Violet Club - Design Features

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 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 lenses. 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 and beryllium 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 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.