(2) rocketback RTLS maneuvers; (3) integrated vehicle health management (IVHM); and (4) adaptive guidance and control (AG&C). Secondary technology risk areas include (1) lightweight structures that can handle the expected loads; (2) robust power, fluid, and actuator systems; (3) advanced assembly and manufacturing techniques; and (4) upper-stage LO2/LH2 engines. Note that application of these technologies carries some risk because they have to be applied to a new vehicle configuration with a flight profile that has never been flown before.

The identified technology items will require various degrees of risk mitigation effort involving both analysis and testing, to achieve the technology readiness level (TRL) needed to proceed to substantial RBS development. Some of these technologies apply principally to reusable vehicles; others also have application to potential future expendable vehicles. The high-risk technologies and their application areas are summarized in Table 3.1.

In the following subsections, the four principal technology risks are discussed, secondary risks are addressed and operational and infrastructure issues, as they relate to the RBS concept, are discussed. The chapter concludes with a summary discussion of RBS risk assessment and mitigation efforts.

3.2 MAIN PROPULSION SYSTEM

There are two main propellant options for the RBS MPS, liquid oxygen/liquid hydrogen (LO2/LH2) or liquid oxygen/rocket propellant (LO2/RP); each has two subchoices for the engine power cycle behind them, open or closed, for a total of four basic options. Because the committee believes that neither a pressure-fed liquid engine nor solid rocket boosters would be tractable options for the RBS, it limits discussion to these four options.

The first options for the RBS MPS are a LO2/LH2 propulsion system using either an open-power cycle or a more efficient but higher pressure closed cycle. The open cycle can be either combustion tap-off or gas generator;

TABLE 3.1 Highest Technology Risks

Risk Area Risk Item Reusable Expendable
Hydrocarbon-fueled Combustion instability X X
booster engine Oxygen rich, staged combustion X X
Power balance X X
Physics-based analytical predictive models X X
Injector X X
Materials/coatings for O2-rich environment X X
Turbomachinery X X
Long-life bearings X
Transients X X
Requirements for vehicle integration X X
Rocketback return to Sloshing/propellant management X
launch site maneuver Plume interactions X
Thermal management X
Deep throttling X
Structural dynamics X
Aerodynamics X
Kinematics and mass properties management X
Integrated Vehicle Reliable/robust sensors X X
Health Monitoring Real-time critical decision making: data to action X X
(IVHM) Identify and develop nondestructive inspection options and quantify reliability X X
System integration into asymmetric vehicle configuration X
Adaptive guidance Integration with IVHM X X
and controls Real-time control algorithms X X
Fast response actuators X
Software verification and validation X X


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