The weapon was manufactured by Vickers Armstrong, the company for whom Barnes Wallis worked. An order was placed with them for 120 Upkeeps, 60 inert casings filled with concrete for practice and the remaining 60 filled with the new, and scarce explosive Torpex, a compound explosive comprising; 42% RDX. 40% TNT and 18% Aluminium powder. Torpex was one of the most powerful of the military explosives then available and was poured warm into the mine casings before setting, not unlike a heavy wax.
By February 28th 1943 Wallis had completed plans for the upkeep and initially he specified a spherical bomb of 7ft 6″ in diameter but to prevent the weapon breaking up on hitting the surface of the water, a particularly high grade of steel was necessary and there was not enough of this available to manufacture the required number of weapons. Consequently he re-designed the bomb to be cylindrical in shape mainly for manufacturing ease. The casing was rolled and welded together and the circular ends were then held in place with angled steel sections. The weapon was then wrapped in mahogany cladding which was held in place by steel bands. This was designed to minimise the shock on the casing when the weapon hit the water. On one end of the mine in the centre of the inner disc was fitted an adaptor which mated onto a driven wheel on the bomb cradle and allowed the weapon to be spun. On the other end was fitted a similar adaptor to hold the bomb in place and allow it to be spun. Underneath this central disc were three fuse pockets each of which were fitted with a standard naval hydrostatic pistol. These were operated by water pressure and once the mine reached a certain depth it would trigger an initiating explosion which detonated the main explosive charge. The test bombs were painted in gloss grey and the live bombs painted in standard aerial munition dark green. The weapon was also fitted with a self destruct charge which is believe to have been based upon celluloid disc/acid time long delay fuse of the same type used in aerial bombs of the period. The idea was that in the event the mine did not explode as planned after a preset delay of several hours it would explode and self destruct.
Due to the size of the bomb two other problems were encountered. The first was how to get the bomb underneath the aircraft so that it could be lifted into the bomb bay. A specially strengthened type B bomb trolley was used to carry the weapon but it could not be moved in sideways under the bomb bay. Consequently a novel solution was found in the form of lifting the tail of the Lancaster into the air by 10 ton Coles crane and then wheeling the bomb on its trolley in from the rear. Despite the modifications to the Lancaster’s bomb bay there was still only a three inch clearance for the weapon to get in. Once situated under the bomb bay the weapon was winched up into position by a pair of ten ton bomb winches situated above the floor of the bomb bay with two sets of wires passing around the mine to give the lift.
Another problem encountered was that the sheer mass of metal mounted on the aircraft affected the compass that the pilot and navigator used to navigate the aircraft. As a consequence of this the compass was “swung” to check its accuracy before the weapon was loaded and a deviation card prepared. The same was done with the mine in place. Once the mine had been dropped the crew would change over from the loaded deviation card to the unloaded deviation card to ensure that the compass remained accurate. Lastly there is generally a problem of balance when spinning any object up to 500rpm. Each mine had a slightly different density of filling and thickness of casing and consequently to stop it vibrating very badly when it was being spun and possibly falling away from the aircraft, each mine was spun on a test rig and balanced in almost the same way a car wheel is when a new tyre has been fitted. Small weights were fitted at the appropriate places on the ends of the casing until the mine could spin without any vibration at all.
Once loaded and fitted in place the mine was initially intended to be spun by a Ford V8 engine but this was not implemented, Instead the hydraulic supply normally used for the mid-upper turret [removed of course on the Type 464 provisioning aircraft] was used to power a Vickers “Janny” hydraulic motor [originally believed to have been designed, for the steering gear of submarines] the speed of rotation being controlled by the W/Op who monitored a rev counter and controlled the hydraulic pressure via a control valve to a gearbox and pulley which drove a rubber belt and spun the bomb via a pulley. When the bomb was to be released two spring loaded arms which held it in place were allowed to flip outwards under spring pressure allowing the mine to drop.
The mine was spun up to speed by the engine some ten minutes before planned release. The mine was spun at 500rpm in an anti-clockwise direction.