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Revision as of 18:23, 15 November 2006
- See fuse for the other meanings of the word.
In an explosive or pyrotechnic device, a fuse (or fuze) is the part of the device that initiates function. In common usage, the word fuse is used indiscriminately, but when being specific (and in particular in a military context), fuse is used to mean a simple pyrotechnic detonating device,as well as a more complicated device, such as a proximity fuse.
Pyrotechnic fuse
The simplest fuse is a length of combustible material which burns from the free end, through a small opening in the casing, and into the device, where it then ignites the bulk material. The fuse thus works to pass fire from the user to the device, which separates the user from the action with both distance and a time delay. Fuses are found in fireworks, model cannons, antique smoothbore firearms, some improvised explosive devices and many forms of pyrotechnics.
A key feature of a pyrotechnic fuse is that it burns steadily using self-contained fuel and oxidizer. This allows it to reliably burn along its entire length at a controlled rate. Fuses with unpredictable burning rates, or that self-extinguish, create hazards in pyrotechnics. The self-containment also passes fire through tight passages and into confined spaces where no air is present.
Fuse is made in various forms:
- Consumer fireworks typically use a small length of Visco fuse, a black powder cored fuse wrapped twice with string and waterproofed with nitrocellulose lacquer, or paper tissue coated with black powder and rolled to keep the black powder in the center.
- Larger fireworks, some commercial applications, and model cannons usually use a fuse which has a core of grains of black powder, wrapped in a sheath of cloth (usually cotton) fibers and then coated with a lacquer, shellac, or more modern protective coating.
- Fuse for blasting (also known as safety fuse), is a larger diameter (typically 1/4 inch or 6.25mm) higher energy, more solidly protected cloth fiber wrapped black powder core fuse. Safety fuse fits into the opening at the end of a pyrotechnic or fuse-ignited blasting cap and is crimped into place. Pyrotechnic blasting caps are still the most common form of detonator used for commercial and military explosives worldwide.
- Some safety fuse has an outer layer of plastic around the cloth and black powder. This fuse can burn reliably underwater once lit.
- Slow match is a very slow-burning fuse consisting of a hemp or cotton rope saturated with an oxidizer such as potassium nitrate. Slow match is 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 match was 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 saturated with nitrate) used for lighting consumer fireworks are a type of slow match.
- 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. It is used today in fireworks construction.
- Quick match or piped match is a type of black powder fuse that burns very quickly, some hundreds of feet per second. It consists of black match 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 match is 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.
- An electric match 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.
- Flying fish fuse 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 spoolette is a delay fuse consisting of a hollow wooden dowel rammed full of black powder. A spoolette is glued into the wall of a fireworks shell and ignited by the lift charge that launches the shell into the air. The spoolette, 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 fuses
A fuse refers to a device used in munitions which is designed to detonate, or to set forces into action to detonate, the charge (or primer) under desired conditions.
Types of fuses include:
- time fuses detonate after a set time by using mechanical, electronic, or chemical timers.
- contact detonators or point detonating fuses explode on impact.
- proximity fuses cause a missile or other munition to explode when it comes within a certain distance of the target. Some proximity fuses utilize radar, sonar, Infrared or television cameras.
- remote detonators use wires or radio waves to remotely command the explosive to function.
- altitude fuses cause a bomb to explode at a certain altitude above sea level by means of an infrared rangefinder, radar, or barometric altimeter
Many weapons have fuse systems to ensure that they do not initiate (go off) before intended. In most cases the munition has to travel some distance before it can detonate. In other cases the bomb, mine or projectile has a fuse that prevents accidental initiation (for example, stopping turning of a small propellor so that the striker-pin cannot touch the detonator even if the weapon is dropped on the ground).
These sort of fuses operate with aircraft weapons where the weapon may have to be dropped 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 fuses still attached, or drop them live by retaining the fuses as the weapons leave the aircraft.
Aerial bombs and depth charges can be nose and tail fused using different detonator/initiator characteristics so that the crew can choose which effect (fuse) 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.
There also exist other fuzing systems.