In a time when most fighters were subsonic and only a handful were supersonic, Chance Vought was contracted to build a Mach 2, submarine-launched, nuclear-armed cruise missile. The result was an airframe so fast that the Navy?s fastest fighter (also built by Vought), the F8U Crusader, could not keep up with it. The attack profile was either by high altitude or on the deck with a pitch up and then a vertical terminal 1,000 mph plus dive. The W27 nuclear warhead could be delivered with pinpoint accuracy because of the Vought developed Internal Navigation System (INS). The Regulus II program was extremely expensive because it included three specially built submarines - two diesel and one atomic boat. At the same time, the Navy was developing a submarine-launched Intercontinental Ballistic Missile (ICBM) - Polaris, a program even more expensive and technically ambitious than the Regulus II. Despite that the Regulus II was within a year of being deployed, the Navy cancelled the program in December 1958 and concentrated on the Polaris. 27 Regulus IIs had been completed. The 21 remaining partial airframes were completed as K2DU-1s target drones in 1960-1962. The drones were used by the Air Force from 1959 through 1962 as a target for the Bomarc surface-to-air missile. The Navy used them from 1959 through 1965 as targets for air-to-air and surface-to-air missiles.