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{{short description|Automatic fuzefuse that detonates an explosive device based on predetermined distance}}
{{Use dmy dates|date=October 2020}}
[[Image:MK53 fuze.jpg|upright=1.35|thumb|Proximity fuzefuse MK53 removed from shell, circa 1950s]]
 
A '''proximity fuzefuse''' (also VT fuze, or fuse<ref>{{Cite news|title=Hopkins Engineer Dies|language=en-US|newspaper=The Washington Post|url=https://www.washingtonpost.com/archive/local/1982/06/25/hopkins-engineer-dies/0b56de1f-3992-4773-b18d-6fdd41f21f7e/|access-date=2020-06-09|issn=0190-8286}}</ref><ref>{{Cite news|last=Sullivan|first=Walter|date=1984-02-08|title=Allen V. Astin Is Dead at 79; Headed Bureau of Standards|language=en-US|work=The New York Times |url=https://www.nytimes.com/1984/02/08/obituaries/allen-v-astin-is-dead-at-79-headed-bureau-of-standards.html|access-date=2020-06-09|issn=0362-4331}}</ref><ref>{{Cite web|last=Birch|first=Douglas|title='The secret weapon of World War II' Hopkins developed proximity fuse|url=https://www.baltimoresun.com/news/bs-xpm-1993-01-11-1993011049-story.html|access-date=2020-06-09 |website=baltimoresun.com|date=11 January 1993 |language=en-US}}</ref>) is a [[FuzeFuse (munitions)|fuzefuse]] that detonates an [[Explosive material|explosive]] device automatically when it approaches within a certain distance of its target. Proximity fuzesfuses are designed for elusive military targets such as airplanes 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 fuzefuse]] or timed fuzefuse.<ref name=Portrait>{{cite journal|last1=Hinman|first1=Wilbur S|title=Portrait of Harry Diamond |journal=Proceedings of the IRE|volume=45|issue=4|date=1957|page=443|doi=10.1109/JRPROC.1957.278430}}</ref><ref>[https://www.youtube.com/watch?v=6-D592VR4RU The Proximity Fuse - Secret Weapon of World War 2]''US Navy''</ref>
 
==Background==
Before the invention of the proximity fuzefuse, 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 fuzefuse. 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;<ref>{{Cite book |last=Kirby|first=M. W.|url=https://books.google.com/books?id=DWITTpkFPEAC&q=20%2C000|title=Operational Research in War and Peace: The British Experience from the 1930s to 1970|date=2003|publisher=Imperial College Press|isbn=978-1-86094-366-9|pages=94 |language=en}}</ref> other estimates put the figure as high as 100,000<ref>{{Citation|title=Engage Veterans {{!}} The Deadly Fuze|url=https://www.pbs.org/video/we-are-veterans-deadly-fuze/|language=en|access-date=2020-06-09}}</ref> or as low as 2,500.{{sfn|Baxter|1968|p=221}} With a proximity fuzefuse, the [[shell (projectile)|shell]] or missile need only pass close by the target at some time during its flight. The proximity fuzefuse makes the problem simpler than the previous methods.
 
Proximity fuzesfuses are also useful for producing [[air burst]]s against ground targets. A contact fuzefuse would explode when it hit the ground; it would not be very effective at scattering shrapnel. A timer fuzefuse can be set to explode a few meters above the ground but the timing is vital and usually requires [[Artillery observer|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 fuzesfuses fitted to such weapons as [[artillery shell|artillery]] and [[mortar shell]]s solve this problem by having a range of set burst heights [e.g. {{cvt|2|,|4|or|10|m|ft|0}}] 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 [[capacitor|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 Curran]], [[W. A. S. Butement|William Butement]], Edward Shire, and Amherst Thomson conceived of the idea of a proximity fuzefuse in the early stages of [[World War II]].<ref name="Brennan, 1968">{{cite journal|last=Brennan|first=James W.|title=The Proximity Fuze ''Whose Brainchild?'' |date=September 1968 |url=https://www.usni.org/magazines/proceedings/1968-09/proximity-fuze-whose-brainchild |volume=94 |issue=9 |pages=72–78 |journal=United States Naval Institute Proceedings}}</ref> 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 fuzefuse 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 fuzefuse, including a photoelectric fuzefuse and a radio fuzefuse, with United States during the [[Tizard Mission]] in late 1940. To work in shells, a fuzefuse needed to be miniaturized, survive the high acceleration of cannon launch, and be reliable.{{sfn|Baxter|1968|p=222}}
 
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 [[United States Army Research Laboratory|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 fuzefuse was completed by Tuve's group, known as Section T, at [[Applied Physics Lab|The Johns Hopkins University Applied Physics Lab]] (APL).<ref>{{cite journal|last=Brown |first=Louis|title=The Proximity Fuze|date=July 1993 |journal=IEEE Aerospace and Electronic Systems Magazine |volume=8|issue=7|pages=3–10|doi=10.1109/62.223933 |s2cid=37799726}}</ref> <ref>{{Cite web|title=Defining Innovations|url=https://www.jhuapl.edu/About/DefiningInnovations|access-date=2022-01-26 |website=www.jhuapl.edu}}</ref> Over 100 American companies were mobilized to build some 20 million shell fuzesfuses.<ref>{{cite book |last=Klein |first=Maury|title=A Call to Arms: Mobilizing America for World War II|pages=651–652, 838 n. 8|year=2013|location=New York|publisher=Bloomsbury Press|isbn=978-1-59691-607-4}}</ref>
 
The proximity fuzefuse 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.<ref>{{citation |last1=Thompson |first1=Harry C. |last2=Mayo |first2=Lida |title=The Ordnance Department: Procurement and Supply |pages=123–124 |location=Washington, D.C. |year=1960}}</ref><ref>{{citation |last=Woodbury |first=David |title=Battlefronts of Industry: Westinghouse in World War II |pages=244–248 |location=New York |year=1948}}</ref><ref>{{citation |last=Parker |first=Dana T. |title=Building Victory: Aircraft Manufacturing in the Los Angeles Area in World War II |page=127 |location=Cypress, California |year=2013 |isbn=978-0-9897906-0-4}}</ref> Admiral [[Lewis Strauss]] wrote that, {{quote|One of the most original and effective military developments in World War II was the proximity, or 'VT', fuzefuse. It found use in both the Army and the Navy, and was employed in the defense of London. While no one invention won the war, the proximity fuzefuse must be listed among the very small group of developments, such as radar, upon which victory very largely depended.{{sfn|Baldwin|1980|p=4}}}}
 
The fuzefuse was later found to be able to detonate artillery shells in [[air burst]]s, greatly increasing their anti-personnel effects.{{sfn|Baldwin|1980|pp=xxxi, 279}}
 
In Germany, more than 30 (perhaps as many as 50){{sfn|Holmes|2020|p=272}} different proximity fuzefuse designs were developed, or researched, for anti-aircraft use, but none saw service.{{sfn|Baxter|1968|p=222}} These included acoustic fuzesfuses triggered by engine sound, one developed by [[Rheinmetall-Borsig]] based on electrostatic fields, and radio fuzesfuses. In mid-November 1939, a German neon lamp tube and a design of a prototype proximity fuzefuse based on capacitive effects [[Oslo Report#background|was received by British Intelligence as part of the Oslo Report]].
 
In the post-World War II era, a number of new proximity fuzefuse 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.<ref>[https://www.youtube.com/watch?v=LyDlq77GVPM Critical Challenge: A History of the Proximity FuzeFuse presented] by Stephen Phillips</ref>
 
===Design in the UK===
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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 defense, but they were soon told to start a second project to develop a range-only radar to aid [[anti-aircraft gun]]s.<ref>{{cite journal |last=Butement |first=W. A. S. |display-authors=et al |title=Precision Radar |journal=J. Inst. Elect. Engrs |volume=73 |issue=part IIIA |date=1946 |pages=114–126}}</ref>
 
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 fuzefuse:
 
{{quote|...Into this stepped W. A. S. Butement, designer of radar sets [[Chain Home Low|CD/CHL]] and [[GL Mk. I radar|GL]], with a proposal on 30 October 1939 for two kinds of radio fuzefuse: (1) a radar set would track the projectile, and the operator would transmit a signal to a radio receiver in the fuzefuse when the range, the difficult quantity for the gunners to determine, was the same as that of the target and (2) a fuzefuse 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.<ref>{{citation |last=Brown |first=Louis |title=A Radar History of World War II |publisher=Inst. of Physics Publishing |year=1999 |at=section 4.4.}}</ref>}}
 
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.<ref name="Brennan, 1968" /> 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 fuzesfuses were then constructed in June 1940, and installed in "unrotated projectiles", the British cover name for solid-fueled [[rocket]]s, and fired at targets supported by balloons.<ref name="Brennan, 1968" /> Rockets have relatively low acceleration and no spin creating [[centrifugal force]], so the stresses on the delicate electronic fuzefuse are relatively benign. It was understood that the limited application was not ideal; a proximity fuzefuse 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 valve]]s (electron tubes) capable of withstanding these much greater forces.<ref>[http://www.pyetelecomhistory.org/prodhist/military/military.html Anti-Aircraft Radio Proximity FuzeFuse (1939–1942) (conceptual and prototype design work)]</ref> 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 [[pentode]]s to function reliably under high pressures until 6 August 1941, which was after the successful tests by the American group.<ref>{{Cite book |last=Frankland |first=Mark |date=2002 |url=https://books.google.com/books?id=ArZ3dc_eq2wC&q=Mark+Franklan,+radio+man|title=Radio Man: The Remarkable Rise and Fall of C.O. Stanley|publisher=IET |isbn=978-0-85296-203-9}}</ref>{{sfn|Holmes|2020|p=304}}
 
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 aid]]s 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 fuzesfuses for bombs and rockets.{{sfn|Baxter|1968|p=222}}
 
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).<ref name="Brennan, 1968" /> Information was also shared with [[Canada]] in 1940 and the [[National Research Council (Canada)|National Research Council]] of Canada delegated work on the fuzefuse to a team at the [[University of Toronto]].<ref>{{cite book |last=Friedland |first=Martin L. |date=2002 |title=The University of Toronto: A History |edition=1st |url=https://archive.org/details/universityoftoro0000frie |url-access=registration |location=Toronto |publisher=University of Toronto Press |pages=[https://archive.org/details/universityoftoro0000frie/page/354 354]–355 |isbn=978-0802044297 }}</ref>
 
===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.<ref name="Brennan, 1968" /> Tuve's group was known as Section T, which was located at APL throughout the war.<ref>{{Cite book |last=Baxter |first=James Phinney |url=https://books.google.com/books?id=57lgAAAAIAAJ |title=Scientists Against Time |date=1946 |publisher=Little, Brown |isbn=978-0598553881}}</ref> 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."{{sfn|Holmes|2020|p=304}}<ref>{{Cite web|date=2015-04-17 |title=Merle Tuve |url=https://www.aip.org/history-programs/niels-bohr-library/oral-histories/3894 |access-date=2020-06-10 |website=www.aip.org |language=en}}</ref> 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 fuzefuse is not dependent on a basic technical idea{{snd}}all of the ideas are simple and well known everywhere."{{sfn|Holmes|2020|p=304}} The critical work of adapting the fuzefuse for anti-aircraft shells was done in the United States, not in England.{{sfn|Holmes|2020|pp=304–305}} Tuve said that despite being pleased by the outcome of the ''Butement et al. vs. Varian'' patent suit, which affirmed that the fuzefuse was a UK invention and thereby saved the U.S. Navy millions of dollars by waiving royalty fees, the fuzefuse design delivered by the Tizard Mission was "not the one we made to work!".{{sfn|Holmes|2020|p=306}}
 
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 (engineer)|Harry Diamond]] and Wilbur S. Hinman, Jr, of the United States [[National Bureau of Standards]] (NBS) to investigate Berkner's improved fuzefuse and develop a proximity fuzefuse for rockets and bombs to use against German [[Luftwaffe]] aircraft.<ref name="Brennan, 1968"/><ref name="USArmy-1963">{{Cite book |title=Research and Development of Material Engineering Design Handbook Ammunition Series: FuzesFuses, Proximity, Electrical Part One (U) |url=http://www.dtic.mil/dtic/tr/fulltext/u2/389295.pdf |publisher=U.S. Army Materiel Command |year=1963 |access-date=26 January 2012 |archive-date=29 March 2018 |archive-url=https://web.archive.org/web/20180329072725/http://www.dtic.mil/dtic/tr/fulltext/u2/389295.pdf |url-status=dead}}</ref><ref name="Cochrane-1976">{{Cite book |url=https://www.nist.gov/sites/default/files/documents/nvl/Measures_for_Progress-MP275-FULL.pdf |title=Measures for progress: A history of the National Bureau of Standards |last=Cochrane |first=Rexmond |publisher=Arno Press |year=1976 |isbn=978-0405076794 |pages=388–399 |access-date=18 June 2018 |archive-date=2 August 2017 |archive-url=https://web.archive.org/web/20170802040136/https://www.nist.gov/sites/default/files/documents/nvl/Measures_for_Progress-MP275-FULL.pdf |url-status=dead }}</ref>
 
In just two days, Diamond was able to come up with a new fuzefuse design and managed to demonstrate its feasibility through extensive testing at the [[Naval Surface Warfare Center Dahlgren Division|Naval Proving Ground]] at Dahlgren, Virginia.<ref name="Hinman-1957">{{Cite journal |title=Portrait of Harry Diamond| last=Hinman | first=Wilbur Jr. |journal=Proceedings of the IRE |volume=45 |issue=4 |pages=443–444 |doi=10.1109/JRPROC.1957.278430 |year=1957}}</ref><ref>{{Cite web |title=Artillery Proximity Fuses |website=warfarehistorynetwork.com |url=http://warfarehistorynetwork.com/daily/wwii/artillery-proximity-fuses/ |access-date=2018-06-18 |archive-date=12 June 2018 |archive-url=https://web.archive.org/web/20180612212300/http://warfarehistorynetwork.com/daily/wwii/artillery-proximity-fuses/ |url-status=dead }}</ref> On 6 May 1941, the NBS team built six fuzesfuses which were placed in air-dropped bombs and successfully tested over water.<ref name="Brennan, 1968"/>
 
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.<ref name="Cochrane-1976" /><ref name="NIST-2018">{{Cite web |url=https://nvlpubs.nist.gov/nistpubs/sp958-lide/059-062.pdf |title=Radio Proximity Fuzes|access-date=18 June 2018}}</ref><ref name="Johnson-1984">{{Cite journal |last1=Johnson |first1=John |last2=Buchanan |first2=David |last3=Brenner |first3=William |date=July 1984 |title=Historic Properties Report: Harry Diamond Laboratories, Maryland and Satellite Installations Woodbridge Research Facility, Virginia and Blossom Point Field Test Facility, Maryland |url=http://www.dtic.mil/docs/citations/ADA175872 |archive-url=https://web.archive.org/web/20170609010919/http://www.dtic.mil/docs/citations/ADA175872 |url-status=dead |archive-date=9 June 2017 |journal=Defense Technical Information Center |language=en}}</ref> The use of the Doppler effect developed by this group was later incorporated in all radio proximity fuzesfuses for bomb, rocket, and mortar applications.<ref name="USArmy-1963" /> Later, the Ordnance Development Division of the National Bureau of Standards (which became the [[Harry Diamond Laboratories]] – and later merged into the [[United States Army Research Laboratory|Army Research Laboratory]] – in honor of its former chief in subsequent years) developed the first automated production techniques for manufacturing radio proximity fuzesfuses at low cost.<ref name="Johnson-1984" />
 
While working for a defense contractor in the mid-1940s, Soviet spy [[Julius Rosenberg]] stole a working model of an American proximity fuzefuse and delivered it to Soviet intelligence.<ref>{{citation |first1=John Earl |last1=Haynes |first2=Harvey |last2=Klehr |title=Venona, Decoding Soviet Espionage in America |page=303}}</ref> It was not a fuzefuse for anti-aircraft shells, the most valuable type.{{sfn|Holmes|2020|p=274}}
 
In the US, NDRC focused on radio fuzesfuses for use with anti-aircraft artillery, where acceleration was up to 20,000&nbsp;{{mvar|g}}, compared to about 100&nbsp;{{mvar|g}} for rockets and much less for dropped bombs.{{sfn|Baxter|1968|p=224}} 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. The T-3 fuzefuse 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 {{USS|Cleveland|CL-55}} tested proximity-fuzedfused 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.<ref name="Brennan, 1968" /><ref>{{cite book |last=Howeth |first=Linwood S. |title=History of Communications-Electronics in the United States Navy |date=1963 |publisher=United States Government Printing Office |lccn=64-62870 |page=498 |url=https://babel.hathitrust.org/cgi/pt?id=uiug.30112064674325;view=1up;seq=530}}</ref>
 
A particularly successful application was the 90&nbsp;mm shell with VT fuzefuse with the [[SCR-584]] automatic tracking radar and the M-9 electronic [[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 fuzefuse used constructive and destructive [[wave interference|interference]] to detect its target.{{sfn|Bureau of Ordnance|1946|pp=32–37}} The design had four or five electron tubes.<ref>{{harvnb|Bureau of Ordnance|1946|p=36}} shows a fifth tube, a [[diode]], used for a low trajectory wave suppression feature (WSF).</ref> 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 fuzefuse. 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.<ref group=notes>The return signal is inversely proportional to the fourth power of the distance.</ref> This reflected signal would affect the oscillator's plate current, thereby enabling detection.
 
However, the [[Phase (waves)|phase relationship]] between the oscillator's transmitted signal and the signal reflected from the target varied depended on the round trip distance between the fuzefuse and the target. When the reflected signal was in phase, the oscillator amplitude would increase and the oscillator's plate current would also increase. But 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 fuzefuse and the target was not constant but rather constantly changing due to the high speed of the fuzefuse and any motion of the target. When the distance between the fuzefuse 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 shift|Doppler-shifted]] from the oscillator frequency by the relative motion of the fuzefuse 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 fuzefuse 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 fuzefuse 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.{{citation needed|date=May 2021}} 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.<ref>Smith, Peter C. Kamikaze: To Die for the Emperor. Pen and Sword, 2014, p.42</ref>
 
The designation VT means 'variable time'.<ref name="DTIC-1946a">{{Cite report|title=Summary Technical Report of the National Defence Research Council|date=1946|chapter-url= https://apps.dtic.mil/dtic/tr/fulltext/u2/221589.pdf|language=en|chapter=Summary of the Work of Division 4|page=1}}</ref> 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.<ref>{{Cite book|last1=Rowland |first1=Buford |last2=Boyd |first2=William B. |year=1953 |title=U. S. Navy Bureau of Ordnance in World War II |location= Washington, D.C. |publisher= Bureau of Ordnance, Department of the Navy |page=279 |url=https://babel.hathitrust.org/cgi/pt?id=uc1.$b671188;view=1up;seq=295}}</ref>
 
===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 fuzefuse, where almost 50,000 test firings were conducted from 1942 to 1945.<ref>{{Cite magazine|title=Request for information about the Isleta Pueblo Ordnance Impact Area |date=8 August 2008 |author=U.S. Army Corps of Engineers |magazine=Isleta Pueblo News |volume=3 |issue=9 |page=12 |url=http://www.isletapueblo.com/uploads/3/0/9/5/3095182/08_august_2008.pdf |archive-url=https://web.archive.org/web/20170326184510/http://www.isletapueblo.com/uploads/3/0/9/5/3095182/08_august_2008.pdf |archive-date=26 March 2017 |url-status=live}}</ref> Testing also occurred at [[Aberdeen Proving Ground]] in Maryland, where about 15,000 bombs were dropped.<ref name="NIST-2018" /> Other locations include Ft. Fisher in North Carolina and Blossom Point, Maryland.
 
[[US Navy]] development and early production was outsourced to the [[Wurlitzer]] company, at [[North Tonawanda Barrel Organ Factory|their barrel organ factory]] in [[North Tonawanda, New York]].<ref>{{cite book|url=https://books.google.com/books?id=FhoEAAAAMBAJ&pg=PT122 |title=Navy presents high award to Wurlitzer men|publisher=Billboard magazine|date=15 June 1946}}</ref>
 
===Production===
First large scale production of tubes for the new fuzes<ref name="Brennan, 1968" /> was at a [[General Electric]] plant in [[Cleveland, Ohio]] formerly used for manufacture of Christmas-tree lamps. FuzeFuse assembly was completed at General Electric plants in [[Schenectady, New York]] and [[Bridgeport, Connecticut]].<ref name="Miller, Men and Volts" >{{Citation |last=Miller |first=John Anderson |title=Men and Volts at War |journal=Nature |volume=161 |issue=4082 |pages=113 |publisher=McGraw-Hill Book Company |location=New York |year=1947|bibcode=1948Natur.161..113F |doi=10.1038/161113a0 |s2cid=35653693 |doi-access=free }}</ref> Once inspections of the finished product were complete, a sample of the fuzesfuses 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.<ref name="NIST-2018" /> These tests included low- and high-temperature tests, humidity tests, and sudden jolt tests.
 
By 1944, a large proportion of the American [[electronics industry]] concentrated on making the fuzesfuses. Procurement contracts increased from [[US$]]60 million in 1942, to $200 million in 1943, to $300 million in 1944 and were topped by $450 million in 1945. As volume increased, efficiency came into play and the cost per fuzefuse fell from $732 in 1942 to $18 in 1945. This permitted the purchase of over 22 million fuzesfuses for approximately one billion dollars ($14.6 billion in 2021 USD<ref>{{Cite web|title=Calculate the Value of $1.00 in 1945. How much is it worth today?|url=https://www.dollartimes.com/inflation/inflation.php?amount=1&year=1945?back=https://www.google.com/search?client=safari&as_qdr=all&as_occt=any&safe=active&as_q=One+dollar+in+1945+inflation+adjusted&channel=aplab&source=a-app1&hl=en |access-date=2021-09-01 |website=www.dollartimes.com}}</ref>). The main suppliers were [[Powel Crosley, Jr.#Crosley's war effort|Crosley]], [[RCA]], [[Eastman Kodak]], [[McQuay-Norris]] and [[Sylvania Electric Products|Sylvania]]. There were also over two thousand suppliers and subsuppliers, ranging from powder manufacturers to machine shops.{{sfn|Sharpe|2003}}{{sfn|Baldwin|1980|pp=217–220}} It was among the first mass-production applications of [[printed circuit]]s.<ref>{{cite book |last1=Eisler|first1=Paul|last2=Williams|first2=Mari |title=My Life with the Printed Circuit|publisher=Lehigh University Press |year=1989|isbn=978-0-934223-04-1}}</ref>
 
===Deployment===
[[Vannevar Bush]], head of the U.S. [[Office of Scientific Research and Development]] (OSRD) during the war, credited the proximity fuzefuse with three significant effects.{{sfn|Bush|1970|pp=106–112}}
* It was important in defense from Japanese [[kamikaze]] attacks in the Pacific. Bush estimated a sevenfold increase in the effectiveness of [[5"/38 caliber gun|5-inch anti-aircraft artillery]] with this innovation.{{sfn|Bush|1970|p=109}}
* It was an important part of the radar-controlled anti-aircraft batteries that finally neutralized the German [[V-1 flying bomb|V-1]] attacks on England.{{sfn|Bush|1970|p=109}}
* 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 fuzesfuses 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, fuzesfuses 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 fuzefuse was sensitive enough to detonate the shell if it passed too close to a seabird and a number of seabird "kills" were recorded.<ref name="Dobinson2001">{{cite book |last=Dobinson|first=Colin|title=AA Command: Britain's Anti-aircraft Defences of World War II |isbn=978-0-413-76540-6 |url=https://archive.org/details/aacommandbritain00dobi |url-access=limited|year=2001|publisher=Methuen |page=[https://archive.org/details/aacommandbritain00dobi/page/n226 437] |via=Internet Archive}}</ref>
 
The Pentagon refused to allow the Allied field artillery use of the fuzesfuses in 1944, although the United States Navy fired proximity-fuzedfused anti-aircraft shells in the July 1943 [[Battle of Gela (1943)|Battle of Gela]] during the invasion of Sicily.<ref>{{cite book |last1=Potter |first1=E.B. |last2=Nimitz |first2=Chester W. |author-link2 =Chester W. Nimitz |title =Sea Power |url=https://archive.org/details/seapowernavalhis0000pott |url-access=registration |publisher =Prentice-Hall |date =1960 |location =Englewood Cliffs, New Jersey |pages =[https://archive.org/details/seapowernavalhis0000pott/page/589 589]–591 |isbn=978-0137968701 |via=Internet Archive}}</ref> After General [[Dwight D. Eisenhower]] demanded he be allowed to use the fuzesfuses, 200,000 shells with VT fuzesfuses (code named "POZIT"<ref>{{cite book|author=Albert D. Helfrick|title=Electronics in the Evolution of Flight |url=https://books.google.com/books?id=EumPJQBViz4C&pg=PA78|year=2004|publisher=Texas A&M UP|page=78|isbn=978-1585444137}}</ref>) 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.<ref>{{cite book|author=Rick Atkinson|title=The Guns at Last Light: The War in Western Europe, 1944-1945|url=https://books.google.com/books?id=FUQ9lEHO0QoC&pg=PA460|year=2013|pages=460–462, 763–764|publisher=Henry Holt and Company |isbn=978-1429943673}}</ref> 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 fuzesfuses with saving Liège and stated that their use required a revision of the tactics of land warfare.{{sfn|Bush|1970|p=112}}
 
Bombs and rockets fitted with radio proximity fuzesfuses were in limited service with both the [[United States Army Air Forces|USAAF]] and USN at the end of WWII.&nbsp; The main targets for these proximity fuzefuse detonated bombs and rockets were [[anti-aircraft]] emplacements and [[Aerodrome|airfields]].<ref name="DTIC-1946b">{{Cite report|title=Summary Technical Report of the National Defence Research Council|date=1946|chapter-url= https://apps.dtic.mil/dtic/tr/fulltext/u2/221589.pdf|language=en|chapter=Summary of the Work of Division 4|page=8}}</ref>
 
==Sensor types==
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The device described in World War II patent<ref name="Kyle, 1950" >{{cite patent
|inventor-last=Kyle |inventor-first=John W
|title=Radio Proximity FuzeFuse
|issue-date=1950-12-04
|patent-number=3152547
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[[Acoustics|Acoustic]] proximity fuzes are actuated by the acoustic emissions from a target (example an aircraft's engine or ship's propeller). Actuation can be either through an electronic circuit coupled to a [[microphone]], or [[hydrophone]], or mechanically using a resonating vibratory reed connected to diaphragm tone filter. <ref name="Hogg-1999">{{Cite book|title=German Secret Weapons of the Second World War|last=Hogg|first=Ian|date=1999|publisher=Frontline Books|isbn=978-1-8483-2781-8|pages=120–122|language=en}}</ref> <ref name="NDRC-1946">{{Cite report|title=Summary Technical Report of the National Defence Research Council|date=1946|chapter-url= https://apps.dtic.mil/dtic/tr/fulltext/u2/221589.pdf|chapter=Chapter 2 Proximity and Time Fuzes|pages=17–18|language=en}}</ref>
 
During WW2, the Germans had at least five acoustic fuzesfuses for [[Anti-aircraft warfare|anti-aircraft]] use under development, though none saw operational service. The most developmentally advanced of the German acoustic fuzefuse designs was the [[Rheinmetall-Borsig]] Kranich (German for [[Crane (bird)|Crane]]) which was a mechanical device utilizing a diaphragm tone filter sensitive to frequencies between 140 and 500Hz connected to a resonating vibratory reed switch used to fire an electrical igniter. The [[Henschel Hs 117|Schmetterling]], [[Enzian]], [[Rheintochter]] and [[Ruhrstahl X-4|X4]] [[Missile|guided missiles]] were all designed for use with the Kranich acoustic proximity fuzefuse. <ref name="Hogg-1999"/> <ref name="Zaloga-2019">{{Cite book|title=German Guided Missiles of World War II|last=Zaloga|first=Steven|date=2019|publisher=Bloomsbury Publishing|isbn=978-1-4728-3179-8|language=en}}</ref>
 
During [[WW2]], the [[National Defense Research Committee]] (NDRC) investigated the use of acoustic proximity fuzesfuses for [[Anti-aircraft warfare|anti-aircraft]] weapons but concluded that there were more promising technological approaches. The NDRC research highlighted the [[speed of sound]] as a major limitation in the design and use of acoustic fuzesfuses, particularly in relation to missiles and high-speed aircraft.<ref name="NDRC-1946"/>
 
[[Hydroacoustic]] influence is widely used as a detonation mechanism for [[naval mine]]s and [[torpedoes]]. A ship's propeller rotating in water produces a powerful hydroacoustic noise which can be picked up using a [[hydrophone]] and used for homing and detonation. Influence firing mechanisms often use a combination of acoustic and [[magnetic proximity fuze|magnetic induction]] receivers.<ref name="Beloshitskiy-1960">{{Cite report|title=Oruzhiye Podvodnogo Udara (Underwater Weapons)|last1=Beloshitskiy|first1=V.P|last2=Baginskiy|first2=Yu.M|date=1960|publisher=Military Publishing House|url=https://apps.dtic.mil/sti/citations/ADA046104|archive-url=https://web.archive.org/web/20201203204734/https://apps.dtic.mil/sti/citations/ADA046104|url-status=live|archive-date=3 December 2020}}</ref> <ref name="Erickson-2009">{{Cite book|title=Chinese Mine Warfare|last1=Erickson|first1=Andrew|last2=Goldstein|first2=Lyle|last3=Murray|first3=William|date=2009|publisher=Naval War College|isbn=978-1-884733-63-5|language=en|pages=12–17}}</ref>
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=== Pressure ===
Some naval mines use pressure fuzesfuses which are able to detect the [[P-wave|pressure wave]] of a [[ship]] passing overhead. Pressure sensors are usually used in combination with other fuzefuse detonation technologies such as [[Acoustics|acoustic]] and [[electromagnetic induction|magnetic induction]].<ref name="Erickson-2009"/>
 
During WW2, pressure activated fuzesfuses were developed for sticks (or trains) of [[bombs]] to create above ground [[Air burst|airbursts]].&nbsp; The first bomb in the stick was fitted with an [[impact fuzefuse]] while the other bombs were fitted with pressure sensitive diaphragm actuated detonators.&nbsp; The blast from the first bomb was used to trigger the fuzefuse of the second bomb which would explode above ground and in this turn would detonate the third bomb with the process repeated all the way till the last bomb in the string.&nbsp; Due to the forward speed of the [[bomber]], bombs fitted with pressure detonators would all explode at about the same height above ground along a horizontal trajectory. &nbsp;This design was used in both the British No.44 "Pistol" and the German [[Rheinmetall-Borsig]] BAZ 55A fuzesfuses.<ref name="Hogg-1999"/> <ref name="NDRC-1946"/>
 
== Gallery ==
<gallery>
Image:MSPO2007-35-01.jpg|120mm [[High Explosive|HE]] [[mortar shell]] fitted with proximity fuzefuse
Image:A01-021A.png|120mm HE mortar shell fitted with [[M734]] proximity fuzefuse
Image:MSPO2007-37-01.jpg|60mm HE mortar shell fitted with proximity fuzefuse
File:PD and Proximity fuze.jpg|A 155mm artillery fuzefuse with selector for point/proximity detonation (currently set to proximity).
</gallery>
 
==See also==
* [[Allied technological cooperation during World War II]]
* [[Artillery fuzefuse]]
* [[Guidance system]]
* [[Guided bomb]]
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== Bibliography ==
* {{citation |last=Baldwin |first=Ralph B. |author-link=Ralph Belknap Baldwin |title=The Deadly FuzeFuse: The Secret Weapon of World War II |publisher=Presidio Press |location=San Rafael, CA |year=1980 |isbn=978-0-89141-087-4}}. Baldwin was a member of the (APL) team headed by Tuve that did most of the design work.
* {{Citation
|last=Baxter
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}}
* {{cite book
|title= VT FuzesFuses For Projectiles and Spin-Stabilized Rockets
|last= Bureau of Ordnance
|date= 15 May 1946
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|last= Sharpe
|first= Edward A.
|title= The Radio Proximity FuzeFuse: A survey
|journal=Vintage Electrics
|year=2003
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==Further reading==
* {{citation |last=Allard |first=Dean C. |title=The Development of the Radio Proximity FuzeFuse |journal=Johns Hopkins APL Technical Digest |date=1982 |volume=3 |issue=4 |pages=358–359 |url=https://www.jhuapl.edu/Content/techdigest/pdf/V03-N04/03-04-Allard.pdf}}
* {{cite web |last1=Allen |first1=Kevin |title=Artillery Proximity Fuses |website=Warfare History Network |url=http://warfarehistorynetwork.com/daily/wwii/artillery-proximity-fuses/ |access-date=4 June 2018 |archive-date=12 June 2018 |archive-url=https://web.archive.org/web/20180612212300/http://warfarehistorynetwork.com/daily/wwii/artillery-proximity-fuses/ |url-status=dead }}
* {{Citation
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|first= Geoffrey
|author-link= Geoffrey Bennett (historian)
|title= The Development of the Proximity FuzeFuse
|journal= Journal of the Royal United Service Institution
|year= 1976
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|last= Collier
|first= Cameron D.
|title= Tiny Miracle: the Proximity FuzeFuse
|journal= Naval History
|year= 1999
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|isbn= 978-1-85367-478-5
}}
* {{Citation |title=FuzesFuses, Proximity, Electrical: Part One |date=July 1963 |id=AMCP 706-211 |series=Engineering Design Handbook: Ammunition Series |publisher=United States Army Materiel Command |url=http://www.dtic.mil/dtic/tr/fulltext/u2/389295.pdf |access-date=26 January 2012 |archive-date=29 March 2018 |archive-url=https://web.archive.org/web/20180329072725/http://www.dtic.mil/dtic/tr/fulltext/u2/389295.pdf |url-status=dead }}
* {{Citation |title= FuzesFuses, Proximity, Electrical: Part Two |id=AMCP 706-212 |series=Engineering Design Handbook: Ammunition Series |publisher= United States Army Materiel Command }}
* {{Citation |title= FuzesFuses, Proximity, Electrical: Part Three |id=AMCP 706-213 |series=Engineering Design Handbook: Ammunition Series |publisher= United States Army Materiel Command }}
* {{Citation |title= FuzesFuses, Proximity, Electrical: Part Four |id=AMCP 706-214 |series=Engineering Design Handbook: Ammunition Series |publisher= United States Army Materiel Command }}
* {{Citation |title=FuzesFuses, Proximity, Electrical: Part Five |date=August 1963 |id=AMCP 706-215 |series=Engineering Design Handbook: Ammunition Series |publisher=United States Army Materiel Command |url=http://www.dtic.mil/cgi-bin/GetTRDoc?Location=U2&doc=GetTRDoc.pdf&AD=AD0389296 |access-date=26 January 2012 |archive-date=8 April 2013 |archive-url=https://web.archive.org/web/20130408130936/http://www.dtic.mil/cgi-bin/GetTRDoc?Location=U2&doc=GetTRDoc.pdf&AD=AD0389296 |url-status=dead }}
* {{cite patent |inventor1-last=Tuve |inventor1-first=Merle A. |inventorlink1=Merle Tuve |inventor2-last=Roberts |inventor2-first=Richard B. |title=Radio Proximity Fuze |country-code=US |patent-number=3166015 |fdate=1943-01-06 |pubdate=1965-01-19 |assign1=United States of America |ref=none}}
 
==External links==
{{Wiktionary}}
* [https://www.youtube.com/watch?v=Dtocpvv88gQ The Secret Invention That Changed World War 2] Real Engineering. Detailed design and operation of the Mark 53 fuzefuse
* Battleship New Jersey, [https://www.youtube.com/watch?v=RTPm_VsK7FI ''Developing the Proximity Fuse''] via YouTube
* [https://archive.org/details/gov.archives.arc.39087 1945 newsreel explaining how it works]
* {{webarchive |url=http://webarchive.loc.gov/all/20140704031301/http%3A//www%2Ehistory%2Enavy%2Emil/faqs/faq96%2D1%2Ehtm |title=Naval Historical Centre – Radio Proximity (VT) FuzesFuses |date=2014-07-04}}
* [http://www.smecc.org/radio_proximity_fuzes.htm The Radio Proximity FuzeFuse – A survey] Southwest Museum of Engineering,Communications and Computation
* [http://www.smecc.org/pfuze.htm Proximity Fuze History] Southwest Museum of Engineering,Communications and Computation
* [http://www.microworks.net/pacific/equipment/vt_fuze.htm The Proximity (Variable-Time) FuzeFuse] – The Pacific War: The U.S. Navy
* [https://web.archive.org/web/20131015083216/http://www.jhuapl.edu/aboutapl/heritage/default.asp The Johns Hopkins University Applied Physics Laboratory]
 
{{DEFAULTSORT:Proximity FuzeFuse}}
[[Category:Science and technology during World War II]]
[[Category:Artillery ammunition]]
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