Optimal Reentry Trajectory Design for Mooncraft Returning to the Earth

This paper presents optimal reentry trajectory for mooncraft to return from the Moon and reentry fly to the Earth.Because the reentry velocity of mooncraft is 10.8km/s,the aerodynamic heating and overload become larger than the one of space vehicle with 7.8km/s reentry velocity.Base on optimal control theory and conjugated gradient algorithm,a lot of reentry flight schemes are presented,including usual optimal reentry trajectory(that is,optimal trajectory with one reentry flight),optimal trajectory with two reentry flights,and optimal trajectory with multiple reentry flights.Some typical results are presented in this paper for those numerical flight simulations.The results shown that the optimal trajectory with two reentry flights can control the mooncraft to fly inside the reentry corridor.It is also easily implemented.Scheme with one reentry flight cannot control the mooncraft to fly inside the reentry corridor.Schemes with multiple(more than 2 times) reentry flights are more complex and more flight time,although they hold better properties of aerodynamic heating and dynamics.Therefore,optimal trajectory with 2 reentry flights is better than the optimal reentry trajectories with one or multiple(more than 2 times) reentry flights.The optimal reentry trajectory for mooncraft returning to the Earth is the trajectory with two reentry flights.