Radio Proximity Fuzes
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The Radio Proximity Fuze - A survey 

By Edward A. Sharpe Archivist, SMEC (c) (now SMECC c.2003)
Reprinted from Vintage Electrics Volume 2 #1


In this fast paced, high tech era of warfare where bombs are smart, and a pilot can hit something the size of a football at near supersonic speeds, we take you back to an earlier, more simpler time. But, this was a time when many lives were lost, and a war was to happen that did not end as swiftly as Desert Storm’, the war against Iraq to reclaim Kuwait in the early part of 1991. 

The dilemma.

During all of the wars that proceeded, and during the beginning of World War II, we needed a method for detonating a shell near the target. If we were to detonate the shell either too early, or later after the shell had passed through the target, the effect would be minimal.

In the earlier days of warfare, many of these shells had been manually timed, which gave the gunner one more thing to contend with, in addition to calculating the correct course, speed, range, bearing, and position angle. Of course, any error in calculations resulted in the target escaping damage. 

Early in the days of World War II, fire-control radar was developed, which helped with many of the calculations that must be considered for a successful launch of a shell towards the target. The accepted method of shelling prior to the marvel of radar, was to saturate the area, wasting a large number of shells and most of the time resulting in the escape of the target. Fire control Radar was to solve the problems of positioning the guns, but now, on to our problem of when to detonate the shell.

The burst range for a 5 inch shell is 70 yards, and we must have our target within this region when the explosion occurs. It took the inventiveness of the Navy, and civilian American scientists to invent the technical marvel of the Proximity Fuze. It took the electronics industry in the United States to manufacture a reliable and compact system.

Early attempts to solve the dilemma.

For a decade prior to World War II, the Navy’s Bureau of Ordnance had thought of building an infrared fuze which could be triggered by the heat developed by an aircraft engine. Due to the complicated engineering problems this project was never implemented.

In the summer of 1940 aircraft technology had improved by many countries, and the international situation started by Hitler’s invasions made the United States take a look at developing a fuze which would detonate a projectile when in proximity of an aircraft. In July of that year, a group consisting of members of the National Defense Research Committee and the Navy Department Council for Research, decided that the development of such a fuze was possible by using either electronic or photoelectric devices.

There were no holds barred as to the techniques to be investigated! A month later, the Bureau of Ordnance gave the fuzes top priority over all projects that it had requested the National Defense Research Committee to look into.

What a surprise it was for the NDRC, when they learned that two of our largest electronics manufacturers were providing the British with thousands of vacuum tubes and photoelectric cells. This led the committee to believe that the components were being used for some type of proximity fuze.

After the arrival, in September 1940, of the British Technical Mission, headed by Sir Henry Tizard, the NDRC received a report from the British that, although they were consuming supplies, they had not made a workable fuze. The Tizard mission claim to fame was in bringing a magnetron to the United States. This early magnetron was to be used as a pattern that set us into production of better radar equipment! (See McMahon’s Radar section).

During August 1940, Section T of the National Defense Research Committee was established under Dr. M. A. Tuve of the Carnegie Institution. Research was to be conducted at the laboratory of the Department of Terrestrial Magnetism of the Carnegie Institution, Washington.

People for this effort were recruited from all over the United States. The work on the proximity fuze was very secret, and when interviewing Dave Smith, son of K. D. Smith, Dave tells us that although his father was involved with Proximity Fuze development. "It was never discussed at home." Even after the war was over Dave said that anything connected with the war effort on the fuze, as well as his fathers efforts on Radar were never talked about in detail. What we do have, however, is the picture included in this article of K. D. Smith’s NDRC Identification card while involved with the VT Fuze project. K. D. was to be presented with an award after the end of World War II, and this may be seen in Volume #2 of Vintage Electrics, where K. D. Smith’s life at Bell Laboratories and the his war efforts are documented.

In November 1940, the Bureau of Standards joined section T on the project and for a few months both of these activities conducted independent research, each working on a variety of devices applicable to a wide range of projectiles.

Since the Navy’s basic and urgent requirement was for a fuze for anti-aircraft projectiles, fired from rifled guns, the work of the two activities was separated in July 1941. Thereafter, Section T devoted its entire energies to this problem, while the Bureau of Standards concentrated on influence fuzes for non-rotating projectiles.

In November 1941, the Bureau of Ordnance contracted with the Crosley Co. to conduct independent research in fuze construction under the technical supervision of the National Defense Research Committee. This industrial concern was expected to provide realistic engineering design rather than development. Meanwhile, the National Defense Research Committee contracted with many other companies and universities. 

The growth of the project was so great that it required increased administrative support. In March 1942, it was placed directly under the Office of Scientific Research and Development, which contracted with Johns Hopkins University to provide for its administration. The secret classification of the project necessitated the provision of secure space for this. The University established the Applied Physics Laboratory at Silver Spring, Maryland, a suburb of Washington D. C. 

During the early months acoustic, thermal, electrostatic, and magnetic types were studied, but were found to be unsatisfactory. Considerable emphasis was placed on the utilization of photoelectric cells and one was practically completed in early 1941, but the cells failed to withstand the centrifugal force developed by the rotating projectile, as well as working in only daylight! Although optical and magnetic methods were not well suited for shells, they found use in 4 1/2 inch rockets and mines. (See the next article.)

A call to development

In early 1941, all contractors supported by Navy funds were directed to concentrate on the development of an electronic fuze.

One method that was experimented with used radio waves transmitted from the ground. These radio waves would be reflected by the target and received by the fuze. Once the radio waves were at a sufficient level, the fuze would activate causing the shell to explode. 

Another method that was more logical and became the accepted means, was to develop a fuze which was capable of obtaining its own intelligence and of using it to ignite the shell. When assembled this fuze consisted of four major parts: A miniature radio transceiver, complete with amplifier and capacitor; a battery; an explosive train; and the necessary safety devices. The theory was that the fuze transmitter, alone, would not produce sufficient signal intensity, to trigger a thyratron tube switch. However, as the projectile approached a target the radio waves reflected by the target would gradually increase and come more and more into phase with the fuze-generated signal. Once the signal level was high enough, the fuze would know that the shell could do a maximum amount of damage, and the thyratron tube switch would be triggered releasing the energy in a charged capacitor and thus igniting the shell. 

It was a brilliant concept! To convert it to a workable device required the development of radio components rugged enough to withstand an acceleration force 20,000 times stronger than normal earth’s gravity and a centrifugal force set up by approximately 500 rotations per second. Once these specifications were met, it was necessary to shoe-horn all of this electronics along with batteries and detonator into a space approximately the size of a pint milk bottle.

With American lives being lost daily, The electronics manufacturers became very inventive! Had the requirement for miniature components of the required ruggedness been submitted to any electronic equipment manufacturer during peacetime, he would have most probably shaken his head and declared them far beyond the engineering capabilities of his staff. But, the issue was saving the lives of our soldiers, and this pushed the developers beyond the normal labors that would have been undertaken normally.

Miniaturization had already been seen in electronic hearing aids, but the ruggedness needed was not an essential requirement of that field.

During the development period, the tubes were handmade by engineers of the Western Electric, Raytheon, Hytron, Erwood, and Parker-Majestic Companies. Although varying in quality periodic tests conducted through the latter half of 1941 offered promise. Constant test and re-design were of paramount importance anytime weakness was discovered in any part of the fuze.

The big test.

On January 29, 1942, a group of fuzes with miniaturized components and dry cell batteries, built on a pilot production line, were installed in standard 5-inch anti-aircraft projectiles and fired from a 5-inch anti-aircraft gun. At the end of a 5-mile trajectory 52 percent had successfully activated themselves by proximity to water. Although this appeared to be a low percentage, the protection given by these shells was far greater than that achieved by saturation shelling. The Bureau directed the Crosley Corp. to commence pilot production of the fuzes without delay. The name that was assigned was the ‘VT Fuze’, with the VT standing for variable time.

One of the key items that had to be developed during this project was a reliable battery. It was found that a small dry cell battery, although providing power, would fail to withstand the shock of gunfire. Another large problem was short battery life under shipboard storage conditions in the South Pacific. 

A parallel research effort to develop improved dry cells and a wet battery, wherein the electrolyte would be kept separated from the electrode until after the projectile was fired, was concentrated at the Cleveland, Ohio plant of the National Carbon Co. The outcome of this research was the development of a cylindrical battery resembling a fountain pen. The way this battery worked was ingenious! The electrolyte was contained in a glass ampule at the center of a cylindrical cell of thin plates. Upon the firing of the projectile the shock breaks the ampule, the electrolyte is released and the centrifugal force generated by the rotation of the projectile forces the liquid between the plates and activates the battery. This battery was ready for experimental testing in February 1942.

Development of the fuze continued concurrently with the pilot production at the Crosley Corporation plant. In April 1942, firing tests in which the new battery was utilized were conducted successfully. A small plane suspended from a barrage balloon was used as the target. Success! But safety and self-destruction devices needed to be added to the fuze before it was formally ready to go to war.

In another test, similar to the one conducted on January 29, it was found that reliability of the fuze technology resulted in 70 percent of the shells detonated. The next logical step was to conduct a shipboard firing test.

The VT Fuze gets sea legs

On August 12, 1942, the first pre-combat service tests were made by the newly commissioned U.S.S. Cleveland. The commander, Capt. S. E. Burroughs, USN, had the ship on a shake down cruise in the Chesapeake Bay. Radio controlled planes (drones) were used as targets. The Gunnery Officer, Lt. Commander. Russell Smith, USN, was an experienced fire-control officer. His gun crews consisted of approximately 10-percent experienced personnel and the remainder were newly enlisted. Smith, with his nucleus of experienced personnel, worked hard before and during the shakedown period to train his fire control and gun crews and achieved magnificent results. The tests were scheduled for a period of 2 days and were to be conducted under simulated battle conditions. All three available drones were destroyed early on the first day while going through all possible evasive maneuvers, by the bursts of only four proximity fuzed projectiles. This was an amazing event to all who witnessed it! Here was a device which would force enemy aviators to be more respectful!

Specifications are created 

Following the Cleveland tests fluid specifications, which permitted incorporation of later developments, were drawn up for mass production of the fuze and manufacture was commenced. Those produced were shipped to the Ammunition Depot, Mare Island, Calif., for assembly into anti-aircraft projectiles. Samples of these were flown back daily to the U.S. Naval Proving Ground, Dahlgren, Va., for verification of quality.

Combat, the ultimate test!

In the middle of November 1942, 5,000 rounds of proximity-fuzed projectiles in storage at Mare Island were rushed to Noumea for distribution to ships of a task force in the southwest Pacific. The first ship to introduce them to the enemy was the U.S.S. Helena. On January 5, 1943, four Japanese bombers attacked the task force and the Helena downed one with the second salvo of proximity-fuzed ammunition.

The secret must be kept!

Realizing that the details of the fuze must be kept from the enemy, the Combined Chiefs of Staff issued a ban against its use in any area where duds or live shells might be recovered by the enemy. During World War II, the Japanese were famous for being able to copy captured radar equipment, and the Americans did not wish this fuze copied and used against the allied forces. This restricted the fuze’s usage to naval warfare and also prevented it from being used in naval shore bombardment of enemy-held territories.

The production lines crank up

Following the Crosley Corp. contract, production was increased to great numbers. Beginning in September 1942, newly established facilities commenced production of the ruggedized miniature tube in large quantities. In October 1942 an average of 500 tubes were being manufactured daily. After the fuze had been proven in combat the expansion of manufacturing facilities was rapidly increased. By the end of 1943 almost 2 million had been delivered. By the end of 1944, 87 contractors, operating 110 plants, were manufacturing parts of the fuze which at that time were being delivered at the rate of 40,000 per day. Procurement contracts increased annually from $60 million in 1942, to $200 million in 1943, to $300 million in 1944 and were topped by $450 million in 1945. Of course, as volume increases cost decreases, and the cost per fuze that had started at $732 in 1942 dropped to $18 in 1945. This permitted the purchase of over 22 million fuzes for approximately $1,010 million.

Many companies involved

Fuze assembly was concentrated in the plants of the Crosley Corp., the Radio Corporation of America, Eastman Kodak Company, and the McQuay-Norris Company. Mass production of the ruggedized miniature tubes had to be limited to Sylvania Electric Products, Inc., since they proved to be the only firm capable of combining quality and quantity. Cost of tubes declined with increased production from $5.05 in 1942 to $0.40 in 1945.

K. D. Smith, whose collection resides at the Southwest Museum of Electricity and Communications, obtained many of these miniature Sylvania tubes following the completion of World War II. He collected them as a memento of his involvement with the Proximity Fuze project with the NDRC. You may view some of the actual tubes that were the heart of the electronics that made this project a success.

A striking combat success!

During 1943 approximately 9,100 rounds of proximity-fuzed and 27,200 rounds of time-fuzed 5-inch anti-aircraft projectiles were fired. Fifty-one percent of the hits on enemy planes were credited to VT-fuzed projectiles. The proximity Fuze equipped shells success in repelling air attacks against fleet units reached its peak when a task group in the Pacific reported the destruction of 91 of 130 attacking Japanese planes. This high level of effectiveness was to save many servicemen’s lives from the onslaught of Kamikaze attackers. Had not these Samurai minded pilots been removed from the air, they would have rammed their planes onto the decks of our navy vessels causing the death of many servicemen. The VT Fuzed shells were also used with great success in the Mediterranean and Atlantic theaters.

Security restrictions removed

During 1944 the intense warfare in the European theater of operations necessitated the lifting of the ban against the use of the fuze where it might be recovered by an enemy. On 12 June 1944 the first V-1 "buzz bomb" fell on London marking the start of Hitler’s massive effort to level the city by rocket. The all-out valiant effort of the Royal Air Force was not able to devise a good defense against this new weapon.

The Combined Chiefs of Staff reluctantly agreed upon the necessity of using the proximity fuze in the defense of London. Large numbers of anti-aircraft guns were moved to the channel coast where they could fire at the bombs over water. Success in destroying the V-1 rocket bombs by gunfire increased proportionally with the increase in the use of VT-fuzed projectiles. In the last month of the terrifying 80 days, 79 percent of the bombs engaged were destroyed as compared with the 24 percent destroyed during the first week of the attacks. On the last day of large-scale attacks only 4 Of 104 bombs succeeded in reaching their target. Some of the 100 destroyed are credited to the Royal Air Force and to the barrage balloons, but the majority of the V-1’s were victims of proximity-fuzed projectiles. There was little profit to the enemy with such a small percentage of success so Hitler turned the weapon on the port of Antwerp, which at that time was vital to the Allied supply lines. In the autumn of 1944 the devastating damage wrought while the Allies were redeploying anti-aircraft guns threatened to close the port. As the number of guns firing the proximity fuze increased, the damage decreased and the Allies were able to move their guns closer and to assume the offensive against the aerial targets. The defense of Antwerp resulted in the Combined Chiefs of Staff removing all bans against the use of the fuze which was most fortunate for the allied soldiers fighting there.

In late December 1944, von Rundstedt launched a counterattack which developed into the Battle of the Bulge. The use of the fuzes entered a new field, that of artillery fire against ground forces. The results of this usage was devastating to German troops and put fear into their hearts. No longer were their foxholes heavens against shrapnel burst, for with the use of the "funny fuze," as it was termed by General Patton, the shrapnel bursts occurred before the projectiles hit the earth, and high-velocity fragments rained down on the German attackers!

Electronic countermeasures

A move to develop countermeasures against proximity fuzes stemmed from the Germans, who during the "Battle of the Bulge," captured an Army munitions dump that contained a large number of the new radar proximity-fused shells. Concerned that the Germans might attempt to copy the proximity fuze, the Research Division of the Aircraft Radio Laboratory at Wright Field, along with the help of the RLL, was called in begin the development of jamming equipment. Lieutenant Jack Bowers, an engineer with the Aircraft Radio Laboratory at Wright Field, recounted the following to Alfred Price:

"The proximity fuse had been a closely guarded secret on our side. Even though we had been working on countermeasures for a long time, we at Wright Field had never heard of the device. Now we were asked to investigate, on a crash basis, the possibility of a jammer to counter the fuse. We asked why such a jammer had not been developed earlier, and were told that the developing agency had conducted tests and concluded that the fuse could not be jammed! We worked on the problem, and within two weeks, a jammer had been built which would detonate the proximity fuses prematurely."

Since the body of the shell served as the antenna for the radar proximity fuse, it limited the frequency spread of the transceiver from 180 to 220 MHz. The APT-4, a high powered jammer, already covered that part of the spectrum. A motor-driven tuner was added to sweep the jamming transmitter’s signal up and down the band theoretically covered by the fuze. Several modified APT-4’s were installed in a B-17, and a top priority full scale test was arranged at Eglin to see whether the countermeasures would be effective.

Price, in another interview with Lieutenant Ingwald Haugen, one of people involved with the test, Haugen tells him:

"For the firing test, the Army sent a battery of 90 mm anti-aircraft guns. These were emplaced near Eglin. We had requested that during the test the guns would fire VT (proximity fused) shells with spotting charges, so that when the fuses operated, the shells would burst with only a puff of smoke. We were told this was not possible. The VT fuse was about 1 1/2 inches longer than the normal mechanical fuse and it would not fit in a shell carrying a spotting charge. So, we were going to have to use live high explosive VT fused shells for the test. As a safety measure, the guns were to be offset by a small angle, initially 30 mils (about 1.7 degrees), later decreased to 12 mils (about .6 Degrees)."

"It was the sort of test that would never be allowed today under the prevailing flight safety guidelines. At the time, however, there was a war on, and the small risk to our one aircraft had to be weighed against the far larger risk to our whole bomber force if the Germans used such a weapon against us. We who were to fly the test were confident we would be all right - we hoped that the jamming would work as planned, and if it didn’t, the offset fed into the guns would burst the shells at least 240 feet away from us at a range of about 20,000 feet."

"The test lasted about 3 months, during which about 1,600 VT shells were fired, individually, in our direction. Sitting in the fuselage of the B-17, the two RCM operators could pick up the radar transmissions from the shells coming up. The VT fuse radiated CW (continuous wave) signals, but the projectiles would often yaw a little in flight. This, in combination with the spin of the shell, would modulate the signal. We in the back could not see out, but the pilots and the navigator would get a kick out of watching the shells burst well below, or if there was a late burst because the jamming had taken some time to sweep through the shell’s frequency, it might explode close to our altitude. The general conclusion of the test was that, modified to radiate CW swept across the VT fuse band, the APT-4 jamming could significantly reduce the effectiveness of the proximity fused AA shell."

The war ends...

The proximity fuze was a valiant contributions of American scientists, engineers, and manufacturers to the winning of the Second World War. Although security prevented the developers and manufactures from receiving the praise they so well deserved during the early years, they were to have full payment in the knowledge of their own great contributions.

General Benjamin Lear, USA, described the VT Fuze as "...the most important new development in the ammunition field since the introduction of high-explosive projectiles."

General George Patton, USA, also paid tribute to the Fuze developers stating, "I think when all armies get this shell we will have to devise some new method of warfare."

Patton’s prophecy might well have come true except that within the year, the success story of the VT Fuze was dimmed by the development of atomic weaponry. The Atomic Bomb was a far greater and more damaging concentrated explosive power than the world had ever seen. Even this development necessitated the continued use of the proximity fuze in the control of when the ‘A-Bomb’ was to detonate. 


History of Communications-Electronics in the United States Navy By Captain L. S. Howeth, USN (Retired) 1963.

The History of US Electronic Warefare By Alfred Price, Publisher: Association of Old Crows.

'The K. D. Smith Collection' at the Southwest Museum of Electricity and Communications, Phx. AZ. 

In addition, many conceptional views were provided by footage and narration of film from the World War II era. Views were also been provided to me over the past 39 years by those that I came in contact with, that either worked on the Proximity Fuze, or were users of the device. - Edward A. Sharpe Archivist, SMEC.

This article was done in the 1990's, Information in the UK was since then de-classified showing they had a larger part in the process I have been told.

If someone would like to write an addendum to this article please contact me......ed sharpe


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