Optical and Magnetic Fuzes
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Optical and Magnetic Proximity Fuzes - a Survey 

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

In the previous article, we studied how the radio, or often called Radar, proximity fuze operated and learned the history of its development. In this article we are going to examine both the optical and the magnetic proximity fuze designed by Bell Laboratories employees during World War II.

Optical Proximity Fuzes 

Unlike early attempts of optical methods we discussed in the earlier article that were tried on 5" shells, this fuze was able to withstand greater ‘G’ forces than some of the earlier experimental models. Also, since this fuze was to be used on rockets, there was not the centrifugal force caused by the rotation of the projectile to contend with. This rotating was caused when a shell was fired from a rifled barrel.

As the name indicates, the optical proximity fuze is a device on a projectile which operates on the light signal produced by the target as the projectile approaches it.

There were three basic parts of the Optical Proximity Fuze, they are: a toroidal lens, a photocell, and an amplifier. The lens as part of the conical nose of the rocket. This lens was arranged to collect light from all directions during its line of flight, and to focus it upon the photocell tube. The photo-sensitive cell then would transform the light into electrical energy which is then sent to an amplifier.

No amplifier output is present until there is a sudden change in the amount of light entering the lens. This change was produced when the rocket approached the target and the light present to the photocell increased. The amplifier output developed a voltage that would then trigger a thyratron tube which, in turn, caused detonation of an explosive charge in the rocket. To operate the fuze, the change in the amount of light entering the lens needed to be just a small percentage of the total light regardless of the ambient light level from dawn to dusk.

This fuze was also provided with a method that would prevent the amplifier from operating and a firing pulse being generated until after the rocket had been fired and is well on its way towards the target. Another consideration for safety was to equip the fuze with safety features designed to prevent premature operation should the rocket, prior to firing, was dropped accidentally. Another novel feature was the self-destruction arrangement, whereas, if the projectile should miss the target, it would explode before reaching the ground. This safety feature was found to be very desirable, especially if the rocket would land back into to your own territory.

As noted in The Proximity Fuze, a Survey, many experiments were made on optical fuzes, both in England and in our country, before the Bell Laboratories Optical Proximity Fuze was developed. In 1942 Dr. Alexander Ellett, Chief of Section E of the National Defense Research Committee (NDRC) in Washington, assigned the Laboratories the task of developing for the Army Ordnance Department a working design of an optical fuze to fit on the 4 1/2-inch rocket. These Fuze’s were intended to be used against aircraft, as well as being mounted on rockets fired from aircraft. Collaborating with the engineers of the National Bureau of Standards, the Apparatus and Transmission Development Departments at the Bell Laboratories jointly undertook the design and development of such a fuze.

The two main objectives to be met in the design process were that the fuze had to fit the nose of the rocket, and to make it capable of withstanding the force of acceleration, which was 1,000 times the force of gravity. The other consideration was that the design of the fuze had to lend itself to easy mass production at a low cost.

There had been little precedent to guide the designers in the production of a photocell, a lens, electronic tubes and other circuit components which could withstand the large forces of acceleration previously mentioned. What was available, however, was the vast expertise of the Bell Telephone Company’s designers experience with materials that had been used in the production of telephone equipment. This knowledge base consisted of: the processing of plastics, die casting, impregnating compounds and electrical wiring. 

F. A. Zupa, who during the war during the war was in charge of the apparatus group engaged in the design and development of proximity fuzes, rocket-firing mechanisms and magnetic mines at Bell Laboratories, provides us with a more technical description of how the fuze worked.

"The toroidal lens is an integral part of the nose piece, the entire part being made of optically clear methyl methacrylate, commercially known as lucite or Plexiglas. The curvature of the toroidal lens was designed to transmit only the light which came through a narrow angle, throughout its circumferential surface, and to have the focal axis at any point around the lens lie on a conical surface. It was manufactured by injection molding to the final dimensions, and no polishing of the lens surface is required after the molding. The portions of the surfaces that had to be opaque to light were coated with a black finish by spraying. Close cooperation between the Laboratories and the Manufacturing Department was required to determine the correct molding time and temperature to produce this part to the required accurate dimensions. The choice of opaque finish presented some difficulties because a number of the common lacquers were found to be destructive to the lucite, the destructive action being known as crazing. A similar difficulty was encountered in the choice of a waterproofing compound, which had to be applied at the junction of the lens piece and housing to protect the photocell from moisture."

"To obtain the desired sensitivity to light when the projectile is in the most effective position with respect to the target, the glass tube portion of the photocell was made opaque to light except for a slit suitably located with respect to the lens. Many designs were conceived for providing such a slit opening, but the search was for a simple and durable construction. As finally adopted, the glass tube is first completely covered with the opaque finish and then the slit is produced by cutting away part of the finish. This technique was new, and it required rather skillful development work before it was reduced to a simple manufacturing process. The photocell and the lens were held in proper relation to each other by securing both parts to a molded phenol plastic part, which accurately positioned the photocell cathode in the focal plane of the toroidal lens. With this arrangement the photocell cathode was made to "see" the target at the angle required to place the target in the densest part of the fragmentation pattern when the projectile exploded."

Shockproof mounting for the amplifier consisted of the components being individually mounted in holes in an oil impregnated wooden block. In addition, many of the component parts were potted in a ductile wax to hold them in place. The advantage of mounting components in a permanently fixed manner was important to decrease any chance of capacitance coupling or regeneration in the amplifier circuit. The variable characteristic values of the miniature amplifier tubes were compensated for by preselecting the tubes and matching them with suitable grid-bias resistors and by-pass condensers before these parts of the fuze reached the assembly line.

Large quantities of the optical proximity fuzes were manufactured by the Western Electric Company, and the product satisfactorily met the rigid specification requirements. A sample number of each group of 1,000 fuzes was tested by Signal Corps engineers before each lot was approved for acceptance. The fuze was not adjustable and although it had to function only once, it had to fire the first time. The effectiveness of this fuze was indeed a testimony of the quality standards that the Bell System was noted for!

Magnetic Proximity Fuzes.

Another form of proximity fuze that was developed at Bell Laboratories personnel during World War II, was a fuze that detected changes in the earth’s magnetic field produced by the presence of ships. The work on fuzes for magnetic mines was an important part of this work in which the extensive knowledge and long background of experience with magnetic alloys, particularly permalloy, were of the utmost importance.

During the war, G. W. Elmen, the inventor of permalloy, was called out of his Bell Laboratories’ retirement by the Navy to work at the Naval Ordnance Laboratory on magnetic mine fuses. Back on the job, Mr. Elmen became actively connected with the Navy’s development program on magnetic mine fuses, and throughout the war he worked jointly with the engineers at Bell Telephone Laboratories In addition, he had the help of three other retired Bell Laboratories’ employees as associates during his development work at the Naval Ordnance Laboratory on mine fuse work. These former Bell Laboratories employees were J. F. Toomey, E. Montchyk, and J. N. Reynolds.

Naval mines, during World War II, were important offensive weapons. One method used to lay mines was accomplished by dropping them from airplanes into enemy waters. This magnetic mine, equipped with its associated proximity fuse mechanism, manufactured by Western Electric Company, was the most modern magnetic mine of the United States Navy. The mines were manufactured throughout the war by Western Electric, which was the part of the Bell system that conducted any mass manufacturing. Bell Laboratories dreamed and designed, Western Electric built the dream in quality!

This mine worked very simply. When a steel vessel passed over it, the steady magnetic field of the earth surrounding the mine is altered, first shifting slowly away from the normal steady condition as the ship approaches, and then drifting back to normal as the ship recedes. The fuze mechanism recognizes the passing ship whenever it detects a magnetic disturbance with a slow flux change, and causes an explosion when the ship is over the mine. The mine was provided with a novel anti-sweep feature, protection for counter-mining, and was equipped with a mechanical memory so that it may be set to blow up a certain numbered ship in a convoy, rather than to just destroy the first ship.

The principle indeed was simple, but the development and manufacture of the magnetic proximity fuze was not as easy. The vital elements of the mine fuse were the search coil that detects the feeble magnetic influence of an approaching ship, and the magnetic amplifier which increases the strength of the feeble detected signal about a million times. Both the search coil and the magnetic amplifier require permalloy of excellent quality and precise manufacture to operate satisfactorily.

Sensitive as the most delicate jeweled instrument, the electronic mine fuse was constructed very ruggedly. Mines equipped with these fuzes were dropped into the sea from airplanes several thousand feet in the air. Amazingly enough, these mines containing the magnetic proximity fuse was used successfully in mines laid by planes from altitudes up to 30,000 feet in a free fall without parachutes. These free-falling mines could be aimed more accurately because the drift without parachutes was much less.

An amazing example was the test mechanism without explosive charge that was dropped from over 10,000 feet. It struck the shore instead of the water, and although the mine case broke and the contents were strewn over a vast area, the fuse mechanism was intact and was found to operate after this rough treatment.

After a mine is dropped into the water, it is made alive by the usual arming devices and begins to search for the presence of ships. As a ship approaches, the change in the earth’s field generates a voltage in the search coil, and the resulting signal current upsets a delicate balance in the magnetic amplifier circuit, firing the mine.

Magnetic proximity fuzed mines were used in operations that cut Japanese life lines and ruined the shipping-dependent economy at the close of the war. This strategic mining blockade, called "Operation Starvation," was undertaken late in March of 1945. The ports of Kure, Hiroshima, Tokayama, Sasebo Naval Base, and Shimonoseki were mined by B-17 superfortress to prevent Japanese naval units from participating in the defense of Okinawa. The blockade was extended later in the campaign to major shipping lanes between industrial cities which depended largely on water transportation for their goods. Shipping was cut to 10 per cent of normal within two months. Heavily used and direct shipping routes to the continent of Asia were then severed by mining the ports of northwestern Honshu.

The last phase of the operations was an intensification of the existent blockade around major shipping centers in Japan, plus additional mine laying in Fusan and other Korean ports. During this final phase, the Japanese shipping that was sunk by mines has been estimated to exceed 300,000 tons.

The development of magnetic mine fuses at Bell Laboratories was carried out under contract with the Navy Bureau of Ordnance. The work was done in close cooperation with the Naval Ordnance Laboratory. In making an appraisal of mine operations by our naval fighting forces, Admiral Nimitz has said:

"The technical planning and operational execution of aircraft mining on a scale never before attained has accomplished phenomenal results and is a credit to all concerned."

It was not only the Americans that were to praise the magnetic proximity fuze! Japanese naval authorities had the following to say about the Magnetic proximity fuzes of the U.S. Navy, as reported in a publication of the Naval Ordnance Laboratory:

"The detonators show superior construction and speak well of the ability of the specialists and the manufacturers. Furthermore, the application of new fundamental principles to mines shows the skill and farsightedness of the technical experts which was far beyond that of those in Japan at the time. That is to say, the mine fuse ... circuit using a small type of glow tube (cold cathode tube developed by the Bell Telephone Laboratories and manufactured by the Western Electric Company) is indeed a clever idea."

With respect to earlier mine fuses on which the Laboratories did extensive development work with the Naval Ordnance Laboratory and the Leeds & Northrup Company, the following was said by the Japanese experts:

"It is clear that these detonators are an application of designs by telephone communication engineers, and the fact that they were perfected with telephone materials speaks well for those specialists in the application of their knowledge. There were no mine technicians in Japan comparable to those in America, and the display of such ability by America was the occasion for surprise among the mine specialists in the Japanese Navy."

F.A. Zupa, Bell Laboratories Record February 1947.
H. 0. SIEGMUND Bell Telephone Laboratories Record Magazine July 1947.. 

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