.
A
proximity fuze (or fuse) 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. It is estimated that it
increases the lethality by 5 to 10 times, compared to these other fuzes.
Before the invention of the proximity fuze, detonation was induced by direct contact, a timer set at launch or an altimeter. All of these earlier methods have disadvantages. The probability of a direct hit on a small moving target is low; a shell that just misses the target will not explode. A time- or height-triggered fuze requires good prediction by the gunner and accurate timing by the fuze. If either is wrong, then even accurately aimed shells may explode harmlessly before reaching the target or after passing it. At the start of
The Blitz, it was estimated that it took 20,000 rounds to shoot down a single aircraft. Other estimates put the figure as high as 100,000 or as low as 2,500 rounds for each aircraft. With a
proximity fuze, the
shell or missile need only pass close by the target at some time during its flight. The proximity fuze makes the problem simpler than the previous methods.
Proximity fuzes are also useful for producing
air bursts against ground targets. A contact fuze would explode when it hit the ground; it would not be very effective at scattering shrapnel. A timer fuze can be set to explode a few meters above the ground but the timing is vital and usually requires
observers to provide information for adjusting the timing. Observers may not be practical in many situations, the ground may be uneven, and the practice is slow in any event. Proximity fuzes fitted to such weapons as
artillery and
mortar shells solve this problem by having a range of set burst heights [e.g. 2, 4 or 10 m (7, 13 or 33 ft)] above ground that are selected by gun crews. The
shell bursts at the appropriate height above ground.
The idea of a proximity fuse had long been considered militarily useful. Several ideas had been considered, including optical systems that shone a light, sometimes
infrared, and triggered when the reflection reached a certain threshold, various ground-triggered means using radio signals, and
capacitive or inductive methods similar to a
metal detector. All of these suffered from the large size of pre-WWII electronics and their fragility, as well as the complexity of the required circuitry.
British military researchers at the
Telecommunications Research Establishment (TRE)
Samuel C. Curran,
William A. S. Butement, Edward S. Shire, and Amherst F. H. Thomson conceived of the idea of a proximity fuze in the early stages of
WW2. Their system involved a small, short range,
Doppler radar. British tests were then carried out with "unrotated projectiles," in this case rockets. However, British scientists were uncertain whether a fuze could be developed for anti-aircraft shells, which had to withstand much higher accelerations than rockets. The British shared a wide range of possible ideas for designing a fuze, including a
photoelectric fuze and a
radio fuze, with United States during the
Tizard Mission in late 1940. To work in shells, a fuze needed to be miniaturized, survive the high acceleration of cannon launch, and be reliable.
The first reference to the concept of
radar in the UK was made by
W. A. S. Butement and P. E. Pollard, who constructed a small
breadboard model of a pulsed radar in
1931. They suggested the system would be useful for the
coast artillery units, who could accurately measure the range to shipping even at night. The
War Office proved uninterested in the concept and told the two to work on other issues.
In
1936, the
Air Ministry took over
Bawdsey Manor to further develop their prototype radar systems that would emerge the next year as
Chain Home. The Army was suddenly extremely interested in the topic of radar, and sent Butement and Pollard to Bawdsey to form what became known as the "Army Cell". Their first project was a revival of their original work on coast defense, but they were soon told to start a second project to develop a range-only radar to aid
anti-aircraft guns.
As these projects moved from development into prototype form in the late 1930s, Butement turned his attention to other concepts, and among these was the idea of a proximity fuse:
...Into this stepped W. A. S. Butement, designer of radar sets CD/CHL and GL, with a proposal on 30 October 1939 for two kinds of radio fuze: (1) a radar set would track the projectile, and the operator would transmit a signal to a radio receiver in the fuze when the range, the difficult quantity for the gunners to determine, was the same as that of the target and (2) a fuze would emit high-frequency radio waves that would interact with the target and produce, as a consequence of the high relative speed of target and projectile, a Doppler-frequency signal sensed in the oscillator.
In
May 1940 a formal proposal from Butement, Edward S. Shire, and Amherst F.H. Thompson was sent to the British Air Defence Establishment based on the second of the two concepts. A
breadboard circuit was constructed and the concept was tested in the laboratory by moving a sheet of tin at various distances. Early field testing connected the circuit to a
thyratron trigger operating a tower-mounted camera which photographed passing aircraft to determine distance of fuze function.
Prototype fuzes were then constructed in
June 1940, and installed in "
unrotated projectiles", the British cover name for solid-fueled
rockets, and fired at targets supported by balloons. Rockets have relatively low acceleration and no spin creating
centrifugal force, so the loads on the delicate electronic fuze are relatively benign. It was understood that the limited application was not ideal; a proximity fuze would be useful on all types of artillery and especially anti-aircraft artillery, but those had very high accelerations.
As early as September 1939,
John Cockcroft began a development effort at
Pye Ltd. to develop tubes capable of withstanding these much greater forces. Pye's research was transferred to the United States as part of the technology package delivered by the
Tizard Mission when the United States entered the war. Pye's group was apparently unable to get their rugged pentodes to function reliably under high pressures until 6 August 1941, which was after the successful tests by the American group.
Looking for a short-term solution to the valve problem, in 1940 the British ordered 20,000 miniature tubes from
Western Electric Company and
Radio Corporation of America that were intended for use in
hearing aids. An American team under Admiral
Harold G. Bowen, Sr. correctly deduced that the tubes were meant for experiments with proximity fuzes for bombs and rockets.
In
September 1940, the
Tizard Mission travelled to the US to introduce their researchers to a number of UK developments, and the topic of proximity fuses was raised. The details of the British experiments were passed to the
United States Naval Research Laboratory and
National Defense Research Committee (NDRC). Information was also shared with
Canada in 1940 and the
National Research Council of Canada delegated work on the fuze to a team at the
University of Toronto.
