Radio Proximty (VT) Fuzes

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Pacific Historian
Jun 4, 2005
Orange County, CA
Today marks the 65th anniversary of the adoption of the VT fuze. Just getting this fuze to work, let alone producing it in quantities and stored in severe enviornments, was an immense achievment by the US.

.....The original development of the radio proximity fuze for projectiles was undertaken by Section T, NDRC, at the Department of Terrestrial Magnetism of the Carnegie Institution of Washington. The problem of devising circuits to detect proximity of objects was simple enough, and it appeared that fuzes could be made to operate on the same principle, provided circuits could be made small enough to be contained in a projectile and rugged enough to withstand firing from a gun. Right from the start, it appeared that the development of vacuum tubes sufficiently rugged this purpose would be the most difficult problem to solve. Late in 1940, experiments were made with commercial vacuum tubes mounted in blocks and dropped on concrete or armor plate to test for ruggedness. A surprising degree of ruggedness was evident and it seemed reasonable to hope that the problem of developing rugged vacuum tubes was not insurmountable. As a very small size was also required for these tubes, investigations were made with small hearing-aid types of tubes them commercially available. Among these were Raytheon and Hytron hearing aid types. As glass breakage of the tube was also a problem, investigations were started on methods of potting the tubes to protect the glass. Likewise, work was started on improvement of electrode structures and methods of mounting to achieve better mechanical strength.

It was soon decided that the best way to test tubes and other components for ruggedness was to actually fire then from a gun and recover them to examine for extent and causes of failure. Early in 1941, experiments were carried out in which tubes were mounted in blocks in a 5"/38 projectile arranged for parachute recovery. Other means of recovery firing were also undertaken. A smooth bore gun was made out of a piece of gas pipe and set up in a farm yard for testing of tubes and components. This gun was fired vertically and the projectiles, which were homemade, fell back in the field where they could be recovered and disassembled. This gun was later superseded with an Army 37mm gun used for recovery firing.

Concurrently, circuit work was carried out in the laboratory. Also, functioning oscillators were mounted in projectiles and fired in attempts to get functioning in flight. Both the 5"/38 and the 37mm guns were used in these tests. Radio receivers were used in an attempt to hear the signal from the oscillator during flight. As a source of power for the unit in the 5"/38 projectile, special batteries built by the National Carbon Company for the bomb fuze were used. For the unit in the 37mm projectile, a special battery was built using National Carbon Company's minimax cells for B-power and pen-light cells for A-power. At about the end of April 1941, an oscillator fired in the 37mm gun was actually heard throughout flight.

By June 1941, circuit work had been carried to the point where a circuit of sufficient sensitivity and small enough size to be contained in a fuze could be made. The circuit consisted of an oscillator, a two-stage audio frequency amplifier, a thyratron, and an electric detonator developed by Hercules Powder Company connected in the thyratron output in such a fashion that it would initiate the explosive detonation. A dry battery built by the National Carbon Company and similar to the unit used in the 37mm test projectile was used as a source of power. Switches, known as set-back switches and developed by Section T, were used in the fuze to close the battery circuits upon firing of the projectile. An electrical arming delay was incorporated in the circuit to prevent arming of the fuze until after the tube filaments had heated and the unit had quieted down after the initial impact of firing. The oscillator radiated a radio frequency signal. Some of the energy from this radiated field would be reflected back from any target in the vicinity of the projectile in such a fashion as to react upon the oscillator, causing an audio frequency signal which was then amplified by the amplifier and used to trigger the thyratron. The electric detonator in the thyratron output circuit initiated detonation of the auxiliary detonator and hence the explosive charge. At this time development had progressed to the point where a complete mechanical design of a proximity fuze was laid out.

In order to improve facilities for recovery firing, a test field was set up at Stump Neck, Maryland, where a 57mm gun was mounted for recovery firing. This gun was selected because it was the smallest gun which fired a projectile large enough to contain a fuze of the size necessary to accommodate the required components. Special recovery projectiles were developed for this gun which could be used to carry the complete fuze or to carry any of the components being tested for improvements in ruggedness. The projectiles fired from this gun were arranged to carry a small smoke puff to indicate operation of the fuze and detonation of the electric detonator.

By September 1941, a complete fuze had been made to ride throughout flight and function properly at the end of the trajectory. Troubles at this time were primarily premature functioning of the fuze caused by mechanical breakage, by microphonic disturbances from the tubes and the circuit, and voltage fluctuations from the battery. Considerable vertical firing was done of tubes and refinements were made in tube design which ultimately led to satisfactory tubes. Circuit designs were modified by such means as shaping the amplifier response to minimize microphonic noises. Refinements of the battery were directed toward more rigid construction, more positive contact, etc. to minimize spurious voltages from these sources. The cannon primer was refined in strength so that it could be made sufficiently rugged. This amounted primarily to modifications in design of the bridge wire and bridge wire support.

In September 1941, tests of complete fuzes were started at Naval Proving Grounds, Dahlgren, in the 5"/38 projectile. Early Dahlgren tests were not very successful primarily because of extreme premature failures. At this time a double filament triode tube was being used as an oscillator, and it was discovered that beats between these two filaments set up microphonic noises within the audio frequency pass band of the amplifier and were probably the cause of much of this premature trouble. Consequently, the oscillator tube was then changed to a single filament type.

In the fall of 1941 the Sylvania Company was brought into the tube program and contributed greatly toward the development of improved types of tubes. Throughout this same period considerable work was done toward refining quality of glass on the miniature tubes and improved methods of potting of the tubes to overcome glass breakage failures. During this period work was started at RCA on the development of metal envelope miniature tubes to overcome the glass breakage failures, However, improvements in manufacture and in methods of mounting glass tubes eventually overcame these tube failures and the metal tube development was subsequently abandoned.

By January 1942 a test had been conducted at Dahlgren which gave slightly better than 50% successful performance which was considered to be adequate to bring a manufacturer in the program. Up until this time all manufacture of test fuzes had been carried out by Section T facilities and by the Erwood Company which was brought into the program in the fall of 1941. At this time a development contract was given to the Crosley Corporation with a view toward ultimate production.

Throughout all this early development period, considerable question remained in the minds of many people that the position of bursts of proximity fuzes of this type around an airplane target might not be properly located to cause maximum or even any damage from projectile fragments. Accordingly, considerable study was made of proper amplifier frequency response curves, etc., with a view toward achieving the proper positioning of influence or proximity bursts. Likewise, the University of Michigan had been brought into the program and had been doing small scale model work to study these various features. From laboratory investigations, it appeared that proper directionality or positioning of bursts had been achieved but in the spring of 1942 it was decided to conduct a test against a full-scale model to ascertain the effectiveness of proximity bursts.


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So is there some information on shell detonation delays, ranges and shell type? It would be interesting to see ideal detonation delay/range vs shell type. And also delay/range variability vs shell type.

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