SSX

American SSTO VTOVL orbital launch vehicle. The X-Rocket was a VTOVL SSTO design by Maxwell Hunter II at Lockheed in the late 1980's. The 227 metric ton vehicle was powered by clustered RL10 engines. Internal reviews at Lockheed rejected the feasibility of the vehicle. After Hunter retired he worked with Gary Hudson to refine the design as the SSX. This was briefed by the pair to Space Defense Initiative Organization (SDIO) officials in 1988. It was largely through their efforts that the US government funded the DC-X demonstrator in the 1990's.

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 ordered 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.