In the realms of top-secret projects of world war 2 there are a couple which most people

will have heard of, the development of the Atomic bomb and Radar but there was another

that was ranked as equally secret and important and its effect on the war could be said to

be greater than the atomic bomb and yet few outside of the military knew about it until

after the war. This is the story of the proximity fuse, one

of the best-kept secrets of World War 2 and regarded by some as the 3rd most important

technological development after the Atomic bomb and Radar. This is they worked and how

this simple principle changed the outcome of the war.

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During the early German air raids on Britain, an anti-aircraft gunner is reported to have

said that shooting down an aircraft in the night sky was akin to trying to hit a fly

in a darkened room with a peashooter. According to reports of the time, early on

in the war, it took about 1000 shells to bring down one aircraft, with the advent of radar

fire control and variable time fuses this was said to be around 500 shells but these

are very low estimates, some sources say it could be as high as 20,000 shells, either

way it was very inefficient and seen by some as just harassment of the german bombers rather

than an effective weapon. The device that controls when the shell, rocket

or bomb explodes is a called fuse and on a shell for example is fits into the top of

the shell and different types were available depending on how you wanted the shell to operate.

At the beginning of the war anti-aircraft shells used either direct contact fuses which

only worked if they hit the target or mechanical variable time or VT fuses, these triggered

a timer once the shell was fired and then detonated after a predetermined time. This

allowed the gunners to place the detonations at a specific altitude, one which would be

determined through observation and radar and where the enemy aircraft were flying in at.

This was far from ideal because each one had to be set by hand to the correct time before

use and then 10 - 15 would need to be fired to confirm the altitude settings. These could

be seen by the enemy which could move up or down to avoid them. If the timing of the fuse

was out by a second either way it would result in the shells detonating harmlessly hundreds

of meters above or below the target. The problem wasn’t new and various ideas

were put forward in the 1930s for ways of getting shells to detonate when they got close

to a target. The British had worked on radar-guided shells

which could be exploded by a radio signal when they were seen to be near an aircraft

on a radar screen but this proved was too difficult to accurately control.

What was needed was a way for the shells themselves to sense the aircraft and then detonate when

they were close enough. Another idea the British came up with the idea of using a tiny transmitter

in the fuse of the shell. This would transmit a constant radio signal,

this would radiate outwards and be reflected off of anything like an aircraft that came

within range. The reflected radio waves would be picked

by the body of the fuse which was the antenna, the difference between this and the transmitted

signal creates a beat frequency which increases as the shell and fuse get closer to the target.

This is amplified, filtered and when it reached a preset amplitude it triggered an electronic

switch, a thyratron which started the detonation process.

As the amplitude of the thyratron trigger signal was proportional to the distance from

the target it could be tuned for a certain distance, usually about 6 - 20 meters before

it detonated the shell, showering the target in shrapnel.

There was just one problem, the acceleration forces experience by a shell at setback or

when it fired are in the region of 20,000 G but that’s not all, the shells are also

creating very large centrifugal forces. Now today this wouldn’t be a problem with

our modern solid-state transistors but back then all they had were glass vacuum tubes

which were large and delicate and could break easily if you dropped a radio on the floor,

let alone exposing them to 20,000 G. Although the British did develop miniature

ruggedised tubes, this was still a major problem so most of the British work was moved towards

fuses for rockets and bombs which had G forces of less than 100.

The British plans for the fuses were also part of the Tizzard mission which gave the

US many top-secret British project plans for further development and production in case

England was invaded by Germany. Over in the US, in 1940, scientists at Johns

Hopkins Applied Physics Laboratory working with both the Navy and Army research labs

had come up with a new modified design with separate transmitter and receiver circuits

but crucially they had access to miniature vacuum tubes which were initially made for

hearing-aid amplifiers that could fit into a top pocket.

To test the effect of the acceleration, they placed a fly into an empty shell and fired

it vertically. When they recovered the shell, the fly was nowhere to be seen, it was only

when they inspected it very carefully they found a slight residue left on the inside

of the shell and that was all that remained of the fly.

With such extreme accelerations, the mass of the electronic components became the limiting

factor, the smaller they were the better. Together with the miniature tubes and smaller

ruggedised resistors and capacitors, the total mass reduction was in the order of ten times

that of a convention tube design. Even things like the solder for the joints

which would normally be made from lead and tin could become an issue with such G forces

so special solder was used that was only available from England.

Four vacuum tubes were used in the design to both transmit and receive the signal, amplify

it and then trigger the detonation, some of the tubes were optimised with a planar electrode

designs which made them more sturdy. You might well have thought that wrapping

the glass tubes & electronics in a shock-absorbing layer would be enough but this could induce

vibrations with multiple harmonic resonances which could shatter the glass so the whole

assembly would need to be as solid as possible so they were potted with a special wax that

set hard. Now having an electronically controlled fuse

that was sensitive to almost anything around it would be a major problem when being handled

on the battlefield, the last thing you want was it detonating in the gun barrel or if

it were dropped, so there were five safety features built-in to the design.

Firstly normal dry cell batteries would be a problem, they had a limited shelf life and

fuses needed to be ready for use without any further work required once they left the factory.

Instead of a normal dry cell battery, they used an ampule of acid which broke on firing

and activated the battery, the spin imparted by the rifled barrel then made sure that the

acid was distributed within the battery. The squib, a small electrical operated explosive

that triggered the detonator was shorted out with a mercury switch so that it wouldn’t

work until the fuse started rotating at speed as travelled along the gun barrel and an out

of line powder train between the squib and the detonator only came into line once the

shell was spinning. To stop premature detonation in the barrel

or as the shell passed over friendly forces, a time delay was incorporated into the capacitor

discharge circuit that fired the squib. This gave it enough time for the shell to be far

enough away before it could detonate. To stop the fuse which was top secret at the

time from falling into enemy hands a mechanical spin switch was activated by the rotation

of the shell. If the squib discharge capacitor was charged and the shell had not come near

an enemy target, the spin would slow down and this would trigger the switch to self

destruct the shell and fuse. In fact, up until the D-day landings, the

use of proximity fuses was only allowed over water so if any did fail they would be lost

in the sea. After the Japanese attack at Pearl harbour

in 1941 testing and production of the fuses were given a top priority and work continued

through 1942. Although up to 20% of the shells fired failed to work, the others proved to

be very effective against target drones, so much some that range commander where the tests

were being performed complained they were destroying too many of his drones.

After a great deal of testing and refinement, they were ready to be used in the Pacific

against Japanese aircraft. On the 5th of Jan 1943, the cruiser Helena

using 5-inch shells with proximity fuses was the first to shoot down a dive bomber as it

was approached the ship with two salvos of shells. Soon they were being used by many

of the ships in the region with spectacular results.

In May 1945 the destroyers Evan and Hadly were off the coast of Okinawa when over 150

kamikaze aircraft attacked both ships. Using proximity fuses all the attacking aircraft

were shot down with just 6 partially destroyed aircraft or parts of them getting through

to hit the ships. According to the official report, the horizon from the east to the northwest

was filled with burning planes. After the war, the Japanese stated that the

uncanny accuracy of the US anti-craft guns to destroy their dive-bombers without actually

hitting them was one of the reasons why they started using Kamikasi attacks and though

they suspected something like a proximity fuse was being used they didn't know how the

Americans were doing it. The proximity fuses also work at low altitudes

as they would detect the ground just as well as an aircraft as they fell to earth so they

could be used to detonate above the heads of the enemy on the battlefield.

This removed the safety provided by trenches and fox holes. With a normal artillery barrage,

the shells exploded when they hit the ground, expending most of their energy and shrapnel

upwards. Troops would be protected if they were dug in even they were quite near a shell

blast. The Japanese on many pacific islands withstood

huge artillery bombardments for weeks using this method. But now with shells that could

be set to detonate just 5-10 meters above the ground, troops in open trenches or fox

holes would be hit by the shrapnel exploding just above them.

This was used by the Allies during the Battle of the bulge when the Germans used the dense

forests in the area for cover during the attack. One report stated that after an artillery

barrage with proximity shells, the forest looked like a giant scythe had cut the tops

of the trees off, destroying everything above 40 feet or 12 meters and killing many of those

under the barrage. On the open battlefield, that same tactic

was used with lines of synchronised artillery fire. Smaller mortars at the front then larger

guns behind and then howitzers from the rear, firing staggered barrages over allied troops

to all explode above the german positions at the same time. They could also be used

to attack the far side of a hill where enemy forces would be hiding with similar results.

This was so terrifying that many of the Germans abandoned their positions because of the effectiveness

of the airbursts above them. Bombs dropped from planes using proximity

fuses were used against anti-aircraft batteries to great effect to demoralise the gun crews

over Italy, silencing the guns allowing the US Airforce to proceed uncontested.

As the allies advanced through Germany, from the Rhine to Berlin, allied anti-aircraft

fire with proximity shells downed over 1000 enemy aircraft.

Proximity fused shells were also used with great effect against the V1 flying bomb attacks

on London. In one day, of the 104 V1’s launched, 68 were shot down with Proximity shells, 14

by the RAF, 2 by barrage balloon cables and 16 suffered a mechanical failure during their

flight. After the war and during the trials that followed,

the Germans like the Japanese revealed that they had no idea that proximity weapons were

being used against them. The Germans did develop a variety of different types of proximity

fuses for bombs and rockets but few went into production and no one considered making them

for shells which the allies used to such a great advantage because they didn’t believe

the electronics could survive the forces involved. Over 20 million proximity weapons were used

during the war. The effect that proximity fuses had on the outcome of world war 2 both

in the pacific and in Europe was one of the best-kept secrets of the war until they were

eventually revealed several years later and the technology would go on to create the guided

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