Proximity Fuze

A proximity fuze (also called a Variable Time (VT) fuze; shells using these fuzes can be seen referred to as "VT shells") was a fuze designed to detonate an artillery shell automatically when the distance to target became smaller than a preset value or when the target passed through a given plane.

Proximity fuzes operated on different sensing principles, including:

• radio frequency sensing

• optical sensing

• acoustic sensing

• magnetic sensing

Radio Frequency Sensing

Radio frequency sensing was the main sensing principle for VT shells.

During the Second World War, the principle worked as follows:

The shell contained a radio transmitter which used the shell body as an antenna and emitted a continuous wave of roughly 180 - 220 MHz. As the shell approached a reflecting object, an interference pattern was created. This pattern changed as the distance to the object diminished and every half wavelength in distance, the transmitter was in or out of resonance. This caused a small oscillation of the radiated power and consequently the oscillator supply current of about 200 - 800 Hz, the Doppler frequency. This signal was sent through a band pass filter, amplified, and triggered the detonation when it exceeded a given amplitude.

Previously, detonation of artillery shells required direct contact with the target, or else a timer set at launch, or (in the case of anti-aircraft shells) an altimeter. The immediate effect of the proximity fuze was that shells only had to pass close by a target in order to detonate, rather than make direct contact (hence the name).

The proximity fuze was invented in the UK in 1940, but developed mainly by the United States in collaboration with the British. The US Office of Scientific Research and Development (OSRD) credited the VT shell for three significant outcomes:

• VT shells enalbed a more effective defence against Japanese kamikaze attacks

• VT shells were also effective against German V1 and V2 Rockets (also known as "buzz bombs")

• VT shells played an important role in the US campaign in The Ardennes, where its effectiveness on troops in the open was demonstrated

From George G. Blackburn's Where The Hell Are The Guns?:

The buzz-bomb menace, which, for several weeks at the beginning, seriously reduced production of goods and services vital to the prosecution of the war (according to the diary notes of the Chief of the Imperial General Staff, General Alan Brooke) was brought under control by the timely arrival on the ack-ack (anti-aircraft) gunnery scene of the proximity fuse - a Canadian-American invention based on a British idea. Shells from the 3.7-inch ack-ack guns, which would have sailed past a few feet from a buzz bomb, were caused to burst and send lethal fragments into the buzz bombs' vitals when a tiny radio-transmitter bounced back from the pilotless monster.

The Americans developed the tiny radio transmitter and receiver. Canada supplied the design for a dependable power supply, which had to have long storage life but produce instant power to the little radio transmitter as the shell left the gun. Regular tiny batteries, with their limited shelf life, were useless, but scientists at the National Research Council in Ottawa, working with industrial scientists in Toronto, produced the answer: a battery that would remain dormant until activated by acid from a tiny vial broken within the fuse by the shocking wham of the propellant charge sending the shell on its way.¹

Optical sensing

Optical sensing was also developed in the Second World War, using 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 shell exploded. These fuzes were mainly used for anti-aircraft work. More modern fuzes (such as in air-to-air missiles) used lasers to trigger the detonation.

Acoustic sensing

Acoustic sensing used a microphone in a missile and was still in development by the Germans when the Second World War ended.

Magnetic sensing

Magnetic sensing was used in naval mines and torpedoes, triggered by magnetic detection of the masses of iron used in ship hulls.

Notes

1. Blackburn, George G. Where The Hell Are The Guns? (McClelland & Stewart, Inc., Toronto, ON, 1997) ISBN 0771015046 Appendix J