NOMINAL TRAJECTORIES FOR THE AEROASSISTED FLIGHT EXPERIMENT

This paper is concerned with the determination of nominal trajectories for an AFE type spacecraft flying with constant angle of attack and v ariable angle of bank. A simulated GEO-to-LEO aeroassisted orbital tra nsfer is considered. With reference to the atmospheric pass, a decompo sition technique allows one to separate the longitudinal motion from t he lateral motion 1!. For fixed control, an eigenvalue analysis shows the existence of strong intrinsic instability in the longitudinal mot ion and near neutrality in the lateral motion. While stability can be artificially induced via feedback control, the effectiveness of a feed back control scheme depends on control margin availability; in turn, t his enables the spacecraft to cope with dispersion effects due to navi gation errors, uncertainties in the atmospheric density, and uncertain ties in the aerodynamic coefficients. In light of this situation, the generation of good nominal trajectories is a compromise between perfor mance (low characteristic velocity, low peak heating rate, low peak dy namic pressure, high minimum altitude) and control margin. Generally s peaking, performance deteriorates as control margin improves. The abov e compromise can be achieved sequentially as follows. In the atmospher ic entry phase, the sequence of bank angles 190 deg, 0 deg, 90 deg is employed; in this phase, the objective is to achieve a specified minim um altitude consistent with good values of the characteristic velocity , the peak heating rate, and the peak dynamic pressure. In the atmosph eric exit phase, the sequence of bank angles 90 deg, 90 deg, 90 deg is employed; in this phase, the objective is to achieve the prescribed o rbital inclination and longitude of the ascending node, while ensuring sufficient control margin so that the spacecraft can reach the specif ied LEO apogee following the atmospheric exit.