Future Responsive Access to Space Technologies (FAST), Pathfinder, the RBD, and RBS full-scale development. (Note that the AFRL-funded FAST program has been completed.) This phased technology development suggests that the maturation of the HCB, Advanced Low Maintenance Structures, IVHM, adaptive AG&C flight software and operable thermal protection systems, which was initiated under the FAST program and will need to continue under subsequent programs (e.g., Pathfinder, RBD, and RBS). The Pathfinder program will also initiate development of capabilities for executing the rocketback RTLS maneuver and for handling propellants management and slosh dynamics, agile responsive ranges and payload separation dynamics. The RBD program will continue the development of the above listed technologies and initiate development of capabilities for rapid mission planning, green/operable RCS propellants, operable power systems for batteries and APU, non-pyro separation, and advanced ground processing automation. The cost benefits of these technologies will be quantified during testing of the full-scale RBS Y-vehicles.

Attaining the goals of the RBS program critically depends on successful demonstration of the relatively large number of widely varying technologies and capabilities. However, it is often difficult to assess the challenges associated with overcoming certain technical barriers until these are studied and characterized over a period of time. Having said this, it seems at the outset that executing the RTLS rocketback maneuver without damaging the booster and ORSC engines will be critical to the success of this program because such a maneuver has never been demonstrated at the scale of the RBS. Executing the RTLS rocketback maneuver will be challenging because it will depend on developing capabilities for considerable hydrocarbon rocket thrust throttling, handling the sloshing and dynamics of the propellants, controlling complex aerodynamics that may involve interactions of the rocket exhaust plume with the system’s aerodynamics, and providing adequate thermal protection.

Additionally, the attainment of “reusability,” which is critical to achieving the expected cost savings, will require development of a reliable IVHM system. Such an IVHM system has to be capable of monitoring the health of critical components of the reusable part of the system. To be effective, the development of the IVHM would logically be undertaken in concert with the development of other system components such as the structure and the propulsion system. However, it is not clear at this point how this will be accomplished if the RBS is to employ legacy propulsion systems such as proposed with the use of the AJ-26.

Finally, development of an adaptive guidance and control system will be critical to the operation of the unmanned, autonomous reusable booster that will need to operate in a complex environment with inherent uncertainties in vehicle performance.

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