Guidance For The Powered Descent Of Lunar Spacecraft

Powered descent is a very important part in the soft landing mission.And moreover,guidance and control,which directly determines the success or failure of the soft landing mission,is one of the key techniques to achieve accurate landing.Therefore,the guidance method for the powered descent of lunar spacecraft is studied in this paper.The main contents and results are as follows:Firstly,the process of powered descent is analyzed in detail,and the corresponding coordinate system is defined.Based on the force analysis and the motion analysis,the dynamic model is established.Secondly,the optimal nominal trajectory guidance is proposed,which includes two parts: the optimal nominal trajectory design and the nominal trajectory tracking.Optimal nominal trajectory design problem can be attributed to the optimal control problem,which uses optimal fuel consumption as the performance index.The direct collocation method can be used to transform this optimal control problem into a nonlinear programming problem.In the trajectory tracking part,the control command is obtained by correspondingly adding the nominal control sequence obtained when solving the nonlinear programming problem and the closed-loop correction part.Simulation results show that the optimal nominal trajectory guidance method proposed in this paper can effectively control the spacecraft to land near the expected landing position with less fuel consumption in the condition of the initial bias existing.Thirdly,in order to improve the robustness of the guidance law to initial errors and process disturbances,the predictive guidance method is proposed.Based on the Kepler orbit mechanics,the landing position and velocity of the spacecraft are predicted according to the current flight state,and the corresponding remaining flight time is predicted.The tangential and normal decoupling closed-loop guidance laws are designed.The tangential guidance law,based on the predicted landing velocity deviation and the remaining flight time,forms tangential thrust to control the velocity of the spacecraft in order to reduce the landing velocity deviation.The normal guidance law,based on the predicted landing position deviation,forms normal thrust to change the flight direction of the spacecraft in order to eliminate the landing position deviation.Simulation results show that the predictive guidance method proposed in this paper is easy to be realized on line and it can guarantee the spacecraft landing with high accuracy.Finally,this paper further studies guidance method for lunar maneuvering flight in response to our country's lunar exploration needs.A simple nominal trajectory guidance method is proposed.Simulation results show that the designed control algorithm can effectively eliminate the trajectory deviation caused by the initial navigation bias and the mass change during the flight,and realize the tracking of the nominal trajectory.