Proximity fuze: Difference between revisions

{{short description|Automatic fuze that detonates an explosive deviceat based ona predetermined distance}}

[[Image:MK53 fuze.jpg|300pxupright=1.35|thumb|Proximity fuze MK53 removed from shell, circa 1950s]]

A '''proximity fuze''' (oralso fuse'''VT fuze'''<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/|date= 1982-06-25|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> or "variable time fuze") is a [[Fuze (munitions)|fuze]] that detonates an [[Explosive material|explosive]] device automatically when theit distanceapproaches towithin thea targetcertain becomesdistance smallerof thanits a predetermined valuetarget. Proximity fuzes are designed for elusive military targets such as planes,aircraft and missiles, as well as ships at sea, and ground forces. They provide a moreThis sophisticated trigger mechanism thanmay the common [[contact fuze]] or timed fuze. It is estimated that it increases theincrease lethality by 5 to 10 times, compared to thesethe othercommon fuzes[[contact fuze]] or timed fuze.<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>[{{Cite AV media |url=https://www.youtube.com/watch?v=6-D592VR4RU |title=The Proximity Fuse - Secret Weapon of World War 2]''US Navy''|date=2012-10-15 |last=Museum of Our Industrial Heritage |access-date=2024-06-24 |via=YouTube}}</ref>

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 [[Thethe 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.<ref>{{harvnbsfn|Baxter|1968|p=221}}</ref> With a proximity fuze, the [[shell (projectile)|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.

British military researchers at the [[Telecommunications Research Establishment]] (TRE) [[Samuel Curran|Samuel C. Curran]], [[W. A. S. Butement|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 [[World War II]].<ref name="Brennan, 1968">{{Citationcite 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 |publisherjournal=United States Naval Institute Proceedings}}</ref> Their system involved a small, short range, [[Doppler radar]]. British tests were then carried out with "unrotated projectiles," in(the thiscontemporary British term for caseunguided 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.<ref name="Baxter 1968 222">{{harvnbsfn|Baxter|1968|p=222}}</ref>

The [[National Defense Research Committee]] assigned the task to the physicist [[Merle Tuve|Merle A. 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 fuze was completed by Tuve's group, known as Section T, at [[Applied Physics Lab|The Johns Hopkins University Applied Physics Lab]] (APL).<ref>{{citationcite 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 fuzes.<ref>{{citationcite book |last=Klein |first=Maury|title=A Call to Arms: Mobilizing America for World War II|pages=651–652, 838n8838 n. 8|year=2013|location=New York|publisher=Bloomsbury Press|isbn=978-1-59691-607-4}}</ref>

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.<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|Adm. Lewis L. Strauss]] wrote that, {{quote|One of the most original and effective military developments in World War II was the proximity, or 'VT', fuze. 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 fuze must be listed among the very small group of developments, such as radar, upon which victory very largely depended.<ref>{{harvnbsfn|Baldwin|1980|p=4}}</ref>}}

The fuze was later found to be able to detonate artillery shells in [[air burst]]s, greatly increasing their anti-personnel effects.<ref>{{harvnbsfn|Baldwin|1980|pp=xxxi, 279}}</ref>

In Germany, more than 30 (perhaps as many as 50)<ref>{{sfn|Holmes, |2020|p. =272</ref>}} different proximity fuze designs were developed, or researched, for [[anti-aircraft]] use, but none saw service.<ref name="{{sfn|Baxter |1968 |p=222" />}} These included acoustic fuzes triggered by engine sound, one based on electrostatic fields developed by [[Rheinmetall-Borsig|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 [[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 fuze systems were developed, includingusing radio, optical, and other meansdetection 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 Fuze presented] by Stephen Phillips</ref>

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 couldto accurately measure the range to shipping even at night. The [[War Office]] provedwas uninterestednot interested in the concept, and told the two to work on other issues.<ref>{{cite journal |last1=Butement, |first1=W. A. S., and |first2=P. E. |last2=Pollard; "|title=Coastal Defence Apparatus", ''|journal=Inventions Book of the Royal Engineers Board'', |date=Jan. 1931}}</ref><ref>{{cite book |last=Swords, |first=S. S.; ''|title=tech. History of the Beginnings of Radar'', |publisher=Peter Peregrinus, Ltd, |year=1986, pp. |pages=71–74}}</ref>

In 1936, the [[Air Ministry]] took over [[Bawdsey Manor]] in [[Suffolk]] to further develop their prototype radar systems that would emergeemerged 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.'', vol. |volume=73, |issue=part IIIA, |date=1946, pp. |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 fusefuze:

{{quote|...Into this stepped W. A. S. Butement, designer of radar sets [[RadarChain inHome World War II#British ArmyLow|CD/CHL]] and [[GL Mk. I radar|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.<ref>{{citation |last=Brown |first=Louis |title=A Radar History of World War II |publisher=Inst. of Physics Publishing |year=1999 |locationat=section 4.4.}}</ref>}}

In May 1940, a formal proposal from Butement, Edward S. Shire, and Amherst F.H. ThompsonThomson 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 fuzes were then constructed in June 1940, and installed in "[[unrotated projectile]]sprojectiles", 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 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 valve]]s (electron tubes) capable of withstanding these much greater forces.<ref>[http://www.pyetelecomhistory.org/prodhist/military/military.html Anti-Aircraft Radio Proximity Fuze (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|date=2002|publisher=IET |isbn=978-0-85296-203-9|language=en}}</ref><ref name="Holmes 2020 304">{{Cite booksfn|last=Holmes|first=Jamie|url=https://books.google.com/books?id=fyKixgEACAAJ|title=12 Seconds of Silence: How a Team of Inventors, Tinkerers, and Spies Took Down a Nazi Superweapon|date=2020|publisher=Houghton Mifflin Harcourt|isbn=978-1-328-46012-7|pagesp=304|language=en}}</ref>

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]] that were intended for use in [[hearing aid]]s. An American team under Admiral [[Harold G. Bowen, Sr.]] correctly deduced that the tubesthey were meant for experiments with proximity fuzes for bombs and rockets.<ref name="{{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 fuze 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>

===ImprovementDevelopment 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, notwhich 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 |language=en}}</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."<ref name="{{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 fuze is not dependent on a basic technical idea{{snd}}all of the ideas are simple and well known everywhere."<ref name="{{sfn|Holmes |2020 |p=304"/>}} The critical work of adapting the fuze for anti-aircraft shells was done in the United States, not in England.<ref>{{Cite book sfn|last=Holmes |first=Jamie |url=https://books.google.com/books?id=fyKixgEACAAJ |title=12 Seconds of Silence: How a Team of Inventors, Tinkerers, and Spies Took Down a Nazi Superweapon |date=2020 |publisher=Houghton Mifflin Harcourt|isbn=978-1-328-46012-7 |pagespp=304–305 |language=en}}</ref> Tuve claimedsaid 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!"<ref>.{{sfn|Holmes, |2020|p. =306</ref>}}

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 fuze and develop a proximity fuze for rockets and bombs to use against the German [[Luftwaffe]] aircraft.<ref name="Brennan, 1968" /><ref name=":1USArmy-1963">{{Cite book |url=http://www.dtic.mil/dtic/tr/fulltext/u2/389295.pdf |title=Research and Development of Material Engineering Design Handbook Ammunition Series: Fuzes, 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=":2Cochrane-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 fuze 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=":0Hinman-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/ |titleaccess-date=Artillery2018-06-18 Proximity|archive-date=12 FusesJune 2018 |websitearchive-url=https://web.archive.org/web/20180612212300/http://warfarehistorynetwork.com/daily/wwii/artillery-proximity-fuses/ |language=en |accessurl-datestatus=2018-06-18dead }}</ref> On 6 May 1941, the NBS team built six fuzes 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 first all solid-state{{when|date=October 2019}}{{clarify|reason=solid state would imply no vacuum tubes, and the transistor was 10 years away. If there was really a solid state version, we should talk about it|date=October 2019}} radio doppler proximity fuze, which employed the [[Doppler effect]] of reflected radio waves using a diode detector arrangement that they devised.<ref name=":2Cochrane-1976" /><ref name=":3NIST-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=":4Johnson-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 fuzes for bomb, rocket, and mortar applications.<ref name=":1USArmy-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 fuzes at a low cost.<ref name=":4Johnson-1984" />

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.<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 fuze for anti-aircraft shells, the most valuable type.<ref>{{sfn|Holmes, |2020|p. =274</ref>}}

In the US, NDRC focused on radio fuzes for use with anti-aircraft artillery, where acceleration was up to 20,000&nbsp;{{mvar|g}}, as opposedcompared to about 100&nbsp;{{mvar|g}} for rockets and much less for dropped bombs.<ref>{{harvnbsfn|Baxter|1968|p=224}}</ref> 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 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 {{USS|Cleveland|CL-55}} 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.<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 fuze with the [[SCR-584]] automatic tracking radar and the M-9[[M9 electronicGun 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.

The Allied fuze used constructive and destructive [[wave interference|interference]] to detect its target.<ref>{{Harvnbsfn|Bureau of Ordnance|1946|pp=32–37}}</ref> The design had four or five electron tubes.<ref>{{Harvnbharvnb|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 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.<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.

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 shift|Doppler-shiftshifted]]ed 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.{{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=":93DTIC-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|access-date=26 November 2020|archive-date=12 November 2020|archive-url=https://web.archive.org/web/20201112015503/https://apps.dtic.mil/dtic/tr/fulltext/u2/221589.pdf|url-status=dead}}</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>

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.<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 fireddropped.<ref name=":3NIST-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 their [[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>

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. Fuze tea assembly was completed at NestléGeneral 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 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.<ref name=":3NIST-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 fuzes. 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 fuze fell from $732 in 1942 to $18 in 1945. This permitted the purchase of over 22 million fuzes 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.<ref>{{Harvnbsfn|Sharpe|2003}}</ref><ref>{{harvnbsfn|Baldwin|1980|pp=217–220}}</ref> 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|year=1989|publisher=Lehigh University Press |year=1989|isbn=978-0-934223-04-1}}</ref>

[[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.<ref>{{Harvnbsfn|Bush|1970|pp=106–112}}</ref>

* It was important in defense from Japanese [[Kamikazekamikaze]] 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.<ref name="Bush p109" >{{Harvnbsfn|Bush|1970|p=109}}</ref>

* 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.<ref name="{{sfn|Bush p109" />|1970|p=109}}

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 that were destroyed 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.<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|isbn=978-0-413-76540-6 |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 fuzes in 1944, although the United States Navy fired proximity-fuzed anti-aircraft shells duringin the July 1943 [[Battle of Gela (1943)|invasionBattle of SicilyGela]] 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 fuzes, 200,000 shells with VT fuzes (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 fuzes with saving Liège and stated that their use required a revision of the tactics of land warfare.<ref>{{Harvnbsfn|Bush|1970|p=112}}</ref>

Bombs and rockets fitted with radio proximity fuzes were in limited service with both the [[United States Army Air Forces|USAAF]] and [[United States Navy|USN]] at the end of WW2WWII.&nbsp; The main targets for these proximity fuze detonated bombs and rockets were [[anti-aircraft]] emplacements and [[Aerodrome|airfields]].<ref name=":94DTIC-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|access-date=26 November 2020|archive-date=12 November 2020|archive-url=https://web.archive.org/web/20201112015503/https://apps.dtic.mil/dtic/tr/fulltext/u2/221589.pdf|url-status=dead}}</ref>

|url=httphttps://wwwpatents.google.com/patentspatent/US3152547A/en?idoq=ygNzAAAAEBAJus3152547

}}</ref> works as follows: The shell contains a micro-[[transmitter]] which uses the shell body as an [[antenna (radio)|antenna]] and emits a continuous wave of roughly 180–220&nbsp;MHz. As the shell approaches a reflecting object, an interference pattern is created. This pattern changes with shrinking distance: every half wavelength in distance (a half wavelength at this frequency is about 0.7&nbsp;meters), the transmitter is in or out of resonance. This causes a small cycling of the radiated power and consequently the oscillator supply current of about 200–800&nbsp;Hz, the [[Doppler effect|Doppler]] frequency. This signal is sent through a [[band-pass filter]], amplified, and triggers the detonation when it exceeds a given amplitude.{{cn|date=April 2023}}

Some modern [[air-to-air missile]]s (e.g., the [[ASRAAM]] and [[AA-12 Adder]]) use [[laser]]s to trigger detonation. They project narrow beams of laser light perpendicular to the flight of the missile. As the missile cruises towards its target the laser energy simply beams out into space. As the missile passes its target some of the energy strikes the target and is reflected to the missile, where detectors sense it and detonate the warhead.

[[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=":95Hogg-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=":96NDRC-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|access-date=26 November 2020|archive-date=12 November 2020|archive-url=https://web.archive.org/web/20201112015503/https://apps.dtic.mil/dtic/tr/fulltext/u2/221589.pdf|url-status=dead}}</ref>

During WW2, the Germans had at least five acoustic fuzes for [[Anti-aircraft warfare|anti-aircraft]] use under development, though none saw operational service. The most developmentally advanced of the German acoustic fuze 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 500Hz500&nbsp;Hz 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 fuze. <ref name=":95Hogg-1999"/> <ref name=":97Zaloga-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 fuzes 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 fuzes, particularly in relation to missiles and high-speed aircraft.<ref name=":96NDRC-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=":98Beloshitskiy-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=":99Erickson-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>

[[File:Luftmine (LM).jpg|thumb|rightupright|German World War II magnetic mine that landed on the ground instead of the water.]]

{{main|Magnetic proximity fuze|Magnetic pistol}}

Some naval mines use pressure fuzes which are able to detect the [[P-wave|pressure wave]] of a [[ship]] passing overhead. Pressure sensors are usually used in combination with other fuze detonation technologies such as [[Acoustics|acoustic]] and [[electromagnetic induction|magnetic induction]].<ref name=":99Erickson-2009"/>

During WW2, pressure activated fuzes 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 fuze]] while the other bombs were fitted with pressure sensitive diaphragm actuated detonators. &nbsp; The blast from the first bomb was used to trigger the fuze 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 No44No.44 “Pistol”"Pistol" and the German [[Rheinmetall-Borsig]] BAZ 55A fuzes.<ref name=":95Hogg-1999"/> <ref name=":96NDRC-1946"/>

<gallery heights="200">

ImageFile:MSPO2007-35-01.jpg|120mm120&nbsp;mm [[High Explosive|HE]] [[mortar shell]] fitted with proximity fuze

ImageFile:A01-021A.png|120mm120&nbsp;mm HE mortar shell fitted with [[M734]] proximity fuze

ImageFile:MSPO2007-37-01.jpg|60mm60&nbsp;mm HE mortar shell fitted with proximity fuze

File:PD and Proximity fuze.jpg|A 155mm155&nbsp;mm artillery fuze with selector for point/proximity detonation (currently set to proximity).

* {{Citationcite book

|volume= OP 1480

* {{citation |last=Allard |first=Dean C. |title=The Development of the Radio Proximity Fuze |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 |ref=none}}

* {{cite web |last1=Allen |first1=Kevin |title=Artillery Proximity Fuses |website=Warfare History Network |url=http://warfarehistorynetwork.com/daily/wwii/artillery-proximity-fuses/ |websiteaccess-date=Warfare4 HistoryJune Network2018 |accessarchive-date=412 June 2018 |archive-url=https://web.archive.org/web/20180612212300/http://warfarehistorynetwork.com/daily/wwii/artillery-proximity-fuses/ |url-status=dead }}

|author-link= Geoffrey Bennett (historian)