Referring to the enemy's power supplies, he wrote (as Axiom 3): "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, and 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".

[13] 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.

[19][f] In the operational version of Upkeep, known by its manufacturer as "Vickers Type 464", the explosive charge was Torpex, originally designed for use as 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 30 feet (9 m); and its overall weight was 9,250 lb (4,200 kg), of which 6,600 lb (3,000 kg) was Torpex.

[21] 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.

[citation needed] 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.

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.

[citation needed] This naval aspect was later to be pressed by a minute issued by British prime minister Winston Churchill, in February 1943, asking "Have you given up all plans for doing anything to Tirpitz while she is in Trondheim?

[citation needed] From November 1942, development and testing for Highball continued alongside that of Upkeep, including the dropping of prototypes at both Chesil Beach and Reculver.

A prototype with an altered design of casing strengthened by steel plate, but empty of inert filling or explosive, was dropped on 30 April and emerged "quite undamaged".

[30] Further testing was carried out by three modified Mosquitoes flying from RAF Turnberry, north of Girvan, on the west coast of Scotland, against a target ship, the former French battleship Courbet, which had been moored for the purpose in Loch Striven.

[33] With this in mind, 618 Squadron had been formed on 1 April 1943 at RAF Skitten, near Wick, in northeastern Scotland, to undertake "Operation Servant", in which Tirpitz would be attacked with Highball bouncing bombs.

By this time Courbet had been designated for use as a Gooseberry breakwater for the invasion of Normandy, so the old battleship HMS Malaya, then in reserve, was used instead (also moored in Loch Striven).

Aiming Highball required a different method from Upkeep; the problem was solved by Wallis's design of a ring aperture sight fixed to a flying helmet.

[39] In January 1945, at the Vickers experimental facility at Foxwarren, near Cobham, Surrey, a Douglas A-26 Invader medium bomber of the USAAF was adapted to carry two Highballs almost completely enclosed in the bomb bay, using parts from a Mosquito conversion.

[42] In July 2017, two Highballs were successfully recovered[43] from Loch Striven in a joint operation by teams from East Cheshire Sub-Aqua Club[44] and the Royal Navy.

After Operation Chastise, German forces discovered an Upkeep bomb intact in the wreckage of the Lancaster commanded by Flt Lt Barlow, which had struck high tension cables at Haldern, near Rees, Germany, and crashed.

[49] In 2011, a project led by Dr Hugh Hunt of the Department of Engineering, University of Cambridge, re-created the bouncing bomb and tested it in a Dambusters-like raid.

[50][51][52][53][54][excessive citations] The filming of the documentary was itself documented as part of the Ice Pilots NWT reality television series that follows Buffalo Airways in season 3 episode 2 "Dambusters".