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Wednesday, February 19, 2003 10:20

Developing the Proximity Fuze, and Its Legacy

Dr. William T. Moye

            Development of the proximity fuze during World War II required a major effort in terms of money and coordination, as well as science and engineering.  Success gave impetus to other projects, such as computers.  Similar work has continued through the years, producing critical weapons and systems so valuable to troops deployed to Southwest Asia during Operation Desert Shield/Desert Storm.

            The 7th Air Force dropped the first bombs with so-called VT (Variable Time) or radio proximity fuzes on Iwo Jima during February 1945, against antiaircraft defenses and radio and radar installations.  In the final attacks on the Japanese mainland during July and August 1945, about one-third of all bombs dropped by Navy carrier-based aircraft were proximity fuzed general-purpose, fragmentation, and incendiary devices.

            The U.S. kept design of VT fuzes a very closely guarded secret, although production occupied most of the U. S. industrial capacity in electronics and plastics.  Indeed, the military did not clear the fuze for general operational use until December 4, 1944 for fear the enemy might recover and copy essential parts.

            In today's parlance, the program had a distinct "purple" tinge.  The Army paid civilian scientists at a civilian agency to develop a product used by the Navy and the Air Force (the Army Air Force), and the civilians accomplished the feat in close collaboration with industry and universities.  At the same time, the Navy was financing work by the Johns Hopkins Applied Physics Laboratory to produce radio proximity fuzes for its own weapons, as well as for the Army's field artillery.

            The proximity fuze for bombs, rockets, and mortars was developed by Division 4 of the National Defense Research Committee (NDRC), Office of Scientific Research and Development (OSRD), under the chairmanship of Dr. Alexander H. Ellett.  The heart of Division 4, the central laboratory, was a group at the National Bureau of Standards (NBS) under the leadership of Harry Diamond, a group later known as the Ordnance Development Division.  Working with Diamond were Wilbur S. Hinman, Jr., Allen V. Astin, Cledo Brunetti, Jacob Rabinow, and others.  Diamond and his associates had already accomplished pioneering feats at NBS, especially in the field of radio, developing radiosondes and automatic weather stations, as well as the instrument landing system for aircraft.

            The Navy and Johns Hopkins faced a different and perhaps a more difficult set of problems in developing the VT fuze for spinning shells, but Army advancement of the VT fuze for bombs, rockets, and mortars ranks as a major engineering feat, and the same people (or their successors) produced the fuze for the Patriot missile that was such a critical element during the Gulf War.  In fact, the Diamond successors are still turning out fuzes for Air Force bombs and Navy rockets, as well as Army mortars.

            Since World War I, the military had sought a fuze that would cause an air burst close to the target, rather than on impact, both to increase the effect of blast and to overcome the protection provided by foxholes and shallow trenches.  Moreover, such an influence or proximity fuze could be used against enemy aircraft - both those flying over population centers and those flying over surface ships.  Germany and Japan had research programs, but Great Britain was more aggressive in its search.  Several U.S. patents were awarded during the 1930s for fuzing devices.

            The great difficulties lay not so much in formulating the concepts but in determining the mechanical design detail and developing the tiny but rugged electrical and mechanical components, and also in organizing industrial personnel and facilities to produce the fuzes on a large scale.  After all, the task required constructing a miniature radio transmitting and receiving set so small that a man's hand covers most models.

            The effort began in the summer of 1940, with NDRC and the Navy sponsoring work at the Department of Terrestrial Magnetism (DTM) of the Carnegie Institution of Washington under Dr. Merle A. Tuve.  In September, Professor John D. Cockroft, a pioneer British researcher, as a member of the so-called Tizard Mission to Washington, briefed Navy officials and Tuve on the British project.  Thus, by the fall of 1940, Tuve's researchers at DTM were investigating photoelectric, acoustic, and radio fuzes and had begun testing electronic tubes for ruggedness.  In November, Dr. Ellett, from the University of Iowa, came to Washington to work with Tuve and, a few weeks later, to initiate the work at NBS on proximity fuzes for bombs and rockets.  In December, Ellett secured the services of Diamond and Hinman from the Radio Section at NBS.

            The Diamond/Hinman team quickly realized that a fuze utilizing the Doppler effect of reflected radio waves was the most promising concept, and they devised a diode detector arrangement that acted when the amplitude of the reflected signals exceeded a predetermined value.  A series of field tests of crude models proved the principles, culminating in successful bomb drops in April and May 1941.  During succeeding months, the team put its efforts into developing and refining electronic circuits, mechanical switches, and safety mechanisms, and in improving the ruggedness of the components and mountings.

            Meanwhile, under great pressure from the Navy, the Tuve group at DTM concentrated more and more on development of the radio fuze for shells.  In July 1941, in order to free up personnel and space for that effort, NDRC moved the DTM photoelectric bomb fuze group over to the NBS, where another group was already actively working on the problem.  In November, the Navy decided to prepare for full-scale production.  Early in 1942, the Tuve group was placed directly under the OSRD, and Johns Hopkins replaced the Carnegie Institution as the prime contractor.

            The Ellett group continued its work at NBS as Section E of Division A of the NDRC, with the Diamond/Hinman team serving as the central laboratory for the group.  In December 1942, NDRC was reorganized, and Section E became Division 4, with Ellett as chief and principal liaison between NDRC and NBS.  At that same time, the Diamond/Hinman team was reorganized and enlarged as the Ordnance Development Division, with about 200 people devoting their energies to developing a proximity fuze for rockets and bombs.  By the end of the war, over 400 persons were assigned to the project.

            By interservice agreement, the Navy developed and obtained the shell fuzes needed by both services, and the Army developed and procured fuzes for bombs, rockets, and mortars.  Signifi-cantly, Navy interest in and support for the shell fuze program was strong and steady, while Army support for bomb and rocket fuzes was not as strong and not nearly as consistent.

            In May 1942, the Army stated its first definite and urgent requirements for a specific fuze, a VT fuze for the new 4.5-inch rocket to be used against German aircraft.  The Diamond/Hinman team completed a design in two days.  NBS and Westinghouse produced initial model lots on small-scale production lines, and the team tested 55 of these fuzes at Fort Fisher, North Carolina, in June.  In fact, a photoelectric fuze developed by another NBS team with the assistance of Bell Telephone Laboratories was also successfully tested at that time, and procurement specifications for both were completed in September 1942.

            About 400,000 of each type were manufactured during 1943, but none were used in combat as intended, largely because the threat of massed formations of German bombers had subsided.  So Diamond and the NBS group found themselves with an excellent fuze but no weapon upon which it might be used.  Production was tapered off during 1943, and the photoelectric project was suspended all together.

            Despite its disappointment, Division 4 and the Ordnance Development Division concentrated on an enlarged program of bomb fuze development.  NBS started small-scale production of models in April 1943, and, in May, NDRC contracted with the Zell Corporation of Baltimore to set up a pilot line.

            Developers had to overcome several engineering problems.  Bomb fuzes had to perform reliably when cooled to the very low temperatures encountered by high-altitude bombers, and the dry battery used in the rocket proved ineffective.  Investigators found the solution in attaching a miniature generator to a wind-driven turbine in the arming system, the spinning turbine generating enough power to operate the proximity fuze.  Tests were begun in May 1943, and specifications were completed in November.

            Army and Navy decisions in July 1943 gave impetus to the project, specifying the types of bombs for which fuzes were required.  The War Department authorized limited procurement in the fall of 1943, even before testing was complete.  Interest climbed in the spring of 1944, with preparations for the invasion of Europe.  However, the Joint Chiefs of Staff forbid use of the fuze over land until October 1944.

            Even then, Army Air Force distrust of the fuze delayed use.  In the spring of 1944, the final development testing at Eglin Field, Florida, had proven somewhat disappointing, leading the Air Force to assert that a high percentage of early bursts nullified any presumed superiority of proximity-fuzed bombs over conventionally-fuzed clusters.  Ellett, believing the test regimen to be flawed, finally succeeded in scheduling another set of tests at Eglin in the fall of 1944, this time conclusively showing the great advantage of the air burst.  Unfortunately, the earlier adverse report received wide circulation, and wary commanders in Europe held off operational use of the fuze.

            In contrast, in late 1944, E. L. Bowles, expert consultant to the Secretary of War, toured the Pacific area, explaining new technical advances.  He discussed the proximity fuze and clarified misunderstandings from the earlier tests.  Following the Bowles visit, the 5th and 7th Air Forces requested that trial lots of the fuze be shipped by air, together with experts to advise on use and handling.  A successful demonstration was held at Saipan on 22 January for representatives of the 7th Air Force and its various bomber groups.  In February, in missions over Iwo Jima, the 7th Air Force dropped proximity-fuzed 500-lb general-purpose bombs and 260-lb fragmentation bombs.

            Reaction to the initial use was very favorable, and orders were placed for large quantities, which were shipped by boat and did not arrive until just before the end of the war.  In the meantime, the 7th Air Force successfully employed the small quantities of fuzes shipped by air (approximately 3,000 all together) in its bombing campaigns against Palau, Marcus Island, the Ryukyu Islands, and the Japanese mainland.

            As the bomb fuze went into full-scale production, the Navy requested adaptation of the fuze for use on its five-inch rockets, both the Aircraft Rocket (AR) and the High-Velocity Aircraft Rocket (HVAR).  Production of both air-to-ground and air-to-air versions began in December 1944 and reached a peak just before the surrender of Japan in August.

            Development of VT fuzes for trench mortars reflected some of these same highs and lows, stops and goes.  In the fall of 1943, when the Army officially requested that Division 4 undertake the venture, the Diamond group was concentrating on the bomb fuzes, so contracts were let with a group at the University of Florida who produced a design with a small loop antenna.  In the spring of 1944, the Diamond/Hinman team turned its attention to designing a fuze only one-third the size of those in bombs and rockets.  The subcontractor, Globe-Union Incorporated, found a way to produce a considerable part of the circuit by painting conducting material onto ceramic plates and blocks, the "so-called printed circuits."

            Testing of mortar shells began at Blossom Point, Maryland, in April 1944 and at Clinton, Iowa, in May 1945.  Production was delayed somewhat while the Ordnance Department emphasized the delivery of the rocket fuzes.  Then, in June 1945, a production schedule was established for 100,000 a month, followed by additional procurement projects for another 300,000 fuzes per month - based on the contingency that the war in the Pacific would last well into 1946 or 1947.

            Such wartime projects as the development of the proximity fuze produced revolutionary devices and uses and demands in many areas, especially electronics and plastics, but others as well.  Thus, after the war, Diamond and his electronics group at NBS helped to construct a digital electronic computer to support the Bureau of the Census.  In the 1950s, personnel from Diamond Ordnance Fuze Laboratories (DOFL) continued to pioneer the microminiaturization of electronic components, contributing achievements in such fields as etched wiring, printed components, and photolithographic fabrication.

            Meanwhile, the Ordnance Development Division and its successors, DOFL and Harry Diamond Laboratories, continued to provide fuze development support to new Army missile programs.  Much of the actual fuze engineering is now being conducted by detachments reporting directly to Picatinny [Armament Research, Development, and Engineering (RDE) Center] and Huntsville [Missile RDE Center].

 

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