Chastise: The Dambusters Story 1943. Max Hastings

under-secretary of state: ‘Sir, I have the honour to state that consideration has again been given recently to the possibility of breaching one or more of the important canals in North West Germany.’ The engineer concerned was referred to ‘as Mr B.N. Wallis of Vickers’. The writer signed himself ‘your obedient servant’.

From the £2,000 budget then allocated to Wallis’s activities by the MAP, money was found to buy from Birmingham City Council a small dam at Nant-y-Gro in Powys, North Wales, rendered redundant by the construction of a larger replacement. A key figure in the experiments that followed was Arthur Collins, a scientific officer in Harmondsworth’s ‘Concrete Section’, who made a breakthrough. For years it had been assumed, not least by Barnes Wallis, that an enormous explosive charge would be necessary to destroy a dam such as the Möhne. Yet experiments convinced Collins, who in turn persuaded Wallis, that a relatively small charge might achieve a wholly disproportionate result if it was detonated sub-aqueously and close to the target, using a timer or a hydrostatic pistol: it could thus harness the power of the water mass to channel the force of the blast. Here was the phenomenon identified as a threat back in 1939 by the German official responsible for his country’s north-western dams. Both Collins and Wallis became increasingly fascinated by the physics of explosions, and especially by the scope for harnessing the power of water, and indeed of earth, dramatically to increase the impact of underwater or underground explosions – the ‘conservation of suspended energy’ that would eventually make possible Operation Chastise.

      In the course of 1941 and 1942, Wallis pursued enquiries about Germany’s dams through patent agents in Chancery Lane, and about hydro-electric control mechanisms via an engineering firm in Kilmarnock. In April 1942 – Holy Week, as it happened – experiments assisted by his children, using marbles projected into an old galvanised washtub on the terrace outside his home at Effingham, shifted his attention from deep-penetration ‘earthquake’ charges towards the notion of much smaller spherical bombs, bowled – in cricketing parlance – or ricocheted – to use Wallis’s original choice of word – towards German dam walls. Here, he was thinking in a fashion not dissimilar from Finch-Noyes and Pemberton-Billing. He envisaged two related, but different weapons: a larger model for attacking dams, later codenamed ‘Upkeep’, as it will hereafter for convenience be called; and a smaller version, to be codenamed ‘Highball’, for use against shipping.

Sir Charles Craven, a former Royal Navy submarine officer who was now chairman of Vickers, did not explicitly bar Wallis’s spare-time work on futuristic weapons. He emphasised, however, that it must not interfere with the engineer’s day job, developing the Windsor bomber. In post-war evidence to the Royal Commission on Awards to Inventors, Wallis stated that ‘the inception of the [bouncing bomb] was the result of private experiment and work outside the scope of his normal employment and that this work was carried out against the wishes of his employers’. He subsequently expanded on this theme, saying that ‘had he not persisted in his efforts to interest the authorities in the face of continued discouragement and even contrary to the wishes of his own Directors, the attack on the dams would never have been made’. In the narrative that follows, it should not be forgotten that, until the last stage of the development of Wallis’s revolutionary weapons, his work on them represented, in the stern view of his employers, a spare-time indulgence.

      3 FIRST BOUNCES

      In the late spring of 1942, Barnes Wallis reported to the MAP and the Air Ministry that he believed he could overcome a critical problem – accurately to deliver a charge from a fast-moving bomber against a target protected with anti-torpedo nets – by bouncing a bomb across the water in the fashion he had explored with marbles on his terrace at Effingham. Moreover, a century and a half earlier Vice-Admiral Horatio Nelson and his fellow 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