The weapon was developed in the greatest of secrecy. It was born in a physics lab before the war, but during the war became a massive project: led by physicists; employing tens of thousands on detailed tasks whose application they did not know; secured by barriers, unsmiling military police, security clearances and teams of counterspies; and encompassing a wide range of industrial effort. It was shared under the most stringent security guidelines with Britain alone, and gave the nations of the Anglosphere combat power unimagined before the war.
Its developers, led by a man whom they all came to admire enormously, were a cross-section, not of society, but of the academy: more diverse ethnically than American society at large, scarily smart, and sometimes, endearingly (or irritatingly) eccentric.
To take it from the blackboards of physicists to the Arsenal of Democracy, new precision needed to be achieved in old technology, new technology entirely needed to be invented, and the frontiers of miniaturization pushed far beyond the 1941 state of the art. The weapon was useless if it could not be delivered to the close proximity of its intended target, so it had to be shrunk, shrunk, shrunk from the initial conceptual models, which would never have fit inside the delivery system.
It required new methods of testing and evaluation to be developed, to prove that it really would work when put to the test. Once it passed these tests, its effect on the enemy was devastating.
Exposing it became a major objective of Soviet spies and the traitors who had sold themselves to them, including some who would be caught and punished, like Julius and Ethel Rosenberg, and some who were too highly placed for suspicion to stick, like Lend-Lease head Harry Hopkins, who tried and failed to have samples sent to his real superiors where his true loyalties lay: the Soviet Union.
We’re referring not to the hoary old story of the Manhattan Project, but the equally old, but much less-known, tale of the proximity fuze, or as its WWII cover name called it, the Variable Time, VT, fuze. That cover name has stuck and prox fuzes are still often called VT, despite the fact that it was intended as a deliberate obfuscation of what the fuze really did: solve an “impossible” gunnery problem of the 20th Century.
The Problem Was Hitting Moving Aircraft
Now, if you’ve ever cocked a cannon in Army or Marine artillery, you know a bit about the uses of VT; we’d like to ask all of you to be quiet, we’ll get to those shortly. But they’re not why VT was invented. The problem was anti-aircraft gunnery. In 1941 every military in the world was shooting at aircraft or preparing to do so, and they were missing. Just about all the time.
Airplanes don’t just sit still and let you whack them with your 88, you see. They move around. AA guns had compensating sights, and gun batteries had fire-direction computers, that could calculate where the airplane would be when the shell got to it — more or less.
(These computers were analog computers, with gears and knobs and whiz-wheels instead of circuits and programs, but they were faster and more reliable than the first many generations of digital technology that would replace them a few decades hence).
There was a little imprecision (not much) in the computer. There was a little more (again, not much) in the laying of the gun. Naval dual purpose and AA, and Army anti-aircraft guns, of the period were extremely accurate, but the problem still remained that for every shell delivered adequately to a vital structure of a Heinkel 111 or Aichi D3A, a much larger quantity, to steal a line from Maxwell Smart, “missed it by that much.”
A miss was as good as a mile, as the saying of the period went. In the initial effort to make a near miss somewhat more hazardous to enemy aircraft, various sorts of fuzes were conceived. The most common was a simple time fuze, and a lot of effort went into arranging the fire direction computer to support the optimum setting of an air defense battery’s time fuzes for effect on target. (Terrestrial artillery tried to use time fuzes to secure airbursts against troops in trenches, foxholes, or open-topped bunkers, too). An antiaircraft alternative was a barometric fuze, detonating at a preset altitude. This required fuze-setters to know the altitude of enemy aircraft (leading to the development of height finding radars, but also leading to the use of airplanes to shadow attackers and report their altitude to defenses). It also required them to know or have a way to dial in to the computer the ambient air pressure.
Winston Churchill describes this problem in Volume 2 of his History of the Second World War, (p.395), and notes: “[A]n aeroplane end on is a difficult target and a contact fuze will work only on impact.” Britain was working to develop such a fuze in the Battle of Britain, and while they were able to bring a mockup into the Cabinet Room to show Churchill, miniaturizing it was proving difficult, maybe impossible. So the British were stuck with the state of the art: impact, time and pressure fuzes.
These fuzes, the state of the art at war’s outbreak, would also be as far as the state of the art of Imperial Japan or Nazi Germany ever got. Many thousands of them had to be fired to ensure a hit on bombing aircraft. The watchword of the bomber theorists was, from the days of Douhet, “The bomber will always get through.” And it’s not easy to find an example of an aerial attack turned by time or baro fuzes alone, even in the sanguinary early days of RAF Bomber Command: the bombers took some hits, but most of them came through, unless they were hit by fighters, too.
How the VT Worked
The VT is essentially a small radar inside an artillery fuze. Initial concepts were wide open: photoelectric, acoustic, active radio (radar principle), or passive radio (detecting aircraft engine ignitions). In the end, after some experiments with photoelectric fuzes, active radio was chosen. Teh fuze radiated a radio wave, and if it received a reflection back, which it would if it were close to a target, pow! Here’s how E.D. MacAlister explained it to scientist Ralph B. Baldwin, who was just joining the project in 1941:
It’s really simple in theory but extremely difficult to convert into practice. The fuze is simply a specialized radio set. There’s a battery whose electrical energy is released by setback, the shock of firing from a gun. This battery furnishes three different electric voltages: one for the filaments of the vacuum tubes, one for the plates, and one for the grids. One of the tubes is an oscillator. In the nose of the fuze is a metallic cap, which together with the rest of the shell acts like a dipole. The oscillator tube thus has an antenna and emits a high-frequency radio wave in particular directions from the shell.
This continuous radiowave surrounds the moving shell, and when the shell pass is close to a target, the letter reflects a small amount of radio wave energy back to the fuse where it is detected by the same to been sent out the wave in the first place.
The plate voltage is “modulated” by the reflected wave, which is at slightly different frequency than the outgoing wave due to the relative motion of the shell and target. Thus, a beat note is set up and the plate voltage varies in frequency within the audio range of a few to a few thousand cycles per second. This audio frequency voltage variation is then passed through a three-tube amplifier.
When the period of the audio frequency wave and also its amplitude or intensity are exactly right, a thyratron tube, serving as a switch, is discharged. It completes a circuit that releases an electrical charge, which meanwhile has been stored in a condenser. The surge of electricity goes through a tiny wire in an electric squib, much like a dynamite cap.
The wire gets hot and the explosive in the squib goes off. This tiny explosion sets off about a cubic inch of a sensitive explosive called tetryl in the auxiliary detonator (auxdet to the Navy, booster to the army), which is at the bottom of the fuze.
This explosion sets off the explosive loading of the shell and it bursts the steel shell body into many hundreds of high velocity fragments. From the time the fuze says ago, the shell travels less than one foot before it bursts.
Now, if that sounds tough to follow, relax: even Baldwin found it “pretty heavy going.” But we include it here as (1) an illustration of the complexity of this pre-transistor electronic innovation, and (2), because we can’t imagine a simpler or clearer technical explanation of the fuze’s working than McAlister’s.
How the VT Was Developed
It started as an academic exercise, and began at Columbia University, but later found a permanent home at the Johns Hopkins University Applied Physics Lab in Baltimore, which was established (in part, to accomplish this task) in 1942. While the Manhattan Project was full of brilliance, the guy at the top of the APL and therefore the APL’s proximity fuze effort was Norwegian-American scientist Dr. Merle A. Tuve, the APL’s founder.
First they had to decide how to do it. They worked in parallel on optical and radio fuzes at first. It soon became apparent that the radio-frequency fuze was a success, and optical fuze development was cut off. (Unlike radio fuzes, optical fuzes worked only in daylight conditions).
The developers had to demonstrate the individual parts of the system; then, miniaturize them; then ruggedize them. From starting work circa 1940, Tuve had a working system within a couple of years. Decades later, the members of the team remembered the day when one of the proximity fuzes, fired from a Navy 5″ gun out over the water, worked for the first time. At the time, nobody made a note of the date! But the occasion was never forgotten. That day, only one of several test shots worked, the others detonating early or plowing into the water without detonating at all. But that one shot proved that the system was feasible. It had passed from science into the realm of engineering somewhere along the way.
This is all the more remarkable when you consider that the transistor, and all semiconductors, were over 15 years in the future: the fuze would have to work with the then-known electronic parts: vacuum tubes, capacitors, diodes and resistors, all powered by batteries. All these parts had to be ruggedized to survive the harsh environment of a cannon shell, with 20,000-g acceleration, 25,000-RPM rotation, and wildly varying temperatures all part of the fuze’s eventful last few seconds of existence.
The hardest parts to ruggedize were the batteries, which you might not expect, and tubes, which stands to reason. Their flimsy wires and glass enclosures were not optimum for high acceleration forces. When the scientists had tubes with 90% reliability, they still couldn’t relax: since the system needed three tubes to work (oscillator, amplifier, and thyratron), 90% reliability of any individual tube meant fuze that wouldn’t work even three-quarters of the time, and that even if every other component was 100% reliable. (The three tubes would have to be 96.6% reliable for the tubes to be overall 90% reliable, again assuming 100% function of everything else).
The lab aimed for an overall 80% reliability of fuzes in the field, which they came to learn meant they could accept only a very low failure rate of the tubes. Even an 80% reliable VT fuze was a godsend to the Navy.
Final and near-final fuzes were tested against mockups of Japanese and German aircraft, and against drones. In April 1942, an unmanned Piper Cub was hung from a balloon hundreds of feet over the water at Parris Island and shot at with live fuzes, but explosive charges replaced by a black-powder marking charge that would make a visible puff of smoke, but not fragment the shell. 20% of the shots would have been hits.
On 12 August 1942, a drone imitating a torpedo bomber “attacked” the new cruiser, USS Cleveland, in a live-fire test. A first drone failed before coming into range. The second bore in on the ship’s beam. Cleveland’s 5″/38 battery shredded the drone, taking it down with under ten rounds fired. A second drone was shot down just as quickly. The drones were the usual anti-aircraft targets, and they never got shot down. The drone officers had to report to the test officer that they were out of targets. The next day, they had located one more, which was set to emulate a level bomber, and attacked fruitlessly with time fuzes. The battery loaded the VT fuzes and destroyed the drone. End of test.
The crew of Cleveland was counting on one more liberty before joining the war, but they’d just seen the successful test of the Navy’s latest secret weapon. The Navy ordered the ship to drop off the APL scientists and technicians, but to keep sailing for the Pacific. The fuzes, already in pilot production, were suddenly a hot item in Naval supply channels.
The Fuze Goes to War
By October 1942, production was up to 500 fuzes a day and they were being flown from the factories to West Coast and Pacific depots. Production ultimately involved five companies performing final-assembly duties, with components coming in from over 100 factories belonging to some 87 businesses. Most of the component makers didn’t know what they were making parts for.
This fuze setter was part of the equipment in a 5″ dual-purpose mount. The fuze setting was directed from the fire director down to the secondary battery plotting room down to the mount. Wartime secondary armament instructions (.pdf) from the USS Massachusetts (battleship)..
On 5 January 1943, off Guadalcanal, the first Japanese airplane ran afoul of VT fuzes fired by the aft 5″ battery of the light cruiser USS Helena. Only three salvos were required to down the Aichi D3A “Val” dive bomber. The carriers Enterprise and Saratoga were also equipped with the new technology at that point, and from that point on the Japanese began having a harder time sinking American ships.
(Aside: Helena, CL-50, was famous for her gunnery, which would lead to her sinking. Japanese destroyers used her muzzle flashes to target her in July, 1943 and slammed three torpedoes into her, causing the ship to break into three pieces and sink with a loss of over 180 lives. Most of the crew did survive, although some were not rescued for days. A Helena sailor’s remains were found as late as 2006 on one of the straits islands).
Along with the fuzes, the Navy hit up the APL for new gun directors that would be optimized for VT, not time, fuzes. These went to sea for the first time with the battleship USS Missouri, and were effective against kamikaze attackers.
The VT Fuze Joins the Army
The highest priorities for fuze development were Naval, and so the Mk 32 was the first made, soon followed by equivalents for Royal Navy applications. But the VT Fuze was developed for Army anti-aircraft applications almost as soon as the Navy had it in production; one of the Cleveland’s drones was killed with an early prototype of the Army 90mm AA fuze screwed via adapter into a Navy 5″ shell. The Army fuze came from miniaturization developments that led to the Mk 45, and the British soon got forces for their Army AA guns as well (just in time for the V-1 buzz bomb attacks). But the AA application was only one application for proximity fuzes.
The more fruitful application, and one that would be key in several late-1944 battles, was creating a guaranteed airburst at an optimum height over enemy troop positions. The munition was, in a word, murderous to troops that didn’t have substantial overhead cover. LTG George S. Patton gleefully reported on the devastation wrought on German troops caught in the open by VT-fuzed barrages during the Battle of the Bulge.
At first, the VT fuzes were such secrets that they were removed from artillery positions during visits of foreign VIPs or American or British reporters. By war’s end, the effect of the fuze had let the cat out of the bag.
How the VT Fuze Spread to Other Nations
The Germans had heard rumors of the fuze, but never got hold of a working copy; they thought they were dealing with a “fuze with an electric eye,” and they tasked their spies to find out. The spies (Frederick Duquesne, Herman Land, and Lilly Stein of the 33-member Duquesne spy ring) never found the detailed information for the still-under-development fuze before a double agent and an FBI sting led to them being rolled up in 1941. The picture shows Duquesne, right, a retread WWI saboteur, with double agent William Sebold, left, a naturalized American who had immediately gone to the FBI when a Dr. Renken of the Abwehr (real name, Nickolaus Ritter) approached him in 1939, and was taken by FBI agents through a two-way mirror (note the clever positioning of the calendar so as to be in frame). The Deutsche Wehrmacht never got the secrets of the American fuze. Captured documents showed that the Germans had been trying to develop such a fuze throughout the war, but had made less progress by 1945 than the APL made by 1942.
The US shared the VT fuze with Britain from the beginning of development. After the initial development for the 5″/38 dual-purpose naval gun, and concurrent with development for additional US AA weapons, 4 Royal Navy calibers were added to the development schedule. Later, after the development of land fuzes for the US Army, 6 more for the British Army were added. By war’s end, VT fuzes had been adapted to 28 different shells. The British had developed their own conceptual fuze as early as 1940, but they’d gotten stuck on miniaturizing and hardening the electronics, and welcomed the US project.
The Japanese never understood why they launched so many kamikaze Special Attack aircraft to achieve such meager results. With a few well-known exceptions where attacks saturated US defenses, the entire Special Attack program was a waste of resources and of men’s lives, and prox fuzes are one reason why. Of course, Japan was beaten by the spring of ’44, but wouldn’t believe it until they’d suffered the most absolute naval defeat since ancient history: while the Navy’s had some 2,500 vessels, most of them were small craft; all capital ships were on the seafloor, and the largest ship left fully combat-worthy was a cruiser.
The Russians got wind of the fuze and tried to come by it legitimately, first, by requesting it from Lend-Lease. They were turned down. Then, they tried to put pressure on through Harry Hopkins, to bend the rules (Hopkins frequently did this for his Soviet masters) and that didn’t work, either, because the subordinate officers wanted it in writing. Meanwhile, they tasked espionage assets, and the couple that came through for them with the VT fuze design would go on to fame — Julius and Ethel Rosenberg. Well, fame and Old Sparky at Sing Sing.
And there’s irony for you: far more Americans have been killed by Soviet copies and improvements of American VT fuzes than have been killed by Soviet copies and improvements of American nukes. But the spies who gave up both weren’t even charged for the little, seemingly inconsequential, war-winning little weapon. If they had been, they would never have received the death penalty. But if you truly understand the weapon, they deserved to fry for this as much as anything.
Baldwin, Ralph B. The Deadly Fuze: Secret Weapon of World War II. San Rafael, CA: Presidio Press, 1980.
US Navy. Radio Proximity (VT) Fuzes. Naval Historical Center Website. Retrieved from: http://www.history.navy.mil/faqs/faq96-1.htm