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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.
A single-rocket SRB-X would carry less payload weight than the Shuttle, but would offer the advantage of economy to a user desiring to put less than a full Shuttle load into an orbit that would preclude sharing payload bay space with another user. The three-rocket configuration would carry approximately the same weight as the Shuttle to LEO, however this configuration would have a much greater capability to GEO.