The proximity fuze is a type of artillery or mine warfare fuze. In the best-known Second World War application, it comprises a radio installed in the nose of an artillery or missile warhead that detects an enemy plane, or the ground, and explodes at exactly the right time.
Before radio proximity fuzes, there were initially electromechanical magnetic influence mines used against ships.
Second World War
It replaced the manual timer that usually fired too soon or too late. In ground action, a shell that exploded too high above the ground does less damage, and one that explodes after it hits the ground has its impact absorbed by the dirt.
The British had invented the fuze in 1940 but lacked the massive industrial capacity needed to produce it in quantity, so shared the blueprints with civilian researchers at Johns Hopkins University under contract to the U.S. Navy. Utilization of the Doppler effect of reflected radio waves was the most promising concept, so researchers devised a diode detector arrangement that acted when the amplitude of the reflected signals exceeded a predetermined value. The basic components were a vacuum tube (six inches long and three inches in diameter) a battery, and a radio transmitter and receiver, all of which have to be rugged enough to withstand 20,000 Gs when shot out of a gun at high velocity. After the shell is fired and begins rotating, a chemical reaction produces an electrical charge which in turn arms the shell and sends out a radio impulse. The return signal, reflected from the target, detonates the shell prior to impact and produces the devastating effects.
Naval anti-aircraft gunnery was a trade off between the typical 5"-38 caliber gun , with their long range, and 20mm Oerlikon (and other) and 40mm Bofors short-range guns with a high rate of fire. The breakthrough came in 1943 with the introduction of the proximity fuze (or "variable time (VT) fuze") in 5" shells. It had a radio and receiver and when a signal bounced back, it was in range and exploded. The effect was to make the target 50 times bigger and thus much easier to hit. The fuzes played a decisive role in defeating the Japanese Kamikaze attacks of 1944-45.
The fuzes were used in land-based artillery in the South Pacific in 1944. They were incorporated into bombs dropped by the U.S. Air Force on Japan in 1945, and they were used to defend Britain against the V-1 attacks of 1944, achieving a kill ratio of about 79%. (They were ineffective against the much faster V-2 missiles.) There was no risk of a dud falling into enemy hands. The Pentagon decided it was too dangerous to have a fuze fall into German hands because they might reverse engineer it and create a weapon that would destroy the Allied bombers, or at least find a way to jam the radio signals. Therefore they refused to allow the Allied artillery use of the fuzes in 1944. The Germans started research in 1930 but never invented a working device. General Dwight D. Eisenhower protested vehemently and demanded he be allowed to use the fuzes. He prevailed and the VT fuzes were first used in the Battle of the Bulge in December 1944, when they made the Allied artillery far more devastating, as all the shells now exploded just before hitting the ground.
By 1944 a large proportion of the American electronics industry concentrated on making the fuzes. Procurement contracts increased from $60 million in 1942, to $200 million in 1943, to $300 million in 1944 and were topped by $450 million in 1945. As volume increased, efficiency came into play and the cost per fuze fell from $732 in 1942 to $18 in 1945. This permitted the purchase of over 22 million fuzes for approximately $1,010 million. The main suppliers were Crosley, RCA, Eastman Kodak, McQuay-Norris and Sylvania.
Current airbursting proximity fuzes use solid-state electronic components, radar rather than simple signal strength, and computer control. They are hardened against electronic countermeasures that could cause predetonation or failure to detonate.
Electronic miniaturization allows their use in much smaller calibers, certainly in 40mm. Mortars in the 80-120mm range began to use both proximity fuzes as well as computer-controlled aerodynamic guidance fins. They are common for howitzer warheads, although other fuzing arrangements are used when surface or subsurface detonation is desired.
- Sharpe, "The Radio Proximity Fuze" (2003)