Proximity fuze
A proximity fuze is a fuze that detonates an explosive device automatically when it approaches within a certain distance of its target. Proximity fuzes are designed for elusive military targets such as aircraft and missiles, as well as ships at sea and ground forces. This sophisticated trigger mechanism may increase lethality by 5 to 10 times compared to the common contact fuze or timed fuze.
Background
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. 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 above ground that are selected by gun crews. The shell bursts at the appropriate height above ground.
World War II
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 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". 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. 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. 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 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. 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 stresses 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 thermionic valves 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 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. 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.
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. Tuve's group was known as Section T, which was located at APL throughout the war. 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 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.
Given their previous work on radio and radiosondes at NBS, Diamond and Hinman developed the proximity fuze which employed the Doppler effect of reflected radio waves. The use of the Doppler effect developed by this group was later incorporated in all radio proximity fuzes for bomb, rocket, and mortar applications. Later, the Ordnance Development Division of the National Bureau of Standards developed the first automated production techniques for manufacturing radio proximity fuzes at low cost.
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.