Bouncing bomb
A bouncing bomb is a bomb designed to bounce to a target across water in a calculated manner to avoid obstacles such as torpedo nets, and to allow both the bomb's speed on arrival at the target and the timing of its detonation to be predetermined, in a similar fashion to a regular naval depth charge. The inventor of the first such bomb was the British engineer Barnes Wallis, whose "Upkeep" bouncing bomb was used in the RAF's Operation Chastise of May 1943 to bounce into German dams and explode underwater, with an effect similar to the underground detonation of the later Grand Slam and Tallboy earthquake bombs, both of which he also invented.
British bouncing bombs
After the outbreak of the Second World War in 1939, Wallis saw strategic bombing as the means to destroy the enemy's ability to wage war and he wrote a paper entitled "A Note on a Method of Attacking the Axis Powers". Referring to the enemy's power supplies, he wrote : "If their destruction or paralysis can be accomplished they offer a means of rendering the enemy utterly incapable of continuing to prosecute the war"Barnes Wallis's April 1942 paper "Spherical Bomb – Surface Torpedo" described a method of attack in which a weapon would be bounced across water until it struck its target, then sink to explode under water, much like a depth charge. Bouncing it across the surface would allow it to be aimed directly at its target, while avoiding underwater defences, as well as some above the surface. Such a weapon would take advantage of the "bubble pulse" effect typical of underwater explosions, greatly increasing its effectiveness: Wallis's paper identified suitable targets as hydro-electric dams "and floating vessels moored in calm waters such as the Norwegian fjords".
Both types of target were already of great interest to the British military when Wallis wrote his paper ; German hydro-electric dams had been identified as important bombing targets before the outbreak of World War II, but existing bombs and bombing methods had little effect on them, as torpedo nets protected them from attack by conventional torpedoes and a practical means of destroying them had yet to be devised. In 1942, the British were seeking a means of destroying the German battleship, which posed a threat to Allied shipping in the North Atlantic and had already survived a number of British attempts to destroy it. During this time, the Tirpitz was being kept safe from attack by being moored in Norwegian fjords, where it had the effect of a "fleet in being". Consequently, Wallis's proposed weapon attracted attention and underwent active testing and development.
On 24 July 1942, a "spectacularly successful" demonstration of such a weapon's potential occurred when a redundant dam at Nant-y-Gro, near Rhayader, in Wales, was destroyed by a mine containing of explosive: this was detonated against the dam's side, underwater, in a test undertaken by A.R. Collins, a scientific officer from the Road Research Laboratory, which was then based at Harmondsworth, Middlesex.
A.R. Collins was among a large number of other people besides Barnes Wallis who made wide-ranging contributions to the development of a bouncing bomb and its method of delivery to a target, to the extent that, in a paper published in 1982, Collins himself made it evident that Wallis "did not play an all-important role in the development of this project and in particular, that very significant contributions were made by, for example, Sir William Glanville, Dr. G. Charlesworth, Dr. A.R. Collins and others of the Road Research Laboratory". However, the modification of a Vickers Wellington bomber, to the design of which Wallis himself had contributed, for work in early testing of his proposed weapon, has been cited as an example of how Wallis "would have been the first to acknowledge" the contributions of others. Also, in the words of Eric Allwright, who worked in the Drawing Office for Vickers-Armstrongs at the time, "Wallis was trying to do his ordinary job as well as all this – he was out at the Ministry and down to Fort Halstead and everywhere"; Wallis's pressing of his papers, ideas and ongoing developments on relevant authorities helped ensure that development continued; Wallis was principal designer of the models, prototypes and "live" versions of the weapon; and, perhaps most significantly, it was Wallis who explained the weapon in the final briefing for RAF crews before they set off on Operation Chastise, to use one of his designs in action.
A distinctive feature of the weapon, added in the course of development, was back-spin, which improved the height and stability of its flight and its ability to bounce, and helped the weapon to remain in contact with, or at least close proximity to, its target on arrival. Back-spin is a normal feature in the flight of golf balls, owing to the manner in which they are struck by the club, and it is perhaps for this reason that all forms of the weapon which were developed were known generically as "Golf mines", and some of the spherical prototypes featured dimples.
It was decided in November 1942 to devise a larger version of Wallis's weapon for use against dams, and a smaller one for use against ships: these were code-named "Upkeep" and "Highball" respectively. A third version, code-named "Baseball", was also planned for use by MTBs or MGBs of the Royal Navy Coastal Forces, but "never saw the light of day" Though each version derived from what was originally envisaged as a spherical bomb, early prototypes for both Upkeep and Highball consisted of a cylindrical bomb within a spherical casing. Development, testing and use of Upkeep and Highball were to be undertaken simultaneously, since it was important to retain the element of surprise: if one were to be used against a target independently, it was feared that German defences for similar targets would be strengthened, rendering the other useless. However, Upkeep was developed against a deadline, since its maximum effectiveness depended on target dams being as full as possible from seasonal rainfall, and the latest date for this was set at 26 May 1943. In the event, as this date approached, Highball remained in development, whereas development of Upkeep had completed, and the decision was taken to deploy Upkeep independently.
In January 1974, under Britain's "thirty year rule", secret government files for both Upkeep and Highball were released, although technical details of the weapons had been released in 1963.
Upkeep
Testing of Upkeep prototypes with inert filling was carried out at Chesil Beach, Dorset, flying from RAF Warmwell in December 1942, and at Reculver, Kent, flying from RAF Manston in April and May 1943, at first using a Vickers Wellington bomber. However, the dimensions and weight of the full-size Upkeep were such that it could only be carried by the largest British bomber available at the time, the Avro Lancaster, and even that had to undergo considerable modification in order to carry it resulting in the Avro Lancaster "B III ". In testing, it was found that Upkeep's spherical casing would shatter on impact with water, but that the inner cylinder containing the bomb would continue across the surface of the water much as intended. As a result, Upkeep's spherical casing was eliminated from the design. Development and testing concluded on 13 May 1943 with the dropping of a live, cylindrical Upkeep bomb out to sea from Broadstairs, Kent, by which time Wallis had specified that the bomb must be dropped at "precisely" above the water and ground speed, with back-spin at 500 rpm: the bomb "bounced seven times over some 800 yards, sank and detonated".In the operational version of Upkeep, known by its manufacturer as "Vickers Type 464", the explosive charge was Torpex, originally designed for use in torpedoes, to provide a longer explosive pulse for greater effect against underwater targets; the principal means of detonation was by three hydrostatic pistols, as used in depth charges, set to fire at a depth of ; and its overall weight was, of which was Torpex. Provision was also made for "self-destruct" detonation by a fuze, armed automatically as the bomb was dropped from the aircraft, and timed to fire after 90 seconds. The bomb was held in place in the aircraft by a large pair of calipers, or triangulated carrying arms, which swung away from either end of the bomb to release it. Back-spin was to begin 10 minutes before arriving at a target and was imparted via a belt driven by a Vickers Jassey hydraulic motor mounted forward of the bomb's starboard side. This motor was powered by the hydraulic system normally used by the upper gun turret, which had been removed. Close contact with the dam was necessary to obtain the maximum effectiveness from the explosive.
Height was checked by a pair of intersecting spotlight beams, which, when converging on the surface of the water, indicated the correct height for the aircraft – a method devised for the raid by Benjamin Lockspeiser of the Minister of Aircraft Production, and distance from the target by a simple, hand-held, triangular device: with one corner held up to the eye, projections on the other two corners would line up with pre-determined points on the target when it was at the correct distance for bomb release. In practice, this could prove awkward to handle, and some aircrews replaced it with their own arrangements, fixed within the aircraft itself, and involving chinagraph and string.
On the night of 16/17 May 1943, Operation Chastise attacked dams in Germany's Ruhr Valley, using Upkeep. Two dams were breached, causing widespread flooding, damage, and loss of life. The significance of this attack upon the progress of the war is debated. British losses during the operation were heavy; eight of the 19 attacking aircraft failed to return, along with 53 of 113 RAF aircrew. The breach resulted in the deaths of roughly 1600 civilians, including around 1000 prisoners and slave laborers. Upkeep was not used again operationally. By the time the war ended, the remaining operational Upkeep bombs had started to deteriorate and were dumped into the North Sea without their detonation devices.