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
Samuel Curran,
William Butement, Edward Shire, and Amherst Thomson at the
Telecommunications Research Establishment (TRE) conceived of the idea of a proximity fuze in the early stages of
World War II. Their system involved a small, short range,
Doppler radar. British tests were then carried out with "unrotated projectiles" (the contemporary British term for unguided 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 the United States during the
Tizard Mission in 1940. To work in shells, a fuze needed to be miniaturized, survive the high acceleration of cannon launch, and be reliable. The
National Defense Research Committee assigned the task to the physicist
Merle Tuve at the Department of Terrestrial Magnetism. Also eventually pulled in were researchers from the
National Bureau of Standards (this research unit of NBS later became part of the
Army Research Laboratory). Work was split in 1942, with Tuve's group working on proximity fuzes for shells, while the National Bureau of Standards researchers focused on the technically easier task of bombs and rockets. Work on the radio shell fuze was completed by Tuve's group, known as Section T, at
The Johns Hopkins University Applied Physics Lab (APL). Over 100 American companies were mobilized to build some 20 million shell fuzes. The proximity fuze was one of the most important technological innovations of World War II. It was so important that it was a secret guarded to a similar level as the
atom bomb project or
D-Day invasion. Admiral
Lewis Strauss wrote that, The fuze was later found to be able to detonate artillery shells in
air bursts, greatly increasing their anti-personnel effects. In Germany, more than 30 (perhaps as many as 50) different proximity fuze designs were developed, or researched, for anti-aircraft use, but none saw service. These included acoustic fuzes triggered by engine sound, one developed by
Rheinmetall-Borsig based on electrostatic fields, and radio fuzes. In mid-November 1939, a German neon lamp tube and a design of a prototype proximity fuze based on capacitive effects was received by British Intelligence as part of the
Oslo Report. In the post-World War II era, a number of new proximity fuze systems were developed, using radio, optical, and other detection methods. A common form used in modern air-to-air weapons uses a
laser as an optical source and time-of-flight for ranging.
Design in the UK The first reference to the concept of radar in the United Kingdom 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
coast artillery units to accurately measure the range to shipping even at night. The
War Office was not interested in the concept, and told the two to work on other issues. In 1936, the
Air Ministry took over
Bawdsey Manor in
Suffolk to further develop their prototype radar systems that emerged 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 defence, 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 fuze: In May 1940, a formal proposal from Butement, Edward Shire, and Amherst Thomson was sent to the British Air Defence Establishment based on the second of the two concepts. 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 electron tubes intended for use in
hearing aids from
Western Electric Company and
Radio Corporation of America. An American team under Admiral
Harold G. Bowen, Sr. correctly deduced that they 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).
Development in the US Prior to and following receipt of circuitry designs from the British, various experiments were carried out by Richard B. Roberts, Henry H. Porter, and Robert B. Brode under the direction of NDRC Section T Chairman Merle Tuve. As Tuve later put it in an interview: "We heard some rumors of circuits they were using in the rockets over in England, then they gave us the circuits, but I had already articulated the thing into the rockets, the bombs and shell." As Tuve understood, the circuitry of the fuze was rudimentary. In his words, "The one outstanding characteristic in this situation is the fact that success of this type of fuze is not dependent on a basic technical ideaall of the ideas are simple and well known everywhere." The critical work of adapting the fuze for anti-aircraft shells was done in the United States, not in England. Tuve said that despite being pleased by the outcome of the
Butement et al. vs. Varian patent suit, which affirmed that the fuze was a UK invention and thereby saved the U.S. Navy millions of dollars by waiving royalty fees, the fuze design delivered by the Tizard Mission was "not the one we made to work!". A key improvement was introduced by
Lloyd Berkner, who developed a system using separate transmitter and receiver circuits. In December 1940, Tuve invited
Harry Diamond and Wilbur S. Hinman, Jr, of the United States
National Bureau of Standards (NBS) to investigate Berkner's improved fuze and develop a proximity fuze for rockets and bombs to use against German
Luftwaffe aircraft. In just two days, Diamond was able to come up with a new fuze design and managed to demonstrate its feasibility through extensive testing at the
Naval Proving Ground at Dahlgren, Virginia. On 6 May 1941, the NBS team built six fuzes which were placed in air-dropped bombs and successfully tested over water. The use of the Doppler effect developed by this group was later incorporated in all radio proximity fuzes for bomb, rocket, and mortar applications. While working for a defense contractor in the mid-1940s, Soviet spy
Julius Rosenberg stole a working model of an American proximity fuze and delivered it to Soviet intelligence. It was not a fuze for anti-aircraft shells, the most valuable type. In the US, NDRC focused on radio fuzes for use with anti-aircraft artillery, where acceleration was up to 20,000 , compared to about 100 for rockets and much less for dropped bombs. In addition to extreme acceleration, artillery shells were spun by the rifling of the gun barrels to close to 30,000 rpm, creating immense centrifugal force. Working with
Western Electric Company and
Raytheon Company, miniature hearing-aid tubes were modified to withstand this extreme stress according to ideas delivered by
Van Allen, who joined the APL in 1942. The T-3 fuze had a 52% success against a water target when tested in January, 1942. The
United States Navy accepted that failure rate. A simulated battle conditions test was started on 12 August 1942. Gun batteries aboard cruiser tested proximity-fuzed ammunition against
radio-controlled drone aircraft targets over
Chesapeake Bay. The tests were to be conducted over two days, but the testing stopped when drones were destroyed early on the first day. The three drones were destroyed with just four projectiles. A particularly successful application was the 90 mm shell with VT fuze with the
SCR-584 automatic tracking radar and the
M9 Gun Director fire control computer. The combination of these three inventions was successful in shooting down many
V-1 flying bombs aimed at London and Antwerp, otherwise difficult targets for anti-aircraft guns due to their small size and high speed.
VT (Variable Time) The Allied fuze used constructive and destructive
interference to detect its target. The design had four or five electron tubes. One tube was an oscillator connected to an antenna; it functioned as both a transmitter and an
autodyne detector (receiver). When the target was far away, little of the oscillator's transmitted energy would be reflected to the fuze. When a target was nearby, it would reflect a significant portion of the oscillator's signal. The amplitude of the reflected signal corresponded to the closeness of the target. This reflected signal would affect the oscillator's plate current, thereby enabling detection. However, the
phase relationship between the oscillator's transmitted signal and the signal reflected from the target varied depended on the round trip distance between the fuze and the target. When the reflected signal was in phase, the oscillator amplitude would increase and the oscillator's plate current would also increase. However when the reflected signal was out of phase then the combined radio signal amplitude would decrease, which would decrease the plate current. So the changing phase relationship between the oscillator signal and the reflected signal complicated the measurement of the amplitude of that small reflected signal. This problem was resolved by taking advantage of the change in frequency of the reflected signal. The distance between the fuze and the target was not constant but rather constantly changing due to the high speed of the fuze and any motion of the target. When the distance between the fuze and the target changed rapidly, then the phase relationship also changed rapidly. The signals were in-phase one instant and out-of-phase a few hundred microseconds later. The result was a
heterodyne beat frequency which corresponded to the velocity difference. Viewed another way, the received signal frequency was
Doppler-shifted from the oscillator frequency by the relative motion of the fuze and target. Consequently, a low frequency signal, corresponding to the frequency difference between the oscillator and the received signal, developed at the oscillator's plate terminal. Two of the four tubes in the VT fuze were used to detect, filter, and amplify this low frequency signal. Note here that the amplitude of this low frequency 'beat' signal corresponds to the amplitude of the signal reflected from the target. If the amplified beat frequency signal's amplitude was large enough, indicating a nearby object, then it triggered the fourth tube – a gas-filled
thyratron. Upon being triggered, the thyratron conducted a large current that set off the electrical detonator. In order to be used with gun projectiles, which experience extremely high acceleration and centrifugal forces, the fuze design also needed to utilize many shock-hardening techniques. These included planar electrodes, and packing the components in wax and oil to equalize the stresses. To prevent premature detonation, the inbuilt battery that armed the shell had a several millisecond delay before its electrolytes were activated, giving the projectile time to clear the area of the gun. The designation VT means 'variable time'. Captain S. R. Shumaker, Director of the Bureau of Ordnance's Research and Development Division, coined the term to be descriptive without hinting at the technology.
Development The anti-aircraft artillery range at
Kirtland Air Force Base in New Mexico was used as one of the test facilities for the proximity fuze, where almost 50,000 test firings were conducted from 1942 to 1945. Testing also occurred at
Aberdeen Proving Ground in Maryland, where about 15,000 bombs were dropped.
Production First large scale production of tubes for the new fuzes Once inspections of the finished product were complete, a sample of the fuzes produced from each lot was shipped to the National Bureau of Standards, where they were subjected to a series of rigorous tests at the specially built Control Testing Laboratory.). The main suppliers were
Crosley,
RCA,
Eastman Kodak,
McQuay-Norris and
Sylvania. There were also over two thousand suppliers and subsuppliers, ranging from powder manufacturers to machine shops. It was among the first mass-production applications of
printed circuits.
Deployment Vannevar Bush, head of the U.S.
Office of Scientific Research and Development (OSRD) during the war, credited the proximity fuze with three significant effects. • It was important in defense from Japanese
kamikaze attacks in the Pacific. Bush estimated a sevenfold increase in the effectiveness of
5-inch anti-aircraft artillery with this innovation. • It was an important part of the radar-controlled anti-aircraft batteries that finally neutralized the German
V-1 attacks on England. • It was used in Europe starting in the
Battle of the Bulge where it was very effective in artillery shells fired against German infantry formations, and changed the tactics of land warfare. At first the fuzes were only used in situations where they could not be captured by the Germans. They were used in land-based artillery in the South Pacific in 1944. Also in 1944, fuzes were allocated to the
British Army's
Anti-Aircraft Command, that was engaged in defending Britain against the V-1 flying bomb. As most of the British heavy anti-aircraft guns were deployed in a long, thin coastal strip (leaving inland free for fighter interceptors), dud shells fell into the sea, safely out of reach of capture. Over the course of the German V-1 campaign, the proportion of flying bombs that were destroyed flying through the coastal gun belt rose from 17% to 74%, reaching 82% during one day. A minor problem encountered by the British was that the fuze was sensitive enough to detonate the shell if it passed too close to a seabird and a number of seabird "kills" were recorded. The Pentagon refused to allow the Allied field artillery use of the fuzes in 1944, although the United States Navy fired proximity-fuzed anti-aircraft shells in the July 1943
Battle of Gela during the invasion of Sicily. After General
Dwight D. Eisenhower demanded he be allowed to use the fuzes, 200,000 shells with VT fuzes (code named "POZIT") were used in the Battle of the Bulge in December 1944. They made the Allied heavy artillery far more devastating, as all the shells now exploded just before hitting the ground. German divisions were caught out in open as they had felt safe from timed fire because it was thought that the bad weather would prevent accurate observation. U.S. General
George S. Patton credited the introduction of proximity fuzes with saving Liège and stated that their use required a revision of the tactics of land warfare. Bombs and rockets fitted with radio proximity fuzes were in limited service with both the
USAAF and USN at the end of WWII. The main targets for these proximity fuze detonated bombs and rockets were
anti-aircraft emplacements and
airfields. ==Sensor types==