Green Bamboo Yellow Sun Stage 1 and Stage 2 were the original designations. Stage 1 was intended as an interim design to carry a one megaton
Green Bamboo warhead of the "layer-cake" type thought similar to the Soviet
Sloika and the US
Alarm Clock concepts. These hybrid designs are not now regarded as truly
thermonuclear, but were then thought to be a stepping-stone on the route to a fusion bomb. Stage 2 was to follow when a true thermonuclear warhead based on the Green Granite design became available. To produce the required yield, the implosion of the fissile core had to be extremely uniform. This required a complex 72-point explosive system, and led to a very large weapon overall. The resulting diameter of Green Bamboo determined the diameter of both Yellow Sun and the
Blue Steel missile. The launch of
Sputnik 1 coincided with ongoing negotiations between the US and UK about nuclear technology, and the sudden shock of an apparent Soviet superiority swept aside lingering US concerns about the UK after the
Suez Crisis. These negotiations would lead to the
US–UK Mutual Defence Agreement in 1958. With access to US designs, the original plan to use Green Granite for Stage 2 was abandoned as a new thermonuclear warhead would be available before Granite would be. This meant the need for a powerful interim design to fill this time period was also considerably less important. The decision was made to cancel Green Bamboo and replace it with a simpler concept.
Green Grass After Green Bamboo was abandoned a decision was made to use the Interim Megaton Weapon known as
Green Grass in the Yellow Sun casing and designate it as Yellow Sun Mk.1 until better warheads were available for a Mk.2. Green Grass was of similar layout to Green Bamboo, although it was not thermonuclear, being a very large unboosted pure
fission warhead that was based in part on the core of the
Orange Herald device tested at
Grapple, with some of the implosion and firing features of Green Bamboo. The
modulated neutron initiator was
Blue Stone. Twelve Green Grass warheads were fitted in the much larger, older casings derived from Blue Danube and known as "
Violet Club". These twelve warheads were later transferred to the Yellow Sun Mk.1 casings and supplemented by further warheads totalling 37. Green Grass yield was originally stated to the
Royal Air Force (RAF) as 500
kilotons of TNT equivalent (2.1
PJ), but the designers' estimate was later revised downwards to 400 kt of TNT. The Green Grass warhead was never tested. It used a dangerously large quantity of fissile material – thought to be in excess of , and considerably more than an uncompressed
critical mass. It was kept subcritical by being fashioned into a thin-walled spherical shell. To guard against the accidental crushing of the core into a critical condition, the shell was filled with 133,000 steel ball-bearings, weighing . In a conflict, these would have had to be removed before flight. The RAF thought it unsafe (see ).
Red Snow Red Snow was the US W28 warhead used in the
US Mk-28 nuclear bomb. This was anglicised to adapt it to British engineering practices, and manufactured in Britain using British fissile materials. For further information see the "Deployment" section below. ==Deployment==