The Douglas GAM-87 Skybolt (AGM-48 under the 1962 Tri-service system) was an air-launched ballistic missile (ALBM) developed by the United States during the late 1950s. The basic concept was to allow US strategic bombers to launch their weapons from well outside the range of Soviet defenses, as much as from their targets. To do this in an air-launched form, a lightweight thermonuclear warhead was needed. Initially, the W47 from the Polaris missile was selected, but it was later replaced by the W59 from the Minuteman missile. The UK joined the Skybolt program in 1960, intending to use it on their V bomber force. When the design added a star tracker in addition to its inertial navigation system (INS) this meant that it could only be carried externally where the tracker could see the sky. This requirement along with the required ground clearance on takeoff limited it to the Avro Vulcan bomber. A number of design decisions in the W47 led the RAF to question its safety, so they intended to use their own Red Snow warheads. This was a heavier warhead and would reduce the range to about , meaning the bombers would have to cross the Soviet coastline to attack Moscow. Testing began in 1962 and was initially marked by a string of failures. These failures, along with a lack of mission after the successful development of submarine-launched ballistic missiles (SLBMs), led to its cancellation in December 1962. The UK had decided to base its entire 1960s deterrent force on Skybolt, and its cancellation led to a major disagreement between the UK and US, known today as the "Skybolt Crisis". This was resolved by a series of meetings that led to the Royal Navy gaining the UGM-27 Polaris missile and construction of the s to launch them. History Background The US Air Force had built up an enormous fleet of strategic bombers during the 1950s, only to see them threatened by the possibility of a surprise attack by Soviet ICBMs. As the US bombers were located at only a small number of air bases, a relatively small fleet of missiles could attack all of them at once. The US had been developing its own ICBMs as well, but early models, like the SM-65 Atlas, required some time to prepare to launch from their surface launchers and were vulnerable to sneak air attack. A carefully timed attack from Soviet bombers