'The secret weapon of World War II' Hopkins developed proximity fuse

At dawn over Guadalcanal 50 years ago, two Japanese dive bombers plunged toward the cruiser USS Helena and ran smack into the future of warfare.

Until that January morning, ships without air cover were sitting ducks. Anti-aircraft fire was frustratingly inaccurate. With ammunition that exploded on impact, even the best gunners had to fire about 2,500 rounds on average to score a hit. Timed fuses that exploded a set number of seconds after firing worked a little better, but not much.


"Almost no one ever hit an airplane with the old-fashioned fuses," recalled Dr. James A. Van Allen, the discoverer of the Earth's radiation belt, who worked on fuses at the Applied Physics Laboratory of Johns Hopkins University during the war. "It would be just a sheer stroke of luck to hit anything."

But the Helena had more than luck that morning. It carried one of history's first smart weapons -- anti-aircraft shells armed with Hopkins' new proximity fuse, an electronic device designed to detect its target and detonate if it flew within about 75 feet.


That weapon was a closely guarded secret. So the pilots of the nimble Aichi 99 bombers may have felt they had the advantage as they screamed toward the Helena, an island of guns and steel as long as two football fields.

Within 90 seconds, two gun crews, firing an estimated 50 to 60 shells, brought down both planes. (Not all the shells fired had proximity fuses, but the ship's chief gunnery officer gave the new fuse credit for both kills).

The proximity fuse obviously didn't have the awesome power of the atomic bomb, which helped shape global politics after the war. It didn't have the broad military and civilian application of radar. But some think it was just as crucial to the Allied victory as the other two devices.

"In my opinion this is the real secret weapon of World War II," said Ralph B. Baldwin of Naples, Fla., a scientist who worked on the fuse project and later wrote a book about it. "It shortened the war drastically. And at the end of the war the general staffs of Japan and Germany didn't know what had hit them."

It helped gun crews destroy hundreds of dive bombers, torpedo planes and kamikazes in the Pacific. It blasted hundreds of German V-1 rockets in mid-flight over the English Channel and the newer V-2s over Allied-occupied Antwerp, Belgium.

On a foggy night during the Battle of the Bulge, Gen. George S. Patton's troops lined up and began shelling German tank crews. Most shells with impact fuses would have detonated on the ground. But Patton's men fired shells with proximity fuses designed to detonate about 10 feet above the ground, creating lethal storms of shrapnel.

"The new shell with the funny fuse is devastating," General Patton wrote to the war department. "I'm glad you all thought of it first."

The fuse was developed by hundreds of civilians working under a Navy contract, first at Washington's Carnegie Institution and, after March 1942, at the newly created Applied Physics Laboratory.


When Merle A. Tuve, chief physicist at Carnegie, persuaded federal officials to back an effort to develop the proximity fuse in August 1940, he picked up where many others had quit in frustration.

A feasibility study produced "one fairly thick book of failures and one thin book of possibilities," said Elmore Chatham of Silver Spring, a radio engineer and retired Applied Physics Laboratory employee who worked on the top-secret project.

Sifting through the slim possibilities, researchers concluded that the best bet was a miniature device that could bounce radio waves off nearby objects.

But radios had not been built small enough to fit in the tip of a projectile. And radio components -- particularly glass vacuum tubes -- were too fragile to survive being fired from guns.

Dr. Van Allen, now a 78-year-old professor at the University of Iowa, worked on creating rugged tubes. "It was an enormous challenge," he recalled in a recent telephone interview from his office in Iowa City.

Then in his late 20s, Dr. Van Allen was made a Montgomery County deputy sheriff so that he could legally carry a .45 automatic to protect shipments of experimental fuses on their way to area proving grounds.


Working with a Massachusetts firm that built miniature electronic tubes for hearing aids, Dr. Van Allen helped find new materials to cushion the glass where it was set on its base. Then researchers solved the trickier problem of keeping the fragile tungsten filaments inside the tubes from snapping when the projectiles were fired.

The scientists then developed a device called a "mousetrap spring" that looked, naturally, like a mousetrap. This enabled the structure supporting the filament to absorb the shock of firing, but kept the thin metal strip taut.

To test the tube designs, researchers took them to rural testing grounds -- such as Stump Neck, Md., along the Potomac -- and fired them vertically out of special guns. The tubes were then dug out of the ground and analyzed.

By January 1942, researchers had their shock-proof vacuum tube -- later patented by Dr. Van Allen and two colleagues. That summer, they tested proximity-fuse-tipped shells that hit their targets about half the time. By early 1943, about 5,000 fuses had been produced.

By the end of the war, five separate manufacturers and 80 subcontractors around the country were producing 40,000 fuses day -- using the labor of 80,000 people.

The Carnegie Institution was the center of fuse work only at the outset. It soon became clear that an independent group was needed to oversee development and manufacture of the new device. So, in early 1942, Dr. Tuve, a Hopkins graduate, approached the Hopkins trustees to ask them to sponsor what he called the Applied Physics Laboratory.


But Dr. Tuve and his colleagues couldn't tell the Hopkins board anything about the laboratory's $1 million military contract. The trustees said no.

"Then there was some hocus-pocus behind the scenes, and the next thing we heard of, President Roosevelt had gotten word to the president of the school," said Mr. Chatham. "He [Hopkins President Isaiah Bowman] went over and twisted the arm of the trustees, and they all got in lock step in creating APL, even though they didn't know what the hell it was."

Secrecy was crucial. The new fuse could easily be foiled by a jamming signal. "You could defeat it by putting a 50-watt transmitter in an airplane," Mr. Chatham said.

During the invasion of Italy, he said, the Navy broadcast spurious radio signals while firing anti-aircraft shells with proximity fuses. The Americans feared that someone using a sophisticated receiver might notice that each time the highly accurate shells were fired, they produced a signal like a hum on radio sets.

Not until late in the war were proximity fuses used in ground combat, for fear that a recovered dud would give away the secret.

Each of the 22 million fuses built during the war had a serial number, and the devices were shipped in sealed containers.


Mr. Chatham said that when he visited Navy ships to help train crews to use the new fuses, he carried technical documents in a briefcase equipped with lead weights, so he could toss it overboard if threatened with capture.

During the war, Dr. Van Allen, Mr. Chatham and others from APL visited Navy ships on active duty to help sell the new shells and train gunnery officers in their use.

Not all the sailors welcomed the new weapon. The biggest fear, Dr. Van Allen said, was that "somehow they would explode within the gun." (A mechanism in the fuse kept them from firing until seven-tenths of a second after they were fired. By that time, they were far from the gun.)

During the battle of the Philippine Sea, in June 1944, the device that Dr. Van Allen helped invent may have saved his skin. He was standing on the bridge of the USS Washington when a Japanese kamikaze bore down on the battleship.

The Washington erupted with gunfire, including anti-aircraft shells tipped with proximity fuses.

"I saw at least two or three 5-inch shell bursts in the vicinity of the plane, and then the plane dove into the water several hundred yards short of the ship," he said. "It was so close I could make out the pilot of the plane."