A possible next-generation launch vehicle may be a fully reusable, single-stage-to-orbit (SSTO), manned aerospacecraft. The class of hypersonic vehicle configurations with this capability reflect highly integrated airframe and propulsion systems. Since these vehicles are neighter conventional aircraft nor rockets, the mission analysis requires care. And in this regard, fuel-optimal trajectories are of special interest. The specific problem addressed is the determination of such trajectories for a generic, hypersonic lifting-body configuration, powered with SCRAMjet propulsion. The focus is on the SCRAMjet-powered phase of the mission, and the use of the energy-state-approximation method is explored. This method is justified when the system exhibits a particular multi-time-scale behavior, and such behavior is shown to exist for the vehicle and mission phase under investigation. Furthermore, solutions obtained via energy-state assumption are compared to a fully-dynamic solution obtained using a non-linear programming routine.
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