Reentry trajectory optimization using direct collocation method and nonlinear programming

Direct collocation method has been widely used for trajectory optimization. In this paper, the application of direct optimization method (direct collocation method & nonlinear programming (NLP)) to reusable launch vehicle (RLV) three-dimension trajectory optimization is introduced. Firstly, the model of trajectory optimization control problem to RLV reentry trajectory is established and appropriate environmental models were chosen and implemented, where the motion equation is a three degree-of-freedom model. Mass, density and reentry angle are also considered. Performance is selected to minimize the sum of heat, overload and dynamic pressure. The control variables are attack angle and bank angle. During the flight, reentry vehicle is subjected to the heating rate, overload and dynamic pressure constraints. Terminal state variables constraints are path angle, altitude and latitude constraints. Then, the optimal control problem is discretized into nonlinear programming problem using direct collocation method. The state variables and control variables are selected as optimal parameters at all nodes and collocation nodes. Parameter optimization problem is solved using the SNOPT software package. The simulation result demonstrates that the direct collocation method is not sensitivity on reentry initial conditions. And it also shows that the optimal solutions of trajectory optimization problem are in real-time. Therefore, the direct collocation method is a viable approach to RLV reentry trajectory optimization problem.