NEAR-OPTIMAL CONTROL OF ALTITUDE AND PATH ANGLE DURING AEROSPACE PLANE ASCENT

Altitude and flight-path-angle control during the posttransonic airbre athing segment of aerospace plane ascent is addressed, with objectives to minimize fuel usage and respect the vehicle flight envelope. Based on a time-scale separation between energy/mass and altitude/path-angl e dynamics, the altitude/path-angle control problem is viewed in a sin gular perturbation framework as an initial boundary-layer problem. A f eedback law approximating the minimum-fuel initial boundary layer cont rol is obtained by solving a neighboring-optimal problem. To facilitat e this derivation, the state constraint that is active on the slow sol ution is modeled in the boundary Layer using an appropriate penalty fu nction. The neighboring-optimal feedback law performs well as long as temporary constraint violations are acceptable in the boundary layer. An alternate Linear feedback law is derived with gains calculated to r educe constraint violations, but this law leads to increased fuel usag e. Numerical results are presented for a Lifting-body configuration of an aerospace plane and a Mach 8 flight condition. The results show th at fuel usage and control activity are reduced when the peak dynamic p ressure is allowed to increase. Differences in fuel usage are small fo r the vehicle model employed.