Target in Sight by Martin Bleasby AGAvA

Watford Or Bust

17th January 2009

Pamela Shields uncovers the local connections behind one of World War II’s most daring exploits.

Thanks to the plethora of media coverage we all know that 2008 marked the 65th anniversary of the legendary Dambusters Raid – the most daring feat in aircraft history – but the local connections largely went unnoticed. Readers may be surprised, therefore, to learn that the first test on how to breach the biggest dam, the Möhne, was carried out in secret right here in Watford.

When Hitler’s military ambitions became all too clear the British government drew up a plan to paralyse the dams in the Ruhr Valley. Air Intelligence had reported that water used in the whole of Germany was only three times that of the Ruhr and that the bulk – thirty billion gallons – was from one reservoir alone, contained by the Möhne Dam. The reservoir, which supplied hydro-electric power and water to industry, would soon be supplying factories turned over to the manufacture of (to use a phrase from a very different war) weapons of mass destruction. Britain’s objectives were really quite straightforward: cut water supplies; cause flooding and damage to industrial plants, rail tracks and waterways; disrupt the supply of water for use in inland waterway systems.

When the government’s Bombing Committee Paper 16 was finally circulated it described the construction and siting of the dams, and reported that to deter torpedoes the Germans had spread nets over the water in front of the dams. The Möhne was also very well protected by anti-aircraft guns. The British plan to generate a man-made tsunami was now on the table; the problem was that no one had any idea how to carry it out.

Enter Mr Barnes Wallis Assistant, Chief Designer at Vickers Armstrong Aviation. Considered a genius by his peers, Wallis was living proof that not going to university need be no drawback in life. After leaving school he was an apprentice engineer in Blackheath before starting work with a shipbuilder on the Isle of Wight. When he was 25 the opportunity to work for Vickers arose, and he stayed with the company and its successor, British Aircraft Corporation, until he retired. He worked on the Vickers Wellesley and Wellington, using his geodesic invention (as now seen in The Eden Project) which provided a lighter, stronger frame compared to conventional constructions.

Through a mutual friend, Wallis met Dr Norman Davey of the Government Building Research Station (now BRE) in Bucknalls Lane, Garston, near Watford. Just before Christmas 1940 at the height of the Blitz, when it was looking extremely likely that we might all soon become German citizens, Barnes Wallis visited Dr Davey in his office at Watford for a top secret meeting. A report by Dr Davey of this historic day is in BRE archives.

Today, Dr Davey is best remembered in Hertfordshire for the restoration of Romano-British wall-paintings at Verulamium (St Albans) which launched the systematic study of wall-painting in Roman Britain, but when he met Barnes Wallis he was revered as the structural engineer who designed the world famous acoustic dome in the Royal Albert Hall. It was while working at Garston that Dr Davey became interested in archaeology and became involved with Sir Mortimer Wheeler and his excavations at Verulamium. These convoluted connections lead almost directly to the Dambusters raid.

Barnes Wallis and Dr Davey decided that the best way to test where to hit the Möhne, and how big a bomb was needed to breach it, was to build a scale model. Over the next two years there would be many other tests at many other locations but this first one was very important because it demonstrated what could and could not work.

Using plans and technical data published in Germany when the dam was built in 1913, Dr Davey designed a replica of the Möhne. During January and February of 1941, in severe winter weather and in extreme secrecy, a 3’ high, 42’ long, 1/50 scale model was built behind the Building Research Station (part of Bricket Wood). The research had been painstaking, and no detail was spared. Garston Research Station cast two million specially manufactured-to-scale miniature mortar blocks (10.2 x 7.6 x 5.1mm), which were taken to the secret location by hand in buckets. Dr Davey built his model across a stream so that the upper face of the dam could be submerged to simulate actual conditions. It took his team seven weeks to build. Miniature explosives were used in the tests, and it was concluded that to breach the dam it would take 6000lb of explosive detonated against the wall thirty feet down when the reservoir was full.

Wallis knew that conventional bombs were not accurate enough to hit the target. Needing a new kind of device to breach the dam from the air he came up with an ingenious idea: a bomb that would bounce over the torpedo nets.

It had been known since the 1800s that cannon balls increased range when bounced on water but Wallis needed a bomb that would not only bounce over water but would hit the target and then sink, before exploding on par with an earthquake. (The term ‘Barnes Wallis’ is used in golf fo a shot that bounces over a water hazard).

He began working on a bomb with back spin which if dropped low (which turned out to be 60’) at an angle of seven degrees from the horizontal, at 220 mph would skip over the surface of the water in a series of bounces before hitting the dam wall. After impact it would roll down the under-water face. Once it reached a certain depth it would trigger an explosion which would detonate the main explosive charge; the shockwave would demolish any target above and for some considerable radius.

Barnes Wallis carried out initial experiments in his garden, using his children’s marbles fired from a home-made sling across tin baths of water, but in the end opted for a cylindrical as opposed to spherical bomb. Known in British history as the Bouncing Bomb, the Germans called it a revolving depth charge. Two years after the Garston tests, Wallis attended a meeting at Vickers Armstrong and convinced the Ministry of Aircraft Production that the concept of skipping a bomb onto the dam face was feasible. His report was read by Sir Arthur (Bomber) Harris who said it was…'tripe… not the smallest chance of it working'.

Wallis was hauled over the coals by the Chairman of Vickers who told him to stop making a nuisance of himself and drop the whole idea. Wallis offered to resign. Two days later, however, he was summoned to another meeting with the Ministry of Aircraft Production and told that the Chief of Air Staff wanted his project to go ahead.

Historians still argue over its success. Eight of the nineteen planes used in the raid were lost; fifty-three men died; three were captured and sent to camps. There’s no doubt, though, about the astonishing heroism of the young crews, or the devastation they wreaked. It took seven thousand men just four months to rebuild the dam wall but the power station at the base was never re-built.

Barnes Wallis’s Bouncing Bomb was kept secret in Britain until well after the war. In Germany it was examined the morning after the raid and, as a result, an inferior German version, code-named Kurt, was designed for use against British shipping, although never deployed.

In 1997 there was speculation that part of the BRE site was to be redeveloped, and that the Möhne dam model might be under threat. The Barnes Wallis Memorial Trust offered to home it in the Yorkshire Air Museum, where it could be placed on public view. Although the bid was unsuccessful, attention was drawn to the fact that the model was not in good repair and should be listed – and it has now been declared a ‘scheduled monument’.

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