Violet Club - Conception, Design and Development

Conception, Design and Development

In 1953, shortly after the Americans tested a thermonuclear weapon in 1952, followed by the Soviets with Joe 4, and before the UK government took a decision in July 1954 to develop a thermonuclear weapon, the Atomic Weapons Research Establishment at Aldermaston was asked about the possibilities for a very large pure fission bomb with a yield of one megaton. This study referred to the Zodiak Mk.3 bomb, but progressed no further than a rudimentary study. At this time studies were also started that ultimately led to a decision in 1954 to develop a thermonuclear weapon, and the design studies were split into two tracks because the British at that time had not yet discovered the Teller-Ulam technique necessary to initiate fusion. One track led to an intermediate design, the so-called Type A thermonuclear design, similar to the Alarm Clock and layer cake hybrid designs of other nuclear powers; although these designs are now regarded as large boosted fission weapons, and no longer regarded as thermonuclear weapons that derive a very large part of their energy from a fusion reaction, designated by the British as Type B, but as hybrids. The British hybrid weapon was known as Green Bamboo, weighed approx 4,500 lb (2,045 kg) and its spherical shape measured approx 45 inches diameter with a 72-point implosion system. This Green Bamboo weapon was intended as the warhead for all projected British strategic delivery systems of the period; the Yellow Sun Stage 1 air-dropped bomb, and the Blue Steel air-launched stand-off missile. The large girth of these weapons was designed to accommodate the diameter of Green Bamboo's implosion sphere.

A variant with a smaller implosion sphere, fewer explosive lenses, and some other changes was intended for the Blue Streak missile that was unable to carry Green Bamboo. However, the reduced size was achieved by reducing the size of the surrounding HE layers, and this resulted in less compression at the fissile core when detonated, and a reduced nuclear efficiency, and yield; meaning that less of the fissile material was consumed before the bomb blew itself apart. To compensate for the reduced compression more fissile material was required to maintain yield at the desired level, and this in turn made excessive demands on scarce and expensive fissile material. Estimates computed from reliable sources of actual core cost and cost per kilogram of HEU put the core sizes of Green Bamboo and Orange Herald as 98 kg and 125 kg respectively, although some other published (and unverified) sources claim lower figures of 87 kg and 117 kg respectively. This design later became known as Orange Herald, and was tested at Christmas Island, yielding 720 kt. These two weapons, Green Bamboo and Orange Herald were intended as the two hybrid weapon predecessors of the first British thermonuclear weapons, based on the Granite design series that began with Short Granite, Purple Granite, Grapple X, Grapple Y, and Flagpole and Halliard at Grapple Z.

Delays and failures in the Granite programme and the abandonment of Green Bamboo without a test, left a gap in the programme and an emergency capability weapon to fill that gap was devised from elements of both Green Bamboo and Orange Herald; being known as Knobkerry, or Green Grass, and the Interim Megaton Weapon. There were also other factors involved in the decision to build an Interim Megaton Weapon. One being that the UK HEU production programme was by 1957 producing quantities of HEU for which there was no immediate need; and the Chiefs of Staff were reluctant to see it continue to languish in stores, unused for weapons, when it was being produced at great cost. The Green Grass warhead containing perhaps 70–86 kg of HEU (although there are no reliable declassified sources for this figure) was therefore hurriedly produced and installed in a modified Blue Danube casing, to be known as Violet Club, until a better solution based on the Yellow Sun casing could be produced, known then with Green Grass installed, as Yellow Sun Mk.1. Only five Violet Club weapons were produced, known by the RAF description of Bomb, Aircraft, HE 9,000 lb HC, and they were stripped of their Green Grass warheads for transfer to the better casings when these became available.