Ascent Trajectories of Hypersonic Aircraft: Operability Limits Due to Engine Unstart

A generic waverider-type hypersonic aircraft that undergoes an ascent trajectory has been modeled using a first-principles reduced-order model. Two types of operability limits are added that represent boundaries on the aircraft trajectory map (of vehicle altitude versus Mach number). These boundaries are associated with engine unstart and ram–scram transition. The predicted unstart boundary is to be avoided; the ram–scram transition is a condition through which the aircraft must fly, but it is useful for the control system to know when this transition is approached to account for possible sudden changes in thrust and moments. The model shows that unstart occurs if the aircraft flies too high, too slow, or at too great of an acceleration. The unstart limit can be avoided by selecting a trajectory having sufficiently large dynamic pressure or a low vehicle acceleration. Optimizing these factors avoids an excessive value of the fuel–air ratio that is required for trim. The model also identifies an engine inlet geometry that avoids unstart. To assess the model, the computed results are compared to some available experiments.