No man's land
No man's land is a term for land that is unoccupied or is under dispute between parties that leave it unoccupied due to fear or uncertainty. The term was originally used to define a contested territory or a dumping ground for refuse between fiefdoms...

, 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
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

 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.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. 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
Inertia is the resistance of any physical object to a change in its state of motion or rest, or the tendency of an object to resist any change in its motion. It is proportional to an object's mass. The principle of inertia is one of the fundamental principles of classical physics which are used to...

 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.

Most artillery contact fuzes act immediately, although some may have a delay. 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.

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. 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

Fuzes for air-dropped bombs have generally used an internally mounted inertia fuze, triggered by the sudden deceleration on impact. Owing to the risk of an aircraft crash, or even the need to land with an undropped bomb still on board, these are protected by sophisticated safety systems so that the fuze can only be triggered after it has been dropped intentionally.

The German Stachelbombe (nose-spike bomb) or stabo of WWII was a standard bomb, from 50kg to 500kg, modified for use from low altitude. To avoid the risk of ricochet
A ricochet is a rebound, bounce or skip off a surface, particularly in the case of a projectile. The possibility of ricochet is one of the reasons for the common firearms safety rule "Never shoot at a flat, hard surface."-Variables:...

 from the ground, a nose spike was fitted, the intention being that this would penetrate first and anchor the bomb against bouncing — without this, there was a risk of the dropping aircraft not only missing the target, but also being damaged by its own weapon. As the German electric fuzes had an arming delay after dropping, and the bombs were dropped at such low altitude as to leave insufficient time for this to arm, they were also sometimes fitted with additional contact fuzes on the tips of these nose spikes.

Similar devices were employed by Russian forces, in a similar ground attack role using the Il-2 Sturmovik.

Fat Man
Notable examples of air-dropped bombs that did use contact fuzes include the Fat Man
Fat Man
"Fat Man" is the codename for the atomic bomb that was detonated over Nagasaki, Japan, by the United States on August 9, 1945. It was the second of the only two nuclear weapons to be used in warfare to date , and its detonation caused the third man-made nuclear explosion. The name also refers more...

 atomic bomb dropped on Nagasaki
is the capital and the largest city of Nagasaki Prefecture on the island of Kyushu in Japan. Nagasaki was founded by the Portuguese in the second half of the 16th century on the site of a small fishing village, formerly part of Nishisonogi District...

. The bomb was intended for air burst
Air burst
An air burst is the detonation of an explosive device such as an anti-personnel artillery shell or a nuclear weapon in the air instead of on contact with the ground or target or a delayed armor piercing explosion....

 detonation and was fitted with both radar height-finding and barometric fuzes. As the device was so secret, and the risk of informative fragments or plutonium
Plutonium is a transuranic radioactive chemical element with the chemical symbol Pu and atomic number 94. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, forming a dull coating when oxidized. The element normally exhibits six allotropes and four oxidation...

 being recovered after a failed drop was considered to be unacceptable, it was fitted with supplementary contact fuzes that were only intended to destroy the weapon beyond recognition. Four AN-219 piezo-electric impact fuzes were fitted to the nose of the bomb casing.

The BLU-82
The BLU-82B/C-130 weapon system, known under program "Commando Vault" and nicknamed "daisy cutter" in Vietnam and in Afghanistan for its ability to flatten a forest into a helicopter landing zone, is a 15,000 pound conventional bomb, delivered from either a C-130 or an MC-130 transport aircraft....

 was a large conventional explosive bomb, used to make helicopter landing clearings in forests. The intended fusing was an extremely low air burst of only a few feet, so as to maximize the clearance effect and minimize cratering. The fuze was a mechanical impact fuze on a 38 inch nose spike.

Other fuzes
The contact fuze is set off when a series of connected crush switches that are placed on the exterior nose of the ordnance device make contact with the ground. The contact with solid ground activates the interior firing circuits which leads to the detonation
Detonation involves a supersonic exothermic front accelerating through a medium that eventually drives a shock front propagating directly in front of it. Detonations are observed in both conventional solid and liquid explosives, as well as in reactive gases...

of the ordnance 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.
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