The inspiration for the bouncing bomb, which some say is more legend than truth, came on the backyard patio at the Wallises’ Effingham home in the spring of 1942. Easter vacations brought the entire Wallis family together – Barnes and Molly and their four children, as well as their two adopted nephews. The gathering sparked backyard games, among them daughter Elizabeth’s pastime of flicking marbles across the surface of a water tub. Perhaps taking note of the physics, not just the play, Wallis considered the action of the marble:  if thrown at a low angle, the marble would skin across the water in the tub and come to rest against the side of the tub; if thrown at high velocity, the marble might skip over the tub wall to the patio surface beyond. For scientist Wallis, the marble bouncing on the water became less a game and more a potential solution to a problem.

I had the idea of a missile, which if dropped on the water at a considerable distance upstream of the dam, he wrote, would reach the dam in a series of ricochets.

In reaching the conclusion, Wallis might have considered the work of noted nineteenth-century gunnery expert General Sir Howard Douglas. Writing in Treatise on Naval Gunnery in 1855, Douglas analysed the ricocheting action of a cannon ball on water, provided the surface was smooth and the trajectory below three or four degrees. Thus on the backyard patio Wallis enlarged the children’s game by devising a catapult, firing marbles at a low angle off the surface of the tub and watching them repeatedly bounce across the water and over a piece of string strung above the water. The game and the science came together to give Wallis a means of designing a bomb that bounced over the surface of the water – and, most importantly, over the anti-torpedo nets – to reach a target.

Wallis described his discovery to his friend Group Captain Winterbotham, who was intrigued but cautioned Wallis to take more time working on the idea before revealing it, Wallis did share the anecdotal trials with George Edwards, a colleague at Vickers, Edwards was an experimental manager by profession and a talented cricketer in his spare time; he shared with Wallis his spin- bowling techniques, illustrating that the more backspin he applied when throwing a cricket ball, the higher it bounced when it hit the ground. Not entirely sold on the technique , Wallis took his catapult and sample spheres to nearby Silvermere Lake. He discovered that a ball that was unspun when it hit the surface of the lake might skip a handful of times, but one shot while spinning skipped thrice as many times. In May, Wallis returned to his writing table to design and describe a spherical bomb with a double skin – the inner skin containing the explosive, the outer providing a layer of air around the charge.

I discovered that you can make a sphere run along the surface of water in a series of bounces, if you impart to it before it is released a very high degree of . . . backspin, he explained later. Curiously enough . . . with backspin or undercut on it, a sphere hitting the ground will bounce backwards, but if dropped on the water, it will run forwards.

One piece of the puzzle was still missing, the question Wallis had posed since the beginning: how much charge and at what distance from the dam would generate the equivalent of an earthquake sufficient to weaken and breach the dam? The answer emerged thanks to A.R. Collins’s group and its scale model testing at the Road Research Laboratory and ultimately, at the Nant-y-Gro Dam in Wales.

And so, on 24 July 1942, the team of army engineers suspended a five-hundred-pound anti- submarine mine on a scaffold almost eight below the surface of the Nant-y-Gro reservoir. With high speed film running and scientists, including Barnes Wallis, gathered to view the results, the engineers detonated the mine. With the explosion, the surface of the water suddenly whitened as the impact of the blast pushed water skyward into a spout. The size of the spout caught Collins off guard, and his finger momentarily slipped off the movie camera button. Moments later, a hole appeared in the dam on the air side of the wall and water gushed through with tremendous force. Collins could now apply his results to a gravity dam of any size or height – in other words, he now knew that. 7500 pound contact charge detonated thirty feet below the reservoir surface could blow a hole through about forty-four feet of dam wall. This was comparable to the Mohne and Eder Dams, and it wouldn’t require a monster bomb for delivery.

The ripple effect, both literally and figuratively, gave Barnes Wallis’s theories the credence they had not enjoyed since he had first shared the idea of breaching the Ruhr River dams with Collins, Winterbotham, and others back in 1940. The Nant-y-Gro experiments confirmed that a bomb, acting like a depth charge but weighing much less than Wallis’s original ten-ton earthquake bomb, and therefore well-within the carrying capacity of the four-engine Lancaster bombers rolling off the assembly line that very autumn, would deliver the devastating blow to Nazi-Germany’s industrial heart that the British Air Ministry  and all its various departments had been debating for nearly a decade.

The whole success of the operation, Wallis explained later, depended on inventing a weapon which could go hopping along the surface of the water until it struck a the freeboard of the dam, the part projecting above the surface, and sank and gradually paddled itself . . . up against the dam face.

Remarkably, while A.V. Roe was indeed rolling Lancasters off its production line at Woodford, Barnes Wallis’s bomb prototype remained little more than Elizabeth’s ricocheting marbles in the family’s backyard at Effingham. By October 1942, however, Wallis had gone to an engineering company to help him construct spheres – the casings for the bouncing bomb – that were four foot six inches in diameter. He also had a Wellington bomber modified with an underside hydraulic device that rotated the sphere just before it was released to ensure that it would bounce when it hit the water.

As smoothly as the planning was going, there remained administrative and political turbulence all around him. Wallis had learned that Lord Cherwell continued to downplay the value of a raid on the Ruhr dams to Prime Minister Churchill. And there appeared to be a tug-of -war going on between the Royal Navy and the Air Force for ownership of the weapon; eventually, two versions of Wallis’s bomb were under consideration – a lighter bomb to be delivered by Mosquitoes, code-named “Highball,” and a larger heavier weapon to be dropped by the Lancasters, code+named “Upkeep.”

February proved pivotal to the entire dams raid scenario, not so much on the technical front as in the political one. When it became clear that development of Upkeep would require not only design modifications to Bomber Command’s Lancasters but also the assignment of a full squadron of them to the operation for a number of weeks, Air Chief Marshal Harris reacted. He called the idea tripe of the wildest description. Harris then entreated those at the Air Ministry and MAP to block any plan to reduce Bomber Command’s capability by assigning bombers to Upkeep. Further on 18 February, Harris wrote to Air Chief Marshal of Air Staff Sir Charles Portal complaining about panacea mongers careering around the Ministry of Air Production recommending the sidelining Harris’s precious Lancasters. He ripped apart every aspect of the Upkeep proposal – the claims of its easy integration with Lancaster technology, the promise of its ballistics and the timeline for delivery – and concluded that I am prepared to bet my shirt . . . that it would never work.

Meanwhile, at the corporate offices of Vickers on 23 February, Wallis’s ego and project took more criticism from his boss Sir a Charles Craven. The CEO berated Wallis for, in Air Chief Marshall Linell’s  words,

• making a nuisance of himself  at the Air Ministry
• offending Air Staff
• damaging Vickers corporate interests

After his dressing down, Wallis offered to resign. “Mutiny!” Craven shouted are him, punctuating his accusations by pounding his fist on the desk. Wallis left the room to compose himself, sensing that the chain of command, from Vickers’s Craven to the Air Ministry’s Linelll and Portal, had turned thumbs down on the entire dams raid concept.

In fact, however, Portal had received credible endorsement of Upkeep prior to Harris’s bet my shirt memo by way of the Air Vice Marshal of Intelligence at the Air Ministry, Frank Inglis. Through an equally respected intelligence channel, Air Intelligence, Inglis had obtained strong support, for Wallis’s Air Attack on Dams document and trials film. The author reminded Inglis of earlier correspondence about an invention that could strike any target where there was from a quarter of a mile of approach water surface. He added that Prime Minister Churchill had been involved (an aside that was completely untrue), and he wondered whether the Air Ministry and the Chief of Air Staff (Portal) knew just how far the development of the invention had progressed. He added that there was great danger the a Royal Navy might use this extraordinary weapon against enemy shipping prematurely instead of within a coordinated RAF plan, in effect implying that it was Inglis’s duty to get Air Attack on Dams to Portal pronto. The letter had come from none other than Group Captain Winterbotham, from one intelligence officer to another. And it worked.

On 19 February, the day after Harris’s “bet my shirt” memo, CAS Portal wrote back to the head of Bomber Command, offering his respect for the Air Marshal’s opinion of technical and operational matters and admitting that Highball and Upkeep might come to nothing. Nonetheless he added, he didn’t feel as if he should refuse Air Staff interest in the weapons. That day Portal also saw Wallis’s trial film and assured Harris he would assign only three Lancasters to the project, but he also told a Harris, If you want to win the war, bust the dams!

On 25 February, Barnes Wallis, just days earlier fearing that all hope was lost for Upkeep, learned that the bouncing bomb was suddenly back in the picture. Mysteriously within a week, what had been three Lancaster bombers assigned to Upkeep became thirty. On 26 February, Wallis was in the office of Air Vice Marshal Francis Linnell At MAP in London. He had claimed the bouncing bomb could be ready in two months. Could this be done? Linnell asked Wallis. Yes, Wallis said, it could.

That experimental spherical device, falling from the belly of a Wellington bomber and hopping over the surface of the Fleet off Chesil Beach- Upkeep- was the very thing that Wing Commander Guy Gibson watched on film on 24 March 1943, at Barnes Wallis’s lab inside the Burhill clubhouse. In the presence of Mutt and Jeff acquaintances that day, in those few seconds of viewing those top-secret motion pictures, Gibson witnessed the culmination of years of research, campaigning, politicking, and trial and error, accomplished mostly by a patriotic scientific intent on driving Germany out of the war the best way he knew how. However, for both Gibson and Wallis, there remained huge obstacles to accomplishing the military objectives that would become known as Operation Chastise. For the scientist, much refinement of the device lay ahead; for the wing commander, there was the task of finding the skilled airmen to deliver the blow. They each had just over seven weeks in the spring of 1943 to pull it off.

Courtesy of: