Also, if the RBS development were to be pursued using the staged development approach as recommended by this committee, each of the technology risks would be matured to a much higher technology readiness level prior to committing to full-scale vehicle development and flight certification, as compared to the above RLV programs. This approach contrasts dramatically with the manner in which technology risks were addressed in the previous RLV programs. In these programs, the major technology risks (i.e., the reusable SSME and solid rocket motor for the space shuttle; the combined-cycle engine for NASP, and the ultra-lightweight structure with hydrogen fuel for the X-33) were all developed in parallel with the full-scale vehicle development.
With these considerations, there are clear differences between the RBS concept and previous RLV programs. And, while efficient reusability remains an elusive goal for launch, the committee believes that the RBS concept represents a logical compromise between fully reusable and fully expendable systems that is technically achievable in a well-structured program. So, while the committee does not believe that the business case of the RBS concept can be closed at this time, it is important to continue to mature the underlying technologies.