Fuze

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See also: fuse

A fuze is an electronic, pyrotechnic or mechanical system that initiates the detonation of a warhead, bomb, or other piece of ordnance. It may be as simple as a device that senses contact with the target, or as complex as a computing system that counts the number of airspaces penetrated by a bomb entering a building and triggering the detonator on a specific floor.

A fuse is a specifically pyrotechnic device, which, as part of a fuzing system, commonly conducts a flame to a heat-sensitive detonator. Fuses may also propagate a detonation rather than a combustion wavefront, triggering shock-sensitive explosives.

Unfortunately, "fuse" and "fuze" are often interchanged, but they are different concepts.[1] Ordnance manuals tend to use the distinction, but not always define it.

Basic fuzes

Basic fuzes trigger on some mechanical event, such as impact, the application of pressure (e.g., weight on a land mine, the release of pressure (e.g., the breaking of a trip wire) or the tilting of a rod. More complex fuzes are based on one or more timers, as, for example, one that initiates some number of seconds after a projectile is fired from a gun, and causes the warhead to burst in the air.

Impact fuze mechanisms further differentiate on the placement of the trigger, most often the nose, frequently the base so there is a slight delay, and also a protruding rod to cause an airburst. The fuse may act "instantaneously", or with a mechanically, and eventually electronically, set delay.

Proximity fuze

For more information, see: Proximity fuze.

Proximity fuzes were the first electronic fuzes, although the earliest were more electromechanical. Magnetic influence fuzes, most often used against ships but also against targets such as tanks, trigger due to the change in magnetic field induced by a large mass of ferrous metal. Early magnetic influence mines were electromechanical rather than electronic.

Introduced in the Second World War, electronic proximity fuzes triggered when they come within a certain distance of a reflector of radio waves, such as a moving aircraft, or a certain height over ground.

The first proximity fuzes did not have any computer-controlled elements, and indeed did not necessarily use recognizable radar technology — they simply transmitted an unfocused radio beam and triggered when they received a signal of predefined power. Such fuzes, therefore, could be predetonated by electronic warfare.

Modules of Marx 8x and BLU series

Intelligent fuze

Modern bombs and shells, as shown on the left, are modular, with various machined wells for fuzes and fuze components. The main fuze, including detonating explosives, often is in the tail group, with sensors in the noze. Safety and arming elements of a bomb are in the hardback. There may be a wind turbine to provide power.

In the drawing to the right, a U.S. FMU-139 fuze is shown, symbolically, within a common bomb case.

The FMU-139 proper can be set to detonate the bomb charge on impact, or with a slight delay that allows some surface penetration. If more complex penetration is needed, the FMU-157/B would replace it. If it is desired to have the bomb burst in the air, before it hits the ground, the DSU-33 proximity sensor is screwed into the nose. This sensor, as well as the main fuze, go to the safety switch, which may be a Mark 122 or FZU-38. Internal cabling connects these two or three components.[2]

If the bomb is intelligent, such as a Joint Direct Attack Munition, the safety switch becomes part of a MIL-STD-1760 physical interface between the aircraft and bomb. The -1760 interface also assigns a bus address for the bomb on a shared MIL-STD-1553 bus, over which targeting information can be passed to the bomb's navigation system.

Advanced fuzes are computer-controlled and use a variety of sensors. Those used in warheads have special challenges; artillery fuzes have to take tens of thousands of times the force of gravity when fired, and fuzes intended to penetrate multiple layers of concrete also must withstand great stresses. Bomb warheads do not have the same firing shock as artillery shells, but, if they have to penetrate rock or concrete, also need to be immensely strong.

Precision penetrators

One advanced system for penetrating buildings or buried structures is the FMU-157/B Hard Target Smart Fuze (HTSF).[3] It uses accelerometers to detect both the shock of hitting solid structure, but also detecting when it reemerges into a "void". A bomb such as a Joint Direct Attack Munition could hit a building at a specific aiming point above a command post inside, but the HTSF would then count floors penetrated to explode as it enters a specific general's office on the fifth floor. It senses hard layers penetrated, voids entered, and distance traveled.

Naval warfare

The availability of computers allow such selective fuze actions as detonating on hearing a specific ship propeller sound. Computer control can also delay detonation to wait for some number of events, so the fuze might ignore sacrificial low-value targets and actuate only for the protected target.

References