by Brian Ford
THE QUEST FOR A SUPER BOMB
Barnes Wallis was a crisp and authoritative figure with a warm, avuncular manner when I met him in the 1970s at the BBC in London. His proposal as World War II enveloped Europe was to construct increasingly massive bombs. Wallis argued that the best way to bring the German war machine to its knees was to disrupt its capacity for weapons production, and he had in mind the huge factories in the Ruhr. He realized that the Nazis would seek to construct massively reinforced concrete bunkers that would be impregnable to conventional explosives, and proposed huge bombs that would bring about the desired level of destruction. Wallis decided on a 10-ton bomb that would be dropped from high altitude and demolish the most heavily reinforced construction by penetrating deep inside the ground before it detonated and erupted from beneath. In 1940 he revealed his proposal for a 22,400lb (10,200kg) bomb that could even bring down the massive dams on which German industry relied.
The Air Ministry conceded that the idea was viable, but it failed because there was no aircraft large enough to carry such a weapon. Barnes Wallis responded by designing just such an aircraft, the Victory bomber. It was designed to be 96ft (23m) long with a wingspan of 172ft (52.4m), designed to fly at an altitude of 34,000ft (10,000m) at a cruising speed of 352mph (566km/h). This was an astonishing concept: the American B-245 Liberator in comparison cruised at 214mph (344km/h) at a maximum altitude of 28,000ft (8,534m). Barnes Wallis’s new aircraft would carry a gun turret with four weapons, but it was otherwise undefended since it would easily out-fly any other aircraft in existence. Nonetheless, in May 1941 the project was rejected. The Air Ministry concluded that the bomber would have only one purpose, and expensive fixed-mission projects were not something the British government wished to support. Furthermore, it was calculated that the aircraft was so revolutionary that it was unlikely to be operational before the end of the war.
The refusal of the Air Ministry to give the go-ahead to the Victory bomber left Barnes Wallis with the ambition of finding alternative ways to destroy the German factories in the Ruhr. This had been an aim of British Intelligence since about 1937, and sketchy plans to bomb the dams above the factories were in mind prior to the outbreak of hostilities. The central focus was the Möhne dam, which was known to be well defended, but which Barnes Wallis knew could be destroyed with his proposed 10-ton bomb, if only there was a plane to carry it. The only feasible alternative was to use large torpedoes that could detonate against the dam wall itself, but the Germans were aware of the risk and had the dams of the Ruhr well protected by heavy-duty submerged nets. Wallis walked along the sea-shore one day, skipping stones across the surface, and suddenly thought of the notion of designing a bomb that would similarly ricochet across the water in the reservoirs — above the protective nets — and impact on the concrete dam itself. His proposal was for a cylindrical bomb, spinning backwards to the direction of travel, which would be dropped from a low-flying aircraft. The backward spin — coupled with the forward speed of the bomb — would cause it to bounce across the surface. If it struck the dam wall, the backwards spin would cause the bomb to hug close to the dam as it sank, and a pressure-sensitive hydrostatic fuse would detonate it at the optimum depth. The backspin would also slow the forward motion of the bomb with each skip, so that it would fall behind the aircraft and reduce the chances of damaging the low-flying plane that had dropped it.
The Royal Air Force was sceptical at first, but Barnes Wallis persisted and produced movie films of his successful experiments. In early June 1942 he experimented with a mine suspended on scaffolding to test the effect of underwater detonations. He destroyed the disused Nant-Y-Gro dam near Rhayader, in central Wales, with a submerged 279lb (127kg) mine, to help ascertain the way a dam might behave under attack. Dummy versions of the bombs were then dropped from a modified de Havilland Mosquito B Mark IV aircraft off Chesil Beach, near Weymouth Bay in southern England, but they burst apart on hitting the water and Wallis asked Wing Commander Guy Gibson if they could be launched from a lower altitude — 50ft instead of 120ft (15m instead of 36m). Gibson agreed and there were subsequent tests of dummies on the dams in the Elan Valley, Wales. In February 1943 the Air Ministry finally accepted the scheme and it was resolved to bomb the dams of the Ruhr in springtime, when the water in the reservoirs was at its highest. The mission was code named Operation Chastise and was given to No. 617 Squadron flying out of RAF Scampton, Lincolnshire, with Gibson as the leader. The Lancaster bombers carried out extensive training and numerous dummy runs, until they were accustomed to flying over the kind of landscape that they would encounter on the raid and made several successful practice runs along the Upper Derwent Valley in Derbyshire. This was dangerous work; the flying was under a full moon at night, no more than 60ft (18m) above the surface and they knew that the target would be some 400 miles (640km) away from their base. The pilots became proficient at straight and level flight only some 30ft (less than 10m) above the ground — little more than the height of a house.
The bombers were mechanically adapted to carry the weapons and were fitted with electric motors to set them spinning, prior to the drop. Barnes Wallis knew that it was vital for the bombs to be launched a precise distance from the dam wall, or they would simply skip over the top. To facilitate this he designed a simple Y-shaped wooden sight which the bomb aimer would use to line up the two ends of the dam. Paired downward pointing lights were carefully positioned, so that the two beams met when the plane was at exactly the correct height above the surface of the water. With the dam in the bomb-aimer’s sights, and the beams correctly aligned, the bomber was certain to fly within the very narrow tolerances that the mission demanded. As the teams kept practising, many of them worked out their own ways to adapt the apparatus to suit their own preferences. One answer was to tie string on struts on the windscreen, pulling it back like the elastic on a catapult, to give the range of the towers at either end of the dam.
The attack finally began on 16 May 1943. The outward flight maintained very low levels throughout. Several planes were lost, including one that crashed into high tension cables at 11.50pm, 2.5 miles (4km) north-east of Rees in the Lower Rhine area of Germany. This was the consequence of the low-level approach, of course, as was the experience of another Lancaster that actually skimmed the surface of the Zuider Zee in the Netherlands, ripping the bomb from its brackets and spraying seawater into the fuselage before the pilot managed to recover. Gibson dropped his bomb first, but the aim was poor and it fell away from the target. A second aircraft was hit by flak and the bomb overshot the target, blowing up beneath the Lancaster that had dropped it. Gibson thereafter flew to draw fire as other Lancasters dropped their bombs, until eventually one struck home and the Möhne dam collapsed with a roar. In contrast, the Eder dam was not well defended: the Germans had assumed it would be impossible to attack as it was situated in a deep valley. Pilots made repeated passes until they were confident that the dam could be mined, and only then dropped their bombs. The Eder dam was eventually breached and — with both dams destroyed — the German factories downstream were severely damaged. The nearby Sorpe dam lay in a winding valley that was unsuitable for the bouncing bombs, and was instead attacked by dropping the bombs as conventional inertial weapons; even though it was hit, it was not significantly damaged by the raids. As a result of the raid, hydroelectric power generation from the dams was interrupted, and a considerable propaganda victory was scored. The supremacy of British secret weapons research was widely celebrated in the UK, though the Nazis also claimed success in minimizing the interruption to power generation, and in the fact that the British could not bring down the Sorpe dam.
After the dambusters
Although the raids were seen as an astonishing and heroic success in Britain, there were tragic civilian losses. At least 1,650 people died in the subsequent floods, most of them Allied prisoners and forced labourers held in Nazi prison camps. At least 500 were female Soviet prisoners. This raid was one of the incidents which led to A
rticle 56 of the amendment to the Geneva Convention agreed in 1977, which outlawed attacks on dams ‘if such attack may cause the release of dangerous forces from the works or installations and consequent severe losses among the civilian population’. What was regarded at the time as a heroic act of immense difficulty would now be categorized as a war crime against helpless civilians. Nonetheless, it is difficult to see how the Ruhr could have been disabled otherwise, especially when viewed under the pressures of a world war. When news of the civilian losses became known there was much distress in London and Barnes Wallis was medicated as he was otherwise unable to sleep. The Allies did not follow up the raids; conventional high-level bombing would have deterred the Germans from restoring the dams to use, but no plans for this were ever brought into play. Although Barnes Wallis wrote that Germany had been dealt a blow from which it would take years to recover, by 27 June that same year full water output had been restored, and the hydroelectric plant was back to generating power. The greatest damage was actually to the domestic residences and prison camps, and also to German agriculture. Farms and livestock were washed away and took years to recover.
And the long-term repercussions? There were smaller versions of bouncing bombs planned by the British, for use against shipping. The Highball bomb was fitted to a cradle underneath a Mosquito B-IV, though it never came into service. Although bouncing bombs did not become a feature of post-war military strategy, the legend of the Dambusters raid was perpetuated in books and films. The 1955 black and white British movie The Dam Busters became, and remains, one of the most successful films about the war.[3]
The German bouncing bomb
There was an immediate response to the bomb by the Germans. After the Lancaster crashed from hitting high-tension power lines, the intact mine was removed from the wrecked plane by the local troops, who initially thought that it was a reinforced auxiliary fuel tank. Once its true nature was realized it took just ten days for the German engineers to draw up detailed blueprints of all the design features and they set out to build a bouncing bomb of their own. The first constructed was code named Kurt and was a 850lb (385kg) bomb built at the Luftwaffe Experimental Centre in Travemünde. The initial trial was from a Focke-Wulf Fw-190 but the importance of backspin was not recognized by the designers, and the bomb leaped high in the air after release, posing a danger to the aircraft. The German engineers later produced a similar bomb fitted with a rocket booster, which could strike the surface of water at speed and skip for some distance; but it was not a success and in 1944 development work was finally stopped.
The fact that the Germans found an intact bomb was due to a vital factor overlooked by the British designers. As we have seen, these were essentially mines fitted with depth charges. The bomb that overshot — because it was never immersed in water — was never going to explode, and so it was recovered intact. A conventional time fuse should have been fitted, and then the weapon would have functioned as conventional bomb if it overshot the dam. And the Germans missed something equally crucial — the fact that the bombs were spinning. It was the backspin that gave the bouncing bombs their awesome ability to ricochet so far across the water. This remained a military secret long after the war; indeed, you will note that there is no mention of spin even in the movie of the Dambusters. Although Barnes Wallis advised on the film, and it is painstakingly accurate in many respects, he was prohibited from releasing this vital piece of information and the public never knew.
The drawings and diagrams were ultimately all lost, and little technical detail remained. In 2011 Ian Duncan, a director with the British documentary company Windfall Films, recreated a scaled-down version of the bouncing bomb, with Dr Hugh Hunt of Cambridge University in charge of the experiments. They began logically (as did Barnes Wallis) with small spheres leading onto increasingly large projectiles, ending up with a half-size bouncing bomb with which they successfully targeted a purpose-built dam. The physics proved interesting: just as Barnes Wallis had calculated, the lower the bomb was dropped, the further it travelled.
The Americans had tried to make use of this principle immediately after World War II. Because they were sent every British military secret, their designers were aware of the need for backspin, and they also knew that a low launch altitude helped maximize the trajectory of the spinning mine. They copied the British design of the Highball weapon, renaming it Baseball. Initial investigations were promising, so — to maximize the distance the bomb would travel — they decided to launch it at 25ft (7.6m), less than half the altitude of the British Dambusters. This was such a success that the officials reckoned the pilot should fly even lower and see how far the bomb went this time. As the plane sped above the water at the perilously low level of 10ft (3m) the bomb was dropped and bounced perfectly — so much so that it smashed up through the fuselage, completely severing the aircraft’s tail. The plane flew on momentarily and then smashed into countless fragments as it hit the water at speed. The surviving film of the incident makes the whole event so obviously predictable, and one can only sympathize with the compliant pilot who either thought it would be good idea at the time or was simply following orders.
The search continues
Meanwhile, Guy Gibson’s 617 Squadron remained together and they were subsequently given the opportunity to deliver Barnes Wallis’s later weapons. The Cookie 5-ton bomb was carried by Lancaster bombers and used with great effect to attack submarine pens in France and German warship bases in the fjords of Norway. Although it proved a success, it was no more than a vast, conventional blast bomb. Barnes Wallis had in mind a very different secret weapon which would penetrate the ground and deliver such powerful shockwaves that it would bring down buildings and bunkers for a considerable distance around. Whereas a conventional bomb (no matter how large) did its damage through air-blast, Barnes Wallis’s revolutionary new bomb would generate a miniature earthquake, by setting up huge ground waves of energy. These could demolish a building from below.
Other solutions were sought to increase the penetrating power of high-explosive bombs. Towards the end of the war, a rocket-assisted high-impact bomb was conceived by the Royal Navy’s Captain Edward Terrell as an alternative answer. The rocket could give a smaller bomb the velocity needed to penetrate thick concrete. The weapon weighed only 4,500lb (2,000kg) and could be dropped from a safe altitude of 20,000ft (about 6,000m). When it had descended to 5,000ft (1,500m) a barometric fuse would fire a rocket motor in the tail. This accelerated the bomb to give it a final speed of 2,400ft/s (730m/s). This secret weapon was first carried under the wings of B-17 Flying Fortress bombers used by the 92nd Bomb Group on 10 February 1945 against the S-boat pens at IJmuiden, Netherlands. Altogether, 158 of these so-called Disney bombs were used operationally by the end of the war in Europe.
Barnes Wallis scaled down his proposals for his gravity-assisted penetrating bomb, and in 1944 designed instead the 12,000lb (5,400kg) Tallboy bomb, which could be carried by the current bombers. Later in the war, the Avro Lancaster improved to such an extent that it could just support a 10-ton payload and so, as we shall see, the 22,000lb (10,000kg) Grand Slam bomb was finally put into production. It was a secret weapon of unprecedented power. As in the case of the Tallboy bomb, the Grand Slam was spin-stabilized by its fins and was built with a thick, heavy steel case to allow it to penetrate deep layers of the ground unscathed. Dropped from high altitude, it would impact at nearly the speed of sound. During manufacture, hot liquid Torpex explosive was poured in to fill the casing and this took a month to cool down and solidify. Torpex (named because it had been developed as a TORpedo EXplosive) had more than 150 per cent the force of TNT. The finished bomb was so valuable that aircraft that could not drop their weapon in an abortive mission were ordered to return to base and land with the bomb intact, instead of jettisoning it over the open sea. Barnes Wallis had planned to create a 10-ton weapon in 1941, but it was not until June 1944 that the bomb was ready for use. It was first dropped on the Saumur rail tunnel from Lancaster bomb
ers of 617 Squadron. No aircraft were lost on the raid, and one of the bombs bored 60ft (18m) through the rock into the tunnel, blocking it completely. These massive ‘earthquake’ bombs were also used on the great concrete stuctures that the Germans were building to protect their rocket storage bunkers and submarine pens, and caused considerable damage. The Valentin submarine pens at Bremen, Germany, were made with reinforced concrete roofs some 23ft (7m) thick yet they were penetrated by two Grand Slam bombs in March 1945.
Ultimate penetration bombs
These ground-penetrating bombs are among the secret weapons that have gone on to give rise to present-day developments. Remote guidance was added to the Tallboy bomb by the United States during the Korean War. The resulting weapon was the 12,000lb (5,400kg) Tarzon bomb, used with devastating effect against a deep underground control room near Kanggye. Bunker buster bombs were also dropped at the Ali Al Salem Air Base, Kuwait, in 1991 as part of Operation Desert Storm. At the outbreak of the First Gulf War none of the NATO forces possessed such a weapon, so some of the original Barnes Wallis bombs were brought out of museums and used as templates for the construction of 2-ton bombs. They were laser guided by the United States forces and proved highly effective.
During the late 1990s a nuclear bomb was being designed by the United States for use in tactical warfare. Known as the Robust Nuclear Earth Penetrator it underwent extensive design and development even though the use of nuclear weapons was prohibited by international agreement. Work on the project continued until it was finally cancelled by the Senate in 2005. Meanwhile, in 2007 the Boeing Company announced that they had carried out successful tests of their Massive Ordnance Penetrator (MOP) weapon at the White Sands Missile Range, New Mexico. This bomb, also known as the Big Blu and Direct Hard Target Strike Weapon, is a 30,000lb (14,000kg) penetration bomb designed to be delivered by a B-52 Stratofortress or a B-2 stealth bomber against heavily protected subterranean targets. This is a project for the United States Threat Reduction Agency, and is designed to hit the ground at supersonic speeds so that it can penetrate deeply prior to detonation. Most of the mass is in the casing, not the explosive component. All of his stems from the work of Barnes Wallis during World War II, so once again the legacy of these secret weapons remains with us to this day.
