Torpedoes couldn’t do the job. Ordinary bombs dropped from planes couldn’t either. But Barnes Wallis bouncing bombs held promise.
During World War II, German aircraft pounded Britain, dropping a hail of bombs on it cities. To turn the tide of the war, Britain needed to cripple Germany’s bomb factories. Most of them were in the Ruhr Valley, a low-lying area fed by rivers and protected by dams. Destroying the dams and flooding the region seemed to be the best way to bring the German war machine to its knees.
But how? The dams were protected by torpedo nets. Bombs dropped from planes tended to roll forward and miss their targets. A more accurate bomb, one that could dodge the torpedo nets and still hit its mark, was needed.
If anyone could produce one, it was Barnes Wallis, a well-known aircraft designer and scientist. As he pondered the problem, Wallis remembered a childhood game, skipping stones across the surface of the lake. If stones pitched at just the right angle bounced and hopped across the water, was it possible to do the same to a bomb? Release it at just the right angle, make it skim across the water, bounce over the protective torpedo nets, and land at the base of the dam where it could do the most damage?
Wallis started a series of tests in his laboratory. Using a small catapult, he fired marbles across a tub of water. They skimmed the water, but bounced in all directions – spherical shapes, he discovered, moved in unpredictable ways. Next he carved a series of fat, cigar-shaped models and fired these across the water. These flatter, barrel-like shapes worked better. Adding a bit of back-spin as the models left the catapult improved their accuracy even more and prevented them from plowing into the water..
The idea of bouncing explosives seemed far-fetched. But Wallis ignored his critics. He filled notebooks with detailed measurements and calculations, as he eliminated some designs and perfected others. Finally satisfied that he had enough information, he set about building life-sized bombs to do the job. They were monsters. Weighing as much as two cars, the bombs were almost 2 meters long and more than a meter thick. Each one carried 2722 kilograms of high explosives.
To give the bombs back-spin as they left the plane, a motor rigged to a chain rotated them just before release. Churning at a speed of 500 revolutions per minute, the spinning bombs would hit the water and skip across it rather than plunging down.
Dropping the bombs at just the right angle and height was critical. The problem was solved with two simple flashlights, one fixed in the nose of the plane, the other in the tail. The flashlights were angled downwards so that when the plane was at a height of 18 meters, their beams crossed. When that happened, the pilot knew he was at the proper height to release the bombs.
On May 16, 1943, under the cover of darkness, nineteen planes carrying skip bombs left Britain. Flying low, the planes swooped over Holland and into German territory. As they approached the dams along the Ruhr Valley, the pilots released their bombs, sending them spinning and skipping across the water.
Barnes Wallis’ crazy idea worked. The skip bombs destroyed two dams, flooding the valley, washing away factories and roads, bringing power and transportation to a standstill. The German war ground to a halt.
This story was originally published in Extreme Science: Science in the Danger Zone under the title Bombs That Bounce. It has been adapted & updated for this blog.
For Further Reading:
Imperial War Museum: The Incredible Story Of The Dambusters Raid
History Learning Site: The Damnbusters