Optimal Design Of Lunar Exlporer Soft Landing Orbit Using Fininte Thurst Control

The trajectory design of lunar soft landing plays a key role on lunar exploration of China. The lunar soft-landing trajectory can be divided into three segments, i.e., transfer orbit segment, Keplerian orbit segment, and propulsive landing orbit segment. This thesis studies the trajectory design during the transferring segment and the landing segment.During the transferring segment, this thesis first applies virtual satellite concepts to orbit transfer and presents a relative motion dynamics model, secondly studies the geometry relation between the true anomalies of the transfer orbit and the virtual satellite orbit and the initial relative state which supplies the initial value for the propagation of the relative orbit dynamics, thirdly calculates the velocity increment and fuel wastage of impulsive transfer mode which are used as initial values to iteratively give the fixed finite thrust angle, fourthly analyzes the feasibility of minimum fuel mode in practice, finally optimizes the finite thrust angle and the initial firing angle by using a least-square method. Finally, The simulation results were present.During the landing segment, two main orbit control modes: minimum fuel mode and precision landing mode are investigated. For the former mode, the thrust angle of a constant thrust is optimized to ensure the minimal fuel wastage by using the Pontryagin maximum principle. As for the second, a PD controller was designed based on the decoupled landing dynamics which is obtained by using the converse systemic principle. Finally, The simulation results were present.