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 ( or physics package ) was known as " Green Grass " ( in Yellow Sun Mk . 1 ) and " Red Snow " ( in Yellow Sun Mk . 2 ) . The ENI or electronic neutron initiator ( generator ) was Blue Stone . Design . The casing was some long , in diameter . The Mark 1 version with the Green Grass warhead weighed . The Mk . 2 version with the lighter Red Snow warhead had ballast added to maintain overall weight , ballistic and aerodynamic properties , and avoid further lengthy and expensive testing , and changes to the electrical power generating and airburst fusing equipment . Unlike contemporary U . S . bombs of similar power it did not use a parachute to retard its fall , instead , the blunt nose was intended to slow the fall of the weapon sufficiently to permit the bomber to escape the detonation , and ensured that Yellow Sun did not encounter the transonic and supersonic shock waves that had caused much difficulty with barometric fusing gates that had plagued an earlier weapon , Blue Danube . Electrical power was supplied by duplicated ram - air turbines located behind the twin air intakes in the flat nose . The earlier Blue Danube design had relied on lead – acid batteries that had proven to be both unreliable and to require time - consuming pre – flight preparation . Warheads . 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 JOE . 4 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 Granite design became available . The diameter of Green Bamboo determined the 48 inch diameter of both Yellow Sun and the Blue Steel missile . 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 . Five Green Grass warheads were fitted in larger , older casings derived from Blue Danube and known as Violet Club . These five 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 – considerably more than an uncompressed critical mass . It was kept subcritical by being fashioned into a thin - walled spherical shell . To guard against 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 have been removed before flight . The RAF thought it unsafe . Red Snow . Red Snow was the US W - 28 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 . Deployment started in 1959 - 60 . The RAF Service designations were " Bomb , Aircraft HE 7000 lb HC Mk . 1 " or " Bomb , Aircraft HE 7000 lb HC Mk . 2 " . Yellow Sun Mk . 1 was intended as an " emergency " weapon , and had not been engineered for reliable long - term stockpiling . It was always envisaged that a Mk . 2 version would be available later fitted with a true thermonuclear warhead derived from the Granite type tested at Grapple , or an American type made available after the 1958 Anglo - US Bilateral Agreement . It was carried only by RAF V bomber s . In September 1958 a decision was made to abandon the Granite type warheads intended for Yellow Sun Mk . 2 ( and Blue Steel , and Blue Streak MRBM ) and instead adopt the US W - 28 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 and known as Red Snow . Red Snow was both more powerful , lighter and smaller than Green Grass . It was always envisaged that the Yellow Sun bomb casing would be adapted for successor warheads to minimise unessential development time and cost . Yellow Sun Mk . 2 entered service in 1961 , and remained the primary air - dropped strategic weapon until replaced with WE . 177B in 1966 . Although the first British designed thermonuclear weapon to be deployed , Yellow Sun was not the first to be deployed with the RAF . US Mk - 28 and Mk - 43 thermonuclear bombs and others had been supplied to the RAF for use in V bombers prior to the deployment of Yellow Sun . Some bombers of the V - force only ever used American weapons supplied under dual - key arrangements .