Hunter had worked with Hudson on the Phoenix series in the early 1980's. He then joined Lockheed Missiles and Space Company and attempted to interest Lockheed's Advanced Development Division in a revised version of the Phoenix dubbed the X-Rocket (or X-OP, for experimental-operational). Advanced Development Division studied the design and confirmed the basic concept. Hunter was then free to promote the concept to government officials. However Lockheed oredered an independent review of the X-Rocket by its Missile Systems Division, which designed the Trident submarine-launched ballistic missiles. Using parametric analysis which was derived their experience with solid-propellant ballistic missiles, Missile Systems Division concluded that the vehicle would have zero payload. A further brief review by the USAF Aerospace Corporation was also unfavorable.
Lockheed dropped the X-Rocket after these reviews, and Hunter retired. As an independent consultant, he renamed the vehicle concept SSX (Spaceship Experimental). In December 1988 Hunter and Gary Hudson briefed the ad hoc Citizen's Advisory Council on National Space Policy. The general concept was endorsed by the Council and High Frontier, a Washington-based lobby for the Strategic Defense Initiative (SDI) program. Hunter and High Frontier convinced SDI and other national officials that an impartial study should be initiated to finally determine the feasibility of SSTO.
The result was an Aerospace Corporation in the spring of 1989. It concluded that the basic idea was feasible, with a few disagreements over technical approaches. This endorsement paved the way for the initiation of the SDIO SSTO program.
LEO Payload: 9,000 kg (19,800 lb) to a 300 km orbit at 28.00 degrees.
AKA: Space Ship Experimental; X-Rocket; X-OP.
Status: Study 1988.
Gross mass: 227,000 kg (500,000 lb).
Payload: 9,000 kg (19,800 lb).
Height: 18.30 m (60.00 ft).
Diameter: 6.10 m (20.00 ft).
Span: 10.40 m (34.10 ft).
Apogee: 300 km (180 mi).