Trajectory optimization for a fixed trim re-entry vehicle using direct collocation with nonlinear programming

This work examines the feasibility of using a direct solution method to the fixed-trim reentry vehicle optimal control problem as part of a reentry guidance scheme. A procedure was developed to calculate locally optimal trajectories for fixed-trim atmospheric reentry vehicles. A four degree-of-freedom vehicle model was introduced and appropriate environmental models were chosen and implemented. Software was developed to discretize the optimal control problem using a direct collocation method. The resulting parameter optimization problem was solved using the MINOS non-linear programming software package. The resulting collocation guidance software was tested using data for the Kistler K-l vehicle system and an independent vehicle simulation. Mass, wind, density, and entry angle dispersions were considered, as were various strategies for updating the trajectory during flight. The results demonstrated that the collocation method is a viable approach to the reentry vehicle guidance problem. The collocation method enforced the vehicle equations of motion to a useful degree of accuracy using as few as 10 nodes, and the resulting control histories yielded acceptably small final position errors.