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. SRB-X would accept Shuttle designed payloads, and offer a Shuttle compatible launch environment.
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.
Another major drawback to an SRB derived booster is the lack of engine out capability. Because solid propellant cannot be throttled once it is lit, control and abort capability during boost would be limited.
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.
The concept has been revived as of mid-2005 as a candidate in NASA's Exploration Systems Architecture Study (aka the "60 day study"). The use of the SRB as a sole lower stage has been pushed by ATK-Thiokol, the current maker of the SRB on its website www.safesimplesoon.com .