Royal Navy commanders had shown the way, exploiting the technique of bouncing cannonballs across the sea to pummel French warships. At the end of May, Wallis set off with his secretary, former British ladies’ rowing champion Amy Gentry, for Silvermere Lake near Cobham to test the potential of using a catapult, much more sophisticated than a child’s toy, to bounce small projectiles down a test tank. In the course of these experiments they found that, if a golf-ball-sized object was backspun on release, it would ‘ricochet’ far more vigorously. Vickers’ experimental manager George Edwards, a keen cricketer, later claimed credit for this idea, but the evidence suggests that Wallis developed it himself, and merely had later conversations about it with Edwards.
The eventual form of Upkeep was that of a large, cylindrical naval depth-charge. Until late April 1943, however, Wallis envisaged its shape as almost or absolutely spherical, the huge canister containing the charge being encased in an outer shell of wood. It was also at times described as a mine, which became part of its cover story in official correspondence and later news coverage. Since legend, however, knows the dam-busting weapon as a bomb, that is how it will continue to be described in this narrative.
Wallis told Fred Winterbotham that he saw every reason to believe that the new weapon’s destructive principles would prove as applicable to enemy shipping as to dams, locks and suchlike. Thus, on 22 April 1942 Winterbotham accompanied the engineer to discuss the project with Professor Pat Blackett, the exceptionally enlightened physicist who was scientific adviser to the Admiralty. Blackett, in turn, lobbied Tizard, who despite his opposition to Wallis’s big-bomb project a year earlier was now sufficiently excited to visit him at Burhill on the 23rd. Tizard thereafter supported Wallis’s request for access to two experimental ship tanks at the National Physical Laboratory at Teddington, where he began tests in June which continued over twenty-two days, at intervals until September. If the pace of progress appears slow, it must be remembered that Britain was still conducting its war effort on desperately short commons, while Wallis was earning his bread working on the Windsor bomber.
Although the Royal Navy was perhaps Britain’s most successful armed service of the war, the Fleet Air Arm was its least impressive branch. Despite the much-trumpeted success of a November 1940 torpedo attack on Italian capital ships in their anchorage at Taranto, carried out by antiquated Swordfish biplanes, thereafter British naval aircraft enjoyed few successes. Churchill more than once acidly enquired why the Japanese seemed much better at torpedo-bombing than was Britain’s senior service. Admirals were thus immediately attracted to a new technology which might make the Fleet Air Arm less ineffectual. For months after Wallis’s ‘bouncing bomb’ was first mooted, the RAF sustained institutional scepticism; sailors did more than airmen to keep the concept alive.
Tizard himself attended some tests at Teddington, as did Rear-Admiral Edward de Faye Renouf, a former torpedo specialist who was now the Admiralty’s director of special weapons. Renouf and several of his staff watched a demonstration in which a two-inch sphere was catapulted down a tank, bouncing along the water until it struck the side of a wax model battleship and rolled down beneath its hull. The admiral, a gifted officer recently recovered from a nervous breakdown after a succession of terrifying experiences while commanding a cruiser squadron in the Mediterranean, urged Sir Charles Craven of Vickers to give priority to Wallis’s weapons research. Renouf envisaged a projectile that might be released from the new twin-engined Mosquito light bomber.
That month, May 1942, Wallis produced a new paper incorporating all this research, entitled ‘Spherical Bomb, Surface Torpedo’. His thinking still focused entirely on round weapons, described in a note from Winterbotham to the Ministry of Production as ‘rota-mines’. Wallis’s paper cited earlier work by a German scientist, and also showed that for a bomb to get close enough to a dam to enable the principle of ‘Conservation of Suspended Energy’ to work, it needed to impact upon the water almost horizontally, at an angle of incidence of less than seven degrees, which meant that it must be dropped from an aircraft flying very low indeed: at that time, 150–250 feet seemed appropriate. Wallis envisaged its release from a range of around twelve hundred yards, to allow time for the attacking pilot to turn away and escape before flying headlong over the target and its defences. Not until months later was a requirement accepted for the aircraft to carry its bomb much closer, and thereafter to overfly the objective.
In a further demonstration of the validity of Churchill’s observation that ‘All things are always on the move simultaneously,’ at the Road Research Laboratory Arthur Collins had meanwhile been conducting a succession of tests on two 1:10 scale models of the Nant-y-Gro dam. On 10 May 1942 Wallis and his wife Molly travelled to Wales with Collins’s team to witness experiments on the full-sized dam. These established that if an explosion took place at any significant distance from its wall, the blast was too weak to precipitate a fracture. Collins wrote: ‘A solution to the problem was, however, found almost by chance shortly afterwards.’ His team needed to remove one of the damaged scale models at Harmondsworth, and used a contact charge to shift the concrete. The result was devastation, on a scale unmatched by any ‘near-miss’.
Further tests confirmed the result, and on 16 July Wallis received an invitation to attend a full-scale demonstration a week later. He was nettled by the short notice, and warned a little pompously that he was working under such pressure – presumably on the Windsor bomber – that he would probably be unable to get away. Nonetheless, he was present at Nant-y-Gro when, on the 24th, army engineers blew a 279-lb charge of which the effects were filmed with high-speed cameras brought to North Wales from the Royal Aircraft Establishment at Farnborough. The test explosion proved a triumph, blasting a breach in a masonry construct that was, for practical purposes, a small-scale version of a German dam.
In the following month, Collins submitted a report which concluded that if a charge weighing around 7,500 lb was exploded at a depth of thirty feet against the wall of a dam such as the Möhne, it should be capable of achieving a breach. Such a weapon would not require the creation of a new bomber to carry it, but was within the lifting capabilities of the new Avro Lancaster, subject to appropriate modifications. Thus, suddenly, the most intractable obstacle to an attack on Germany’s masonry dams was removed: it seemed feasible – in theory at least – to convey to the target sufficient explosive to destroy it. Credit for the principal scientific achievements that made possible Operation Chastise should rightfully be shared between Collins, who resolved the challenge posed by the physics of destroying a vast man-made structure, and Wallis, who conceived a technique whereby the necessary charge might be laid from the air with the exactitude indispensable to success.
In the late summer of 1942, a situation obtained wherein Barnes Wallis had devised a revolutionary weapon, of which the scientific principles were agreed by most of the experts who studied them to be sound. The Royal Navy was excited about its possibilities for use by the Fleet Air Arm. Widespread scepticism nonetheless persisted, shared by MAP’s David Pye and his deputy, Ben Lockspeiser, about whether the resources could be justified to pursue a speculative technology that could only be used over water, and which demanded superhuman courage and skill from aircrew who would have to launch it against an enemy. Moreover, every aircraft which carried such a bomb would require expensive modification.
Such reservations were fully justified. Lockspeiser wrote to Tizard on 16 June: ‘It is quite impractical and uneconomic to modify our bombers in large numbers for the special purpose of carrying any particular bomb.’ Nonetheless the Admiralty’s enthusiasm, and the uneasy acquiescence of MAP’s AVM Linnell, sufficed to secure a request for Vickers to fit a Wellington twin-engined bomber to carry a prototype Wallis bomb, of which on 22 July an order for twelve examples was placed with the Oxley Engineering Company. On 25 August Wallis attended a meeting at MAP at which arrangements were agreed for a series of trials to be conducted a month later, at Chesil Beach in Dorset.
It is striking to notice, at this stage, two camps in the service ministries and the defence scientific community about the whole project. One faction believed that Wallis’s weapons were fanciful; would never work. The other cherished wildly over-optimistic fantasies concerning their war-winning potential. Fred Winterbotham wrote to the parliamentary
