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Robust Skip earth entry guidance for a low L/D spacecraft

Atmospheric skip-entry trajectories are designed for the Lunar-return missions for new vehicles with low-lift-to-drag L/D ratios. Moreover, modern requirements, such as the diverse option of downrange, call for flexibility for long ranges. In case of a low L\D vehicle, a controlled skip would be necessary to obtain such requirements. The guidance precision decreases with the long downrange and large error. To solve this problem, a fully integrated guidance algorithm during the skip entry phase of a lunar-return mission is presented. A numerical predictor-corrector (NPC) guidance method is adopted. To simplify the guidance law structure, the NPC entry guidance is decoupled. In the longitude channel, the trajectory is predicted with the nominal bank angle, then the error between the target point and the end point is obtained. The strategy calls for controlling the trajectory by modulation of the vehicle's bank angle. The bank angle used in the skip phase is determined by meeting the range requirement to the landing zone. The problem is formulated as a nonlinear univariate root-finding problem. The NPC algorithms are highly adaptive and the extensive three-degree-of-freedom simulations are performed to evaluate the performance of the proposed approach. The Mont-Carlo simulation results show that the proposed algorithm is sufficiently robust to allow precision landing whose position error is less than 3.0km for the entire range between 2500km and 9000km , meanwhile the results indicate that the proposed guidance algorithm features rapidity and effectiveness, which increase the robustness and reliability of the skip entry.