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Radio Proximity (VT) Fuzes

Radio Proximity (VT) Fuzes and How They Operate

Proximity fuzes are intended to detonate missiles automatically upon approach to a target and at such a position along the flight path of the missile as to inflict maximum damage to the target. Various methods of obtaining proximity operation against a target were investigated: electrostatic, acoustic, optical, and radio. Prime considerations for a proximity fuze were reliability and simplicity. The former was necessary to insure performance under various stringent Service conditions, and the latter, to allow the fuze to be contained in a small volume and to be produced quickly in large quantities. Following initial exploratory investigations, two types of fuzes, optical (photoelectric) and radio, were selected for intensive development. The photoelectric method was selected because it appeared a relatively easy approach to the proximity fuze problem, although the fuzes would be limited to daytime use, unless light sources were provided. The radio method appeared to be more complicated, but it afforded opportunity for reliable performance not only 24 hours a day but under a much wider variety of other conditions than were possible with the photoelectric fuze. The two methods were pursued in parallel until it was definitely established that radio proximity fuzes could be produced to fulfill all requirements. When this stage of development was reached, work on photoelectric fuzes was terminated (October 1943), and the radio method was prosecuted even more vigorously than before.

How a Radio Proximity Fuze Operates

Among various possible types of radio proximity fuzes, an active-type fuzed operating on the doppler effect was selected as being the most promising method.

In a doppler-type fuze, the actuating signal is produced by the wave reflected from a target moving with respect to the fuze. The frequency of the reflected wave differs from that of the transmitted wave, because of the relative velocity of the fuze and the target. The interference it creates with the transmitter results in a low-frequency beat caused by the combination of the transmitted and the reflected frequencies. The low-frequency signal can be used to trigger an electronic switch. Selective amplification of the low-frequency signal is generally necessary.

Operation of the fuze occurs when the output signal from the amplifier reaches the required amplitude to fire the thyratron. For a given orientation of the fuze and target, the amplitude of the target signal produced in the oscillator-detector circuit is a function of the distance between the target and the fuze. Hence, by proper settings for the gain of the amplifier and the holding bias on the thyratron, the distance of operation may be controlled. Distance, however, is not the only factor which requires consideration. Orientation or aspect is very important, particularly against aircraft targets, since operation should occur at that point on the trajectory when the greatest number of fragments will be directed toward the target.

For most missiles, the greatest number of fragments are directed upon detonation approximately at right angles to the axis of the missile. For trajectories which would normally pass by the target without intersecting it, there will be optimum chance of damage if detonation of the missile occurs when the target is in the direction of greatest fragmentation density. However, for trajectories which would intersect the target, the missile should come as close to the target as possible before detonation. Hence the basic requirements for directional sensitivity of a proximity fuze for antiaircraft use are: (1) the sensitivity should be a maximum in the direction corresponding to maximum lateral fragmentation density of the missile, and (2) the sensitivity should be a minimum along the axis of the missile. Directional sensitivity of this type can be obtained by using the missile as an antenna, with the axis of the missile corresponding to the axis of the antenna. With the fuze in the forward end of the missile, such antennas are excited by means of a small electrode, or cap, on the nose of the fuze. Additional control over the sensitivity pattern of the fuze is possible by means of the amplifier gain characteristic.

For use against surface targets, proximity fuzes are designed for an optimum height of burst, depending on the nature of the target and the properties of the missile. These optimum heights vary from 10 to 70 ft for fragmentation and blast bombs and are of the order of a few hundred feet for chemical warfare bombs.

With a fuze intended for ground approach operation, it is desirable to have maximum sensitivity along the axis of the bomb. A short dipole antenna mounted in the fuze transversely to the bomb's axis gives such sensitivity.

It was also found that fairly good ground approach performance could be obtained from fuzes with axial antennas by designing the amplifiers to compensate for the appreciable decrease in radiation sensitivity in the forward direction. For example, steep angles of approach generally mean high approach velocities with higher doppler frequencies. Thus a loss in radiation sensitivity with steep approach can be compensated by an increase in amplifier gains for the higher doppler frequencies.

A miniature triode is used for the oscillator in the fuze, and a pentode for the amplifier. Some fuzes use separate detector circuits wit a tiny diode to provide the required rectification. A miniature thyractron serves as the triggering agent, and a specially developed electric detonator initiates the explosive action.

Energy for powering the electronic circuit is obtained, in the later fuze models, from a small electric generator. This is driven by a windmill in the airstream of the missile. A rectifier network and voltage regulator are also essential parts of the power supply.

The arming and safety features of the radio proximity fuzes are closely tied in with the power supply. This is a natural procedure since an electronic device is inoperative until electric energy is supplied. Arming a radio proximity fuze (generator type) consists of the following operations: (1) either removal of an arming wire which frees the windmill, allowing it to turn in the airstream (bomb fuzes), or actuation of a setback device freeing the drive shaft of the generator, allowing it to turn (rocket and mortar shell fuzes), (2) operation of the generator to supply energy to the fuze circuits, (3) connection of the electric detonator into the circuit after a predetermined number of turns of the vane corresponding to a certain air travel, and (4) removal of a mechanical barrier between the detonator and booster, prior to which explosion of the detonator would not explode the booster. Generally, operations (3) and (4) occur simultaneously by motion of the same device.

Additional safety is provided by the fact that unless the generator of the fuze is turning rapidly the fuze is completely inoperative. A minimum airspeed of approximately 100 mph is required to start the generator turning.

Source: Office of Scientific Research and Development. National Defense Research Committee. Summary, Photoelectric Fuzes and Miscellaneous Projects. vol. 3 of Summary Technical Report of Division 4 [Ordnance Accessories] NDRC. (Washington DC: 1946): 2-3. [declassified 27 Oct. 1960].

Significance and Background of the Radio Proximity Fuse (VT) in World War II

The radio proximity, or VT fuze for artillery shells represents, as will be readily apparent, a major contribution to the successful prosecution of the war in Europe as well as in the Pacific. Its development, production and military use is an outstanding tribute to continuous and effective collaboration by research groups, industrial organizations and the military services.

In ordnance terms, a fuze is that part of an artillery projectile which detonates the explosive charge. An ideal fuze would detonate the shell at the most favorable position to inflict maximum damage on the target.

Early in the war, it became disturbingly evident that speed, maneuverability and heights attainable by modern military aircraft presented a method of attack against which fuzes currently available for antiaircraft guns were relatively ineffective. Even with the improvements in directing antiaircraft gunfire made possible by radar, diminishing probability of hitting elusive attacking planes made the problem of defense against aircraft extremely urgent for a nation involved in the war.

The idea of influence, or proximity fuzes is not unique and was suggested independently by various persons in the United States and abroad long prior to 1940. However, the obstacles in the way of actually developing a fuze of this type seemed insurmountable. Many technically inclined people who have witnessed an antiaircraft demonstration have toyed with the idea of a proximity fuze. The small target area presented by an airplane, together with its isolation in space, practically forced a consideration of a fuze which would detonate in the vicinity of the airplane.

The inherent disadvantages in the time fuze and the contact fuze stimulated this type of speculation. The first type, which detonates a projectile at a specified time after it leaves the gun, has been widely used against airplanes and personnel. However, use of time fuzes requires not only that time of flight from the gun to the airplane be calculated in advance, but that each fuze be "set" for this time. Even a slight error in setting will cause the projectile to explode at a harmless distance from the target.

The value of the contact fuzed projectile as an antiaircraft device is also limited, since it must actually hit its target before it will detonate. As range lengthens, this becomes increasingly difficult.

It has long been recognized by ordnance experts that the efficacy of explosive projectiles would be greatly increased if they could be equipped with fuzes which would be actuated by the influence of a target. For example, an antiaircraft projectile which would automatically detonate when it came within lethal range of an airplane would simplify fire control techniques and would be highly effective.

Although inventors had suggested almost every possible type of proximity fuze, in both prewar and war years, they failed to indicate how the formidable development and engineering difficulties could be satisfactorily overcome. Such fuzes to be useful for artillery purposes would have to be capable of withstanding the shock of acceleration when shot from a gun, in addition to undergoing a high rate of rotation in flight. Many patents on proximity devices were issued in various countries, but these also failed to indicate how the invention would be manufactured.

British scientists were working on proximity fuze devices for rockets and bombs at least as early as 1939. Captured documents indicate that German work on proximity fuze development had begun in the early 1930's, and was still in process when hostilities ended in the European Theatre.

In brief, there is nothing unique about the "idea" of a proximity fuze. The possibility that proximity fuzes of various types might be feasible has been recognized for a long time. The American achievement, accomplished by no other country, was the actual development of a proximity fuze that would function and that could be manufactured by mass-production techniques.

Source: The "VT" or Radio Proximity Fuze: Supplemental Basic Information Prepared by Applied Physics Laboratory, the Johns Hopkins University. (Silver Springs MD: The Laboratory, 1945): 3-5. [Released for publication on 20 Sep. 1945.].

Development of Proximity Fuzes (VT) for Projectiles-VT Fuzes Mks 32 to 60


During the summer of 1940 shortly after the formation of the NDRC [National Defense Research Committee], work was started on the development of a proximity fuze. The initial development was undertaken by Section T of Division A of the NDRC. The initial project was very broad in objective; namely, to develop a proximity fuze of any type (radio, acoustic, photo-electric, electro-static, infra-red, etc.) for rockets, bombs, and projectiles. Such a project was assigned to Section T by the Navy.

At the time this project was started, the primary objective was to provide better defense against aircraft. Methods of using proximity fuzes for this purpose then being considered included use in bombs for air-to-air bombing, use in rockets, and use in projectiles. At about the time this project was started, it was learned that the British had been developing proximity fuzes and had some considered to be fairly promising for use in bombs and rockets. The British had considered fuzes for projectiles, but felt that the technical difficulties in making such fuzes rugged enough to withstand firing from a gun were insurmountable, at least during World War II.

While the original project covered all types of proximity fuzes, for rockets, bombs, and projectiles, a primary interest to the Navy was a proximity fuze for the Navy 5"/38 projectile, as this weapon was the Navy's principal antiaircraft weapon. Section T at the outset considered the development of the projectile fuze as a primary objective, and undertook investigations leading toward achievement of sufficient ruggedness of electronic parts and the like to permit firing from a gun. By the spring of 1941 work on the radio type of projectile fuze had progressed to the point where it appeared to he the most promising type of fuze, and at that time Section T dropped its work on investigation of other types of proximity fuzes and concentrated entirely on the radio-type of projectile proximity fuze. This development ultimately led to the present type of radio proximity fuze for projectiles and is the development with which this report is concerned.

In addition to the U. S. Navy interest in the projectile proximity fuze, the British and the U. S. Army were also interested. Agreements were made that all projectile proximity fuze work would be carried out by the Navy and Section T. The Army had also entered the proximity fuze program, but in line with these agreements, the Army concentrated on proximity fuzes for rockets and bombs. The British had also started some work in Canada on the proximity fuze for British projectiles, and this development was carried out cooperatively with the Section T program. At that time the priorities for projectile proximity fuze development were set up as follows:

(1) U. S. Navy, (2) British Navy, (3) U. S. Army, (4) British Army.

The first development was a fuze, known as the VT fuze Mk 32, for the Navy 5"/38. This development was followed by modifications in design to permit adaptation of the projectile fuze to British Navy guns, U. S. Army guns and British Army guns. At that time the primary objective was to provide better defense against aircraft; and hence, the fuzes being developed were all antiaircraft fuzes.

Some thought was given to use of a proximity fuze to achieve air bursts over ground for anti-personnel work, etc., but it was not until late in 1942 that much emphasis was placed on this use of the proximity fuze. This use of the proximity fuze would necessarily require enormous quantities of fuzes; and hence, its attainment depended upon a design not only small enough to be used in Army projectiles, but also simple enough to produce in very large quantities. The first fuze developed, which was the Mk 32 for the 5"/38, was too large and too difficult to produce to be used for this purpose. Continual effort toward reduction in size and simplification of design finally led to the development of the Mk 45 which was suitable for such use, and from that tine on emphasis was also placed on the field artillery use.

Development and Tests

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.

Accordingly, in April 1942, a test was conducted at Parris Island, North Carolina, in which proximity fuzes in 5"/38 projectiles were fired against a full-scale airplane target suspended beneath a balloon. An analysis of the results of this test appeared to show that the fuze was functioning effectively, although some question remained because the projectiles used were black powder loaded which caused the bursts to be somewhat late and made the burst pattern appear slightly behind the target. However measurements of delay in firing caused by black powder loading seemed to clear up this discrepancy. In later tests of this sort when black powder loading was used, the fuze threads were relieved so that the fuze would be blown out instantaneously rather than delay while sufficient internal pressure was being built up within the projectile.

Throughout all this early development, one important item was that of providing an adequate safety feature to prevent functioning of the fuze until it had traveled a safe distance beyond the muzzle of the gun. The use of an auxdet [auxiliary detonator], of course, provided safety against bore bursts, but was no assurance against bursts just outside the muzzle. This safety feature was to be achieved by incorporating a mechanical clock as one component of the fuze arranged to prevent functioning of the fuze within a time interval of about 3/10 to 5/10 second after firing. All early tests were conducted using fuzes which contained only the RC delay. It was not until the middle of 1942 that a satisfactory safety clock had finally been developed. The design finally developed, known as the Mk 1 clock, was more or less a modification of the Mk 18 time fuze movement. When this development was finally achieved, all the essential components for a satisfactory proximity fuze were available. Accordingly, plans were them made to carry out actual drone firings from a Navy ship.

This test firing of proximity fuzed 5"/38 projectiles against drones was carried out in August 1942 aboard the cruiser [USS] Cleveland [CL-55]. Results of this test were entirely satisfactory and accordingly, full-scale production of proximity fuzes was initiated at the Crosley Corporation in September 1942. Early production was plagued with numerous difficulties but satisfactory material was finally produced. This fuze, which was designated the Mk 32, was delivered to the Fleet during November and December 1942, and the first Japanese plane was shot down with proximity fuzed projectiles by the cruiser [USS] Helena [CL-50] in January 1943.

Early in 1942 it became apparent that the complexity of the proximity fuze was such that to a large extent its successful manufacture would require very careful and extensive quality control procedures. While the Bureau of Ordnance was setting up an organization to handle the proximity fuze program, it did not possess facilities to handle adequately this phase of the program, Consequently, Section T was requested to assume responsibility for quality control as well as for the engineering and development of proximity fuzes. Section T undertook to carry this responsibility and as a result had to expand its facilities considerably. In May 1942, Section T was divorced from the NDRC and was placed directly under the OSRD [Office of Scientific Research and Development], becoming Section T, OSRD. At this time Section T also was moved from the Department of Terrestrial Magnetism of the Carnegie Institution of Washington, which did not have adequate facilities for such an expansion, and was set up in a new laboratory in Silver Spring, Maryland, under The Johns Hopkins University which took a contract with the OSRD to carry the proximity fuze work. This laboratory became known as the Applied Physics Laboratory of The Johns Hopkins University.

As the second priority for proximity fuze development was an antiaircraft fuze for British Navy projectiles, work had been underway since the summer of 1942 for providing a fuze for this use. As British projectiles were smaller in diameter at the nose end where the fuze was contained, this problem amounted primarily to that of shrinking down the size of the Mk 32 fuze to a smaller diameter.

In the original British requirement, it was intended to include in the fuze an adjustable self-destruction mechanism in order to avoid having duds fall back down around friendly installations. For this purpose, design was started on a mechanical clock which incorporated this feature. As a result of this requirement, the original mechanical design of the British fuze, which was finally termed the Mk 33, was somewhat different from the mechanical design of the Mk 32 although the fundamental assembly of the various parts was pretty similar to that of the Mk 32. This clock development did not progress very rapidly and consequently it was finally abandoned and the British Mk 33 fuze was produced without this feature being included.

In general, all work on the British fuze paralleled the work on the Navy's Mk 32 fuze. About the fall of 1942 a contract was placed with the Radio Corporation of America for production of these fuzes, and shortly after the first of the year 1943, Eastman Kodak Company was also brought into the program on this fuze. Early work on the Mk 33 was rather unsatisfactory and although production was carried along at a small rate, acceptable material was not available for sometime. In about May 1943 an emergency program was set up to iron out the remaining difficulties in the Mk 33 fuze with the hope of obtaining satisfactory material before the end of the summer of 1943. By September of that year the fuze was in fairly satisfactory production and deliveries were commenced to the British. This fuze was designed specifically for the British 4".5 gun which was carried aboard aircraft carriers. In addition, it was contemplated that the fuze would also work in the British 5".25 Navy gun, but because of more severe treatment of the fuze in this gun, the fuze was not at that time satisfactory for use in the 5".25 British gun.

In addition to the development of the Mk 33 fuze for the British, another fuze known as the Mk 41 was also produced. This latter fuze was designed primarily for the British 4" gun carried aboard destroyers, and differed from the Mk 33 in that its size was still smaller. This was necessary because the 4" projectile was too small to accommodate the Mk 33 fuze and still leave sufficient quantity of explosive. The design of the fuze was more or less similar to that of the Mk 33 except that the mechanical clock rear fitting safety device was replaced by a newer fitting which contained a mercury switch to provide the arming delay. Likewise, the firing condenser which provided the RC electrical delay in the firing circuit of the thyratron and which had been mounted in a block down with the mechanical clock rear fitting had to be placed up in the amplifier section of this fuze.

A second difference between these fuzes and the U. S. Navy Mk 32 was that the dry battery which was used in the Mk 32 fuze could not be made small enough to fit into the Mk 33 and Mk 41 fuzes and still retain adequate life characteristics. Such a small dry battery was produced and used experimentally, but its shelf life was not greater than two or three months, which was entirely inadequate for this fuze.

Accordingly, along with fuze development for the British, work was also started on a new type of battery known as the reserve battery which was a wet battery containing the active electrolyte in a glass ampoule [small bulbous glass vessel hermetically sealed] within the battery which did not permit the battery to become activated until that ampoule had been broken at setback and the electrolyte had been distributed over the battery plates. This development was carried out at the National Carbon Company under supervision of Section T, OSRD. One of the principal problems in the development of this battery was to provide a means of supporting the ampoule so that the battery would be sufficiently rugged for normal handling and yet permit the ampoule to break on setback. This was ultimately accomplished by means of various sorts of breaker mechanisms mounted below the battery ampoule.

The development of the Mk 41 fuze was also somewhat later in being accomplished than that of the Mk 33 fuze because the British 4" gun for which this fuze was designed subjected the fuze to appreciably higher setback than did the other British guns. Mechanical difficulties in this fuze were finally overcome by November 1943, and these fuzes were also supplied to the British.

Among the various problems peculiar to the fuze development for the British were the following: The fuze was originally designed so that it depended to a considerable extent upon the potting medium, namely, cerese wax for support of the oscillator bundle. This seemed to work quite satisfactorily in cold weather when the wax was hard, but under high temperatures the wax softened enough so that this support was inadequate. This was overcome by rigidly fastening the oscillator assembly to the plastic nose. The rear fitting safety device used in the Mk 33 fuze, which was essentially a scaled-down version of the Mk 1 safety device used in the Mk 32, did not operate at the projectile spins encountered. Testing of these rear fittings had all been done at higher spins than were encountered in projectiles and did not show up this trouble. Later when the failure was discovered, a slight modification of the fittings and a change in testing procedure to check the units at spins comparable to projectile spins overcame this trouble. Raytheon tubes were used in the models built by Eastman Kodak Company and were giving considerable trouble. This was overcome by improvements in supporting of the tube elements in the micas, by improved welding techniques, and by other similar modifications. It was also discovered that upon being fired, the soldered connection between the antenna cap of the fuze and the radio circuit frequently melted permitting the connection to separate, causing intermittent connections which resulted in prematures. It was subsequently learned, although not at this time fully appreciated, that this melting was caused by generation of heat from air friction as the projectile traveled through the air. At this tine a procedure for waterproofing this fuze was tried which consisted of coating the fuze nose with a wax made of a mixture of cerese wax and vistanex. This wax coating protected the solder joint from heat sufficiently so that this trouble was overcome.

Throughout this entire period of development, work was also underway on reducing the size of the fuze still further. By the spring of 1943 a new model, smaller than the Mk 41, had been designed and work was underway toward proving it in for ultimate use. This new smaller fuze, which was later termed the Mk 45, appeared to be small enough and simple enough to manufacture to meet the requirements of use in Army field artillery. Consequently the Army began to become quite interested in using this fuze for obtaining air bursts against personnel, etc. with howitzers in addition to use as an antiaircraft weapon. By September 1943 successful tests had been achieved with this fuze and it was started in production at the Crosley Corporation.

The first model being produced was a model for the Army 90mm antiaircraft gun. In addition, models were being developed for use in all various Army howitzers. Production facilities were being expanded in order to produce the enormous quantities required for such Army uses and quantity production eventually got underway on these various models of the Mark 45. Up until this time, because of security considerations, it had been decided that proximity fuzes should not be used where there was any chance of a dud falling on enemy territory and being recovered by the enemy. Consequently, stock piles were being accumulated with the intent of committing these fuzes to use at some future opportune tine. Finally, in December 1944, a decision was reached to release proximity fuzes for general unrestricted use and these fuzes were committed to use in Europe with outstanding effectiveness.

In parallel with all these developments on the British and the Army fuzes, continual refinements were being made in U. S. Navy fuzes. One of the very first problems to become evident on the U. S. Navy Mk 32 fuze was that its life in storage was inadequate. Most of this life failure resulted from the fact that the dry batteries used in this fuze deteriorated after several months in storage under hot, tropical conditions. Consequently, steps were taken to use the so-called reserve battery which has a more permanent life in this particular fuze. In about May 1944, Mk 32 fuzes including the reserve battery were made ready for delivery to the Fleet.

In addition to this life problem, difficulties also became apparent in the use of these fuzes against torpedo plane attacks low over waves. It was apparent that signals returned back from waves to the fuze when the fuze was fired at low trajectories could also trigger the fuze prematurely and under certain conditions could even prevent the fuze from arming at all. To overcome this problem, a circuit was devised known as the wave suppression feature to reduce the sensitivity of the fuze to these spurious wave noises. This circuit, which is in effect an automatic volume control (or AVC) circuit, was incorporated in the Mk 32 fuze. Mk 32 fuzes incorporating this feature were produced during May 1944 and delivered to the Fleet. The first fuzes including AVC were made with the original type of dry battery subsequent to this, a new model fuze known as the Mk 40 was supplied which incorporated both AVC and the reserve battery.

As development of the Mk 45 fuze progressed, it became apparent that would be desirable to utilize this fundamental design in certain of the U. S. Navy fuzes. This fuze was the first fuze small enough to be included in the U. S. Navy 3"/50 projectile. Consequently, a Mk 45 fuze was produced for the 3"/50 in about May of 1944. This fuze was delivered to the Fleet, but was never very satisfactory and its production was ultimately discontinued. A new fuze, known as the Mk 58, was designed for the 3"/50 which contained more or less the standard Mk 45 design with the addition of a wave suppression feature to permit use of this fuze low over waves. The Mk 58 fuze was delivered to the Fleet in November 1944.

Likewise, work was being carried out toward adapting the Mk 45 design fuze to the U. S. Navy 5"/38. Such a fuze, known as the Mk 53, was finally developed for the 5"/38. When production of this fuze was commenced in about August 1944, considerable difficulty was experienced with melting of the solder on the nose cap similar to that trouble encountered on the British Mk 33 and Mk 41 fuzes. On this fuze, no water-proofing wax could be used because of difficulties of use of such wax in fuze pots, etc. Consequently, the nose connection failure could not be cured in such a simple fashion. A number of schemes were devised to overcome this difficulty. One method which enjoyed only moderate success was to cement a plastic cap over the external tip of the fuze nose, thus covering the soldered joint and insulating it from heat. Another scheme which was worked out was to use a welded connection at that joint. This seemed like a straight-forward solution, but welding techniques which would not damage the plastic had to be devised. A third method which has been the most successful was to devise an oscillator circuit which did not use an antenna cap but contained only a small wire loop, entirely enclosed in the plastic nose, for the antenna. This model was known as the "capless model" and that design principle-was later extended to practically all of the other fuzes. This so-called "capless" design was very satisfactory but had some limitations with regard to frequency ranges which can be achieved.

Although production of the British Mk 33 and Mk 41 fuzes was terminated early in 1944, it was contemplated that British requirements would again need to be met along toward the end of 1944. The British were particularly anxious to get new models of fuzes incorporating the refinements included in U. S. Navy fuzes and particularly the AVC wave protection. Consequently, development was also undertaken on two more fuzes known as the Mk 56 and the Mk 60 which were modifications of the fundamental Mk 45 design and were intended specifically for the British Navy guns. The Mk 56 was to be used in the British Navy 4" and 5".25 guns and the Mk 60 to be used in the British Navy 4" guns and some of the 4".7 guns. These fuzes were essentially similar to the Mk 53 and the Mk 58 U. S. Navy fuzes and the development program here amounted primarily to am extension of the development on the corresponding Navy type. The Mk 56 fuze was finally produced and delivered in the fall of 1944. This fuze was also plagued with the nose cap solder-melting failure of the original Mk 53 and in the British case first attempts at curing this were made by using the cerese and vistanex wax coating and later by trying cemented-on plastic caps. The Mk 60 fuze was delivered to the British in the so-called "capless" design and hence was free of this difficulty.

Additional Navy developments led to the Mk 47 fuze and the Mk 59 fuze. Both of these were again extensions of the basic Mk 53 design to a different caliber projectile. The Mk 47 was designed for the U. S. Navy 6"/47 gun and differed from the Mk 53 only in its circuit which had to be modified slightly from the Mk 53 to match the different projectile size. The Mk 59 fuze was designed specifically for the U. S. Navy 5"/54 projectile and again differed from the Mk 53 only by a slightly different contour and a somewhat different rear fitting safety device to permit operation at the 5"/54 projectile spin.

After the development of the Mk 45 and the extension of that design to all other types of projectiles, the primary differences in the different types of fuzes lay in different circuit arrangements necessitated by different projectile sizes and in different operating characteristics of the rear fitting safety device to accommodate different projectile spins. In addition, there were two distinct types of batteries used in all these fuzes, One being an AA [Anti Aircraft] battery, the other being a howitzer battery. The difference here was that the howitzer battery had to be capable of being activated at considerably lower setbacks than was required in the AA type of fuze. Later on, battery development progressed to the point where a universal battery was available that was sufficiently rugged for handling in any projectile, and yet capable of having its ampoule broken and the battery being activated at even the low charges of the Army howitzers.

Present Status

At the present time the U. S. Navy has radio proximity projectile fuzes for the 5"/38, 6"/47, 5"/54 and 3"/50 guns. These fuzes are all basically the same design, differing only in the minor modifications required to adapt then to the different projectile sizes. All are primarily antiaircraft fuzes although they may be used for shore bombardment to produce air bursts. The 5"/38 fuze is the only one that has seem very extensive use in service, but the effectiveness of the other Navy fuzes is probably comparable to that of the 5"/38 fuze. Analyses of action reports indicate that the effectiveness of radio proximity fuzes as compared to mechanical time fuzes in antiaircraft fire is about the order of a 3-to-1 improvement for the proximity fuzes. These effectiveness figures, of course, are dependent upon such matters as fire control accuracy, etc. and represent the effectiveness obtained with presently used equipment.

The U. S. Army has radio proximity projectile fuzes for the Army 90mm gun, the 75mm, 105mm, 155mm, 8", and 240mm howitzers. Models have been developed for the 120mm gun, 155mm gun, and 75mm AA gun. The AA guns have AA fuzes incorporating a self-destruction feature. The howitzers all have fuzes which are very effective for producing air bursts.

The British Navy has been supplied with AA fuzes for its principal AA guns. The British Army has been supplied with AA fuzes for its 3".7 AA gun and with artillery fuzes for its principal howitzers.

The principal weakness of all of the U. S. Navy fuzes is that their life in storage is not adequate. Replacement of the original dry battery used in the Mk 32 fuze by the reserve battery has overcome the weaknesses in life characteristics caused by battery deterioration. However, the fuze itself deteriorates because of inadequate protection against moisture and humidity, particularly under tropical storage conditions. This failure is pretty definitely pinned down to deterioration of condensers used in the electrical circuits, resulting from entrance of moisture. A primary job now underway is that of improving fuze waterproofing in order to overcome this source of life failure. A second weakness in these fuses is that the AVC or wave suppression feature is not as completely successful as would be desired. The present AVC fuzes do permit use moderately low over waves, but further improvements in wave protection are desirable. A third feature of these fuzes which needs to be improved is that the present rear fitting safety device incorporating the mercury switch rear fitting is very dependent upon projectile spins for operating time limits; and hence, none of the present rear fittings can be used in different projectiles at widely different projectile spins. This requires a special rear fitting for each particular use and also prevents one fitting in one fuze being used in a single projectile at different charges; for example, the 5"/38 at service or reduced charges. Likewise, the problem of holding manufacturing tolerances is quite critical. Overall performance scores on these fuzes, when new, average from 70% to 80%, but premature failures are still higher than would be desirable.

Production on U. S. Army fuzes has now been terminated. However, considerable developments are still underway. The Army has on hand a large backlog of current types of fuzes which are usable but possess some inherent limitations which these new developments hope to overcome. One of the principal limitations of U. S. Army fuzes is that there is no positive means of preventing prematures from occurring over the heads of Army troops. Likewise, air observation planes are in constant danger when proximity-fuzes are fired near them. These fuzes, likewise, are all relatively vulnerable to enemy countermeasures. While this failure has not been encountered yet, it no doubt will in the future and considerable work is underway toward improving the protection of these fuzes against enemy countermeasures. Some models were produced which were relatively less vulnerable than the first models, but even these are still more vulnerable than they should be.

All work has been terminated on fuzes for the British. The British themselves are undertaking considerable work on proximity fuzes of their own, but if the U. S. Navy should again undertake development of proximity fuzes for British projectiles, these developments would closely parallel developments in U. S. Navy projectiles.

From the standpoint of production, one of the primary problems is that very careful and extensive quality control procedures are necessary to maintain satisfactory fuze quality. This is true because present manufacturing specifications, laboratory tests, etc. are not adequate to guarantee satisfactory fuze performance in service use. In addition to all laboratory tests, actual firing tests are relied upon to determine fuze performance; but it has not yet been possible to --completely simulate all aspects of service conditions in laboratory, handling and stowage, or firing tests. As a result, it is necessary to exercise very careful control over component quality, and general manufacturing quality, in addition to the meeting of all manufacturing specifications and tests, in order to assure fuze quality.

Because of these difficulties, fuzes have thus far been procured on a basis of payment for all fuzes produced in accordance with the present specifications, regardless of whether ultimate service performance could be guaranteed, and quality has been maintained by careful follow-up of the quality control procedures. These quality control procedures have thus far been carried out by the Applied Physics Laboratory, The Johns Hopkins University, but steps have been taken to transfer this responsibility plus responsibility for recommending acceptance for service use of fuzes to the Naval Ordnance Laboratory. In addition, work is also underway to devise specifications which will guarantee ultimate fuze performance, so that the responsibility for producing fuzes satisfactory for ultimate service use can be placed upon the manufacturer.

Note: British gun bore usage places the decimal directly under the inch symbol, a combination not readily available. In the above text the combination, as in 4".5 gun, will be used to most closely approximate the original.

Source: Dilley, N. E. "Development of Proximity Fuzes (VT) for Projectiles - VT Fuzes MKS 32 to 60, Inclusive (General Description)." chapter 1 of The World War II Proximity Fuze: A Compilation of Naval Ordnance Reports by the Johns Hopkins University Applied Physics Laboratory. (Silver Spring MD: The Laboratory, 1950): 1-12. [declassified 16 Jun. 1976].


Published: Mon Nov 06 15:46:39 EST 2017