Sir Barnes Wallis was a genius engineer who designed a very special bomb during World War II. The idea was that it would bounce across water and destroy German dams along the Ruhr Valley, causing massive flooding and damage to water and hydroelectricity supplies.
Partly thanks to the 1955 film The Dam Busters, the story behind Operation Chastise, which took place on May 16 and 17 in 1943, has become a familiar war time tale. But Wallis’s actual working calculations were lost (fittingly perhaps, in a flood in the 1960s). So what do we know about the complex science behind the bouncing bombs?
We know that the Germans considered their dams to be a potential target for their enemies, and placed torpedo nets in front of the structures to protect them. And to bust a dam, Wallis realized that peppering it with lots of small bombs wouldn’t work. It would be the difference between throwing a handful of sand at a window, and then doing the same with a rock.
Wallis figured that to do serious damage, a single four tonne bomb had to be detonated right up against the dam wall at a depth of about 30 feet below the water. In those days, high altitude bombing accuracy wasn’t good enough to deliver such a bomb bang on target. The idea of bouncing it across the water towards the dam like a skimming stone was inspired.
In early experiments a few things became clear. First, for the bomb to bounce it had to be spinning – with backspin. Just like that a delicate backspin dropshot in tennis, which causes the ball to hover just over the net.
Wallis worked out that a bomb with backspin would be levitated by what is known as the Magnus effect countering the downward pull of gravity and ensuring that it struck the surface of the water gently. If the bomb hit the water too hard, it would detonate prematurely, causing damage to the aircraft above, but no damage to the dam.
Spin therefore meant that the bombs could be delivered from a manageable height. Flying at 60 feet was already dangerously low, but without backspin the Lancaster bombers would have to have flown even lower and faster.
In Wallis’ earliest experiments he worked with marbles and golf balls and it was obvious that his bomb would be spherical. But because