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

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!


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