A proximity fuze is a fuze that detonates an explosive device automatically when the distance to the target becomes smaller than a predetermined value. Proximity fuzes are designed for targets such as planes, missiles, ships at sea and ground forces. They provide a more sophisticated trigger mechanism than the common contact fuze or timed fuze.
British military researchers Sir Samuel Curran and W. A. S. Butement developed a proximity fuze in the early stages of World War II under the name VT, an acronym of "Variable Time fuze". The system was a small, short range, Doppler radar that used a clever circuit. However, the UK lacked the capacity to develop the fuze, so the design was shown to the US during the Tizard Mission in late 1940. The circuitry needed to be miniaturized, the fuze had to survive the high acceleration of cannon launch, and it needed to be reliable. Development was completed under the direction of physicist Merle A. Tuve at The Johns Hopkins University Applied Physics Lab (APL).
The proximity fuze is considered one of the most important technological innovations of the war. It was so important that it was a secret guarded to a similar level as the atom bomb project or D-Day invasion. It was also fiendishly tricky to manufacture. Adm. Lewis L. Strauss wrote that, "One of the most original and effective military developments in World War II was the proximity, or "VT," fuse. It was of incalculable value to both the Army and Navy, and it helped save London from obliteration. While no one invention won the war, the proximity fuse must be listed among the very small group of developments, such as radar, upon which victory very largely depended." The fuse was later found to be equally useful causing artillery shells to explode in air bursts, greatly increasing their anti-personnel effects.[
The Germans were supposedly also working on proximity fuses in the 1930s, based on capacitive effects rather than radar. Research and prototype work at Rheinmetall were halted in 1940 to devote available resources to projects deemed more necessary. In the post-WWII era, a number of new proximity fuse systems were developed, including radio, optical and other means. A common form used in modern air-to-air weapons uses a laser as an optical source and time-of-flight for ranging.