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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 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 secretary at the Ministry of Production on 14 September 1942, speculating about what Wallis’s bombs might achieve: ‘If this new weapon is intelligently used, e.g. for simultaneous attacks on all German capital ships and main hydro-electric power dams, there is little doubt but that Italy could be brought to a complete standstill and that industry in Germany would be so crippled as to have a decisive effect on the duration of the war … To attain this result much preparation and careful planning are clearly required and meanwhile I repeat nothing is being done.’ Here was a manifestation of a British yearning, characteristic of its time and place, for some dramatic stroke that might sidestep battlefield slaughter and bring the war to an early closure. Winterbotham’s note took no heed of the Alpine difficulties in the way of his fantasy, prominent among them that its fulfilment would require hundreds of bombers to be modified to carry Wallis’s weapons. Meanwhile its expectations about what these, or for that matter any, bombs might do to the Axis drifted into fairyland.

That autumn of 1942, the bomb project languished. Oxley Engineering experienced difficulties in constructing the test weapons, and in October Wallis was kept at home for several days by illness. Only on 2 December did he at last board a modified Wellington, piloted by veteran test pilot Mutt Summers – the very same who had parachuted more than a decade earlier from one of the engineer’s less successful prototypes – for a trial of the backspin technology. It worked, though no test bombs were dropped.

Two days later, on the afternoon of the 4th, the Wellington took off for Dorset, where on Chesil Beach a camera crew waited to record the trial bomb-dropping. The first two tests, with non-explosive fillings, resulted in the spheres bursting on impact. When a subsequent succession of droppings took place, Wallis watched from the shore. Outcomes suggested that the technologies for releasing the bomb from an aircraft, and for its subsequent bouncing progress, were viable. Yet repeated collisions with the sea at speeds of over 200 mph imposed enormous stresses on the projectile during its bouncing progress. Half-sized prototypes disintegrated. Their begetter undertook modifications and adaptations, still convinced that the principles of his creation – or rather, his instrument of destruction – were sound.

Following these further developments, Wednesday, 20 January 1943 found Wallis ensconced in Weymouth’s Gloucester Hotel, from which he wrote to Molly: ‘I do wish you could come & share this lovely room. It would be just perfect with you here. If you could come tomorrow by the mid-day train, do … Now we are scheduled to start [tests] at 10 a.m. … so I must go to bed. All my love little sweetheart, and come if you can …’

She came. Almost two decades of marriage had done nothing to cool the couple’s passionate romance. Molly, still only thirty-seven, was now responsible for six children, including a niece and nephew whose parents had been killed in the blitz. This brood was surrendered to their nanny while she set forth for Dorset. On 24 January she wrote to the children from Weymouth about her visit to their father:

… A lovely time it’s been. This morning I drove out with him and the others to the nearest point I was allowed [to the bomb tests] & then I got out and walked back. 8 miles of lovely road, high up with the Downs one side & the sea to other. It was sunny & clear. I did enjoy it. Of course it’s quite mad that they shouldn’t let me watch the proceedings seeing as I’ve lived with it since last February & probably know more about it than anyone save Barnes. But it’s quite true – policemen do bob up and turn you away. I suppose the others would say ‘If that wife why not my wife’ little knowing what a very special wife this one is.

Darling Barnes. You should see him among these admirals and air vice marshals patiently explaining and describing to them & they drinking it all in – or trying to. And he’s so quiet and un-assuming none of them could imagine what pain & labour it’s been. How he’s got up in the middle of the night to go up to the study & work summat out. No wonder he looks drawn and tired. I suppose if he were a self-advertiser he’d have been Sir Barnes in the New Year Honours. Oh well, it’d have been a nuisance. But it’s an exciting life & no mistake.

The contrast seems extraordinarily moving between the private domesticity and indeed passion within the Wallis family, and the devastating public purposes which its principal was pursuing. Some might find it repugnant, in the light of what later befell Upkeep’s victims, but it must never be forgotten that Barnes Wallis’s country was engaged in a war of national survival against one of history’s most evil forces. The engineer was straining every sinew, and all his own astonishing gifts, to assist the Allied cause. During that 24 January trial in Dorset which Molly Barnes was not permitted to witness, one dummy bomb achieved thirteen bounces. Next day, another managed twenty. Wooden test spheres, dropped from between eighty and 145 feet, travelled 1,315 yards across the water. Barnes once wrote down for Molly the closing lines of Tennyson’s ‘Ulysses’, among his favourite poems: ‘One equal temper of heroic hearts,/Made weak by time and fate, but strong in will/To strive, to seek, to find, and not to yield.’

Returning to London, the white-haired evangelist now strode through the corridors of ministries proudly clutching a can of 35mm film, showing his weapon skimming the sea. Eagerly, he awaited authorisation to continue with development of both Upkeep – the dam-bursting version – and Highball, the smaller naval bomb. He chafed for a swift commitment, because for optimum effect an attack on the German dams needed to take place in May, when water levels in the reservoirs were at their maximum height after winter rains and snows.

Instead, however, on 12 February 1943 a blow descended. Ben Lockspeiser told Wallis that AVM Linnell had become concerned that the engineer’s labours on his bombs – nobody used the word obsession, but this was obviously in many service minds – was impeding his ‘proper’ work, development of the Windsor bomber. Linnell did not explicitly oppose the bomb scheme – he was too canny a service politician for that. He merely reported on its speculative character to AVM Ralph Sorley, assistant chief of air staff for Technical Requirements, further emphasising the drain that the project was imposing on resources. Once again, it seems worthy of emphasis that Linnell did not thus play the part of a myopic senior officer flying a desk, but was instead assessing Wallis’s project from the viewpoint of a department besieged by competing demands for facilities to develop new aircraft and weapons systems. Meanwhile Syd Bufton, deputy director of bomber operations, told a 13 February meeting at the Air Ministry that he, as an experienced operational pilot, considered it impracticable to drop Wallis’s bombs in darkness, at low level over enemy territory.

Gp. Capt. Sam Elworthy, a Bomber Command staff officer who attended the same meeting, was charged with reporting on its findings to Harris’s headquarters at High Wycombe, which he did on the following day. The consequence was a note on the ‘bouncing bomb’ drafted by AVM Robert ‘Sandy’ Saundby, senior air staff officer to his chieftain. This, in turn, prompted the C-in-C to scribble one of his most famous, or notorious, judgements of the war: ‘This is tripe of the wildest description … There is not the smallest chance of it working.’ And much more of the same.

Wallis now wrote an anguished personal note to Fred Winterbotham, expressing his frustration: ‘We have just worked out some of our results from the last experiment at Chesil Beach, and are getting ranges nearly twice those which would be forecast from the water tank, that is, with a Wellington flying at about 300 miles an hour and dropping from an altitude of 200 feet, we have registered a range of exactly three-quarters of a mile!!’ He said that the problems of constructing prototypes to be carried by a heavy bomber would be easily solved. ‘It follows that sufficient bombs for the Lancaster experiment (if, say, thirty machines were to be used, to destroy simultaneously five dams, that is, six machines per dam to make certain of doing it) can be completed within two or three weeks.’ He added that modifying the Lancasters would be a far more time-consuming process than manufacturing the weapons, and concluded ‘Yours in great haste,’ adding a handwritten scrawl: ‘Help, oh help.’ It was characteristic of the strand of naïveté in Wallis that in calculating the number of aircraft needed to destroy five dams in enemy territory he was heedless of the possibility that the German defences might remove from the reckoning some, if not all, of the attackers.

Winterbotham responded by writing on 16 February to AVM Frank Inglis, assistant chief of air staff for Intelligence. He extravagantly described the bouncing bomb as an invention ‘for which I was partly responsible’. He asserted that the chief of combined operations and the prime minister were enthusiastic, though there is no shred of documentary evidence of Churchill’s involvement at any stage. He then employed an argument often advanced by estate agents: if the Royal Air Force did not snap up this opportunity, the Royal Navy was eager to do so: ‘My fear is that a new and formidable strategic weapon will be spoiled by premature use against a few ships, instead of being developed and used in a properly coordinated plan.’ He urged ensuring that the chief of air staff was briefed, before it was too late.

Despite Harris’s attitude, and airmen’s continuing doubts about the tactical feasibility, on Monday, 15 February, Gp. Capt. Syd Bufton chaired a further meeting at the Air Ministry, attended by Elworthy, Wallis, Mutt Summers and others, in fulfilment of an Air Staff instruction ‘to investigate the whole [dams] operational project’. Wallis delivered a superbly eloquent sales patter. Upkeep, he said, could be released from a height of 250 feet, at a speed of around 250 mph, at a distance from the target of between three-eighths and three-quarters of a mile. Responding to Elworthy’s concern, expressed on behalf of Bomber Command, about a diversion of precious Lancasters for modification, he said that only one aircraft would be needed for full-scale trials, while those used for an Upkeep attack could be restored to normal operational mode within twenty-four hours. He suggested that while the Möhne dam was the most prominent target suited to Upkeep, the Eder – forty-five miles east-south-eastwards – was also vulnerable.

Most of this was debatable, and some of it flatly wrong. Nobody at the meeting pointed out that even if the Eder represented a suitable target for bouncing bombs, it was unrelated to the Ruhr water system, which was supposedly the strategic objective. The aircraft to carry Wallis’s weapons did not require mere modification, but would instead need fuselages purpose-built by Avro, and could not thereafter be readily returned to Main Force duty. Wallis’s persistence emphasised his gifts as a street-fighter. Where his professional passions were engaged, he was a much less gentle, more ruthless man than was sometimes supposed by those who met him casually. On this occasion, his reputation and conviction carried the day. Bufton changed his mind, renouncing the disbelief he had expressed on 13 February to report in the name of the committee: ‘It was agreed that the operation offered a very good chance of success, and that the weapons and necessary parts for modification should be prepared for thirty aircraft.’ It was thought that as long as the attack took place before the end of June, reservoir levels should be high enough to create massive flooding.

Bufton told AVM Norman Bottomley, assistant chief of air staff for Operations, ‘the prospects offered by this new weapon fully justify our pressing on with development as quickly as possible’. Bottomley, who would play an important role in securing the final commitment to the dams raid, was a veteran networker within the corridors of power. Syd Bufton said of him with wry respect: ‘Nobody could play the Air Ministry organ as skilfully as Norman.’ It was Wallis’s additional good fortune that Bufton and Elworthy – a thirty-one-year-old New Zealander of outstanding abilities who eventually became head of the RAF – were original thinkers, open to new ideas in a fashion that Harris was not. They grasped the terrific theatrical impact that the dams’ destruction would make, surely greater than that of yet another assault on German cities. Churchill once said grumpily, ‘I’m sick of these raids on Cologne,’ to which Sir Arthur Harris’s riposte – ‘So are the people of Cologne!’ – was not wholly convincing.

A weakness of the debate about Upkeep, however, was that it focused overwhelmingly on the feasibility of constructing and dropping the bombs; much less on the vulnerabilities of the water systems of western Germany, the Ruhr in particular. Throughout the Second World War, intelligence about the German economy and industries remained a weakness in Western Allied warmaking, and explicitly in the conduct of the bomber offensive.

Just three days after the Air Ministry meeting, on 18 February, following a telephone conversation with Linnell of MAP, who remained a sceptic, Harris wrote a testy note to Portal, his chief, head of the Royal Air Force. Linnell had told him, he said, ‘that all sorts of enthusiasts and panacea-merchants are now coming round MAP suggesting the taking of about thirty Lancasters off the line to rig them up for this weapon, when the weapon itself exists so far only within the imagination of those who conceived it. I cannot too strongly deprecate any diversion of Lancasters at this critical moment in our affairs.’ Wallis’s bomb, in Harris’s view, ‘is just about the maddest proposition … that we have yet come across … The job of rotating some 1,200 pounds [sic] of material at 500 rpm on an aircraft is in itself fraught with difficulty.’

But Wallis had acquired supporters more powerful even than Harris. After a screening of a new batch of films of his tests before audiences that included Portal, First Sea Lord Admiral Sir Dudley Pound and Vickers chief Sir Charles Craven, Pound threw his weight behind the naval version of Wallis’s mine: ‘The potential value of Highball is so great,’ he minuted on 27 February, ‘… that not only should the trials be given the highest priority, but their complete success should be assumed now.’

It is hard to overstate the stress under which the bomb’s begetter existed in those days. He was still spending many hours on the design of the Windsor, Air Ministry specification B.3/42. In the mind of Sir Charles Craven, this was much more important than Upkeep and Highball: contracts for a big new bomber promised immense rewards for Vickers, contrasted with those to be gained from building a few bombs. Moreover, confusingly for Wallis and for the entire Whitehall hierarchy, Craven was intermittently seconded to assist and work in the Ministry, so that it was sometimes unclear to all concerned whether he spoke as the engineer’s employer, or as the voice of officialdom. On 18 February, Wallis worked until 7.45 p.m. at the National Physical Laboratory. Next morning, he met the Admiralty’s director of weapons development, then at 2.30 p.m. saw MAP officials to discuss unspecified aircraft de-icing problems. At 4 p.m. he was back at Vickers, where at 5.30 p.m. there was another screening of his bomb-test films, following which he drove to Dorking with Admiral Renouf. On the next morning, a Saturday, he worked in his office at Burhill, then attended more meetings in the afternoon. On Sunday he was confined to his home at Effingham with a migraine, such as he often and unsurprisingly succumbed to.

Next day, Monday the 22nd, he drove with Mutt Summers to High Wycombe for a personal audience with Sir Arthur Harris. Sam Elworthy claimed credit for persuading the C-in-C to meet the engineer. He wrote to Harris after the war, saying emolliently that ‘your scepticism of what seemed just another crazy idea was certainly shared by your staff’. But the clever group-captain had been impressed by what he heard about Upkeep – and he also saw which way the wind was blowing at the Air Ministry.

Harris was no fool. For all his bombast, he grudgingly acknowledged that he had masters who must sometimes be appeased. He knew that Portal had authorised the modification of three Lancasters to carry Upkeep. While the words ‘whether the Commander-in-Chief of Bomber Command likes it or not’ were never articulated, they were understood. The Chief of the Air Staff wrote to Harris on 19 February: ‘As you know, I have the greatest respect for your opinion on all technical and operational matters, and I agree with you that it is quite possible that the Highball and Upkeep projects may come to nothing. Nevertheless, I do not feel inclined to refuse Air Staff interest in these weapons.’

That morning of the 22nd at High Wycombe, Wallis was subjected to a predictable barrage of invective: ‘What is it you want? My boys’ lives are too precious to be wasted on your crazy notions.’ Yet it is unlikely that Harris would have received Wallis at all had he not already recognised that he would have to give way, and provide resources for an operational trial of Upkeep. Having viewed the films, he professed grudging interest.

Wallis succumbed to a brief surge of optimism. This was shattered, however, on his return to Weybridge. He received an order to present himself immediately at the London office of Vickers, his employers, for an audience with the company’s chairman. Craven, without inviting his visitor to sit down, declared brusquely that MAP’s Linnell had complained that Wallis had become a nuisance; that his bouncing bombs had become a serious impediment to the vastly more important Windsor project. The air marshal had explicitly demanded that Craven call a halt to Wallis’s ‘dams nonsense’.

This was what the Vickers chairman now did. A shouting match followed, in which the designer offered his resignation, and Craven shouted ‘Mutiny!’ They parted on terms of mutual acrimony. Moreover, while Wallis was not often a grudge-bearer, he never forgave AVM Linnell for the part he played in attempting to kill off Upkeep. He went home despondent to Effingham, sincerely determined upon resignation, as was scarcely surprising after the humiliation he had suffered. Craven, whose responsibility was to Vickers, can scarcely be blamed for his behaviour, after being told by the Ministry of Aircraft Production – upon whose goodwill his company depended for orders – that its chiefs were tired of his nagging, insistent assistant chief designer (structures). Why should such people as Linnell, Craven and indeed Harris have accepted at face value the workability of a new weapon which represented a marriage of technologies of extreme sophistication with others of almost childlike simplicity, which when fitted to a Lancaster caused it to resemble a clumsy transport aircraft with an underslung load?

Yet Wallis knew that, whatever Craven said about the MAP’s view of Upkeep, the Admiralty remained enthusiastic about Highball. On 26 February, by previous arrangement he drove to London to attend a meeting that was to be chaired by the now-detested Linnell, to discuss measures to improve the aerodynamics of what some described as ‘the golf mine’ – because of its resemblance to the shape of a golf ball. After Wallis was told that Roy Chadwick of Avro, designer of the Lancaster, would also be attending, he understood that Craven had got things all wrong the previous day: the RAF had not abandoned Upkeep.

When the delayed meeting finally convened at 3 p.m. that Friday, in Linnell’s office at MAP on London’s Millbank, it was to receive tablets from on high. Sir Charles Portal was not only chief of air staff and a former C-in-C of Bomber Command; he had also been among the first enthusiasts for attacking Germany’s dams. He was troubled by doubts about Sir Arthur Harris’s obsession with destroying cities. His reservations were founded not upon moral scruples – no senior wartime airman admitted to those – but instead on uncertainty about its war-winning potential. Portal never summoned the courage to sack Harris, even in the winter of 1944–45, when his subordinate directly defied targeting orders. But the CAS was always at heart a proponent of attacking precision objectives, if means existed to make such a policy work. Now, Barnes Wallis promised to provide these; to make possible fulfilment of the RAF’s 1937–38 dream, of an assault upon Germany’s dams.

The array of brass assembled at the MAP on the afternoon of 26 February 1943 was told that the chief of air staff had given his assent, or rather had issued an order, to proceed with immediate development of Upkeep. Portal wrote: ‘I think this is a good gamble.’ The reaction of Sir Charles Craven, who was also present, is unrecorded. He must have felt privately foolish, if not furious, following his ugly dressing-down of his designer four days earlier. What the CAS demanded, however, Vickers must seek to provide.

Linnell, too, can scarcely have enjoyed announcing – against his own strong personal conviction – the decision to prioritise Upkeep over the Windsor bomber. ‘The requirement for bombs,’ this MAP potentate now said, ‘has been stated as one hundred and fifty to cover trials and operations’ – one hundred and twenty were eventually made. It was further stated that studies of the German dams showed that 26 May – just three months ahead – was the latest date in 1943 on which they could plausibly be attacked. It would thus be necessary to build thirty ‘Provisioning’ Lancasters, as they had been codenamed, and to produce a sufficiency of bombs by 1 May, to provide reasonable time for aircrew training for the operation. The MAP’s budget for research on Upkeep, which in August 1942 had been raised from £2,000 to £10,000, was now further increased to £15,000, and later again on 1 April to a princely £20,000.

While Portal made a personal commitment that enabled Upkeep to be unleashed, it deserves emphasis that he placed a relatively modest bet. Only air-power fantasists could suppose that a single squadron of Lancasters, a maximum of twenty bouncing bombs, would cripple the entire water system of the Ruhr, an aspiration demanding a much larger force even if Wallis’s weapons were half-successful – almost no new weapons system in history has performed better than that. Yet a squadron was all that the Air Ministry would authorise, in the face of Harris’s virulent hostility, a limited supply of aircraft and uncertainty about the viability of Upkeep. British war-makers have for centuries displayed a weakness for ‘gesture strategy’ – deploying a disproportionately small force as a means of displaying interest in fulfilment of a disproportionately large objective. Mass matters, however, to the success of all military operations, and in this case it would be lacking. Though the RAF neither then nor later admitted this, its commitment to an assault on Germany’s dams was marginal, a tiny fraction of the forces that set forth upon almost nightly attempts to burn cities. To borrow a modern phrase, this would be a niche operation.

Wallis’s commitment to Upkeep, by contrast, was total, and now confronted him with a dramatic challenge. At breakneck speed he must convert his theoretical concept into a viable bomb, while work was simultaneously rushed forward on building the modified Lancasters. Avro, the plane’s manufacturers, agreed to fit the necessary electrical release gear, along with strongpoints where the bomb doors must be removed, and hydraulic power for backspin pulleys. Vickers would meanwhile make protruding retainer arms for the Upkeeps, to be attached to the strongpoints; a rotational driving mechanism; and the bombs themselves. All the latter work would be carried out at Weybridge, with Avro dispatching a team to work on the Vickers site. Wallis promised to provide working drawings of the latest version of Upkeep within ten days. Craven expressed concern about whether a revolutionary weapon of such size could be machined within the necessary time-frame.

Before the final decision, some minor obstacles had to be swept away. By coincidence Combined Operations, then headed by the frisky Lord Louis Mountbatten, was proposing an attack on the Möhne, which Mountbatten described as ‘one of the great strategic targets’. Special Operations Executive also suggested an assault by parachute saboteurs. Both bodies’ proposals were now quashed, fortunately for those who might have been charged with implementing them, in favour of what was designated Wallis’s ‘rolling bomb’. It would be the RAF which destroyed the dams of north-west Germany. Or nobody.