Violet Club
Violet Club was a nuclear weapon
Nuclear weapon
A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission or a combination of fission and fusion. Both reactions release vast quantities of energy from relatively small amounts of matter. The first fission bomb test released the same amount...

 deployed by the United Kingdom
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...

 during the cold war
Cold War
The Cold War was the continuing state from roughly 1946 to 1991 of political conflict, military tension, proxy wars, and economic competition between the Communist World—primarily the Soviet Union and its satellite states and allies—and the powers of the Western world, primarily the United States...

. It was Britain's first operational "high yield
Nuclear weapon yield
The explosive yield of a nuclear weapon is the amount of energy discharged when a nuclear weapon is detonated, expressed usually in the equivalent mass of trinitrotoluene , either in kilotons or megatons , but sometimes also in terajoules...

" weapon, and was intended to provide an emergency capability until a thermonuclear weapon could be developed from the 1956-1958 Operation Grapple
Operation Grapple
Operation Grapple, and operations Grapple X, Grapple Y and Grapple Z, were the names of British nuclear tests of the hydrogen bomb. They were held 1956—1958 at Malden Island and Christmas Island in the central Pacific Ocean. Nine nuclear detonations took place during the trials, resulting in...

 thermonuclear tests conducted on Kiritimati
Kiritimati or Christmas Island is a Pacific Ocean raised coral atoll in the northern Line Islands, and part of the Republic of Kiribati....

 (Christmas Island).

Conception, design and development

In 1953, shortly after the Americans tested a thermonuclear weapon in 1952, followed by the Soviets with Joe 4
Joe 4
Joe 4 was an American nickname for the first Soviet test of a thermonuclear weapon on August 12, 1953. It utilized a scheme in which fission and fusion fuel were "layered", a design known as the Sloika model in the Soviet Union...

, and before the UK government took a decision in July 1954 to develop a thermonuclear weapon, the Atomic Weapons Research Establishment at Aldermaston
Aldermaston is a rural village, civil parish and electoral ward in Berkshire, South-East England. In the 2001 United Kingdom Census, the parish had a population of 927. The village is on the southern edge of the River Kennet flood plain, near the Hampshire county boundary...

 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
Joe 4
Joe 4 was an American nickname for the first Soviet test of a thermonuclear weapon on August 12, 1953. It utilized a scheme in which fission and fusion fuel were "layered", a design known as the Sloika model in the Soviet Union...

 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
Yellow Sun
Yellow Sun was the first British operational high-yield strategic nuclear weapon. The name actually refers only to the outer casing; the warhead was known as "Green Grass" and "Red Snow"...

 Stage 1 air-dropped bomb, and the Blue Steel
Blue Steel missile
The Avro Blue Steel was a British air-launched, rocket-propelled nuclear stand-off missile, built to arm the V bomber force. It was the primary British nuclear deterrent weapon until the Royal Navy started operating Polaris missile armed nuclear submarines....

 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.

Design features

Violet Club (and to a lesser degree Yellow Sun Mk.1) was not considered a satisfactory design and suffered from numerous design defects. An implosion design, the fissile core of the weapon was a hollow sphere of highly-enriched uranium (HEU) which was surrounded by a High Explosive supercharge
In theoretical physics, a supercharge is a generator of supersymmetry transformations.Supercharge, denoted by the symbol Q, is an operator which transforms bosons into fermions, and vice versa...

 and 72-lens implosion system. The HEU core was greater than one uncompressed critical mass and to maintain it in a sub-critical condition AWRE fashioned it into a hollow thin-walled sphere. The HEU sphere was collapsed inwards by the supercharge and 72 explosive lens
Explosive lens
An explosive lens—as used, for example, in nuclear weapons—is a highly specialized explosive charge, a special type of a shaped charge. In general, it is a device composed of several explosive charges that are shaped in such a way as to change the shape of the detonation wave passing through it,...

es. However, a fire in the bomb store or a traffic collision on the airfield could easily lead to a partial crushing or collapse of the unremovable uranium shell, and in turn a spontaneous nuclear chain reaction. AWRE responded by inserting (though a hole in the shell) a rubber bag, rather akin to an outsize female condom, and filled this with 20,000 steel ball bearings of 0.375-inches diameter (9.5 mm), weighing 70 kg. Subsequently, the number of these steel balls was increased to 133,000, with a reduction in size to approx 5 mm diameter. The balls were retained in the device by sealing the hole with a plastic bung. The steel balls were intended to prevent a nuclear detonation even if the explosives fired accidentally, or in any conceivable accident. The ball bearings had to be removed through the hole in the bomb casing during flight preparation, and after the bomb was winched into the aircraft. The ball bearings then had to be re-inserted into the lowered and upturned bomb before transport back to the bomb store. Quite obviously, without the ball bearings installed, these weapons were armed and live, and too dangerous to allow to be flown on exercises. Bomber Command exercises demonstrated that flight preparation followed by a scramble take-off could not be reduced below thirty minutes, and on exercises in bad weather and at night a ninety minute scramble was the norm. At least one accident dated 1960 was reported in the press when the plastic bung was removed and 133,000 steel ball bearings exited onto the aircraft hangar floor, leaving the bomb armed and vulnerable. The Royal Air Force were so nervous of the outcome of a fire in storage, that permission was sought to store the bombs inverted, so that a loss of the plastic bung could not end with the steel balls on the floor, leaving the HEU unprotected against a subsequent explosion. Even without the partial nuclear detonation feared by the RAF, there was "a risk of catastrophe"

AWRE's trickery with the steel balls had other unintended outcomes. Even an event as inconsequential as early morning frost was an issue. Violet Club could be loaded into a bomber for up to thirty days on standby while parked overnight on a remote base where the bomb could get very cold. If the steel balls froze together inside the bomb cavity and could not be removed, the bomb was useless. AWRE's solution was to fit the bomb with an electric blanket. Because the bombs were armed before flight, the take-off was hazardous; the bombs could not be jettisoned, and landing with an armed bomb on return to base was too hazardous to contemplate. As a consequence, Violet Club could not be used on an airborne alert, or even flown to a remote dispersal base.

Other design flaws centred on a requirement for a strip-down and inspection at six-monthly intervals, this took three weeks per weapon using AWRE civilian staff. The unstable nature of the bomb required that the work be done in-situ at RAF facilities, causing considerable disruption to operational duties. There were three main reasons for the strip-down and these were deterioration of the rubber bag, corrosion of the steel balls, and deterioration of the HE, which was prone to cracking. Replacement of the HE would cost the RAF in excess of £90,000 adjusted to 2006 prices.

Green Grass/Violet Club was the first UK-deployed weapon to dispense with the crude polonium
Polonium is a chemical element with the symbol Po and atomic number 84, discovered in 1898 by Marie Skłodowska-Curie and Pierre Curie. A rare and highly radioactive element, polonium is chemically similar to bismuth and tellurium, and it occurs in uranium ores. Polonium has been studied for...

 and beryllium
Beryllium is the chemical element with the symbol Be and atomic number 4. It is a divalent element which occurs naturally only in combination with other elements in minerals. Notable gemstones which contain beryllium include beryl and chrysoberyl...

 impact or crush type initiators used in Fat Man and other early US weapons together with the British Blue Danube and Red Beard. Instead it used an Electronic Neutron Initiator or ENI, known as Blue Stone
Blue Stone
Blue Stone was the code name for the Unit 386D ENI or electronic neutron initiator for the first British operational high-yield strategic nuclear weapon, Violet Club.-References:**...

 which had the great advantage of being adjustable, allowing the neutron burst to be triggered at precisely the right moment. The burst heights of Green Grass were optimized for either maximum overpressure without allowing fireball contact with the ground, or to maximise the ground area subjected to a 6 psi overpressure; respectively 3,500 ft and 6,200 ft above ground level, using a barometric fuze backed-up with an adjustable clockwork timer fuze and fail-safe impact fuzes. Most of these mechanisms originated from Blue Danube although the radar altimeter fuze was omitted. The electrical power for these fuzing mechanisms and the firing mechanisms of the 72-lens 45 inch diameter implosion device came from lead-acid accumulators located in the tail of the Blue Danube casing. These were commercially-sourced six volt motorcycle batteries.


Green Grass struggled to meet the Chiefs of Staff requirement for an Interim Megaton [yield] Weapon. It was never tested, and initially, AWRE estimated its yield at 500 kt based on the Orange Herald test of 720kt, and non-nuclear tests of the HE implosion sphere fitted with non-fissile cores. A Mr Challens of AWRE who later became the Director of AWRE then claimed to the Air Staff that
A play-on-words that later returned to haunt AWRE when later estimates revised the yield to 400kt. Challens also stated on behalf of AWRE that
The Royal Air Force were not amused, with Bomber Command Staff officers minuting their seniors with remarks like this one.
Summing up, senior levels of the RAF believed they had been sold a 'lemon', and this further extract from the archives does much to illustrate the uncertain safety of the weapon in the hands of the inexperienced RAF.
The fifth and last Violet Club weapon was due for delivery to the RAF by the end of May 1959 and all were withdrawn by 1960, but in their Yellow Sun Mk.1 form they survived until 1963, when all had been replaced with Yellow Sun Mk.2 with Red Snow warheads installed. They were only used by the Avro Vulcan
Avro Vulcan
The Avro Vulcan, sometimes referred to as the Hawker Siddeley Vulcan, was a jet-powered delta wing strategic bomber, operated by the Royal Air Force from 1956 until 1984. Aircraft manufacturer A V Roe & Co designed the Vulcan in response to Specification B.35/46. Of the three V bombers produced,...



These very large and dirty fission bombs were the largest pure-fission bombs deployed by any state, and unlike their predecessors, Blue Danube and Red Beard, they used HEU as a fissile material rather than plutonium, the reason being primarily economic. The cost of HEU to the Royal Air Force was (at 1958-9 prices) £19,200 per kg, with plutonium priced at £143,000 per kg. Although a HEU weapon needed more fissile material for a given yield than a plutonium weapon, a saving per weapon was of the order of £22.7M at 2006 prices. Thirty-seven Green Grass warheads were built (five as Violet Club) saving the Treasury £840M. The influence of the Treasury on weapons procurement should not be underestimated: it reaches even into weapons design. Clearly, there were economic benefits in building large and dirty U-235 fission bombs, rather than cleaner, but more expensive plutonium weapons, especially given the shortage of plutonium. By 1958 Britain's accumulated production of plutonium was only 472.2 kg and a proportion of that was bartered to the United States in exchange for HEU and other items. Up to 1958, British output of HEU was only 860 kg, while the United States supplied the UK with approximately seven tons of HEU from their less costly production process. So HEU used for Green Grass was purchased cheaply from the U.S. while selling to the U.S. the unwanted and high-priced weapons-grade plutonium.

Knobkerry, alias Green Grass, alias Interim Megaton Weapon, alias Violet Club and Yellow Sun Mk.1 had one other distinction. It was the last entirely British nuclear weapon deployed with the UK Armed Services. The British Operation Grapple
Operation Grapple
Operation Grapple, and operations Grapple X, Grapple Y and Grapple Z, were the names of British nuclear tests of the hydrogen bomb. They were held 1956—1958 at Malden Island and Christmas Island in the central Pacific Ocean. Nine nuclear detonations took place during the trials, resulting in...

thermonuclear weapon tests at Christmas Island in 1957-58 were the end of evolution. There were no more wholly home-grown designs. Britain never deployed a true thermonuclear weapon of wholly home-grown design. All the weapons tested at Operation Grapple were abandoned, because AWRE no longer needed them; although some of their features were undoubtedly incorporated into later weapons. These Granite-type devices were all experimental devices needing to be developed further into reliable Service-engineered warheads at considerable cost in time and money. The U.S. designs offered after 1958 were fully tested and engineered, and cheap to produce. They were manufactured in Britain from British materials and U.S. blueprints. They were British property and there were no American political constraints on their use; they were also a favourable deal for the Treasury; and Violet Club and Yellow Sun Mk.1 bridged the gap until the American designs could be manufactured.

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