Ever heard of bouncing bombs? You have now.
Barnes Wallis, an assistant chief designer, came up with the idea for a bouncing bomb that could be used to target strategic dams in 1942. His idea formed the basis of the Dambusters raid that took place in May the following year, causing major damage to two out of the three targets selected.
His idea was advanced by research from another researcher, A. R. Collins, who discovered that an underwater explosion would have a greater effect if it took place with the device directly in contact with the target.
This later informed one of the most significant design modifications for the bomb – the need to impart backspin on it so that it would (a) fall behind the path of the plane and ensure that the explosion would not impact the aircraft and (b) so that the backspin would ensure the device stuck to the surface of the dam wall.
Barnes Wallis developed his idea at first on a very small scale – with marbles in a bath than with increasingly large models.
The bomb itself (technically a mine) was at first codenamed 'Highball' and interested the navy as a potential weapon against enemy ships. However, it eventually became known as 'Upkeep' after it first got the go-ahead for further development from the Air Ministry on February 26, 1943, and then, on March 17, the final decision was made to form a new squadron to deliver the weapon.
Size
Cylindrical in shape, the bouncing bomb, also known as Highball, measured 60 inches long and 50 inches in diameter. It contained 3 hydrostatic pistols, which measured the hydrostatic pressure of the water as the bomb sank until it was equal to the pressure corresponding to 30 feet depth. The bomb was also fitted with a 90 second time fuse so that if the hydrostatic mechanism failed, the bomb would still detonate.
The bomb weighed a total of 9,250 lb, 6000lb of this weight being explosive charge of TNT and RDX and possibly other materials to improve the efficiency of the explosion.
How did the bouncing bomb work?
There’s some nifty physics behind the dam-busting bombs that were used by the British in WWII. Just like skipping stones, to make a bomb bounce off the water you need to have enough speed and a perfect angle (about 7° in the case of the bomb).
If you get these just right, conservation of momentum means that the water pushes back on the bomb and kicks it up in the air. For multiple bounces, the magic ingredient is spin. Spin the bomb and you stabilise its motion, like a frisbee or a gyroscope. Before being released from specially modified aircraft, the bombs were held by v-shaped arms in the plane and rotated to the speed of 500 rpm by a hydraulic motor and belt drive. The planes had to fly at the low height of 60 feet and to be travelling between 240-250 mph. To help the aircrews obtain these specifications, two spotlights were fitted to the undercarriage of the plane and positioned so that they converged into one at exactly 60 feet.
This helped various countries to attack water-level structures with bouncing bombs.
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