The Creation of the Proximity Fuze | Defense Media Network (2024)

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.

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.

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.

The Creation of the Proximity Fuze | Defense Media Network (2024)


What was the proximity fuse in the Battle of the Bulge? ›

The proximity fuze was developed through British and American cooperation in the early stages of World War II. It was first used against ground troops in the Battle of the Bulge (1944). The advantage was that the gunners could fire shells to explode over troop positions, showering them with deadly shell fragments.

How does a proximity fuze work? ›

The proximity fuzes developed in World War II markedly increased the effectiveness of artillery by triggering the explosion of the shell by its proximity to the target. This was accomplished by including a tiny radar-like radio sender-receiver in the fuze.

How much did the proximity fuse cost? ›

During the course of the war, the proximity fuze program as a whole cost approximately a billion dollars in contemporary money, or about $15 billion today. The cost per fuze fell from $732 in 1942 to as low as $18 in 1945, and over 22 million were produced.

What is the frequency of proximity fuse? ›

Radio frequency sensing (radar) is the main sensing principle for artillery shells. The device described in World War II patent works as follows: The shell contains a micro-transmitter which uses the shell body as an antenna and emits a continuous wave of roughly 180–220 MHz.

What went wrong in the Battle of the Bulge? ›

On the ground, the woods of the Ardennes offered good concealment for the massing German armies and bad weather prevented Allied reconnaissance from spotting them. Although the Allies had decrypted German codes, because most of the communications took place inside Germany, the Allies had few chances to intercept them.

What was the bulge in the Battle of the Bulge? ›

The “bulge” in Battle of the Bulge refers to the shape, as depicted on maps, created by German troops that had wedged westward in the Ardennes through the Allies' front line. The term was coined by Larry Newman, an American war correspondent.

How does fuze work? ›

The flame from the burning of the gunpowder propellant ignited this "fuze" on firing, and burned through to the centre during flight, then igniting or exploding whatever the projectile may have been filled with.

What is the purpose of a fuze? ›

The fuse breaks the circuit if a fault in an appliance causes too much current to flow. This protects the wiring and the appliance if something goes wrong.

How long does a fuze last? ›

How long does a fuse last? If the fuse does not operate then it will typically last 20 - 30 years as long as it is operating within its design limitations. If it is in a higher cyclic operating temperature i.e. very cold or very hot, then this can reduce its life.

How effective were proximity fuses in WWII? ›

The proximity fuze proved three to four times more effective than conventional time fuzes, and night kill-ratios increased by 370%. In 1943, naval guns fired 36,370 antiaircraft rounds.

Do fuses still exist? ›

Fuses and fuse boxes have long provided homes with essential protection against system temperature increases and excessive current flows. Though largely supplanted by circuit breakers and service panel boxes, fuses and fuse boxes can still be found in some older homes—still operating and often code-compliant.

What was the smallest proximity fuze shell? ›

30″/7.62mm(ish). Although the smallest practical HE rounds start at the . 50″/12.5mm range*, which is probably the smallest round in which you could fit a proximity fuse and HE charge and not be utterly useless.

What is the range of a proximity fuse? ›

Fuzes have been designed to operate at dis- tances up to 70 feet against aircraft and up to several hundred feet over ground. At these distances, the reflected signal is a small fraction of a volt.

How far can a proximity sensor work? ›

Capacitive sensing involves charging up plates, so it is somewhat slower than inductive: 10 to 50 Hz, with a sensing distance from 3 to 60 mm. Many housing styles are available; common diameters range from 12 to 60 mm in shielded and unshielded mounting versions.

What voltage do proximity sensors operate on? ›

It is a three-wire sensor with an open collector NPN transistor configured in a normally open mode. It requires a power supply of 10 to 30 volts DC.

What was the major turning point in the Battle of the Bulge? ›

Vith as the “turning point” of the Battle of the Bulge. By slowing the German's advance and defending a vital road junction at St. Vith, General Clarke's leadership during the Battle of St. Vith put the Americans on a path to victory.

How did WWII bomb fuses work? ›

Fuzes have two purposes. The obvious one is to cause the bomb to explode. When the bomb makes impact, the fuze has a spike or electrical circuit that detonates the bomb. If the fuze has a spike, that spike is driven into a small detonation charge that sets off the main bomb charge.

How does an artillery shell fuse work? ›

When the shell was fired the shock of firing set back a detonator onto a firing pin, which ignited the powder ring, when the burn reached the fuze setting it flashed through a hole into the fuze magazine, which then ignited the bursting charge in the shell.

What is a fuse in a missile? ›

A fuze is that part of an artillery projectile which detonates the explosive charge and ideally would detonate the shell in the most optimum position to inflict maximum damage to the target.


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