The production proximity fuze was originally designated the "T3G Device" and then the "VT", which was suggested by the British, since it misleadingly implied "variable time" or "velocity triggered", which told snoops nothing. The fuze sent out a continuous radio signal in the range of 1.67 to 1.36 meters (180 to 220 MHz), and detonated when the shell got within a few wavelengths of a target. A backup self-destruct timer fuze destroyed the shell before it fell back to earth if it missed the target.
Building practical circuitry that could fit into an anti-aircraft shell and survive being shot out of a gun, with accelerations of thousands of gees and spinning at hundreds of revolutions per second, was a major engineering accomplishment, particularly in the days before solid-state electronics. A miniature ruggedized vacuum tube, the "T3", was developed by the Sylvania company and put into massive production. A particularly tricky issue was powering the proximity fuze, since conventional dry cells would drain away in storage.
The answer was to develop a battery that was inert until the shell was fired. The shock of firing broke a glass ampoule, flooding the electrodes with an acid electrolyte, which powered up and activated the fuze. The battery only worked for two minutes, but that was well longer than the lifetime of the shell after firing. The fact that the battery wasn't active before firing also provided an arming mechanism, since the shell wouldn't be fully powered up until a tenth of a second after it was in flight, by which time it would be hundreds of meters away. The device was called a "reserve battery" even though it was the only battery in the fuze, the derivation apparently being from the fact that it was held in reserve until firing.
However, Tuve didn't rely on this feature as the primary arming mechanism since it wasn't entirely predictable. The fuze also featured an ingenious arming mechanism activated by the shell's spin. Instead of a complicated and bulky centrifugal clutch system, the fuze was fitted with a porous cylinder offset from the center, with the core of the cylinder filled with mercury. Under normal conditions, the mercury provided a conductive path that shorted out the fuze circuitry, but when the shell was set to spinning rapidly, the mercury leaked out through the porous material, opening the circuit. The charge time of the capacitors in the circuitry also provided a safety delay.
The proximity fuze was longer than the older timed fuzes and protruded into the interior of the shell, but the greater accuracy more than compensated for the reduction in explosive charge. Although proximity-fuzed shells tended to have a high rate of misfires, for example sometimes being set off by entering heavy cloud, they were still much more effective than timed shells. They were built in a wide range of mark numbers for different types of American and British guns -- the "Mark 53", for example, was for US Navy 127 millimeter (5 inch) guns.
Mark 53 VT fuze
* The proximity fuze project was top secret, with shipments protected by armed guards and the fuzes stored under lock and key. Even when the fuzes were deployed, they were at first restricted to naval forces in the Pacific, where it was unlikely that a dud shell would be recovered by the enemy. There were worries not only that the Axis might be able to duplicate the fuze, but could even generate countermeasures against it, running a sweep of radio waves through the fuze frequency range to set the shells off prematurely. This actually happened by accident in a few cases, when the fuzes were triggered by longwave radars that happened to be on their frequency.
In the summer of 1944, the Germans began firing their "V-1 flying bombs" at London. The V-1 was a small winged missile powered by a pulsejet engine that gave it a distinctive buzzing sound in flight. It flew at high speed on a straight and level trajectory, held on course by a gyroscopic guidance system. The flying bombs did terrible damage to London at first, but fighter and ground defenses were refined and slowly managed to pick off more and more of the bombs. The Americans released SCR-584 radars and M-9 fire control systems intended for the Continent to British 94 millimeter (3.7 inch) anti-aircraft gun batteries, and also set up similar batteries with their own 90 millimeter anti-aircraft guns. The batteries were set up in a screen along the English coast.
Churchill pleaded for the proximity fuze, pointing out that the shells would only fall in the English Channel or on English soil. He got his wish, and the combination of SCR-584, M-9 director, and proximity fuze proved to be the most effective countermeasure against the flying bombs. The straight and level path of the intruders made them relatively easy targets, and after a learning curve, fewer and fewer of the V-1s got through to London. In the end, statistics showed that it took 156 proximity-fuzed shells to kill a flying bomb, which may not sound good except in comparison with the 2,800 conventional anti-aircraft shells required to accomplish the same trick. Incidentally, the proximity fuze had been designed to engage larger flying machines than the V-1, and so the fuzes supplies to the defenders were "recalibrated" following tests against a static V-1 model back in the US.
The TRE also developed a rangefinder to help fighters shoot down the flying bombs at night. It was a simple but clever device, developed on short notice. All it did was optically split the image of the bomb's orange jet exhaust and focus the images so they came together at a range of 180 meters (600 feet), providing the pilot with a glowing indicator that indicated he was in firing range.
The V-1 attacks ended as the Allies overran the launch sites in northern France. Although the Germans tried to continue attacks by air-launching the buzz bombs from Heinkel He-111 bombers, the effort proved expensive in men and airplanes and was abandoned. Unfortunately, as the V-1 flying bomb threat faded out, the Germans began launching V-2 rockets that came hurtling down from space at over 4,800 KPH (3,000 MPH). The rockets came in fast, giving little warning, and were impossible to intercept.
* Although the proximity fuze had been developed for anti-aircraft shells, of course Tuve and his people had always known it could be used for conventional artillery as well. A proximity fuze attached to ground bombardment artillery would allow the shells to burst in the air just before impact, showering the target area with fragments and leaving few places for victims to hide. Demonstrations of howitzer shells with proximity fuzes were performed to Army brass in September 1943. Although the demonstrations were characterized by a good deal of bungling, the Army was still impressed and wanted to get the proximity fuze into the hands of the field artillery as soon as possible.
Most of the field artillery used howitzers, which often used high-angle fire trajectories. That led to a problem in that sometimes small powder charges were used, resulting in low acceleration and spin that defeated the fuze arming mechanisms. Gun crews were told to use heavier powder charges with proximity-fuzed shells.
There was also the worry about the fuzes falling into enemy hands. In fact, the Germans were working on proximity fuzes themselves, mostly for rockets. One issue was that the V-2 missile tended to bury itself before detonating, reducing its effectiveness, and the Germans were also working on anti-aircraft missiles. They experimented with acoustic, optical, and radar proximity fuzes, but the effort was unfocused and went nowhere. An intact VT fuze might have helped them a great deal, but by late 1944 the Reich was obviously on its last legs. There was little chance that the Germans would have the resources or the time to duplicate proximity fuzes if they fell into their hands, and after strong lobbying by fuze advocates, their use was greatly expanded.
During the Battle of the Bulge in December 1944, proximity fuzes were installed on artillery shells for ground bombardment. A backup impact fuze detonated the shell if the proximity fuze failed. Proximity fuzed shells proved devastatingly effective, and shell-shocked German soldiers surrendered in large numbers. Allied brass worried enough about the possibility of the Germans copying the VT fuze to order the development of jamming systems that would cause proximity-fuzed shells to detonate prematurely. The jammers were built, but they were not needed. The Germans simply didn't have the time left to copy the fuze.
