Abstract: The application of direct optimization method (direct collocation method & nonlinear programming (NLP)) to lunar probe soft landing trajectory optimization was introduced.Firstly, the model of trajectory optimization control problem to lunar probe soft landing trajectory was established and the equations of motion were simplified respectively based on some reasonable hypotheses.Performance was selected to minimize the fuel consumption.The control variables were thrust attack angle and thrust of engine.Terminal state variable constraints were velocity and altitude constraints.Then,the optimal control problem was transformed into nonlinear programming problem using direct collocation method.The state variables and control variables were selected as optimal parameters at all nodes and collocation nodes.Parameter optimization problem was solved using the SNOPT software package.The simulation results demonstrate that the direct collocation method is not sensitive to lunar soft landing initial conditions;they also show that the optimal solutions of trajectory optimization problem are fairly good in real-time.Therefore, the direct collocation method is a viable approach to lunar probe soft landing trajectory optimization problem.

Key words: Direct collocation method, Orbit analysis, Soft landing