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It was the sound that terrified Britain. In the pre-dawn hours of 13
June 1944, a loud buzzing was heard over the south coast. The noise, like
the sound of a powerful outboard motor at full throttle, came from a small
aircraft heading towards London. Suddenly, its engine stopped. Moments
later, the eerie silence was shattered by a thunderous explosion.

Arriving a week after the D-day invasion of Normandy, this was the first
of the engineering masterpieces Hitler called Vergeltungswaffen, or revenge
weapons. Built by Volkswagen and soon known as doodlebugs, the pilotless
V-1s looked like 25-foot cigars with wings and could carry a 1-ton warhead
for at least 140 miles. After flying a preset distance, usually towards
London, their jet-like engines shut off and the V-1s plummeted to earth.

Under attack

For many British people, the attacks came as a shock. During the first
years of the Second World War, Britain had built a highly efficient network
of early warning radar posts and quick response fighter units. So effective
was this network that, after the Blitz ended in 1941, German aircraft rarely
challenged the RAF in large numbers over Britain.

But effective as it was against conventional aircraft, the system was
not designed to cope with V-1s. They cruised only a few thousand feet above
the ground, flying at speeds close to 350 miles per hour. Fighter aircraft
could get some – but they flew too low and too fast for anti-aircraft guns
to aim accurately.

Three nights after the first attacks the V-1 campaign began in earnest
as 144 V-1s hit Britain. Nearly half of them exploded in the London area.
In the weeks up to the end of August 1944, some 8600 would be launched from
German positions in occupied Europe. Although V-1s could not carry as heavy
a load as a conventional bomber, they provoked a unique terror. Many Londoners
fled the city, or returned to sleeping in bomb shelters as they had done
during the Blitz.

Top-secret response

In response, British and American forces rushed to bring together the
results of three top secret projects. The first was the world’s first automatic
tracking radar. The second, a revolutionary electronic computing system
that helped to aim guns at fast-moving targets. And the third was a deadly
new attachment to artillery shells called the proximity fuse that dramatically
increased the effectiveness of anti-aircraft fire. These technologies
arrived in the nick of time. That they were ready at all was the result
of a unique scientific collaboration between Britain and the US that began
15 months before America even entered the war.

In September 1940, a small party of British scientists and military
officers arrived in Washington DC. Led by the chemist Henry Tizard, then
head of a government committee to investigate air defence technology, the
group brought with them blueprints, diagrams and prototypes of Britain’s
most important scientific secrets. Tizard’s mission was to persuade a still
neutral US to help develop a new generation of weapons using British ideas
and innovations.

Britain’s most highly prized secret was a compact and powerful source
of microwaves known as a resonant cavity magnetron. A radar operating with
microwaves, which have wavelengths of a few centimetres, could achieve a
much higher resolution than existing radars that operated at much longer
wavelengths. Microwave radar could therefore gauge the position of enemy
targets more accurately and distinguish between tightly bunched aircraft.
But Britain lacked the resources to develop it much further.

When the magnetron was demonstrated to American scientists early in
October 1940, the US military quickly set up a top-secret centre to develop
microwave radar. Called the Radiation Laboratory, and based at the Massachusetts
Institute of Technology (MIT), the centre was staffed mainly by graduate
students and university physicists. By the end of the war, its staff had
swelled to nearly 4000.

With the Blitz on Britain then nearing its height, the highest priority
was given to designing an airborne radar to help night fighters home in
on German bombers. The result of this project was an airborne interception
radar that remained the RAF standard until 1957. Second on the agenda, with
a much smaller team, was a microwave radar that could lock antiaircraft
guns onto and follow enemy planes in any kind of weather, day or night.
Existing radar could only detect an aircraft’s approximate position, after
which anti-aircraft guns let loose static barrages in the hope that a plane
might run into the flak.

Leading the small team to develop a better radar to control guns was
a sharp-tongued physicist from the University of Pennsylvania called Louis
Ridenour, and a former Rhodes scholar from Harvard University named Ivan
Getting, also a physicist. The team’s solution was novel. Their radar comprised
a six-foot-wide parabolic dish with an antenna near the centre emitting
a narrow, cone-shaped beam of microwaves. The cone moved in a small circle,
rather in the way that a spinning top precesses. The motion was arranged
so that when the dish was pointing straight at a target, that target would
send a constant echo back to the receiver. If the target wandered off-centre,
the change in amplitude of the echo signal could be instantly analysed and
fed to servomotors that automatically realigned the radar with the moving
target.

Such a system could plot an aircraft’s elevation and azimuth but determining
the range was not as easy as with other radars. The targets needed to be
tracked with at least ten times’ greater accuracy to within about ten yards.
The group had to design extremely precise new circuitry that could measure
intervals of less than 100 millionths of a second. Within a few months,
the team had built a crude prototype, codenamed the XT-1, on a rooftop at
MIT, with the radar dish mounted on a servo-driven platform, adapted from
a machine gun turret. The first automatic tracking of an aircraft took place
on 31 May 1941, with a telescope attached to the mount to allow observers
to see where the radar was pointing. Even when a plane disappeared behind
a cloud, the radar continued to track it, and the aircraft would emerge
moments later in the centre of the telescope. ‘It was just like magic,’
recalled Getting.

Taking aim

In the meantime, American Telephone and Telegraph’s Bell Telephone Laboratories
in New Jersey had been working on an electronic system that could predict
the position of fast-moving targets and aim a gun in the appropriate direction
– a device known as a gun predictor. The company ran a microwave radar
research and development laboratory second in size and scope only to the
Radiation Laboratory. At the Whippany Radio Laboratory 30 miles west of
New York City, Bell had been working on developing microwave radars with
manual tracking since late 1937.

In May 1940, a 29-year-old Bell engineer called David ‘Parky’ Parkinson
was perfecting an invention called the automatic level recorder, which drove
a recording pen across a strip of paper to chart voltages. Parkinson had
nothing to do with Bell’s radar programme, but he realised that the circuitry
he was developing to control the motion of a recording pen with great
speed and accuracy could do the same for an anti-aircraft gun.

Existing gun predictors were cumbersome mechanical devices. Although
accurate in many instances, they required several operators, who tracked
the target through telescopes attached to an array of cams and gears. As
the telescopes moved to follow the target, the mechanism calculated the
future position of the craft and set the guns accordingly. Such predictors
were too slow for fast, low-flying targets like the V-1s. Parkinson and
his supervisor at the labs, Clarence Lovell, hurriedly drew up the specifications
to turn the recorder into an extremely fast predictor that performed almost
every step electronically. This became known as the M-9 gun predictor.

Originally, Bell Labs’ goal was to design a machine that worked with
existing radar or optical sights. But when the Radiation Laboratory was
set up, the two centres began working together, and before long Ridenour’s
team heard about Parkinson’s work. From that point, shortly after the attack
on Pearl Harbor in December 1941, the XT-1 and the M-9 were designed to
work together.

In this combined design, a series of input potentiometers were incorporated
into the radar set. As the radar moved to track an aircraft, it turned the
shafts of the potentiometers, which produced voltages proportional to the
aircraft’s elevation and azimuth. These signals were then fed to the predictor.

The human touch

Only the rangefinding required human help. Looking at a screen displaying
the target’s distance, operators kept a cross hair in step by turning a
hand crank that was connected to a potentiometer. This set the potentiometer
to produce a voltage proportional to the distance to the target. This voltage,
together with the voltages representing azimuth and elevation that were
generated automatically by the radar set, were fed to the main processing
unit which processed them to yield a value for the craft’s speed and bearing.
This information, still in the form of voltages, was passed on to another
array of preset potentiometers which acted as memory storage for a set of
standard artillery tables. These in turn produced voltages proportional
to the required orientation of the guns, which was fed to the servomotors
that aimed them. The whole setup was an electronic analogue computer.

By 1 April 1942, a tracking radar using the new gun predictor was ready
for trials at the US Army’s anti-aircraft command headquarters at Fort Monroe,
Virginia. Controlling 90-millimetre guns, the machine destroyed dummy targets
towed behind real aircraft using as few as eight rounds – and without visual
contact. The next day the US Army decided to order more than a thousand
units and renamed the radar the SCR-584.

At the same time, a secret team at Silver Spring, Maryland, operating
under the cover name ‘Applied Physics Laboratory’, was working on yet another
legacy of the Tizard mission. The Silver Spring team toiled away on a fuse
built into the nose of artillery shells that could sense targets nearby.
The man behind the project was Merle Tuve, a physicist at the Carnegie Institution
of Washington, a private research organisation created by the steel tycoon
Andrew Carnegie. In 1940, Tuve had been asked by the US government to develop
a proximity fuse, and for this he had been considering acoustic and photoelectric
triggering mechanisms as well as radio triggers. But when the Tizard mission
brought over a miniature radio circuit design by the engineer W. S. Butement,
Tuve adopted it at once as a trigger for his fuse.

The fuse screwed into the front of an artillery shell, where it emitted
a steady, oscillating radio signal. The nose cone functioned as a receiver.
Any object within a few wavelengths of the oscillator would interfere with
the signal and activate the detonator. This was before the days of transistors,
but the device’s neat electrical trick was accomplished with just four valves.

Closing in

Gunners firing shells fitted with proximity fuses would no longer need
to hit enemy aircraft and doodlebugs, or rely on alarm-clock fuses that
exploded at a certain time after firing. The valves were miniaturised and
strengthened so that at least 95 per cent of the mass-produced fuses withstood
the stresses of being fired from anti-aircraft guns, which could involve
an acceleration of up to 20 000 times that of gravity. By the time of the
V-1 attacks in the summer of 1944, the US was churning out several hundred
thousand proximity fuse valves a day.

The V-1s did not catch Britain totally unaware. Military intelligence
already knew the approximate accuracy, speed and range when the first doodlebugs
arrived, and a response of sorts was ready. Fighters, usually cruising at
6000 feet, formed the first and most important line of defence. Early-warning
radars could pick up the V-1s about 100 miles away. Ground controllers waited
until the flying bombs were about half a mile behind the planes, then ordered
the pilots to intercept. At low altitudes, the V-1s were faster than almost
any plane, but pilots could build up speed by diving towards the V-1s. This
gave the fighters at least a sporting chance of shooting the invaders down.

During the first month of attacks, some 3000 V-1s crossed the Channel
towards Britain. Fighters shot down more than 900 while anti-aircraft guns
and barrage balloons, which formed a second ring of defence near London,
accounted for 300. Such a kill rate would have broken most conventional
bomber attacks. But with the mass-produced, unpiloted V-1s the Germans could
continue the attacks almost indefinitely. They were killing or seriously
injuring thousands of people.

In mid-July, a desperate Britain shifted hundreds of anti-aircraft guns
from London to coastal areas: one advantage of shooting over water was that
V-1s that had been hit would not fall on cities or towns, something that
had been a significant cause of casualties. About the same time, the US
president, Franklin Roosevelt acceded to a plea for help by Winston Churchill,
Britain’s prime minister, and granted permission to use the SCR-584 radar
tracker, the M-9 gun predictor and the proximity fuse. Although each technology
had been used in battle before, they had never been used together. The previous
February, a few 584s with M-9 predictors had been drafted in during the
ill-planned Allied landings at Anzio in Italy. However, the fuse had been
considered so vital that its use over enemy territory was banned, to prevent
unexploded shells falling into enemy hands.

Special relationship

At the time of the V-1 attacks, a consignment of proximity fuses had
already arrived in Britain. About three hundred 584 sets were also in Britain
awaiting transfer to the Continent. Following Churchill’s plea, the US lent
some of its radar and predictor equipment to British artillery emplacements
to direct 3.7-inch anti-aircraft guns and assigned others to control American
90-millimetre artillery based along the south coast.

The 584s were housed in a special trailer, with the radar dish mounted
on top. A petrol-fuelled generator parked nearby supplied power, and cables
connected the radar to the main processing unit, which rode on its own small
trailer. Commands from the M-9 predictor were fed to a battery of four guns
positioned about 50 feet away.

The V-1 attacks presented the perfect opportunity to test the three
technologies together. Any dud fuses would fall harmlessly at sea or over
friendly territory so there was little danger of them falling into enemy
hands. Moreover, the doodlebugs flew in a straight line, which made them
relatively easy targets for the predictor.

Teething troubles

But there were plenty of unexpected problems. The equipment was rushed
into service with little time for gun crews to become familiar with it.
When Getting’s deputy, Lee Davenport, visited an American battery he found
that the gun operators were unable to work the machines. ‘Seven or eight
buzz bombs came over within range while I was there,’ remembers Davenport,
‘and the crew could not fire a single shot at any one of them.’ To help
his confused comrades, Davenport and a colleague toured the coast conducting
on-the-spot tutorials from a borrowed ambulance with its red cross painted
out.

As the gunners became more competent, the kill rate improved. By the
fourth week of August, Allied forces were shooting down nearly 80 per cent
of the V-1s they engaged, until Allied troops overran the launch sites and
the attacks stopped. On 28 August, the last day large numbers of V-1s were
launched against Britain, the Germans fired 104 across the Channel. Artillery
hit 68. Fighters shot down 14. Barrage balloons accounted for two more,
and 16 crashed before reaching their targets. Only four hit London. In the
two and a half months since the nightmare had begun on 13 June, anti-aircraft
batteries brought down 1629 V-1s, almost equalling the total hit by fighters.
And after the move to the coast, the guns outperformed aircraft by 50 per
cent – registering some 1300 kills, virtually all with 584-controlled guns
using shells equipped with proximity fuses.

The reign of terror was over.

Robert Buderi is a freelance journalist currently writing a book on
the impact of radar on the war and postwar science and technology.