Yellow Sun

Yellow Sun was the first British operational high-yield strategic nuclear weapon. Developed to contain a variety of warheads, its design trajectory shifted following the 1958 US-UK Mutual Defence Agreement. The initial plan to deploy the interim Green Bamboo and subsequent Green Granite thermonuclear warheads was cancelled. Instead, the United Kingdom adopted the Red Snow warhead for the production version, utilizing the Green Grass fission warhead as an interim measure.

The casing featured a flat nose that induced aerodynamic drag, slowing the weapon's descent to allow the delivery aircraft to escape the detonation zone without requiring a parachute. This flat-nose design also prevented the formation of complex transonic and supersonic shock waves, eliminating barometric fuzing errors. Electrical power was provided by duplicated ram-air turbines located behind twin air intakes in the nose, replacing the unreliable lead-acid batteries of earlier designs.

The Yellow Sun Mk.1 utilized the Green Grass warhead, a large, unboosted pure fission device. It featured a thin-walled spherical fissile core kept subcritical during storage by steel ball-bearings, which had to be extracted before flight. The Royal Air Force considered this safety mechanism hazardous. The warhead had an estimated yield of 400 kilotons and was never tested.

The Yellow Sun Mk.2 carried the Red Snow warhead, a British-manufactured, anglicized variant of the United States W28 thermonuclear warhead. Because the Red Snow was smaller and lighter than the Green Grass, ballast was integrated into the Mk.2 casing. This maintained the ballistic and aerodynamic properties of the Mk.1, eliminating the need for additional aerodynamic testing or modifications to the electrical power and fuzing systems.

The weapon was operated exclusively by the United Kingdom's Royal Air Force, deployed on V bombers. The Mk.1 entered service in 1959 as an emergency weapon, replacing the Blue Danube. The Mk.2 entered service in 1961, replacing the Mk.1 as the primary air-dropped strategic weapon. Retirement of the Yellow Sun began in 1966, when it was replaced by the WE.177.