The SSM-N-9 Regulus II cruise missile is a supersonic guided missile armed with a nuclear warhead, intended for launching from surface ships and submarines of the U.S. Navy (USN). History The limitations of the Regulus I were well known by the time it entered service in 1955, so the Navy issued a specification for a surface-launched supersonic shipborne cruise missile, equipped to carry a nuclear warhead, that had greater range, accuracy and resistance to countermeasures. Development of the Regulus II was well under way when the program was canceled in favor of the UGM-27 Polaris SLBM (Submarine-Launched Ballistic Missile) system, which gave unprecedented accuracy as well as allowing the launch submarine to remain submerged and covert. Prototype and initial production missiles were later converted to KD2U-1 supersonic target drones for the US Navy and the U.S. Air Force, which used the KD2U-1 during tests of the Boeing IM-99/CIM-10 Bomarc SAM (Surface to Air Missile). The SSM-N-9a Regulus II was redesignated as the RGM-15A in June 1963, nearly five years after the missile program had been terminated. At the same time the KD2U-1 target drone was redesignated as the MQM-15A. Some targets equipped with landing gear were redesignated as GQM-15As. Design and development The major drawback of the original Regulus was the use of radio-command guidance, which required a constant radio link with the launch ship / submarine that was relatively easy to interfere with. The earlier missile also suffered from restricted range which required the launch ship to launch the missile close to the target and remain exposed until the missile hit the target. To alleviate these drawbacks, the Regulus II was designed with an inertial navigation system, which required no further input from the launch ship / boat after launch, and a greater range through improved aerodynamics, larger fuel capacity, and a lower specific fuel consumption from its jet engine. Prototype missiles were built, designated XRSSM-N-9 Regulus II, with retractable landing gear, to allow multiple launches, and Wright J65-W-6 engines and Aerojet General booster, which restricted them to subsonic flight. The first flight of the XRSSM-N-9 took place in May 1956. Beginning in 1958, testing was carried out with the XRSSM-N-9a, equipped with the General Electric J79-GE-3 turbojet and a Rocketdyne solid-fueled rocket booster to allow the entire flight envelope to be explored. Evaluation and training missiles with retractable undercarriage were produced as the YTSSM-N-9a and TSSM-N-9a respectively. After