Proximity fuze
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Template:Wiktionary Image:MK53 fuze.jpg A proximity fuze (also called a VT fuze, for "variable time") is a fuze that is designed to detonate an explosive automatically when close enough to the target to destroy it. Developed first for use against airborne missiles, it was later used against ground targets. By sending out radio waves that are reflected by the target and comparing the frequency of the outgoing waves to the incoming waves, a radio proximity fuze uses the Doppler effect to determine its proximity to a target missile. When the incoming waves sharply decrease in frequency, the target is at the closest proximity that can be determined using this method. (The sharp decrease in frequency means that the projectile has just passed the target. Think of the sound a train whistle makes as the train approaches you. The long blast of the whistle changes as it just passes your location.) For use against land targets, the proximity fuze was tuned to detonate the explosive at the optimum height above the ground.
Before the fuze's invention, detonation had to be induced by direct contact, a timer set at launch, or an altimeter. All of these have disadvantages. Getting direct contact with a relatively small moving target is hard (even ignoring the effect of wind); to set a time- or height-triggered fuze one must measure the height of the target (or even predict the height of the target at the time one will be able to get a missile in its neighbourhood). With a proximity fuze, all one has to worry about is getting a shell or missile on a trajectory that, at some time, will pass close by the target. This is still not a trivial task, but it is much easier than previous methods were.
Use of timing to produce air bursts against ground targets requires observers to provide information for adjusting the timing. This is not practical in all situations and is slow in any event. Proximity fuzes remove these problems.
The proximity fuze was developed through a U.S. and British collaboration during World War II. Vannevar Bush, head of the U.S. Office of Scientific Research and Development (OSRD) during this war, credits it with three significant effects. It was important in defense from Japanese Kamikaze attacks in the Pacific. It was an important part of the radar-controlled anti-aircraft batteries that finally neutralized the German V-1 bomb attacks on England. Third, it was released for use in land warfare just in time for use in the Battle of the Bulge, where it decimated German divisions caught in the open. The Germans felt safe from timed fire because the weather prevented accurate observation. Bush cites an estimated 7 times increase in the effect of artillery with this innovation.
Proximity fuzes were also used on the first atomic bombs.
Some modern air-to-air missiles make use of lasers that function as a proximity fuze. They project narrow beams of laser light perpendicular to the flight of the missile. As the missile cruises towards the target the laser energy simply beams out into space. However, as the missile passes its target some of the laser energy strikes the target and is reflected back towards the missile where detectors sense the reflected laser energy and trigger the missile warhead.
References
- Pieces of the Action by Vannevar Bush, William Morrow and Co., inc. 1970
- An account of the development and initial introduction of proximity fuzes is given in The Deadly Fuze by Ralph B Baldwin (UK Edition published by Janes, 1980. ISBN 0-354-01234-6). Dr Baldwin was a member of the Johns Hopkins University team headed by Merle A Tuve that did most of the work.