Proximity Fuzes: A Challenge to Air Power

by JAMES PHINNEY BAXTER

1

IN anti-aircraft fire the purpose of the fuze is to explode the projectile at a point where the maximum number of lethal fragments will pass through the target. Any duck hunter who studied this problem soon reached the conclusion that the best fire-control devices, no matter how accurate their tracking and how superhuman their computers, left much to be desired. In 1940 it was generally estimated that good anti-aircraft brought down one plane for every 2500 rounds. This degree of inaccuracy resulted from poor range finding rather than poor aim, for the existing fire-control systems gave good results in calculating the angles of fire. Both optical and radar range finders, on the other hand, contained a lag which could be diminished but could not be eliminated. The most optimistic of gunners dared not hope for a direct hit with a shell fuzed to burst on contact.

Reliance had to be placed on time-fuzed shells. If these functioned perfectly the shell would burst at an instant when the plane would be within the cone of hurtling fragments. This if, however, remained very big. Even if the range had been estimated correctly, there remained several possibilities of error: first, in the manufacture of the fuze; second, in its manual or automatic setting; and third, in the estimated allowance for “dead time,” which is the interval between the instant the fuze is set and the instant of firing. In practice the explosion might occur anywhere along a thousand feet of shell path, so that, even if all other factors were favorable, the chances of a hit were few.

The stakes of the game were high indeed. The success of the German campaign in Norway had shown what land-based air power could do to checkmate sea power. Advocates of the plane against the capital ship crowed, “I told you so.” The dive bombers roared down from the skies of Holland, Belgium, and France, opening the roads to Dunkirk and Paris. Unless some improved methods could be found to knock them out of the skies, the mobility of fleets and the security of ground forces would be seriously threatened.

Fire-control methods could, as things turned out, be greatly improved, but not to the point desired. The obvious answer to the problem was a fuze operated not by time but by proximity to the target. It was easy enough to see what needed to be done, but incredibly difficult to find the ways and means. The only successful development of proximity fuzes for shells was American, sponsored by the Office of Scientific Research and Development. Except for the development of the atomic bomb, this constitutes perhaps the most remarkable scientific achievement of the war.

When the National Defense Research Committee was established in 1940, the problem of proximity fuzes had already been under consideration for some time in the United States Navy, whose Council for Research was then headed by Rear Admiral Harold G. Bowen, the first Naval officer to serve as a member of the NDRC. It was clear that the airplane constituted a growing threat both to surface ships and to ground forces and installations. Late in July, Charles C. Lauritsen of the NDRC learned that the Western Electric Company and the Radio Corporation of America were manufacturing 20,000 miniature tubes for the British Army. He drew the correct inference that they were desired for experiments with proximity fuzes.

On August 12, Lauritsen and Richard C. Tolman, also of the NDRC, conferred with Commander Gilbert C. Hoover of the Bureau of Ordnance as to work which their division might undertake for the Navy. Radio and other proximity fuzes were discussed. Captain (now Vice-Admiral) W.H.P. Blandy, Chief of the Bureau of Ordnance, was from the first interested in this problem. After further conferences with officials of the Bureau of Ordnance, the first research contract drawn up by the new agency was concluded on an actual-cost basis between the NDRC and the Carnegie Institution of Washington, for “preliminary experimental studies on new ordnance devices” to be undertaken at the Institution’s Department of Terrestrial Magnetism. The DTM became the base of operations of Section T of the NDRC, whose chief, Dr. Merle A. Tuve, had performed with Dr. Gregory Breit in 1925 the celebrated measurements of the height of the ionosphere which led to important consequences in the development of radar.

In September, 1940, it was revealed to the American investigators that the British, in anticipation of air attacks, had been working on proximity fuzes for bombs to be dropped on hostile aircraft by interceptor planes. The British were also working on proximity fuzes for ground-to-plane rockets, and had considered their use as a plane-to-plane weapon. They were experimenting with various types of fuzes, but regarded radio proximity fuzes as the best of all, both for rockets and for bombs. They had thus far, however, had little success, and they deemed the shell problem extremely difficult.

2

IN the United States, as in Great Britain, the early emphasis was on the development of proximity fuzes for bombs and rockets, rather than for shells. A wide range of approaches was considered. The Bureau of Ordnance stressed the fact that unless the triggering pattern were properly related to the fragmentation pattern so that the target was included within the cone of fragments, the fuze would have no value for combat and would simply, in Captain G. L. Schuyler’s phrase, be “the world’s most complicated form of self-destroying ammunition.”

During the first year much of the extensive research work was spent on the elimination of ideas and projects which were shown by laboratory and field tests to be impracticable for quick military application. This was true of the extensive studies of both acoustic and electrostatic fuzes.

Meanwhile the work of Section T, under increasing pressure from the Navy, shifted more and more to the radio fuze for shells. It would be hard to overstate the difficulties of the problem. Open the ordinary radio set on your table and try to imagine how you would fit it, equipped with a power plant and a transmitter as well as a receiver, into the nose of a 5-inch shell in a space about the size of an ice-cream cone. Remember the “setback” or translational force exerted on you when you are standing in a bus which starts suddenly. A bomb dropped from an airplane starts gently enough on its descent. When a rocket is launched, its various components are subject to an initial acceleration of some hundred times the force of gravity (100g). The translational force applied in firing to an anti-aircraft shell, on the other hand, is approximately 20,000 times the force of gravity. An electronic tube weighing less than an ounce must be subjected to a force of over 1000 pounds during the acceleration of the shell in the gun. Most conventional tubes available in 1940 gave a high percentage of failures when accelerated merely to 50 times gravity.

To make the story complete, picture to yourself the immense centrifugal force applied to the radio set when the shell spins at speeds as high as 475 rotations per second. It is not simply a matter of producing tiny tubes whose glass or metal envelopes will not break when subjected to these tremendous forces; their delicate cathodes, plates, and grids cannot be thrown out of alignment without impairing or destroying the performance of the tube.

To meet the Navy requirements, the fuze would have to be sensitive and rapid in operation, but not subject to being triggered by the passage of other shells or by the reflection of radio waves from ground, water, and clouds. It must also be safe to handle and not subject to serious deterioration in storage. Somehow, by some miracle of design supplemented by another miracle of production technique, a radio set compact enough to fit in a shell must be made rugged enough to stand these tremendous accelerations, and it must be manufactured in large quantities.

The first requisite, as Tuve put it to his fellow workers, was to conquer their fear of the gun. The first tests of the ruggedness of some existing vacuum tubes proved surprisingly encouraging. A .22-caliber bullet fired into a lead block, on which was mounted a standard small tube, produced approximately 5000g without damaging the tube. Several types of miniature radio and hearing-aid tubes, mounted in wax, were dropped from the roof of the three-story Cyclotron building at the Department of Terrestrial Magnetism to the concrete driveway below, with less damage than had been expected. Others were tested in centrifuges, and dropped in steel containers against lead and steel blocks. The experimenters then constructed a homemade smoothbore gun out of steel tubing and fired electronic components from it. Contracts for the development of rugged tubes were placed with the Western Electric Company, the Raytheon Company, and the Hytron Corporation. By February of 1941 three types of miniature vacuum tubes had been developed which were rugged enough to withstand firing inside a 5-inch star shell.

For testing these improved handmade electronic components, Section T acquired from the Navy some 37 - and 57-millimeter guns and the use of a field at Stump Neck, Maryland, until it could obtain a testing field of its own at Newtown Neck. At first the projectiles were equipped with parachutes which opened after firing. When these proved unsatisfactory, the shells were fired vertically so that they would land base down and could be recovered by digging.

The first experiments were confined to testing an oscillator alone (radiosonde), to see if it could withstand the acceleration of a 37-millimeter naval gun. When fired vertically at Vienna, Virginia, on April 20, 1941, it could be followed in flight by means of a radio receiver.

The fact that they could still hear the radio signal after the shell landed puzzled the listeners. The matter was cleared up within ten days by the discovery that the ruggedness of the elements tested and the sensitiveness of the receiver were such that the oscillator could be heard half-buried in the ground. When seven 5-inch shells were fired at full velocity at Dahlgren, Virginia, on May 8, over water, Tolman, Lauritsen, and Tuve, listening in a boat, were able to hear signals from two of them as they passed overhead.

The success of these tests, and the progress being made in designing successful circuits, led the late Captain S. R. Shumaker, then Director of the Research and Development Division, Bureau of Ordnance, to request that Section T place a first priority on the development of a radio proximity fuze for shells.

Meanwhile, in November, 1940, Dr. Alexander H. Ellett of the University of Iowa had come to Washington at Tolman’s request, and, after working for ten days with Tuve’s group, had organized, at Tuve’s suggestion, another NDRC group to work on proximity fuzes for bombs and rockets at the Bureau of Standards. This group started work on both radio and acoustic fuzes, but dropped the latter in April.

The development of a radio proximity fuze for bombs at the Bureau of Standards had meanwhile progressed rapidly. The first model, fixed at the top of an 84-foot radio tower at the field station of the Bureau of Standards at Camp Springs, Maryland, indicated the passage of planes which came within 50 feet of the fuze. Other tests were conducted with the model carried in tethered meteorological balloons which were then shot down with a rifle. At the Naval Air Station at Lakehurst, New Jersey, fuze models were suspended beneath a blimp and the wave-form of the fuze signal as shown on an oscillograph was photographed as fighter planes dived past. After two earlier tests in February and March at the Naval Proving Ground at Dahlgren, six bombs containing fuzes of a new model were released separately on April 26, 1941, by a plane flying at 3000 feet. All six functioned properly at heights of from 150 to 300 feet over the water, which corresponded closely to their predicted performance as indicated by the previous laboratory tests of the responsiveness of the fuze. A few weeks after this outstanding success, work began on similar fuzes for rockets.

By May, 1941, Section T had achieved a basic design for a radio proximity fuze for shells, which separated the components into an oscillator and amplifier circuit, battery, safety device, and detonator. The pressure from the Navy for this fuze had grown so great that it was decided to confine all of Section T’s attention to it, and to transfer all work on fuzes for bombs and rockets to Ellett’s section. L. R. Hafstad’s group, working on photoelectric fuzes, consequently moved from the Department of Terrestrial Magnetism to the Bureau of Standards in July. By October the photoelectric fuze they had developed for rockets was superior in many respects to their successful fuze of that type for bombs. They had been aided in both developments by information concerning previous British efforts.

The transfer of its own work on fuzes for bombs and rockets to Ellett’s section at the Bureau of Standards enabled Tuve’s section to drive ahead faster with the program of radio proximity fuzes for shells. It gave them more space for their work at the Department of Terrestrial Magnetism, just at a time when progress in the development of rugged components permitted their project to gain speed.

3

IT WAS one thing for a physicist or an engineer to produce a handmade tube at a laboratory bench, and another thing to teach unskilled women the art of assembling small, rugged tubes on a pilot line or large-scale assembly line. Since the circuits called for four or five tubes, and the failure of any one would destroy the usefulness of the whole system, a standard of 98 per cent performance was indicated. The first attempts to produce the tubes by assembly-line techniques at Sylvania led to a marked drop below this standard, till a campaign of instruction and inspection raised the quality to the required level.

This story repeated itself again and again in the production of VT (“variable time”) fuzes, indicating the necessity of follow-through from the laboratory to the last step in large-scale manufacture. No matter how perfect the laboratory models might be, the whole effort would have come to naught without mass production of a usable device. And never, perhaps, in the history of assembly-line methods have the standards of performance been more difficult to meet.

These requirements of extreme ruggedness applied not only to vacuum tubes but to batteries, condensers, resistors, and the setback switches which kept the dry-battery voltages from the components until the shell was fired. It proved necessary to assign a large part of the staff of the section to the problem of developing suitable batteries and to maintaining satisfactory standards of quality once they were placed in production. The first approach was to develop a miniature dry battery able to stand the shock of being fired from a gun. The first completely assembled batteries, in essentially the same form still used for the Navy’s Mark 32 fuze, were delivered on June 20, 1941.

Two limitations on the effectiveness of this battery forced Section T and the National Carbon Company to a more radical approach to the problem. The first was its short shelf-life, limited to six to twelve months under normal conditions or to three or four in the South Pacific. The second was the difficulty in making dry batteries still smaller for use in smaller shells. Extraordinary ingenuity and long periods of experimentation were required to overcome these major difficulties.

Long shelf-life, possibly reckoned in terms of years, could be obtained if one could eliminate the drybattery “mix” and substitute a liquid electrolyte stored in a suitable container, such as a glass ampoule, to be released at the time of use. The problem was to get an ampoule strong enough to withstand the shocks incident to normal handling of the fuze, yet not so strong that it would fail to break when the shell was fired. The spin of the rotating shell would spread the electrolyte after the container was broken, and the short delay before the battery began to deliver power would constitute an additional safety factor. The road to success was almost as long and difficult as that leading to satisfactory rugged tubes, but the efforts of National Carbon Company engineers and production men and Section T personnel were at last crowned with success, both for 2-inch and 1.5-inch models. Special studies were required to find an electrolyte satisfactory at low temperatures.

When the first complete fuzes were tested over water at Dahlgren during the late summer, much trouble arose from prematures and duds. This difficulty kept Section T members under heavy pressure throughout the autumn, during which the Erwood Company of Chicago undertook the production of experimental models of fuzes and components, and Sylvania and RCA were brought into the tube program. In November, 1941, Captain Shumaker concluded a development contract with the Crosley Corporation for pilot production of radio proximity fuzes and for preparation for fullscale production.

4

WHEN the Japanese flyers scored nineteen torpedo hits on our stricken fleet at Pearl Harbor and two days later sank the Prince of Wales and the Repulse, the sense of urgency which spurred the members of Section T increased. The fuze, which its advocates hoped would improve anti-aircraft fire certainly by a factor of 3, and possibly by a factor of 30, was desperately needed. On December 10 and 11 the Navy renewed the pressure on the NDRC to expedite to the utmost the work of Section T. Already priorities of need had been established for these fuzes, which put the United States Navy first, the British Navy second, the United States Army third, and the British Army fourth.

Captain Shumaker had set the goal of a 50 per cent score at a Dahlgren firing test, using material manufactured by pilot plant workers, as the signal for full approval for large-scale production. His conditions were fully met in tests held on January 29, 1942, when 52 per cent of the units assembled with Erwood components functioned properly. Shortly afterwards the Navy committed $80,000,000 for manufacture of these fuzes. (Up to this time the research undertaken by Section T had cost a little over $1,000,000.)

Now that the Navy had decided on full-scale production, requiring a large extension of Section T’s efforts to follow through from the laboratory bench to the last stage of manufacture, Tuve insisted that the Section be expanded and moved to quarters larger than those available at the Department of Terrestrial Magnetism. It was decided to detach Section T from Division A, place it directly under the Office of Scientific Research and Development, taper off the contract with the Carnegie Institution of Washington, and conclude a new management contract with the Johns Hopkins University, as of March 10, 1942.

Under the new arrangement the Bureau of Ordnance transferred $2,000,000 to the OSRD for expansion of the research on proximity fuzes for shells and assigned Commander (now Rear Admiral) W. S. Parsons, USN, to act as special assistant to Dr. Vannevar Bush in charge of Section T activities. Tuve continued as Chairman of the Section, reporting to Bush through Parsons, with Hafstad as vice-chairman. D. Luke Hopkins of Baltimore, vice-president of the Maryland Trust Company and a trustee of the Johns Hopkins University, became the representative of the University to supervise the administrative details of the contract.

The staff of the Applied Physics Laboratory they created at Silver Spring, Maryland, grew from fewer than 100 in April, 1942, to over 700 two years later. Tuve, Hafstad, Hopkins, and Parsons — and later his relief, Commander (now Captain) C. L. Tyler, USN — constituted one of the ablest and smoothest-working teams that ever sought to translate new scientific ideas into mass-produced devices for combat use. Their drive, enthusiasm, and ability to inculcate team play, secrecy, and standards of highest quality pervaded not merely the central laboratory but the fifty allied establishments, academic and industrial, that shared in this great work.

The Navy-OSRD-Johns Hopkins team enjoyed many advantages. It was able to attract wellqualified technical men for the work, and to maintain flexibility of assignments within the group, thanks to freedom from Civil Service requirements. Purchase of technical equipment and materials could be made without the delays inherent in government purchasing methods. The Bureau of Ordnance not only furnished funds and distinguished personnel but permitted prompt and full access to all necessary information. Thus the technical staff of Section T was kept currently posted as to the immediate needs of the Fleet.

A milestone in ordnance history was passed in April, 1942, at the Marine Corps base at Parris Island, South Carolina. Here a Taylor cub plane covered with aluminum gauze was suspended from a Navy kite balloon in such a way that it would swing with the wind through an arc of perhaps 100 feet in the course of a minute. Against this target were fired 182 5-inch Navy shells with reduced charges equipped with standard VT fuzes made by factory methods. These tests, under conditions approximating service use, were highly encouraging. But before more elaborate tests could be undertaken from a ship at sea, much hard work was required on safety devices, over and above those normally provided for the conventional time and contact fuzes.

A single burst of a VT-fuzed shell close to the muzzle in the early days of the project would quite likely have meant the end of the research as well as the death of some or all of the gun crew. Tuve figured that if personnel and equipment were to be reasonably safe, muzzle bursts should not occur more than once in a million rounds. To get this degree of safety, the fuze had to be provided with both mechanical and electrical safety devices, all subject to severe limitations of space and requiring a high degree of ruggedness.

The safety devices, incorporated in a separate unit known as the rear fitting, went through a long and difficult evolution. The natural attack was to start with the standard clockwork time fuze, and to adapt parts of it to the VT fuze. This was done with shells, but the clockwork mechanism proved too bulky for smaller projectiles. For these a switch was eventually developed consisting of two chambers separated by a porous diaphragm. In the inner chamber, mercury maintained an electric short which acted like the safety mechanism of a revolver. When the shell was fired, the spin of the projectile forced the mercury out of the inner chamber through the porous diaphragm into the outer chamber, removing the short so that the primer could fire. By this means the fuze was “armed,” just as a pistol is made ready to fire by cocking. A wealth of ingenuity was lavished on the device, which represented, in its later models, the highest degree of control yet achieved in the field of powder metallurgy.

A further safety device is incorporated into the auxiliary detonator at the base of each VT fuze, in which the rotation of the shell is relied on to move misaligned explosive charges into alignment. Another highly ingenious device, known as the reed spin switch, serves as a safety device prior to firing the gun, and provides a means of self-destruction for the projectiles that miss their target. A vast amount of effort on these safety features was expended by the personnel of Section T and of various subcontractors — and to such good purpose that the VT fuzes have been the safest ever furnished the armed services.

Satisfactory progress with safety devices paved the way for the first tests of the VT shell fuzes under conditions essentially like those of battle. These took place on the cruiser Cleveland in Chesapeake Bay on August 10 and 11, 1942, with spectacular results. Shells fitted with fuzes produced by Crosley, Sylvania, and Erwood were fired against drones (radio-controlled target planes) which were knocked down one after another with an expenditure of very few shells. These results gave a great impetus to the work, and increased the desire of the Navy for fuze deliveries in quantity at the earliest possible moment.

By September, production had reached 400 per day. As rapidly as possible 5000 rounds of VTfuzed ammunition were accumulated at the Mare Island Ammunition Depot, from which samples were flown daily aeross the country to Dahlgren to make sure that nothing had gone wrong in transit and loading. By the middle of November, 4500 shells were on their way across the Pacific, and by Admiral Halsey’s orders they were distributed at Nouméa to the ships most likely to see early action.

5

ON January 5, 1943, four Aichi 99 dive bombers attacked an American task force, scoring two near misses and a hit on a cruiser. One pilot, thinking he was outside effective anti-aircraft range, flew in a straight path long enough to give the Helena’s after 5-inch battery a perfect setup. Two twin mounts opened fire with VT fuzes, and on the second salvo the Japanese plane crashed in flames.

From that moment a great increase in the safety and mobility of our sea forces was assured, provided Section T and its contractors and the companies working on Navy production contracts could solve the problems of large-scale manufacture. Here American industry came through as superbly as in the production of radar and the atomic bomb. Before the war began, the nation’s entire tube output amounted to no more than 600,000 a day. Before it ended, Sylvania alone, which was producing 95 per cent of the miniature rugged tubes, was turning out more than 400,000 a day.

At the peak, over 10,000 persons were engaged in rugged-tube production. Every tube manufactured was spun in a centrifuge to an acceleration of 20,000g, and hundreds of thousands of them were shot from guns in tests designed purely to control quality. By the end of the war more than 130,000,000 tubes had been produced, and the cost had dropped to less than that of many commercial standard tubes. Similar triumphs in the production of rugged reserve-type batteries were achieved by National Carbon, Eastman Kodak, and the Hoover Company. At the peak of production, with 300 different companies and 2000 different plants at work, nearly 2,000,000 fuzes were manufactured each month. Large scale production had reduced the cost per fuze to between $16 and $23, depending on the type.

The great secrecy required created many problems. Even after contracts were negotiated, only top personnel in the key companies were given basic information. Few subcontractors knew anything important about the project. Fuzes transported by rail from assembly plants were kept under Marine guard. Upon arrival in port, no one was permitted to leave a transport until every VT fuze on board had been accounted for and turned over to the proper authorities.

When the fuzes simplified so greatly the work of the men behind the Navy’s 5-inch guns, the desire of the Army for prompt deliveries naturally grew more intense. Proximity fuzes appealed to the Ground Forces not merely for anti-aircraft fire, for which the targets were less numerous than formerly, but primarily for howitzer fire against troops on the ground. It had long been realized that air bursts would inflict many more casualties on troops in trenches and foxholes than would shells fuzed to explode on contact. The difficulty was to set time fuzes with sufficient accuracy to ensure bursts at the exact range and height desired. It was hard to time, precisely enough, a projectile traveling several hundred feet in a tenth of a second.

Shells fitted with radio proximity fuzes could deliver uniform bursts at the preferred height regardless of variations of terrain, bad weather, or darkness. Tests against targets placed in deep and shallow trenches indicated that the VT fuze, triggered by reflection from the ground, could improve the efficacy of howitzer fire against personnel by a factor of 10 for long-range fire. With the VT fuze, air bursts “follow” the terrain, bursting at the same height from the ground over a hill as they do over a valley. To the Ground Forces the new device seemed a heaven-sent means to open holes in the German lines for our advancing troops.

The danger, however, was that, despite the selfdestroying features incorporated in the VT fuze, the Germans might recover a dud and be able to duplicate the fuze in time to use it against us. If they did, they might blast the Eighth Air Force and the RAF from their skies. If they gave their discovery to the Japanese, it was possible that we might lose one of our greatest advantages. So, though the Navy began production of fuzes for the British and United States Armies in November, 1943, the Combined Chiefs of Staff ruled that VT fuzes could be fired only over water, where there would be no risk of compromising the device.

The arrival, in the autumn of 1943, of secret intelligence that the Germans were preparing to use robot bombs against London and the ports in southern England where the forces destined to invade Normandy would eventually be gathered threatened the success of the great cross-Channel operation. Activity in the OSRD reached fever heat. With the coöperation of Allied intelligence services, a Section T member brought back from London detailed information concerning the buzz bomb six months before the first one was launched at England.

A complete mock-up of the robot bomb or the V-1 was hastily constructed, and was suspended between the two towers on the Section T proving ground operated by the University of New Mexico group near Albuquerque. Full-scale tests proved that the buzz bombs would trigger the VT fuzes, and indicated which model of proximity fuze would function best against them. Under the compulsion of necessity the Combined Chiefs of Staff relaxed their security restrictions to permit the use of VT fuzes against the new German menace. Three months before the first buzz bomb fell on British soil a shipment of VT fuzes arrived in England.

The problem of hitting the robot missiles, traveling at 350 miles or more per hour, was solved by relying as little as possible on the gunner’s skill of hand and eye, and trusting as completely as possible to the accuracy of the new devices which had been developed for anti-aircraft fire. The laurels went to three new weapons, all developed by the NDRC, and all manufactured in the United States. They were the SCR-584 radar, the M-9 electrical predictor, and the radio proximity fuze.

As all three were used in combination, and none would have been so effective without tho others, it is impossible to divide tho credit, which was won in abundant measure by all. When General Sir F. A. Pile, Chief of the British Anti-aircraft Command, sent a copy of his report on these celebrated operations to Dr. Bush, he wrote on the cover: “With my compliments to OSRD who made the victory possible.”

6

IT WAS an extraordinary story. When the V-1 bombs were first launched against London on June 12, 1944, tho anti-aircraft played at first a minor role and interceptor planes carried the chief burden of defense. During the second week of July a large concentration of anti-aircraft guns was effected on the Channel coast where the duds and the early bursts from VT shells would not be dangerous to civilians. This concentration included large numbers of British 3.7-inch guns and five battalions of 90millimeter guns, all under the command of General Pile and all equipped with SCR-584 radar, the M-9 predictor, and the VT fuze, described in Army parlance as the T-98 or Pozit fuze.

In the four closing weeks of the eighty days of V-1 attacks, the shooting steadily improved. In the first week, 24 per cent of the targets engaged were destroyed, in the second 46 per cent, in the third 67 per cent, and in the fourth 79 per cent. On the last day in which a large quantity of V-1’s were launched against British shores, 104 were detected by early-warning radar but only 4 reached London. Some 16 failed to reach the coast, 14 fell to the RAF, 2 crashed, thanks to barrage balloons, and anti-aircraft accounted for 68.

General Lear, Chief of our Army Ground Forces, who regarded the VT fuze as “the most important innovation in artillery ammunition since the introduction of high explosive shells,” pressed strongly for release of the device to the Army. Careful estimates had been prepared as to the shortest possible time in which Germany or Japan might duplicate the fuze, for it would have been a most formidable weapon indeed against our planes in bomber formations. Finally on October 25, 1944, the combined Chiefs of Staff agreed to its release for general use, on a date which was later moved ahead to December 16 to counter the German break-through. Great credit is due to Admiral King for his ability to see the war as a whole and his readiness to expose one of the most closely guarded of Navy secrets in order to help our Ground Forces over the hard sledding ahead.

Dr. Bush visited General Eisenhower’s headquarters for conferences on the introduction of the new fuze into general Army use. On December 16, von Rundstedt launched the last great German drive of the war. The VT fuze did deadly service that day against German planes, and two days later was first used in howitzers to stem the German advance toward the Meuse and the threat to Liège. Observers close to the scene of action agreed that “the terrific execution inflicted and the consternation resulting from night and day bombardment” had contributed materially to halting the advance and hastening the reduction of the salient.

Prisoners of war characterized our artillery fire as the most demoralizing and destructive ever encountered. General Patton wrote to General Levin Campbell, Chief of Ordnance, on December 29: —

The new shell with the funny fuze is devastating. The other night we caught a German battalion, which was trying to get across the Sauer River, with a battalion concentration and killed by actual count 702. I think that when all armies get this shell we will have to devise some new method of warfare. I am glad that you all thought of it first.

The VT fuze proved its worth repeatedly in the days that followed, notably in the crossings of the Rhine and in the defense of the all-important Allied base at Antwerp, against which the Germans laid down a heavy barrage of V-1 bombs. It was used with great effect in the Mediterranean theater and in the heavy fighting on Okinawa and Luzon. Although the Navy continued to control the procurement of VT fuzes and on December 1, 1944, took over the Johns Hopkins contract from the OSRD, by far the largest share of production since the close of 1943 has gone to the United States Army.

The successful development of proximity fuzes for the defense of ships against planes introduced a new factor into the fire-control problem. Because fewer rounds per bird were required, several targets might be engaged simultaneously. Where three or more planes were in range, centralized fire control became inefficient as compared to local fire control, since, because of the much greater probability of hitting, fewer guns needed to be engaged per plane. The reader who has fired into a covey rise of quail knows the importance of picking an individual target.

Studies at Michigan on damage probability had made it clear that the full potentialities of the VT fuze would not be realized unless the Fleet was equipped with local-control gun directors and unless each of these was provided with less cumbersome, more versatile radar equipment to permit blind firing. In the autumn of 1943 the Bureau of Ordnance asked Section T to undertake the development of both manual and power-driven gun directors for local control of fire with proximity fuzes. In these systems radar is used for blind tracking, range, and range rate. Lead angles from the abovedeck components are transmitted to a below-deck computer for combination with director position angles to give the final angles for positioning the gun.

The highly successful manual Gun Director Mark 57, whose development was pushed rapidly at Eastman Kodak Company, was issued to the Fleet late in 1944 and proved of much assistance in improving gunfire against kamikazes. Prototypes were installed on the Missouri and the large cruisers Alaska and Guam and production models later on many other vessels. They are known to have accounted for many a suicide plane, with an astonishing economy of ammunition.

7

THE detonation of torpedoes upon proximity to the underhull of a ship had long been a desired aim. Since the underhull is the most vulnerable part of a ship, a proximity exploder might greatly increase the effective target area and break the ship’s back. The major navies, including our own, had expended lavish efforts on the development of magnetic exploders for torpedoes, but had found the problem a difficult one because of premature firing when the torpedo rolled, pitched, or yawed. When the United States Navy invited its assistance in 1943, Section T proposed a new line of attack, which eventually provided a solution for the earlier difficulties. The proposed basis of operation made the exploder insensitive to minor changes caused by the roll, pitch, and yaw, but on the other hand it called for a much more delicately balanced device.

To solve this problem the OSRD concluded a separate contract with the University of Washington for the establishment of an Applied Physics Laboratory to work in close liaison with the central laboratory of Section T at Silver Spring. To make the design usable in all marks of torpedoes, the most difficult case of all was attempted first, that of the aircraft torpedo Mark 13, which was more subject to roll, pitch, and yaw than the larger submarine torpedoes and had to withstand impacts of the order of 300g from airplane drops.

The Mark 9 Torpedo Exploder, which resulted from these studies, was successfully adapted to aircraftand submarine-launched torpedoes, both steamand electric-driven. It utilizes highly sensitive electrical components, and parts developed for the rugged VT fuze. The unit is equipped with an anti-countermine switch which prevents premature firing by protecting the device from the mechanical shock resulting from the detonation either of other torpedoes in the salvo or of enemy countermine charges. Designs were “frozen” in November, 1944, and production was entrusted to the International Harvester Company. This device was ready for use when Japan surrendered.

Meanwhile, Section E had developed proximity fuzes for bombs and rockets. Those for bombs were first used against Iwo Jima in February, 1945, and later in Italy and against the Japanese mainland. Section E likewise succeeded in the difficult task of developing smaller proximity fuzes for the 81-millimeter mortar to reach the Japanese in foxholes. A highly ingenious system of manufacturing circuit components, such as condensers and resistors, and the connections between them, by new techniques developed in the field of ceramics by Globe-Union, permitted considerable saving of space. By V-J Day an Army production program of 100,000 mortar fuzes per month was getting under way, and would shortly have been quadrupled but for the Japanese surrender. It was believed that these fuzes would have increased the effectiveness of mortar fire by ten to twenty fold.

If one looks at the proximity fuze program as a whole, the magnitude and complexity of the effort rank it among the three or four most extraordinary scientific achievements of the war. Towards the close of hostilities it monopolized 25 per cent of the facilities of our electronic industry and 75 per cent of the nation’s facilities for molding plastics. The job never could have been done without the highest degree of coöperation between American science, American industry, and the armed services. That it was done at all, borders on the miraculous. The results are writ large in the story of the war on land and sea and in the air.