Contact fuze 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 . Artillery fuzes . The British Army ' s first useful impact fuze for high - explosive shell s 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 entanglement s 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 . 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 fuze s 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 50 kg to 500 kg , modified for use from low altitude . To avoid the risk of ricochet 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 . Notable examples of air - dropped bombs that did use contact fuzes include the Fat Man atomic bomb dropped on Nagasaki . The bomb was intended for air burst 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 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 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 . 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 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 .