Violet Club (nuclear weapon)
Violet Club was an interim United Kingdom nuclear weapon, which included a variety of design tradeoffs in its explosive yield enhancement method, physical packaging, but in no way more than in its safety system.  At the most basic, it was a incorporating the Green Grass (nuclear weapon) fission device in the bomb casing from the earlier Blue Danube, which had been proposed, in August 1956, as a way to provide a high-yield gravity bomb before the Yellow Sun family could be available. By March 1958, five had been put into service, but when purpose-built Yellow Sun Mark 1 bombs were available and the Green Grass packages rebuilt into Yellow Sun Mk. 1.
Blue Danube had had a solid "pit" at its center, which could be removed and inserted in flight. Only after the pit was inserted could the weapon detonate. Green Grass, however, could not use the removable pit technique because its core was a hollow spherical shell. Since the core was made of several individually critical masses of highly enriched uranium, and there was a real risk of a radiation burst or explosion if the entire weapon was in a crash or fire; the one-point safe criterion was not met.
To prevent criticality, "The safety solution adopted was to fill the hollow centre of the core with steel ball bearings, which physically prevented any collapse of the core."
The balls were contained in a rubber
bag, which fitted inside the core. Before take-off the balls were removed by draining them through a rubber hose into a bin, followed by removal of the bag; the 2” diameter hole was then closed with a plug containing a section of the explosive lens. Once the balls had been removed, there was a serious risk of a nuclear explosion if the aircraft crashed or the bomb was jettisoned. In the event of the bombing mission being aborted and the Valiant returning safely, the operation was reversed; the bomb was lowered from the bomb bay, rotated so that the drain hole was uppermost, the bag inserted and inflated, and the core refilled with ball
It was found, even on the ground, that the plug at the bottom of the bomb was not completely secure. If, while technicians worked on Violet Club, the plug let go, they would find themselves dancing on a veritable sea of 20,000 0.375-inch ball bearings, with a live atomic bomb over their heads. One may observe that this design was introduced long before Monty Python had been formed and was available for consultation, although a possible role of Heath Robinson has not been declassified.
There was considerable pressure, at the time, for the U.K. to be considered a credible nuclear strategic partner for the U.S. The U.S., and then the U.S.S.R., had demonstrated full fusion weapons. Since Britain had not yet completed development of fusion technology, it needed an alternative technique to make a weapon that at least approached megaton yield. Alternative techniques were called Alarm Clock (nuclear weapon) in the U.S., Layer Cake (nuclear weapon) in the Soviet Union, and  and tamper boosting in the U.K., where the technique had been developed by Keith Roberts.
- Donald McIntyre (2006), The Development of Britain’s Megaton Warheads, University of Chester, MA Dissertation, pp. 29-31
- Richard Moore, The Real Meaning of the Words: a Pedantic Glossary of British Nuclear Weapons, Mountbatten Center for International Studies, UK Nuclear History Working Paper Number: 1, p. 14
- TNA: AVIA 65/1218. Violet Club: Correspondence, quoted by McIntyre
- Carey Sublette, 4.3.3 The Alarm Clock/Layer Cake Design, 4.3 Fission-Fusion Hybrid Weapons, Nuclear Weapons Archive
- McIntyre, pp. 20-27