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The Creation of the Proximity Fuze

The National Defense Research Committee, Johns Hopkins Applied Physics Laboratory and the "funny fuze"

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When the Pearl Harbor attack was over, amid the preparations for war, an accounting of the cost began. The losses to the military were serious, both in the accoutrements of war like ships and aircraft and in personnel, but there were also 48 civilian casualties who are often forgotten. Some of them were killed when attacked while at or near the military bases where they worked, but most were killed in Honolulu by improperly fuzed anti-aircraft shells that exploded when they hit the ground in the city, rather than in the air near Japanese attackers.

It was a tragic indication of the inadequacy of anti-aircraft weapons technology in the face of modern, high-performance aircraft. While slow-moving, high-level bombers flying in formation over a fixed point or on a bomb run presented a workable problem for anti-aircraft gunners, the advent of fast, single-engine dive-bombing aircraft made for a much more difficult problem. Ships of all nations sprouted anti-aircraft weapons to the point that they began to resemble porcupines as the war went on, but this was only a partial solution.

Civilian Casualties Pearl Harbor

Four civilians killed during the attack on Pearl Harbor when their car was hit by errant U.S. anti-aircraft fire. Most of the 48 civilian casualties of the Pearl Harbor attack were killed by dud or incorrectly-fuzed anti-aircraft shells fired by U.S. guns. U.S. Naval History and Heritage Command photo

In 1941, the state of the art in large-caliber anti-aircraft shells consisted of a clockwork time fuze, manually set by a crewmember before firing, that exploded the shell at a preset time and location in space, calculated by primitive gun directors or dead reckoning. Rapidly maneuvering aircraft flying an evasive course, or saturation raids of so many aircraft that the anti-aircraft fire had to be split, made the task of the gunner difficult if not impossible, while fuzing the shells under the strain of battle could end with the same results as at Pearl Harbor. In Europe, the British naval
experience defending against Axis dive-bombers showed that, at least in smaller combatants, more guns were not the answer, as there was an upper limit beyond which the ship would become unstable due to the increased topside weight. Ships experienced air attacks so heavy, defending themselves with anti-aircraft ammunition so relatively inefficient, that some vessels were sunk by aircraft simply because they had no ammunition left to shoot at them. Clearly, a better answer had to be found.

By 1940, study of events in Europe had convinced the U.S. Navy that a better way had to be found to solve this problem, and the president’s National Defense Research Committee (NDRC) was asked to approach the scientific community about developing a fuze that would automatically detonate an anti-aircraft shell when it was within killing range of an aircraft. A “proximity fuze,” as it was termed, was not necessarily a new idea. In fact, it had existed as a concept for some time as a solution to the problem of air attacks, but was considered impossible to achieve with the materials and technology available.

Nevertheless, the NDRC assigned the project to “Section T” of the Department of Terrestrial Magnetism, The Carnegie Institution, under Dr. Melle A. Tuve. The concept was simple enough: essentially the enemy aircraft or target would interrupt some signal from the fuze and the interruption of the signal would cause the fuze to detonate. A modern security lighting system uses a similar photoelectric mechanism to activate floodlights when a person passes in front of the sensor. Section T had three options for the mechanism to produce the signal: photoelectric, acoustic, and radio. The most promising mechanism to use was a radio signal. The problem was in making it work.

Consider the challenges: First, the radio transmitter and receiver had to be housed in the shell itself, which would require a tremendous achievement in producing a fuze tiny enough (by the standards of the time) to be packaged into such a small space. Second, this miracle of miniaturization would have to be tough enough to survive repeated manhandling, exposure to salt air and dirt, and finally being shot from a cannon. Third, it would have to be mass-produced in the millions. Fourth, it had to be done before any of the Axis powers developed their own version, which also meant that, fifth, all of this had to be accomplished in absolute secrecy.

Mark 53 Variable Time (VT) Fuze

A cutaway view of the Mark 53 Variable Time (VT) Fuze. U.S. Naval History and Heritage Command photo

Section T developed a fuze that emitted radio waves continuously from the nose of the shell itself. These were reflected back to a small oscillator. If nothing passed closely enough in front of the fuze, the shell would detonate after a set period of time. If a target passed closely enough to reflect the radio waves with sufficient force, however, the waves would be amplified by vacuum tubes, triggering a thyratron tube that acted as a switch, releasing the energy stored within a tiny condenser, powered by a battery, which set off an electrical detonator, in turn setting off the normal detonating fuze of the Navy’s standard 5-inch shell. All of this happened in fractions of a second, and within a radius of about 70 feet from the target. In essence, Section T had created a tiny, extremely rugged radar set for a projectile.

But as the project met with a string of successes, the need for more space and facilities became pressing, and finally the Office of Scientific Research and Development signed a contract with The Johns Hopkins University to provide lab space, a test site, equipment, and manpower to speed the program. Dr. Tuve renamed the organization the Applied Physics Laboratory, or APL.

The president of Johns Hopkins appointed a member of the Board of Trustees, D. Luke Hopkins, as the university’s representative to APL, and he quickly made it clear that he would do everything in his power to further APL’s work on the fuze. The university leased a three-story building in the little town of Silver Spring, Md., that had formerly housed a garage on the ground floor as well as Social Security Administration offices on the second and third floors, and was well-suited for both lab and office space. The university also equipped the site with a complete suite of lab instruments, machine tools, and other equipment. A test site was located and equipped for field tests of the fuze in Newton Neck, Md., and APL began operations at the new sites in May 1942. By 1944, every inch of space was needed, the staff expanding from 100 to 700 people as work on the fuze progressed.

Testing was underway constantly, as was the search for the highest quality glass, filaments, wire, batteries, and other components that would be fine enough for the close tolerances required of the mechanism as well as strong enough to survive the shock of being fired from a gun. Even when the fuze went into production, APL continued quality control testing.

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