In militarymunitions, afuze (sometimes spelledfuse) is the part of the device that initiates its function. In some applications, such astorpedoes, a fuze may be identified by function as theexploder.[1] The relative complexity of even the earliest fuze designs can be seen incutaway diagrams.
A fuze is a device thatdetonates amunition'sexplosive material under specified conditions. In addition, a fuze will have safety and arming mechanisms that protect users from premature or accidental detonation.[2][3] For example, an artillery fuze's battery is activated by the high acceleration of cannon launch, and the fuze must be spinning rapidly before it will function. "Complete bore safety" can be achieved with mechanical shutters that isolate the detonator from the main charge until the shell is fired.[4]
A fuze may contain only the electronic or mechanical elements necessary to signal or actuate thedetonator, but some fuzes contain a small amount ofprimary explosive to initiate the detonation. Fuzes for large explosive charges may include anexplosive booster.
The word derives from Italianfuso, itself derived form Latinfūsus, spindle, applied to the spindle-shaped tube originally used to detonate a bomb. "Fuse" is attested from 1647.[5]
Some professional publications about explosives and munitions distinguish the "fuse" and "fuze" spellings.[6][7] Where a distinction is made usages are:
Historically, it was spelled with either 's' or 'z', and both spellings can still be found.[11]
The situation of usage and the characteristics of the munition it is intended to activate affect the fuze design e.g. its safety and actuation mechanisms.
Time fuzes detonate after a set period of time by using one or more combinations of mechanical, electronic,pyrotechnic or evenchemical timers. Depending on the technology used, the device mayself-destruct[13] (or render itself safe without detonation[14]) some seconds, minutes, hours, days, or even months after being deployed.
Early artillery time fuzes were nothing more than a hole filled with gunpowder leading from the surface to the centre of the projectile. The flame from the burning of the gunpowder propellant ignited this "fuze" on firing, and burned through to the centre during flight, then igniting or exploding whatever the projectile may have been filled with.
By the 19th century devices more recognisable as modern artillery "fuzes" were being made of carefully selected wood and trimmed to burn for a predictable time after firing. These were still typically fired from smoothbore muzzle-loaders with a relatively large gap between the shell and barrel, and still relied on flame from thegunpowder propellant charge escaping past the shell on firing to ignite the wood fuze and hence initiate the timer.
In the mid-to-late 19th century adjustable metal time fuzes, the fore-runners of today's time fuzes, containing burning gunpowder as the delay mechanism became common, in conjunction with the introduction ofrifled artillery. Rifled guns introduced a tight fit between shell and barrel and hence could no longer rely on the flame from the propellant to initiate the timer. The new metal fuzes typically use the shock of firing ("setback") and/or the projectiles's rotation to "arm" the fuze and initiate the timer : hence introducing a safety factor previously absent.
As late as World War I, some countries were still using hand-grenades with simpleblack match fuses much like those of modern fireworks: the infantryman lit the fuse before throwing the grenade and hoped the fuse burned for the several seconds intended. These were soon superseded in 1915 by theMills bomb, the first modern hand grenade with a relatively safe and reliable time fuze initiated by pulling out a safety pin and releasing an arming handle on throwing.
Modern time fuzes often use an electronic delay system.
Impact, percussion or contact fuzes detonate when their forward motion rapidly decreases, typically on physically striking an object such as the target. The detonation may be instantaneous or deliberately delayed to occur a preset fraction of a second after penetration of the target. An instantaneous "Superquick" fuze will detonate instantly on the slightest physical contact with the target. A fuze with agraze action will also detonate on change of direction caused by a slight glancing blow on a physical obstruction such as the ground.
Impact fuzes in artillery usage may be mounted in the shell nose ("point detonating") or shell base ("base detonating").
Inertial fuzes are triggered when the entity carrying them (for example, atorpedo, air-dropped bomb,sea mine, orbooby trap) experiences a sudden (or gradual, depending on the design) acceleration, deceleration, or impact. In this way they are both similar to and different from impact fuzes. Whereas impact fuzes usually require physical contact with, or an impact against a hard surface, inertial fuzes can trigger from any change of momentum. This allows them to be mounted deep inside the entity carrying them, rather than on its exterior. Designs can be varied. Some can bepassively safe, ignoring all changes of momentum below a certain threshold, thereby functioning similarly to impact fuzes without the limitation of being externally mounted. Other designs can bepassively dangerous, using other energy sources such as gravity or an electrical battery to greatly amplify slight changes in inertia over time. One easy way of visualizing a passively safe inertial fuze is to picture a marble in a bowl - the device is triggered when the marble rolls to the rim of the bowl. In contrast, a passively dangerous inertial fuze would be similar to a marble on a smooth, flat plate. The former uses gravity to actively suppress weak forces acting on the marble, whereas the latter uses gravity to actively amplify them. Passively dangerous inertial fuzes are commonly employed inanti-handling devices.
For comparatively low-velocity munitions such as torpedoes, inertial fuzes of the pendulum and swing-arm types have been used historically. An early example of a pendulum fuze can be seen in the design of theBrennan torpedo. Inertial fuzing is also used in the design ofHESH munitions, since their concept precludes the use of contact fuzing.
Proximity fuzes cause amissile warhead or other munition (e.g. air-dropped bomb,sea mine, ortorpedo) to detonate when it comes within a certain pre-set distance of the target, or vice versa. Proximity fuzes utilize sensors incorporating one or more combinations of the following:radar, activesonar, passive acoustic,infrared,magnetic,photoelectric,seismic or eventelevision cameras. These may take the form of ananti-handling device designed specifically to kill or severely injure anyone who tampers with the munition in some way e.g. lifting or tilting it. Regardless of the sensor used, the pre-set triggering distance is calculated such that the explosion will occur sufficiently close to the target that it is either destroyed or severely damaged.
Detonation can be triggered from distant sources of interaction. This can be accomplished both by mechanical and electronic means. Electronic or electricalremote detonators usewires orelectromagnetic radiation (such as radio waves or infrared light) to remotely command the device to detonate. This can be achieved by either a deliberate command from a transmitter apparatus, or by autonomous transmission of a signal, or sensing of an interruption of a continuous signal. A simple example of an electrical wired remote detonator is known as ablasting machine, a device used since the 19th century. Vast and sophisticated systems likeDead Hand could also technically fall within this category.
Mechanical remote detonators use some type ofmechanical linkage, usually a lanyard ortripwire, but sophisticated rigid linkages are also a possibility. Lanyards have been commonly employed byartillery pull primers.[15] Lanyards continue to be used to trigger primer detonations in modern emplaced artillery, for safety reasons.[16] Such devices are no longer of the friction primer type, using modern impact or electronicprimer ignition systems.
Barometric fuzes cause a bomb to detonate at a certain pre-set altitude abovesea level by means of aradar,barometricaltimeter or aninfraredrangefinder.
A fuze assembly may include more than one fuze in series or parallel arrangements. TheRPG-7 usually has an impact (PIBD) fuze in parallel with a 4.5 second time fuze, so detonation should occur on impact, but otherwise takes place after 4.5 seconds. Military weapons containing explosives have fuzing systems including a series time fuze to ensure that they do not initiate (explode) prematurely within a danger distance of the munition launch platform. In general, the munition has to travel a certain distance, wait for a period of time (via aclockwork, electronic or chemical delay mechanism), or have some form of arming pin or plug removed. Only when these processes have occurred will the arming process of the series time fuze be complete. Mines often have a parallel time fuze to detonate and destroy the mine after a pre-determined period to minimize casualties after the anticipated duration of hostilities. Detonation of modernnaval mines may require simultaneous detection of a series arrangement ofacoustic,magnetic, and/orpressure sensors to complicate mine-sweeping efforts.[17]
The multiple safety/arming features in theM734 fuze used formortars are representative of the sophistication of modernelectronic fuzes.
Safety/arming mechanisms can be as simple as the spring-loaded safety levers onM67 orRGD-5grenade 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 theinertia 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 certainrpm beforecentrifugal forces cause a safety feature to disengage or move an arming mechanism to its armed position. Artillery shells are fired through arifled 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 smallpropeller (unless a lanyard pulls out a pin) so that the striker-pin cannot hit thedetonator 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 overfriendly territory to allow a damaged aircraft to continue to fly. The crew can choose to jettison the weaponssafe by dropping the devices with safety pins still attached, or drop themlive by removing the safety pins as the weapons leave the aircraft.
Aerial bombs anddepth charges can benose andtail 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 ofsafe,nose, ortail 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 tonuclear weapons, and their safety/arming devices are correspondingly complex. In addition toPAL 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. As an additional safety precaution, most modern nuclear weapons utilize a timed two point detonation system such that ONLY a precisely firing of both detonators in sequence will result in the correct conditions to cause a fission reaction[citation needed]
Note: some fuzes, e.g. those used in air-dropped bombs and landmines may containanti-handling devices specifically designed to killbomb disposal personnel. The technology to incorporatebooby-trap mechanisms in fuzes has existed since at least 1940 e.g. the German ZUS40 anti-removal bomb fuze.[18]
A fuze must be designed to function appropriately considering relative movement of the munition with respect to its target. The target may move past stationary munitions likeland mines or naval mines; or the target may be approached by a rocket, torpedo, artillery shell, or air-dropped bomb. Timing of fuze function may be described asoptimum if detonation occurs when target damage will be maximized,early if detonation occurs prior to optimum,late if detonation occurs past optimum, ordud if the munition fails to detonate. Any given batch of a specific design may be tested to determine the anticipated percentage ofearly,optimum.late, anddud expected from that fuze installation.[17]
Combination fuze design attempts to maximizeoptimum detonation while recognizing dangers ofearly fuze function (and potential dangers oflate function for subsequent occupation of the target zone by friendly forces or for gravity return of anti-aircraft munitions used in defense of surface positions.) Series fuze combinations minimizeearly function by detonating at the latest activation of the individual components. Series combinations are useful for safety arming devices, but increase the percentage oflate anddud munitions. Parallel fuze combinations minimizeduds by detonating at the earliest activation of individual components, but increase the possibility of prematureearly function of the munition. Sophisticated military munition fuzes typically contain an arming device in series with a parallel arrangement of sensing fuzes for target destruction and a time fuze for self-destruction if no target is detected.[17]
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citing The Oxford Companion to World War II Edited by: I. C. B. Dear and M. R. D. Foot. Oxford University Press 2001 ISBN 9780198604464
