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Proximity Fuse: The Little Device that Helped Win World War II

Imagine a device so crucial to winning the Second World War that it was developed under secrecy similar to the atomic bomb and the invasion of Europe on D-Day. 

A device that brought together the most astute minds and secretive government agencies from both sides of the Atlantic, with the common aim of halting the Axis juggernaut sweeping across Europe and the Pacific. 

Now imagine that the device was small enough to fit in the palm of your hand, that more than 20 million were manufactured, and that despite its role in winning the war, it has largely been forgotten by all but the most ardent tech geeks and history buffs. 

Background 

Proximity fuses – or VT fuses for “variable time” – cause explosive projectiles to detonate when they get within predetermined distances from their targets.

For example, the proximity fuse in the shell of a 5-inch naval gun might be set to trigger detonation when the projectile gets within 30 yards of an enemy dive bomber. 

VT fuses are packed with lots of intricate little parts like oscillators, amplifiers and ampules, as well as pint-size transmitters and receivers that emit and register radio waves.  

As the shell approaches an object with reflecting qualities, the waves create an interference pattern that bounces back and is picked up by the receiver.  

The distance between the projectile and its target gets ever and ever smaller, until at a predetermined point the fuse triggers detonation. 

Though this is an oversimplification of how VT fuses work, the efficient little devices are particularly effective against aircraft, missiles, naval vessels and other moving targets.

They can also trigger airburst explosions over ground targets like troop formations, stationary aircraft, and vehicle clusters consisting of trucks, tanks and armored personnel carriers. 

Unlike similar weapons with contact fuses that actually need to hit their targets, munitions with proximity fuses are much more effective at spreading shrapnel and submunitions over large areas, which significantly increases the likelihood of damaging, destroying or killing the target or targets.  

These days bombs, missiles, rockets, artillery and mortar shells can be set to explode at varying distances and heights depending on target, terrain and ordinance type. 

This flexibility opens up various new applications for existing weapons, but on the downside, proximity fuses have always been relatively fragile and expensive.

That said, reliability has increased exponentially in recent decades, and though the initial cost is much higher, the increased kill-rate often equates to big cost savings because less units – or just one – are needed to take out each target.  

But though they’re common now, during World War II proximity fuse technology was in its infancy, and even for experienced gunners the chances of hitting a small moving target thousands of yards or even miles away was decidedly small. 

Not surprisingly, tactics were largely based on the timeless American adage that if you throw enough sh*# at the wall, some of it’s bound to stick.  

In gunnery terms, this means that the more lead you hurl into the air the more likely you are to hit something, which is exactly what Britain did during “The Blitz”. 

With the island nation on the brink of collapse, the Luftwaffe relentlessly pounded industrial targets, RAF bases, and urban population centers. 

In what may have been one of the country’s many “darkest hours,” London was bombed around-the-clock for nearly two months from September to October of 1940. 

British gunners and aviators in Spitfires and Hurricanes downed more than 1,000 aircraft and the country ultimately weathered the storm, but bean counters determined that approximately 20,000 ground-fired projectiles had been expended for each downed plane.

Some figures even suggested that the real number may have been five times higher. 

Whatever the case, it was clear that standard gunnery practices and antiaircraft shells of the day weren’t particularly effective, despite the fact that many were fitted with time and altimeter fuses. 

The former detonated at predetermined times after leaving the muzzle, while the latter exploded at a specific altitude that was dialed into the shell before it was fired. 

But if either of these calculations or settings were wrong, these otherwise lethal projectiles would fly within inches of their targets without exploding or doing the slightest bit of harm.  

Both are much more effective aircraft killers than those with contact fuses, but neither can sense their targets nor detonate when in proximity to them. 

What was needed was a new “smart” weapon to stem the tide of the air war, on which Britain’s very survival depended.

Development

Though they didn’t come into their own until the 1940s, proximity fuses weren’t new concepts.

In the ‘30s, optical, infrared, magnetic and radio frequency fuses were on drawing boards across Europe, North America and Japan, but all were limited by the crude, clunky, expensive, unreliable and fragile electrical components of the day. 

Of these, fragility was perhaps the most limiting factor, because fuses in artillery shells would need to withstand forces greater than 20,000 times that of gravity (20,000 Gs) associated with being fired from a cannon and accelerating to thousands of miles per hour in mere fractions of a second. 

By comparison, rockets only produced about 100 Gs, and dropped bombs less than five.  

In addition, barrel rifling caused artillery shells to spin at more than 20,000 rpm, and the resulting centrifugal force usually tore apart delicate components. 

Royal Artillery Museum Woolwich London 144 ( Vt fuze)
Royal Artillery Museum Woolwich London 144 ( Vt fuze). By DAVID HOLT is licensed under CC-BY-SA

With much slower acceleration, lower top speeds and either minimal or no spin, rockets lent themselves to proximity fuses more than artillery shells did, but they were rarely used in the antiaircraft role. 

Hence, artillery fuse development took precedence, and the first prototypes were developed by researchers at England’s Telecommunications Research Establishment before war broke out. 

Using small short-range transmitter/receivers similar to those on Doppler radars, the fuses had promise, but they needed to be miniaturized and strengthened to be practical. 

As early as September 1939 development began on vacuum tubes capable of withstanding much greater forces.

These fuses were originally destined for use on big coastal defense guns that even under optimal daytime conditions weren’t particularly good at hitting distant targets, but funding dried up and scientists were reassigned to other projects that the War Office deemed more pressing.  

However, proximity fuse development got a much needed shot in the arm in 1940 when Britain ordered 20,000 hearing aids from the Western Electric Company and Radio Corporation of America.

When American scientists, engineers and military brass found out about the odd order they considered it unlikely that England was experiencing a wave of sudden deafness, and that the devices were almost certainly being used to develop proximity fuses.

Within months the fuses derived from these hearing aids were fitted in non-rotating solid-fuel rockets that were lethally effective when fired at stationary targets. 

Up until that point development in Britain and America had been conducted autonomously, but that changed with the Tizard Mission in September of 1940. 

Representatives from the UK travelled to the US to share ideas and technology that might help win the war, and the proximity fuse was near the top of the list.

America’s National Defense Research Committee and National Bureau of Standards had been working on proximity fuses of their own, with a focus on their use in anti-aircraft artillery. 

One early design relied on a radar-like device that tracked the projectile while it was in flight, after which an operator would electronically trigger the fuse when the shell got close to its target. 

But though these devices were capable of sensing and destroying their targets some of the time, many became inoperable after firing, and it was until the summer of 1941 that they were able to be fired from cannons reliably, thanks to shock absorbing springs added to the tungsten filaments.  

Improvements & Controversy 

After the transfer of information from the Tizard Mission, much of the additional work done to perfect the proximity fuse was done in the United States

A significant portion of the development was carried out at the Johns Hopkins University Applied Physics Laboratory in Maryland, an ostensibly “private” facility with only one customer – the US Navy. 

The new devices they produced were more reliable, robust and inexpensive than earlier versions, but since the American design was remarkably similar to the ones handed over by Britain in 1941, controversy arose.  

The Americans readily admitted that they’d incorporated certain elements from the British designs into their VT fuses, but the official position was that the new devices were entirely distinct.

Whatever the case, the fact that the US military avoided paying millions in licensing and royalty fees rubbed many Brit scientists the wrong way, though with Europe in chaos it wasn’t exactly an issue worth pursuing.  

In late 1941 the first solid-state radio Doppler proximity fuse was introduced, and it became the most common of all types of proximity fuses that would eventually be put to use in dozens of types of munitions. 

Designated T-3, in mid-1942 the new fuses underwent artillery testing aboard the cruiser USS Cleveland, when the ammunition was fired at drones over the Chesapeake Bay.  

The trials were scheduled to last for two days, but testing was cut short when all of the drones were destroyed in just a few hours on the first morning. 

In one instance three drones were destroyed with just four projectiles used in conjunction with tracking radar and revolutionary M-9 fire-control computers. 

Now the new proximity fuse-equipped shells, radars, and computers had kill rates exceeding 50%, and the US Navy ordered them into full production. 

Deployment

Antiaircraft proximity fuses played significant roles during The Blitz, helping to eliminate most of the threat from German aircraft, V-1 rockets.  

Most of Britain’s anti aircraft defenses were placed in a coastal ring to the island’s north and south, as well as on the east coast along the English Channel. 

Over the course of the V-1 campaign, the percentage of rockets and flying bombs that successfully pentrated this ring of guns dropped from about 75% to less than 20%. 

In fact, the new fuses were so sensitive that a new phenomenon arose, whereby they would sometimes detonate early when they flew too close to seabirds, and a number of innocent feathered creatures were lost to massive airburst explosions during the war. 

At first the fuses were only used in areas where the shells would land in water if they failed to detonate, which meant that they couldn’t be recovered and reverse-engineered. 

American forces were initially prohibited from using them over land as well until General Dwight D. Eisenhower protested.  

Approximately 200,000 shells with proximity fuses were used during the Battle of the Bulge in late 1944, and since they exploded before hitting the ground they proved far more deadly against enemy troops than standard rounds. 

This was especially true when German soldiers ventured out into open ground during inclement weather, which they often did because accurate artillery observation wasn’t possible in low-visibility conditions. 

But despite successes in Europe, it was the large naval engagements in the Pacific between the United States and Japan where the new fuses were used with the greatest success.

The first confirmed kill occurred on January 5, 1943, when a lumbering Japanese Aichi D3A dive bomber was downed by a US Navy task force off Guadalcanal. 

Along with three other identical aircraft, the Aichi had just dropped its bombs, one of which scored a direct hit on New Zealand’s light cruiser Achilles

American gunners on the cruiser USS Helena unloaded with 20 and 40 mm cannons as well as bigger 5-inch guns. 

The shell from one of the 5-inchers exploded near the Aichi and the shrapnel took it down, marking the first time that an aircraft was downed by a shell that was never meant to actually hit its target. 

Other Sensor Types

Radio proximity fuses were the most widely used during the Second World War, but other variants were developed as well. 

Optical sensors were studied in Britain as far back as the mid-’30s.

Primarily designed to be used in bombs that would be dropped by British bombers flying over German bombers in an odd scheme the Air Ministry referred to as “Bombs on Bombers,” they never saw service.

Acoustic proximity fuses that were able to sense the noise of aircraft engines and rocket motors were also developed, as were those that used hydroacoustic influence to detect underwater noises like those produced by ship and submarine propellers. 

Still other fuses detected slight changes in water pressure, like when a heavy ship passed over a submerged sea mine. 

Though Germany may have had more proximity fuses under development during the war than any other country, none were ever used in combat. 

Germany’s fuses were kept under wraps just like the ones in Britain and America, but the Allies became aware of their existence from information contained in the Oslo Report, which was written by German physicist Hans Ferdinand in late 1939 and mailed to the British embassy in Oslo. 

Likewise, details of America’s proximity fuses were forwarded to Soviet handlers by turncoat Julius Rosenberg, though the information didn’t include data about the most critical ones used in artillery shells.

All told nearly 100 American companies manufactured more than 20 million proximity fuses that were used in nearly every theater. 

Early production was approximately 500 units per day, but within a year output had increased to nearly 40,000 per day. 

Procurement contracts increased from 60 million USD in 1942 to nearly 500 million USD in 1945, with total expenditures exceeding 1 billion USD, or about 15 billion USD today.  

Proximity fuse development continued after the war, when other means of target detection became increasingly prevalent, ultimately including cameras and laser detector/range finders like the ones used on rockets, missiles and advanced artillery shells today.  

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