software to accommodate potential subsystem failure modes. Critical is the availability of mature AG&C for incorporation into the RBD to assess booster rapid ground processing, fully retiring Risk 4.

5.1.2 IVHM Development Phase

A preliminary version of IVHM is required to support AG&C during the Pathfinder flight tests. Development of an evolved IVHM version will be needed for the RBD to support both flight tests and the demonstration of ground turnaround operations. Appropriate IVHM sensors will need to be integrated into the design of the new ORSC engine to meet operability requirements for reusability. This partially retires Risk 5. An IVHM version must be available for the RBS-Y booster and demonstrated through the full-scale testing to fully retire Risk 5. Extensive integrated testing of IVHM, combined with AG&C and booster actuators in a simulation laboratory, is essential for developing booster flight software.

5.1.3 RBS Pathfinder Phase

This subscale reusable booster flight test article addresses Risk 1 primarily and Risk 4 secondarily. Its main purpose is to demonstrate the feasibility of the rocketback turnaround maneuver, although at lower Mach numbers and altitude. If Pathfinder can demonstrate vehicle aerodynamic control and propellant control for engine feed during turnaround, that would partially retire Risk 1, and AG&C for flight control would partially reduce Risk 4. To reduce risk to the Pathfinder flight demonstration phase, the RBS development program should consider flying at least two competing concepts. This increases the opportunity for success and is likely to lead to more robust design(s) for the RBD booster. If neither of these Pathfinder vehicles can successfully demonstrate the rocketback turnaround and RTLS maneuver, then the RBS program development approach will need to be reassessed. This is the key on-ramp to committing funding for continued RBS Program development.

5.1.4 Booster Engine Development

Development of a reusable oxygen-rich, staged-combustion (ORSC) hydrocarbon rocket engine for RBS is likely to be conducted in two phases: a U.S. version of the Russian NK-33 (i.e., the AJ-26) to achieve some level of reusability and an upgraded and modernized version of the AJ-26 or a new ORSC engine development. A new ORSC engine improves reusability and incorporates IVHM; which provides input to the AG&C to accommodate in-flight anomalies and enhance operability for rapid booster ground turnaround. AJ-26 Engine Development Phase

This phase is conducted in parallel with the RBS Pathfinder to support the RBS demonstrator phase. Using the NK-33 engine as a point of departure, it develops a completely U.S.-built ORSC LO2/RP-1 rocket engine (AJ-26) that can satisfy the RBD reusability goal. If this U.S. version is not available, a back-up position would be to use several NK-33 engines to perform the RBD flight demonstrations. New ORSC Engine Development Phase

This phase is conducted in parallel with the development of the Pathfinder and RBS demonstrator development. This engine is considered a new development by the U.S. aerospace industry and introduces a new design which requires design, development, test, and evaluation and technology development specifically in materials and processes and incorporation of IVHM sensors. Further development of the AJ-26 could conceivably satisfy this requirement. New material is required to eliminate the coating process utilized on the U.S. version of the AJ-26. The new material is required to be compatible in an oxygen-rich high-pressure environment. An ORSC engine is required to reduce coking, thereby enhancing engine reuse. It is essential that any new material selected will satisfy the LO2 compatibility requirement and be properly characterized as well as reasonable in cost to procure and manufacture.

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