A Beginner’s Guide To The Dam Busting Bouncing Bombs

Everything you need to know about the WW2 weapon that became a symbol of Allied derring-do.

The dam at Moehne Reservoir as it is today

Moehne Reservoir as it is today - one of the dams breached during Operation Chastise.

THE MAN BEHIND THE MISSION

The bouncing bombs used in the famous "Dam Busters" raids were the brainchild of an English engineering genius called Barnes Wallis. He'd already made his name as a key designer behind the early airships, as well as fearsome British bombers used in the war. But he had a real task on his hands when he tackled the question of how to destroy the German dams of the Ruhr valley.

The Ruhr was a top strategic target because it was one of the industrial heartlands of Germany. Smashing the damns would cause massive flooding and wreck the local infrastructure. It would be a mighty blow in the battle against the Nazis. The trouble was, the dams were incredibly strong (they had to be, to withstand all the water), and they were protected by lines of anti-torpedo nets.

Derwent reservoir in the Peak District, site of the testing of the low-level flying needed in the Dam Busters' raid.

Derwent reservoir in the Peak District, site of the testing of the low-level flying needed in the Dam Busters' raid.

THE IMPORTANCE OF MARBLES

Barnes Wallis realised it was logistically impossible to destroy the dams by simply dropping gigantic bombs on them. A more cunning approach was needed, targeting the weakest parts of the dams that were underwater. How? By deploying depth charges, much like the ones used to against submarines back in WW1. And the best way to overcome the anti-torpedo nets? By making the explosives bounce over them, like thrown stones skimming across water.

Wallis put the theory into practice in his back garden using a set of children's marbles and a tin bath filled with water. He wanted to see if they would bounce in the same way that flat stones would. Those marbles would inspire the creation of the bouncing bombs, and the original set used by Wallis in his garden would eventually be sold at auction for over £27,000.

A "MAD" IDEA

To say the military bigwigs were initially "sceptical" would be quite an understatement. Sir Arthur "Bomber" Harris, the boss of RAF Bomber Command, dubbed the concept of the bouncing bomb "tripe of the wildest description" and "just about the maddest proposition as a weapon we have yet to come across".

The maddest proposition as a weapon we have yet to come across

He had a point - the idea of dropping a drum-shaped explosive at exactly the right height, at exactly the right speed, and having it skip over enemy nets before sinking at precisely the right moment to detonate underwater and destroy a dam, seemed like something from a fantasy yarn. Yet, despite Bomber Harris being "prepared to bet my shirt that the bomb will not work", they went ahead anyway.

THE LETHAL LOGISTICS

The bouncing bombs were so innovative and unprecedented, the planes used in the "Dam Busters" raids had to be modified to carry them. Some of their inner armour had to be taken out to make room for the new equipment, and even the bomb bay doors had to go, meaning the barrel-shaped bombs would hang below the aircraft on crutches, in open view.

Motors were also installed to spin the bombs backwards at over 500 rpm just prior to deployment. This was in order to give the bombs their backspin, which was crucial to ensuring they would bounce properly and stay on target. The residual spin would also make the bomb run down the side of the dam and detonate at the right time.

OPERATION CHASTISE

After all this complex preparation, zealously overseen by Wallis himself, the Dam Busters raids finally took place one night in May 1943. Known officially as Operation Chastise, the raids involved 19 bombers and were nail-bitingly dangerous. There was a precise procedure for deploying the bombs: the backspin motors had to be switched on 10 minutes before reaching the target, and the massive bombers had to fly down until they were exactly 60ft above the water. Spotlights were shone down from the bombers to determine the distance - when 60ft was reached, the lights would intersect to create a figure of 8, and that was when the bomb would be dropped.

The raids were a success, with two dams completely breached and a third damaged. The flooding caused widespread devastation, but there were downsides too. Many civilians were drowned, while the British suffered losses too, with eight of the bombers shot down and 53 airmen killed. The deaths would deeply affect Barnes Wallis, but his contribution to the war effort would become the stuff of legend.