Artillery Proximity Fuses: A Major Technological Breakthrough of World War II

October 19, 2020 Topic: History Region: Europe Blog Brand: The Reboot Tags: World War IIU.S. ArmyArtilleryBattle Of The BulgeMilitary History

Artillery Proximity Fuses: A Major Technological Breakthrough of World War II

The use of proximity fuses in the Battle of the Bulge was a milestone for Allied scientists seeking to develop an efficient artillery shell.

Key Point: The creation of the proximity fuse has never drawn as much attention as the development of the atomic bomb or radar.

Early on the morning of December 16, 1944, the commander of the U.S. 406th Artillery Group, Colonel George Axelson, had a difficult decision to make. The Germans had just launched the offensive that would become known as the Battle of the Bulge, and one of their first targets was the 38th Cavalry Squadron, dug in around Monschau, Germany. The lightly armed cavalry troopers needed help, and the commander quickly called for artillery support from the 406th.

Axelson had just the thing: a new, secret artillery shell that had just been issued. The problem was that Allied commander Dwight Eisenhower had not yet given permission to use the weapon. Axelson decided that the emergency trumped the restrictions and ordered his gunners to use the new shell. Minutes later, rounds equipped with a new radio proximity fuse started exploding right over the heads of the attacking Germans. The attack collapsed.

Use of the proximity fuse in the Battle of the Bulge marked a final milestone in one of the most extraordinary scientific efforts of the war, rivaling that of the atomic bomb. Like the Manhattan Project, it involved teams of scientists struggling to overcome technical and physical obstacles in absolute secrecy. An estimated 3 percent of all the physicists in the United States were working on the project at one point.

The “Wizard War”

The proximity fuse—a fuse that would explode just before reaching its target—had long been a dream of gunners. The two existing types of fuses, contact and timed, left much to be desired. A contact fuse literally had to hit its target to work, and a timed fuse depended on the judgment of whoever set the timer. In the Napoleonic and American civil wars, soldiers sometimes disabled timed shells by putting out the fuses after the shell landed

Neither fuse was well suited to dealing with airplanes. An antiaircraft shell had to go off within 100 feet of a plane to be sure of damaging it. Antiaircraft shells with contact fuses had to actually hit a moving aircraft, while timed fuses that were even a fraction of a second off could explode too far away to do any damage. The only practical solution was to fire hundreds of rounds at a target to increase the chance that one of them might hit it.

Getting a more sensitive and technically advanced fuse into antiaircraft ammunition was far from easy. For one thing, the shells were not very big, and the components of the time were not very small. The acceleration generated when the guns fired could shatter things like glass tubes. When World War II began, the technical challenge quickly became a high priority. German efforts never went very far, and the Japanese did not develop and deploy a workable fuse until the end of the war.

British efforts to come up with a proximity detonator were part of what Prime Minister Winston Churchill called the “Wizard War.” The British initially concentrated on photoelectric fuses, which used light-sensitive receptors to determine when a target was close enough to hit. Photoelectric fuses could fit into the larger warheads of antiaircraft rockets and were better known (several inventors had already filed patents on various designs).

Churchill, stressing the enormous importance of getting a proximity detonator in production as soon as possible, was among those pushing the photoelectric fuses. While British scientists also worked on a radio transmitter, the general feeling was that it would take too long to develop. Photoelectric triggers had their own problems, among them a tendency to go off because of light reflection from other sources, and the fact the fuses were of little use at night. By the time the British worked out a production-ready model, their U.S. allies had come up with their own, more effective radio fuse.

NDRC: Building a New Proximity Fuse

American efforts began in 1940 with the creation of the National Defense Research Council. The group, chaired by Carnegie Institute president Vannevar Bush, was to coordinate and direct research efforts on military-related projects. Immediately upon creation, NDRC asked the various services for their wish lists. At the top of the Navy’s list was development of a proximity fuse for its antiaircraft guns.

NDRC turned the problem over to the Department of Terrestrial Magnetism at the Carnegie Institute, and its director, Dr. Merle Tuve. Tuve’s group later became Section T of NDRC’s successor, the Office of Scientific Research and Development. Researchers explored a number of different approaches, including photoelectric, radio wave reflection, acoustic, and ground-controlled fuses.

NDRC pulled in researchers from the National Bureau of Standards to work on proximity fuses for Army ordnance, primarily bombs and rockets. An informal division was set up, with the Navy and Section T working on fuses for rotating projectiles such as shells, and the Army group (later separated into its own division, Section E) developing nonrotating projectiles such as bombs, mortars, and rockets.

Section E’s Army ties meant those researchers had to cope more with internal opposition, specifically from the Army Air Forces. AAF brass, including General Henry “Hap” Arnold, were concerned that development of the fuses would inevitably lead to the enemy getting its hands on them and turning them against Allied planes and pilots. In addition to tight security, this led to an unusual edict from the Combined Chiefs of Staff: there would be no use of a proximity fused weapon anywhere that it could be recovered and reverse-engineered.

Testing the Components of the Fuse

Research started to take off after British scientists visited the United States in September 1940 as part of an information exchange program. The British group, led by Sir Henry Tizard, brought a number of developments, among them a circuit design for a fuse utilizing a radio oscillator. Tuve’s group realized that if the components could be made strong enough to withstand the shock of firing, they could be fitted into a shell for a 5-inch antiaircraft gun. That would allow sending a radio signal from the shell and using the signal’s reflection from the target to trigger the fuse—in essence, a sort of mini radar.

Developing rugged components was a critical obstacle to making a radio fuse.  Section T researchers started testing various means to make sure the glass tubes and circuits could survive the shocks. They mounted tubes in metal blocks and fired .22-caliber bullets at them, put them in lead tubes and dropped them off buildings, and fired them from homemade cannons. At one point, Section T even explored the idea of metal vacuum tubes. However, researchers found that mounting the tubes (which were about the size of a pencil eraser) in blocks of plastic and coating them with wax enabled them to withstand acceleration forces of up to 22,000 gs.

Researchers also had to tackle finding a battery small enough to generate the power for the radio. While cutting down dry-cell batteries (like those in hearing aids) worked, they proved to have a very short shelf life. Realizing that a battery was basically zinc plates immersed in acid, the developers came up with a battery made of a glass ampule filled with acid surrounded by small metal plates. When the shell was fired, the glass broke and the acid covered the plates. While it did not generate power for long (less than two minutes), it was more than enough for the shell’s flight.

Scientists still had to make sure the shell would not go off too soon. If the radio transmitter started too quickly, the gun could reflect the radio wave and the round would go off in the gun barrel, with obviously unfortunate results for the crew. Developers designed fittings that would short-circuit the power to the transmitter for a half second.

The Navy’s Proximity Fuse in Action

Work on the shells moved more quickly than the nonrotating projectiles, in part because of the Navy’s intense interest—a senior officer on the project said every month’s delay in developing a proximity fuse was equivalent to losing a cruiser. Section T had a successful test of a 5-inch shell with a transmitting fuse by June 1941. By the first half of 1942, they had progressed to test-firing over water and at model versions of Japanese airplanes to determine damage patterns.

The first practical test came on August 13, 1942. The cruiser USS Cleveland, equipped with proximity-fused 5-inch antiaircraft shells, destroyed two drones in rapid succession. The results stunned Navy officers, particularly those in charge of the drones. They had never seen any of them totally annihilated before and had no more to send against the Cleveland. They did find a third drone the next day, but the shells brought it down almost immediately.

That was enough for the Navy. By the end of the year, 5,000 rounds of the new ammunition were shipped to the Southwest Pacific, where they were distributed to the carriers Enterprise and Saratoga, and the light cruiser Helena. The proximity fuse entered the war on January 5, 1943. A cruiser and destroyer task force was returning to Guadalcanal in the Solomon Islands after a raid against enemy airfields when Japanese dive-bombers struck. On board Helena, Lieutenant “Red” Cochrane, in charge of the 5-inch antiaircraft guns, opened up on one of the bombers and brought it down with his second salvo.