The liquid rocket booster as an element of the U.S. national space transportation system

Abstract Liquid rocket boosters (LRBs) were first considered for the U.S. Space Transportation System (STS) during the early conceptual phases of the Space Shuttle program. However, solid rocket boosters (SRBs) were ultimately selected for the STS, primarily due to near-term economics. Liquid rocket boosters are once again being considered as a possible future upgrade to the Shuttle. This paper addresses the findings of these studies to date, with emphasis on the feasibility, benefits, and implementation strategy for a LRB program. The principal issue relating to LRB feasibility is their ability to be integrated into the STS with minimal vehicle and facility impacts. Booster size has been shown to have a significant influence on compatibility with the STS. The physical dimensions of the Orbiter and STS support facilities place an inherent limitation on the size of any booster to be used with this system. In addition, excessively large diameter boosters can cause increased airloads to be induced on the Orbiter wings, requiring modification of STS launch trajectory and possible performance losses. However, trajectory and performance analyses have indicated that LRBs can be designed within these sizing constraints and still have sufficient performance to meet Space Shuttle mission requirements. In fact, several configurations have been developed to meet a design goal of providing a 20,000 lb performance improvement to low Earth-orbit (LEO), as compared with current SRBs. Several major system trade studies have been performed to establish a baseline design which is most compatible with the existing Space Transportation System. These trades include propellant selection (storable, hydrogen-oxygen, hydrocarbon-oxygen, and advanced propellants); pump-fed vs pressure-fed propellant feed system design; engine selection (Space Shuttle Main Engine, Titan LR-87, and advanced new engines); number of engines per booster; and reusability vs expendability. In general, it was determined through these trade studies that several options exist for designing a LRB that can be integrated into the STS with manageable impacts on STS facilities and operational procedures. While LRBs offer a potential 40% improvement in Shuttle performance, their most significant benefit is the potential improvements they offer in the area of Shuttle safety. This begins during ground handling operations, where LRBs eliminate the need for large quantities of hazardous solid propellants to be emplaced in the Kennedy Space Center Vehicle Assembly Building. In the pre-launch phase, all LRB engines can be ignited on the launch pad and verified prior to release of the STS. During flight, LRB engines can be shut down on command should the need arise. Further, missions could be aborted safely during the boost phase—an option not available with SRBs. A related benefit of LRBs is their ability to accomplish a mission even if one engine fails, assuming the LRB is designed with sufficient performance margin. An implementation plan has been developed which indicates that LRBs can be operational by 1997. The attractive features of the LRB have prompted NASA to include this booster as a principal element of the agency's long range plan for enhancing STS capabilities through an evolutionary program of block changes. The implementation of LRBs offers an attractive option for developing a safer, more reliable, and better performing STS.