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A burning fuse

A fuze for an artillery shell, circa 1936

In an explosive, pyrotechnic device or military munition, a fuse (or fuze) is the part of the device that initiates function. In common usage, the word fuse is used indiscriminately. However, when being specific (and in particular in a military context), the term fuse describes a simple pyrotechnic initiating device, like the cord on a firecracker, whereas the term fuze^[1]^[2]^[3] is used to indicate a more sophisticated ignition device incorporating mechanical and/or electronic components e.g. a proximity fuze for an M107 artillery shell, magnetic/acoustic fuze on a sea mine, spring-loaded grenade fuze^[4]^[5]^[6], pencil detonator or anti-handling device^[7].

1 History
2 Burning fuses
3 Munition fuzes
4 Gallery of fuzes
5 See also
6 References
7 External links

History

The simplest form of fuse is the burning fuse, believed to date back to the 10th century and originating in China, this simple fuse consisted of lightweight paper filled with loose gunpowder, and served as a means of delaying ignition in fireworks. This simple form of burning fuse can still be found today in many modern pyrotechnics. A version of this simple fuse is called visco fuse, and consists of the burning core coated with wax or lacquer for durability and water resistance. The commercial and military version of a burning fuse referred to as safety fuse (invented by William Bickford) is a textile tube filled with combustible material and wrapped to prevent external exposure of the burning core. Safety fuses are used to initiate the detonation of explosives through the use of a blasting cap.

Fuses

Modern day safety fuses are often used in mining and military operations, to provide a time-delay before ignition, and they more often than not are used to initiate an explosive detonator, thereby starting an explosive chain reaction to detonate a larger more stable main charge. Safety fuses are typically colored black (military) or fluorescent orange (commercial) to distinguish them from detonating cords such as Primacord, which are brightly colored or transparent.

Usage

Fuses are found in fireworks, model cannons, matchlock firearms, some improvised explosive devices and many forms of pyrotechnics.

Types

Burning fuses

A slow match is a very slow-burning fuse consisting of a hemp or cotton rope saturated with an oxidizer such as potassium nitrate. Slow matches are used as a source of fire for manually lighting other devices, such as matchlock guns, or fuses on black powder cannons. Before percussion caps, slow matches were most suitable for use around black powder weapons because it could be roughly handled without going out, and only presented a small glowing tip instead of a large flame that risked igniting powder supplies nearby.
Today's punks (wood splints covered with ground plant pith or dung and then saturated with nitrate) used for lighting consumer fireworks are a type of slow match.
A Black match is a type of fuse consisting of cotton string coated with a dried slurry of black powder and glue. This acts as a simple pass-fire, and was used to fire ancient cannons. They are used today in fireworks construction.
A quick match or piped match is a type of black powder fuse that burns very quickly, some hundreds of feet per second. They consist of black matche covered with a loose paper wrap (pipe). When lit, the flame propagates quickly down the paper pipe from the hot gases produced by the burning powder. Quick matches are used in professional fireworks displays to pass fire nearly instantly between devices that must be physically separated while firing simultaneously, such as a finale rack. Devices which should fire in sequence can be branched from a single master fuse, consisting of quick match spliced onto Visco fuses of various length for time delays.
A visco fuse has a core of black powder with one or more textile overwraps. The outer layers may be coated with wax or nitrocellulose lacquer for water resistance. These fuses are widely used in modern pyrotechnics because they burn at a uniform rate, with an easily visible external flame. Depending on their outer treatment, visco fuses are water resistant and the better quality can burn reliably underwater once lit, since the black powder core provides both its own fuel and oxidant.
A safety fuse consists of a black powder core in a textile tube, covered with asphaltum or other waterproofing agent, and having an outer wrapper of tough textile or plastic. They are made in a standard diameter designed to be crimped into blasting caps.^[8] Once ignited, safety fuses will burn underwater, and have no external flame that might ignite methane or other fuels such as might be found in mines or other industrial environments. Safety fuses are manufactured with specified burn times per 30 cm, e.g. 60 seconds, which means that a length of fuse 30 cm long will take 60 seconds to burn. Manufacturers warn that although every effort is made to insure uniform burn times, safety fuses are subject to variation depending on conditions and should be used with adequate safety margins.
An Igniter Safety Fuse Electric (ISFE) lights a main fuse or device when activated by an electrical current. They typically consist of a pair of wires leading to a thin resistance wire that heats when current is applied. The resistance wire is covered by a bit of pyrotechnic composition that ignites from the wire heating, providing enough fire to reliably ignite the main fuse via a mechanical connection, or the device directly. Estes model rocket motors are lit by a type of electric match. Large fireworks displays are launched with complex timing sequences using a computer that energizes electric matches connected to the individual device fuses.
A flying fish fuse (bumblebees) is an unusual type of component for fireworks. It is made like Visco fuse, but contains a metallic spark composition or other effect instead of black powder. Flying fish can thus perform as a main effect instead of just an initiator. For example, simply lighting a short piece of flying fish on the ground makes it fly through the air, seeming to swim in random directions, while emitting sparks and noise. A aerial shell loaded with many such pieces results in a beautiful myriad of pieces flying and sparking high in the air.
A spolette is a delay fuse consisting of a hollow wooden dowel or a paper tube rammed full of black powder. A spolette is glued into the wall of a fireworks shell and ignited by the lift charge that launches the shell into the air. The spolette, after a delay that allows the shell to reach its top of trajectory, ignites the shell's main effect(s). The tough wood construction ensures that the fuse burns reliably despite the explosive force and acceleration of the launch.
The saucisson was an early form of fuse.

Munition fuzes

Mk 53 Proximity fuze for an artillery shell, circa 1945

British No. 25 time fuze Mk IV, circa 1914, used for star shells

SD2 Butterfly bomb circa 1940 - wings rotate as bomb falls, unscrewing the arming spindle connected to the fuze

As can be seen from the various accompanying diagrams, most countries use the 'z' spelling^[9] ^[10] ^[11] ^[12] ^[13] to distinguish between simple burning fuses and more complicated munition fuzes, which contain mechanical and/or electronic components. The relative complexity of even the most early fuze designs (dating from circa 1915) can be seen in the cut-away diagrams at the end of this article. Modern fuzes for "smart weapons" are invariably computerised i.e. incorporate microprocessors such as gate arrays.

A fuze is a device used in munitions which is designed to detonate, or to set forces into action to ignite, detonate or deflagrate, the charge (or primer) under specified conditions. In contrast to a simple pyrotechnic fuse, a munitions fuze always has some form of safety/arming mechanism, designed to protect the user from premature or accidental detonation^[14].

A munition fuze assembly 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. Fuze assemblies for large explosive charges may include an explosive booster.

Types

Types of fuzes include:

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^[15] (or render itself safe without detonation^[16]) some seconds, minutes, hours, days, or even months after being deployed.
contact detonators or point detonating fuzes detonate immediately on impact with the target.
proximity fuzes cause a missile warhead or other munition (e.g. air-dropped bomb or sea mine) 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 detonators use wires or radio waves to remotely command the device to detonate.
barometric fuzes cause a bomb to detonate at a certain pre-set altitude above sea level by means of an radar, barometric altimeter or and infrared rangefinder.

Combination fuzes

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 occurs on impact, but not later than 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 clockwork, electronic or even a chemical delay), or have some form of arming pin/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.^[17]

Fuze safety/arming mechanisms

The multiple safety/arming features in the M734 mortar fuze are representative of the sophistication of modern electronic fuzes.

Safety/arming mechanisms can be as simple as the spring-loaded safety levers on M67 or RGD-5 grenade 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 the inertia 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 certain rpm before centrifugal forces cause a safety feature to disengage or move an arming mechanism to its armed position. Artillery shells are fired through a rifled 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 small propellor (unless a lanyard pulls out a pin) so that the striker-pin cannot hit the detonator 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 over friendly territory to allow a damaged aircraft to continue to fly. The crew can choose to jettison the weapons safe by dropping the devices with safety pins still attached, or drop them live by removing the safety pins as the weapons leave the aircraft.

Aerial bombs and depth charges can be nose and tail 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 of safe, nose, or tail 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 to nuclear weapons, and their safety/arming devices are correspondingly complex. In addition to PAL 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.

Note: some fuzes, e.g. those used in air-dropped bombs and landmines may contain anti-handling devices specifically designed to kill bomb disposal personnel. The technology to incorporate booby-trap mechanisms in fuzes has existed since at least 1940 e.g. the German ZUS40 anti-removal bomb fuze^[18].

Reliability

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 like land 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 as optimum if detonation occurs when target damage will be maximized, early if detonation occurs prior to optimum, late if detonation occurs past optimum, or dud if the munition fails to detonate. Any given batch of a specific design may be tested to determine the anticipated percentage of early, optimum. late, and dud expected from that fuze installation.^[17]

Combination fuze design attempts to maximize optimum detonation while recognizing dangers of early fuze function (and potential dangers of late 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 minimize early function by detonating at the latest activation of the individual components. Series combinations are useful for safety arming devices, but increase the percentage of late and dud munitions. Parallel fuze combinations minimize duds by detonating at the earliest activation of individual components, but increase the possibility of premature early 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]

Gallery of fuzes

Fuzes fitted to M107 155mm artillery shells, circa 2000	Fuzed 81mm white phosphorus mortar shell in 1980. Note spelling of "fuze" on adjacent boxes	An assortment of fuzes for artillery and mortar shells	British AP Shell Mk XXII BNT (circa 1943) for BL 15 inch Mk I naval gun, showing base fuze
British No. 63 Mk I Time and Percussion fuze, circa 1915 - used in shrapnel shells	British No. 100 Graze Fuze for high-explosive shell, World War I.	Hotchkiss base percussion fuze for artillery shells, circa World War I	British Time Fuze No. 31, World War I, used in trench mortars
British Percussion Fuze No. 110 Mk III, World War I, used in trench mortars	British No. 131 D.A. (Direct Action) Impact Fuze, Mk VI, World War I, used in anti-aircraft artillery	Percussion fuze for an artillery shell	British No. 16 D Mk IV N Base percussion fuze, circa 1936
British No. 45 P Direct Action Impact Fuze, World War I, used in howitzer shells	Cut-away diagram of Japanese Type 99 Grenade showing fuze mechanism. Circa 1939	Cut-away diagram of a US M2A4 bounding mine showing the M6A1 pressure/pull fuze. Circa 1950	USSR pull-fuze designed for booby-trap or anti-handling purposes. Circa 1950s. Detonator assembly is inserted into explosives
Alternative design of USSR booby-trap pull-fuze, usually connected to a tripwire. Circa 1950s	USSR pressure fuze for booby-trap purposes e.g. victim steps on loose floorboard with fuze (connected to TNT explosives) concealed underneath. Circa 1950s	Italian TC/2.4 mine, showing central location of mechanical pressure fuze	German S-mine dating from World War II showing fuze well into which a 3-pronged fuze would be screwed
Fuze for a German S-mine, which would be screwed into the fuze well on the mine