Deadly Accuracy
BEFORE DAWN ON JANUARY 5, 1943, FOLLOWING A bombing raid in the Solomon Islands, an Allied naval task force rendezvoused off Guadalcanal. It was thirteen months after Pearl Harbor, and the United States was struggling in every theater against past neglect of its military forces as well as against powerful and battle-wise enemies. The Americans had gained the upper hand on the strategic island of Guadalcanal and Allied forces were pushing northward into New Guinea, but victory in the Pacific was far from assured.
As daylight spread, the morning calm was shattered by the whine and roar of enemy aircraft attacking. Before the ships’ gunners could respond, four Japanese VaI divebombers had completed their run. Three bombs whizzed past the escort cruiser Honolulu before exploding in the sea. Another struck the New Zealand cruiser Achilles dead-on on number three turret, killing 13 men and wounding 8.
On board USS Helena , Lt. “Red” Cochrane, the son of the chief of the Navy’s Bureau of Ships, commanding the aft battery of five-inch guns, opened fire on a fleeing VaI. American antiaircraft fire had rarely been effective at that distance, and the Japanese pilot must have thought he was home free, since he took no evasive action. As Cochrane’s salvos burst in ugly black smudges, however, the VaI erupted in flames and spiraled into the sea. The VaI was warfare’s first victim of a proximity fuze, a supersecret American weapon whose contribution to the outcome of the war has never been fully credited.
The proximity fuze, which explodes in the air near its target, was thenceforth used with devastating effect against Japanese airplanes in the Pacific, helping ensure U.S. supremacy over the ocean. During the rest of the war, it dramatically reduced the chances of survival for Japanese planes attacking U.S. ships, and it emboldened naval commanders from the Solomons to Okinawa to press aggressively against Japan.
The fuze was later turned against V-1 flying bombs in England with considerable success. In December 1944 it entered use against ground targets, doing service against German infantry formations, often shrouded by fog, during the Battle of the Bulge. German troops had never encountered such destructive and accurate artillery fire.
Before the proximity fuze, antiaircraft crews could only dream of a shell that would automatically detonate within feet of a target. Gunners had to estimate the distance and altitude of the target and hand-set a mechanical time fuze to go off at the right instant, or else they had to score a direct hit with an impact fuze. Most planes moved too fast for a precise time-fuze setting and presented too small a target to hit with an impact fuze. So it took about 2,400 rounds of time-fuzed ammunition to bring down one aircraft. By the end of the war, proximity-fuzed rounds were downing planes six times as efficiently.
The proximity fuze’s secret lay in its electronics, which consisted mainly of a miniature radio transmitter and receiver. The transmitter broadcasts a continuous signal; the signal reflects back from the target to the receiver; and the outgoing and incoming signals set up an interference that is intensified by a small amplifier. When that interference reaches a designated intensity and frequency, the fuze detonates and sets off the explosive in the shell. The process is like determining when you’re near a P.A.-system loudspeaker by listening to the feedback screech when you speak into the microphone.
Other nations had tried to develop such a device, but they had been stymied by the tremendous forces that act on a spinning shell shot from a gun barrel. Designing the thing was only a first step; making it actually work seemed almost impossible. Had the Germans succeeded in their attempt at it, the outcome of the war in Europe might have been different; the Japanese did produce a version for bombs, but they used it only once, late in the war. They manufactured 12,000 of the weapons and stockpiled them for use against the anticipated American invasion of the home islands.
The British wanted a proximity fuze early in the war to counter the Luftwaffe raids on London and other British cities during the Blitz. They also wanted it to protect their navy; the vulnerability to airpower of His Majesty’s warships was tragically demonstrated when Japanese aircraft sank the battle cruiser Repulse and battleship Prince of Wales off Malaya on December 10,1941. No amount of traditional antiaircraft fire could stop those smothering attacks.
The Americans desperately needed it too, for the same reasons, but some Army Air Forces generals, including the AAF’s chief, H. H. (“Hap”) Arnold, advised against pursuing it. They feared that its secrets would be discovered and used against the Allies or that its effectiveness would be compromised by jamming it with radiation of the appropriate frequency. (In fact, the Germans did gather fragments of proximity-fuzed shells during the Bulge, but they couldn’t piece them together before the war ended.) For this reason, when the fuze did emerge, it was used at first only in naval operations, where misses were unlikely to be recovered. Not until October 1944 were proximity-fuzed shells approved for use against land targets on the Continent.
The fuze’s formal American development began in the summer of 1940 under the auspices of the National Defense Research Committee, which had been set up to organize and coordinate military research. The NDRC eventually put two groups to work on versions of the device. One group, known as Section E, was headed by Alexander H. Ellett of the University of Iowa and the National Bureau of Standards. Section E collaborated with the Army on a fuze for nonrotating projectiles, such as bombs, rockets, and mortars, and performed other defense-related work. Section E was separated in July 1941 from Section T, which was led by Merle A. Tuve of the Carnegie Institution. Section T worked with the Navy on a version for rotating projectiles, initially the widely used five-inch shell.
Section T’s task was especially daunting. Its scientists and engineers would have to develop electrical and mechanical components rugged enough to withstand the tremendous forces generated when fuze and shell were blasted from an artillery piece. (A shell can be subjected to 20,000 g’s during firing, and after leaving the gun barrel, it can spin at up to 30,000 rpm.) Moreover, they’d have to make those components small enough to fit into the nose of a shell. Of course, miniaturized circuits and transistors didn’t yet exist.
So great were the challenges facing the builders of a proximity fuze for an antiaircraft shell that many scientists simply didn’t believe it possible, but nonetheless, by late 1940 the British and Americans were working together. In England, W. A. S. Butement had already invented a circuit design that would prove to be the heart of the fuze: a radio-wave emitter known as a Hartley oscillator resistance-coupled to an audio amplifier. British scientists also worked on a photoelectric version of the fuze and a pulse fuze that could be set off by remote control from the ground. However, they found that the photoelectric one could work only in daylight without heavy cloud cover (and could be set off by direct sunlight), while the pulse fuze turned out to be too complicated to use effectively. So the radio fuze got most of their attention.
The first order of business was to develop vacuum tubes that could withstand the violent forces. The British assembled rugged prototypes while American scientists studied off-the-shelf tubes, including some made for hearing aids, that were both small and tough by the standards of the day. Then they began subjecting their tubes to extreme stresses. They fired them from artillery pieces, mounted them on lead blocks and then shot at them with .22s, dropped them from great heights, and spun them in a centrifuge at up to 30,000 g’s. They reinforced the filaments to better withstand shock and spin, and by February 1941 they had developed three types of miniature tube that could survive being fired inside a five-inch shell. By that fall, only 5 percent of tubes were breaking during test firings.
The next hurdle was the development of a power source. First the scientists came up with a miniature dry battery, but its shelf life was too short, especially in the hot, damp South Pacific. Section T’s researchers replaced it with a new wet battery, which stored an electrolyte of chromic acid in a glass ampoule. The ampoule would shatter upon firing, and the spin of the shell would distribute the liquid electrolyte over the battery plates, which had zinc on one side and carbon on the other. Electric current would then activate the radio transmitter within a tenth of a second. The battery had an indefinite shelf life.
Without some sort of safety device, a proximity fuze could activate the instant it left the gun barrel because of its proximity to the vessel it was fired from. In the larger shells, this could be handled with a standard time fuze that kept the proximity fuze from immediately arming; for the smaller ones, scientists had to devise a unique electronic switch. It consisted of two chambers separated by a porous membrane, which was made of chemical filter paper in early versions and later of sintered nickel. Mercury in one chamber maintained an electric short circuit that kept power from reaching the fuze; the spin of the shell forced the mercury through the membrane into the other chamber, removing the short to arm the fuze. Other safety devices were added to provide additional protection.
By May 1941 the basic electronic design for the brains of the radio proximity fuze was complete. It included an oscillator, amplifier circuit, battery, safety device, and detonator. Essentially it amounted to a specialized and extremely tough radio set. The oscillator emitted radio waves from the shell, the approaching target reflected them back, and the nose cap, acting as an antenna, picked them up. The reflected waves were at a slightly different frequency from the outgoing ones because of the relative motion of fuze and target, and the difference created interference in the form of a beat note. This beat note was detected by the amplifier tubes, and at a prescribed frequency and output level, a thyratron tube tripped a switch to close a circuit to release an electrical charge stored in a condenser. The surge of electricity poured through a tiny wire whose sudden intense heat then set off a small explosion; this in turn set off the detonator to explode the shell into lethal fragments. From the moment the fuze activated until the shell burst, the projectile traveled through the air less than a foot.
The first formal tests of the fuze took place August 12, 1942, on Chesapeake Bay. Five-inch guns on the cruiser Cleveland opened fire on a pair of Navy drones and quickly blew them out of the sky. This admirable performance shocked the naval officers in charge of the drones. They were not used to losing their precious craft so quickly in target practice.
By the next month, production of the fuzes for five-inch guns was at 400 a day, and by November, 4,500 proximityfuzed shells were on their way to Adm. William F. Halsey’s fleet in the South Pacific. The Helena had her inaugural kill a few weeks later in 1943.
While continuing to improve the proximity fuze for fiveinch antiaircraft guns, Section T expanded its activities by developing a fuze for antipersonnel weapons. The idea was to enable a shell to detonate just above troops on the ground, where artillery airbursts are deadliest. To achieve this, two main hurdles had to be overcome. First, no one in the Army knew the optimum height for exploding such a shell. Second, the fuze would have to be further miniaturized to fit into the Army’s howitzers, some of which were smaller than the Navy’s antiaircraft guns.
To answer the height question, Army staffers laid boards flat on the ground and exploded shells above them. Then they studied the blast patterns in the wood. They found that the optimum height for a burst varied with the caliber of the shell, from 30 feet and up for the smallest, a 75mm, to 72 feet off the ground for a big 240mm howitzer. As Ralph B. Baldwin, a developer of the Army’s proximity fuze who interpreted data from the first field-artillery tests, wrote in The Deadly Fuze: The Secret Weapon of World War II (1980), this range of requirements “complicated the picture a great deal because we had twenty-nine different muzzle velocities and twenty-nine different rates of spin for these howitzers.”
Developing a proximity fuze for artillery shells, like making one for antiaircraft guns, was an unprecedented challenge, but it was also a natural outgrowth of the art of fuze making, which had started to mature at the beginning of the twentieth century. For centuries, artillery fuzes had been either basic impact mechanisms or simple “powder train” time devices, in which compacted gunpowder burned until its flame reached and ignited an explosive charge. Achieving detonation at a precise moment with a powder train had been more an art than a science, and gunners had had to guess, on the basis of experience, how long to make the fuze. By the turn of the century, engineers and scientists were beginning to craft complex and precise fuzes. Time fuzes became horological works of art, masterpieces of springs, levers, gears, and pinions.
Section E’s work on a device for bombs, rockets, and mortar shells was the least daunting of the proximity-fuze projects, but it had its own great difficulties too. The bomb team quickly realized that a fuze using the Doppler effect with reflected radio waves would be most effective for their purposes, and without having to deal with the concussive or rotational force of a gunshot, they developed a working radio fuze relatively quickly. The fuze had to be strengthened to withstand the vibrations that built up in a bomb casing when dropped from high altitudes and had to be made capable of operation in the subzero temperatures bombers passed through. Section E engineers came up with a power source by attaching a miniature generator to a wind-driven turbine in the nose of the bomb; the turbine would start spinning once the bomb was let loose.
In the fall of 1943, Section E began designing a fuze for mortar shells and, because they had to be small, pioneered in printed-circuit technology in the process. Applying techniques that were already in use by manufacturers of electrical components, engineers were able to print an entire circuit (except for the tubes) on a ceramic base, saving valuable space and reducing the need for soldering. Early tests of the mortar fuze in the spring and summer of 1945 gave encouraging results, but the war ended before they could be put into production.
The proximity fuze was no atomic bomb that left a mushroom-shaped mark on the history of the world; it was kept secret for most of the war and was invisible in action. But studies clearly demonstrated its value. During 1943, 25 percent of the antiaircraft rounds fired by naval five-inch guns were proximity-fuzed (the rest were time-fuzed), but they accounted for 51 percent of airplanes shot down. That means they were three times as effective as the time fuzes. By the end of the war, improvements in the fuzes would make them six times as effective as time fuzes.
James V. Forrestal, Secretary of the Navy starting in May 1944, was unequivocal about the results. “Without the protection this ingenious device has given the surface ships of the Fleet,” he wrote in November 1945, “our westward push could not have been so swift and the cost in men and ships would have been immeasurably greater.” Gen. George Patton put it even more directly: “The funny fuze won the Battle of the Bulge for us.”
The proximity fuze has seen continued improvements over the last half-century. Today it carries the same basic components as during World War II, but it often uses infrared radiation instead of radio, and it can be set to start work just before it reaches the target, making it essentially jamproof. The detonation trigger can be made more sensitive, functioning when the frequency pattern of the return signal matches one stored in its memory for optimum detonation conditions. According to George McNally, of the U.S. Army Armament Research, Development, and Engineering Center, in Adelphi, Maryland, this arrangement “reduces the fuze’s dependence on the strength of the return radio frequency signal,” which “is very sensitive to scattering due to the ground or whatever target the fuze is used against.” The Patriot missile of Desert Storm fame employs a proximity fuze, and some 56 percent of all the artillery rounds fired during the Gulf War had the fuzes.
World War II-era fuzes were “shell-excited”: The entire projectile served as an antenna. This meant that the size of the shell affected the choice of frequencies. Today’s versions use higher frequencies and carry an isolated antenna independent of the shell’s size. “We can design the antenna pattern to match the application, such as looking forward for an artillery or mortar fuze or to the side for air targets,” McNally says.
More and more, artillery and mortar projectiles are employing a multi-option fuze that can work by proximity, by time, or on impact. The soldier firing the weapon picks the desired mode to yield the optimum lethal effect for the particular situation. The Army is developing a replacement for the M-16 rifle that will fire both standard 7.62mm rounds and 20mm grenades; when it shoots the grenades, it will use a laser rangefinder to measure the exact distance to the target and then time the projectile to detonate at that distance. “It will function like a proximity-fuzed round, but without using radio frequencies,” says McNally.
Also in development are artillery rounds that will be guided to their targets by Global Positioning System technology and will adjust their projected impact points using drag rings or movable fins. Future projectiles may also contain sophisticated processors that can see through clutter to destroy targets hidden under trees or bushes.
The story of the proximity fuze goes on, the continuation of one of the least-known sagas of World War II. Even today it gets little more than passing mention in histories of the conflict. Nevertheless, it deserves to be ranked alongside the atomic bomb and radar as one of the top scientific achievements of the war.
