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A proximity fuze (also called a VT fuze, for "variable time") is a fuze that is designed to detonate an explosive automatically when the distance to target becomes smaller than a predetermined value or when the target passes through a given plane. There are different sensing principles:

• radio frequency sensing
• optical sensing
• acoustic sensing
• magnetic sensing
• pressure sensing

Contents 1 Radio frequency sensing 2 Optical sensing 3 Acoustic sensing 4 Magnetic sensing 5 Pressure sensing 6 References 7 See also 8 External links

Radio frequency sensing is the main sensing principle for shells and this is mostly in mind when one speaks of "proximity fuzes".

The WW2 patent works as follows: The shell contains a micro-transmitter which uses the shell body as an antenna and emits a continuous wave of roughly 180 - 220 MHz. As the shell approaches a reflecting object, an interference pattern is created. This pattern changes with shrinking distance: every half wavelength in distance (a half wavelength at this frequency is about 0.7 meters), the transmitter is in or out of resonance. This causes a small oscillation of the radiated power and consecutively the oscillator supply current of about 200 - 800 Hz, the Doppler frequency. This signal is sent through a band pass filter, amplified, and triggers the detonation when it exceeds a given amplitude.

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 shell or 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 invented in the UK in 1940, but developed mainly by the U.S. (with 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 seven times increase in the effect of artillery with this innovation.

Optical sensing was also developed first in WW2, mainly for anti-aircraft missiles. It used then a toroidal lens, that concentrated all light out of a plane perpendicular to the missile's main axis onto a photo cell. When the cell current changed a certain amount in a certain time interval, the detonation was triggered.

Some modern air-to-air missiles make use of lasers. 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.

Acoustic sensing used a microphone in a missile. The characteristic frequency of an aircraft engine was filtered and triggered the detonation. This principle was applied in German anti-aircraft missiles, which were mostly still in development when the war ended.

Naval mines can also use acoustic sensing, with modern versions able to be programmed to "listen" for the signature of a specific ship.

Magnetic sensing can only be applied to detect huge masses of iron such as ships. It is used in mines and torpedoes. Fuses of this type can be defeated by degaussing, using non-metal hulls for ships (especially minesweepers) or by magnetic induction loops fitted to aircraft or towed buoys.

Some naval mines are able to detect the pressure wave of a ship passing overhead.

• 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 0-354-01243-6. Dr Baldwin was a member of the Johns Hopkins University Applied Physics Laboratory (APL) team headed by Merle A Tuve that did most of the work.

• M734

• http://www.history.navy.mil/faqs/faq96-1.htm
• http://www.amc.army.mil/amc/ho/studies/fuze.html
• http://www.smecc.org/radio_proximity_fuzes.htm
• http://www.microworks.net/pacific/equipment/vt_fuze.htm
• http://www.smecc.org/pfuze.htm
• http://www.tubedata.org/unknown_sylvania/050123CL/US3166015.pdf
• http://www.tubedata.org/unknown_sylvania/050123CL/US3113235.pdf
• http://www.jhuapl.edu/aboutapl/heritage/default.asp