SRB-X
The SRB-X was once described as "the single worst shuttle-derived launcher ever proposed". In 1984 the USAF issued a request for proposals for a commercial expendable launch vehicle capable of placing 10,000 lb payloads into GEO are discussed. A fixed-price incentive was included for the delivery and launch of 10 rockets. The government would furnish propellants, oxidizers, and pressurants.
Candidate boosters were the Titan T-34D, a hybrid Atlas Centaur G, and the NASA SRB-X/Centaur. The SRB-X was only in the conceptual stage, and it appeared that readiness for the required first launch by 1988 was not possible. Modifications to the two other candidates are also necessary, however, to meet the GEO capabilities, while the SRB-X is designed to make maximum use of existing launch preparation facilities used for the Shuttle.
In February 1985 the Department of Defense announced that it has selected the Titan 34D-7, rather than the SRB-X as its Complementary Expendable Launch Vehicle.
