Barnes Wallis The Bouncing Bomb That Had to Skip Before It Exploded

In 1942, Vickers engineer Barnes Wallis, who had already designed the R100 airship and the Wellington bomber's geodetic frame, flicked marbles across a tub of water and wrote a paper on a bomb that could skip over torpedo nets, hit a dam wall and sink before it exploded. Backspin kept it against the wall. He proved the explosive idea on a small disused dam in Wales, Nant-y-Gro. Arthur Harris, head of Bomber Command, wrote "This is tripe of the wildest description" on the plan. But the Chief of the Air Staff, Charles Portal, saw the test film and on 26 February 1943 ordered the raid for May. In April the first full-size bombs broke up on the water at Reculver, so the outer casing was dropped: a bare, spinning steel cylinder, released at 60 feet and 232 mph. On 13 May a live test bounced seven times. On the night of 16/17 May 1943, 617 Squadron breached the Möhne and Eder dams. 53 of the 133 aircrew were killed, and about 1,600 people died in the floods, most of them prisoners of war and forced labourers. Wallis was deeply affected by the losses. He gave his entire £10,000 award to his old school, Christ's Hospital, for the children of RAF airmen killed or injured.

The film stills show inert practice bombs at Reculver in 1943 (Imperial War Museum, public domain). The memo is Harris's own note of 14 February 1943 (The National Archives). The wooden catapult is the one Wallis used for scale tests at Teddington; the marbles themselves are not shown.