Onboard planning of constrained longitudinal trajectory for reusable launch vehicles in terminal area

A rapid planning algorithm is developed to generate a constrained longitudinal trajectory onboard for reusable launch vehicles (RLVs) in the terminal area energy management (TAEM) phase. The longitudinal trajectory is planned in the flight-path angle vs. altitude space. This flight-path angle profile is designed with required altitude, flight-path angle and range-to-go, and then optimized as a one-parameter search problem to meet the velocity constraint. Considering the dynamic pressure constraint, a dynamic pressure protection (DPP) method is designed. With the DPP, the highly constrained longitudinal trajectory is generated by tracking the planned flight-path angle profile. Finally, the TAEM trajectory planning algorithm is tested on the X-33 vehicle model in different cases. The algorithm is shown to be effective and robust to generate longitudinal flight trajectories with all constraints satisfied in high precision. In each case, the constrained trajectory is planned within 1 s on a PC, which indicates that the algorithm is feasible to be employed onboard.