Country of Origin: United States of America
Other: 2 1/4 in. diameter x 8 in. tall (5.7 x 20.3cm)
Cap - plastic, body - metal, elecgtronics and glass components
Case - steel
This proximity fuze is typical of those used in artillery shells in the later stages of World War II. The casing of this Mark 58 fuze has been partly cut away to display the microelectronics in the interior, and classified components have been removed, per the label on the stem. This proximity fuze was donated to NASM by Dr. James Van Allen and the University of Iowa in April, 1993.
The proximity fuzes developed in World War II markedly increased the effectiveness of artillery by triggering the explosion of the shell by its proximity to the target. This was accomplished by including a tiny radar-like radio sender-receiver in the fuze. This device depended, in those days before solid state electronics, on the availability of rugged miniaturized vacuum tubes. In 1942 James Van Allen joined the Applied Physics Laboratory of Johns Hopkins University (APL) where he helped develop the required tubes. In 1955, after he had returned to Iowa, he designed the scientific instrumentation that eventually flew on America's first successful satellite, Explorer 1. The instruments in those early spacecraft had to meet some of the same ruggedness and size requirements as those in proximity fuzes. Their design consequently drew on the knowledge that had been gained from designing the microelectronics for the fuzes. Data obtained from Explorer 1 and later satellites led to the discovery of a zone of intense radiation that surrounds the earth that was later named the Van Allen belt.
Gift of James A. Van Allen and the University of Iowa.
DEPARTMENT OF THE NAVY -- NAVAL HISTORICAL CENTER
805 KIDDER BREESE SE -- WASHINGTON NAVY YARD
WASHINGTON DC 20374-5060
Radio Proximty (VT) Fuzes
Radio Proximity (VT) Fuzes and How They Operate
Significance and Background of the Radio Proximity Fuze (VT) in World War II
Development of Proximity Fuzes (VT) for Projectiles - VT Fuzes MKS 32 to 60
Image of Mark 53 VT Fuse
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 am