Proximity fuze

The proximity fuze is used to make naval gunnery and land artillery much more effective, It comprises a radio installed in the nose of a 5" or larger shell that detects an enemy plane, or the ground, and explodes at exactly the right time. 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.

Naval anti-aircraft gunnery was a trade off between 5" guns, with their long range, and 20 and 40 calibre short-range guns with a high rate of fire. The breakthrough came in 1943 with the introduction of the proximity fuze (or "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 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. Utlization 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.

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. 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.

Proximity fuzes or a more modern, transistorized design are still used for U.S. Air Force bombs and Navy rockets, as well as Army mortars.

Bibliography

• Baldwin, Ralph B. The Deadly Fuze: Secret Weapon of World War II. (1980)
• Bennett, Geoffrey. "The Development of the Proximity Fuze." Journal of the Royal United Services Institute for Defence Studies 1976 121(1): 57-62. Issn: 0953-3559;
• Collier, Cameron D. "Tiny Miracle: the Proximity Fuze." Naval History 1999 13(4): 43-45. Issn: 1042-1920 Fulltext: Ebsco
• Moye, William T. "Developing the Proximity Fuze, and Its Legacy," U.S. Army Materiel Command, Historical Office (2003) online edition