In military munitions, a fuze (sometimes spelled fuse) is the part of the device that initiates its function. In some applications, such as torpedoes, a fuze may be identified by function as the exploder. The relative complexity of even the earliest fuze designs can be seen in cutaway diagrams.
A fuze is a device that detonates a munition's explosive material under specified conditions. In addition, a fuze will have safety and arming mechanisms that protect users from premature or accidental detonation. 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.
A fuze may contain only the electronic or mechanical elements necessary to signal or actuate the detonator, but some fuzes contain a small amount of primary explosive to initiate the detonation. Fuzes for large explosive charges may include an explosive booster.
Name The word derives from Italian fuso, itself derived form Latin fūsus, spindle, applied to the spindle-shaped tube originally used to detonate a bomb. "Fuse" is attested from 1647.
Some professional publications about explosives and munitions distinguish the "fuse" and "fuze" spellings. Where a distinction is made usages are: Fuse: Cord or tube for the transmission of flame or explosion usually consisting of cord or rope with gunpowder or high explosive spun into it. (The spelling fuze may also be met for this term, but fuse is the preferred spelling in this context.)"A fuse is a wick or other combustible cord for an old-fashioned explosive. A fuze is for more high-tech explosives: it's a mechanical or electronic device used for detonations." Fuze: A device with explosive components designed to initiate a main charge. (The spelling fuse may also be met for this term, but fuze is the preferred spelling in this context.)
Historically, it was spelled with either 's' or 'z', and both spellings can still be found.
Munition types
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.
Artillery Artillery fuzes are tailored to function in the special circumstances of artillery projectiles. The relevant factors are the projectile's initial rapid acceleration, high velocity and usually rapid rotation, which affect both safety and arming requirements and options, and the target may be moving or stationary. Artillery fuzes may be initiated by a timer mechanism, impact or detection of proximity to the target, or a combination of these. Grenades Requirements for a grenade fuze are defined by the projectile's small size and slow delivery over a short distance. This necessitates manual arming before throwing as the grenade has insufficient initial acceleration for arming to be driven by "setback" and no rotation to drive arming by centrifugal force. Aerial bombs Aerial bombs can be detonated either by a fuze, which contains a small explosive charge to initiate the main charge, or by a "pistol", a firing pin in a case which strikes the detonator when triggered. Article has a great many illustrations and descriptions of bomb fuzes and pistols. The pistol may be considered a part of the mechanical fuze assembly. Landmines The main design consideration is that the bomb that the fuze is intended to actuate is stationary, and the target itself is moving in making contact. Naval mines Relevant design factors in naval mine fuzes are that the mine may be static or moving downward through the water, and the target is typically moving on or below the water surface, usually above the mine.
Activation mechanisms
Time
Time fuzes detonate after a set period of time by using one or more combinations of mechanical, electronic, pyrotechnic or even chemical timers. Depending on the technology used, the device may self-destruct (or render itself safe without detonation) 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 the gunpowder 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 of rifled 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 simple black 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 the Mills 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
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 nearly instantly on the slightest physical contact with the target. A fuze with a graze 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").
Inertia
Inertial fuzes are triggered when the entity carrying them (for example, a torpedo, air-dropped bomb, sea mine, or booby 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 be passively 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 be passively 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 in anti-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 the Brennan torpedo. Inertial fuzing is also used in the design of HESH munitions, since their concept precludes the use of contact fuzing.
Proximity
Proximity fuzes cause a missile warhead or other munition (e.g. air-dropped bomb, sea mine, or torpedo) 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, active sonar, passive acoustic, infrared, magnetic, photoelectric, seismic or even television cameras. These may take the form of an anti-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.
Remote detonation
Detonation can be triggered from distant sources of interaction. This can be accomplished both by mechanical and electronic means. Electronic or electrical remote detonators use wires or electromagnetic 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 a blasting machine, a device used since the 19th century. Vast and sophisticated systems like Dead Hand could also technically fall within this category.
Mechanical remote detonators use some type of mechanical linkage, usually a lanyard or tripwire, but sophisticated rigid linkages are also a possibility. Lanyards have been commonly employed by artillery pull primers. Lanyards continue to be used to trigger primer detonations in modern emplaced artillery, for safety reasons. Such devices are no longer of the friction primer type, using modern impact or electronic primer ignition systems.
Barometric
Barometric fuzes cause a bomb to detonate at a certain pre-set altitude above sea level by means of a radar, barometric altimeter or an infrared rangefinder.
Combinations
A fuze assembly may include more than one fuze in series or parallel arrangements. The RPG-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 a clockwork, 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 modern naval mines may require simultaneous detection of a series arrangement of acoustic, magnetic, and/or pressure sensors to complicate mine-sweeping efforts.Frieden, David R. Principles of Naval Weapons Systems Naval Institute Press (1985) pp.405-427
Safety and arming mechanisms
The multiple safety/arming features in the M734 fuze used for mortars are representative of the sophistication of modern electronic fuzes.