Bombs away: The Dambusters bounce back

Simulated bouncing “bombs” were constructed from scratch, then dropped by aircraft flying just 60 feet above the surface of a lake in British Columbia, Canada, to destroy a 130-feet wide dam. The project was led by Dr Hugh Hunt, from the Department of Engineering, University of Cambridge, and will be the subject of a Channel 4 documentary – “Dambusters: Building The Bouncing Bomb” – at 8pm on Monday, 2 May.

To date, nobody has examined the engineering complexity of the remarkable raid, which took place on the night of 16/17 May, 1943, or managed to prove how it was successfully carried out.

Originally known as “Operation Chastise”, and later immortalised in the 1955 film, The Dam Busters, the raid sent Lancaster bombers from RAF no. 617 squadron to fly dangerously low over reservoirs in the strategically important Ruhr Valley in order to bomb the Möhne and Edersee Dams. At the time, both were thought to be almost impregnable to conventional bombing raids.

Under intense anti-aircraft fire, the crews dropped “bouncing” bombs, specially designed by the British engineer, Sir Barnes Neville Wallis, to skip across the water en route to the target. If released at the right moment, these could bypass the torpedo nets the Germans had placed to defend the dam. On reaching the dam wall, they sank to a predetermined depth and exploded.

While the mission itself has gone down as one of the most iconic episodes in Britain’s wartime story, few details about how the bouncing bomb was built remain. Most of Barnes Wallis’ original calculations, designs and results were lost; many of them in a flood in the 1960s. The physics of “richochet” (the bouncing of objects on water) is quite well understood but actually doing it has been a different matter.

“There’s no massive mystery in a theoretical sense, but the fact that no-one has been able to repeat the mission meant that there was no-one alive who knew whether it was difficult, easy, or indeed possible,” Dr. Hunt said. “The question was really finding out whether anyone could do it again.”

Drawing heavily on a 1976 paper by his Cambridge colleague, Professor Ian Hutchings, which proposed a model for how the bouncing bomb was made, Hunt set to work trying to build one. He started by firing cricket balls from a bowling machine at the Jesus Green open air swimming pool in Cambridge to test Hutchings’ theories. This was gradually scaled up, until much larger imitation bombs were being fired out of a compressed air cannon.

The team of dam engineers, explosive experts, mechanics and pilots then headed for Mackenzie in British Columbia, Canada, where a 30-feet high and 130-feet wide dam was specially built to see if the Dambusters raid could be reconstructed.

Before that could happen, however, the group had to negotiate several engineering hurdles. A mechanism had to be designed to carry the bomb and the device itself had to be balanced so that it did not vibrate.

The biggest challenge was making the bomb itself spin. Barnes Wallis’ original device bounced cleanly and was stabilised because it was rotating at a rate of 500 revolutions per minute (RPM) when it hit the water. For the reconstruction team, to do the same thing meant either repeating the inventor’s strategy of spinning it during the flight – which is logistically complex – or setting it spinning on the runway before takeoff, which might lead to the RPMs falling too low before the aircraft reached the drop zone.

The group opted to set their bombs spinning before take off. To keep them turning, Hunt, who worked closely with his PhD student, Hilary Costello, designed a shield, rather like the windscreen on a vintage sports car. This was custom-designed to deflect air around one side of the device. The movement of the air kept the bomb spinning so effectively that it was still turning at 1,000 RPM when it was dropped.

The shield was developed and optimised with the aid of the Wind Tunnel in the aerodynamics laboratory in Cambridge, primarily with a view to spoiling the aerodynamic lift due to spin (Magnus effect) so that there was no risk of the bomb rising up and hitting the plane on release.  During these tests, the team found that a cut-down version of the shield helped significantly to keep the bomb spinning during flight

Even then their problems were not over. During the first drop in Canada, the bombs were tangled up on release and the mission looked to be a failure. “It was one of those things,” Hunt said. “The theory looks nice and easy , but once you do things for real, it’s never that simple. There were a lot of glum faces.” Further inspection revealed that a release cable was five inches too short. This could not be lengthened, but two tie bars were replaced - each measuring 2.5 inches longer. On the next flight, the bomb bounced perfectly and after a few more test runs the dam was destroyed.

Not everything could be reconstructed faithfully. So few Lancaster bombers survive that the team had to use World War II vintage DC4 aircraft instead. The dam itself was also one third the scale of those attacked in Germany – although the rest of the project was scaled accordingly to make it realistic.

For Hunt, however, this only served to emphasise the remarkable nature of what Barnes Wallis and the pilots of 617 squadron, achieved. Of the 133 hand-picked air-crew in 1943, which comprised pilots of many nationalities, including members of the Canadian, Australian and New Zealand air forces, 53 lost their lives in the Dambusters raid.

“Our pilots had no-one shooting at them, the engineers could use things like bowling machines to test their theories, and the whole thing was only at one third scale – and even then it was hard enough,” Hunt said. “You compare that with the original challenge – for Barnes Wallis and for the pilots – and you realise what an amazing achievement it was.”