Geometry and time updaters-based arbitrary-yaw iterative explicit guidance for fixed-thrust boost back of vertical take-off/vertical landing reusable launch vehicles

The typical return flight profile of vertical take-off/vertical landing (VTVL) reusable launch vehicles (RLVs) mainly comprises flip maneuver phase, boost back phase, grid fins deploy phase, among which the boost back phase plays a determinant role in accurate landing of reusable launch vehicles. In order to achieve pinpoint terminal precision in the boost back phase, a geometry and time updaters-based arbitrary-yaw iterative explicit guidance method is presented in this paper. By adopting an analytic motion predictor and abandoning small yaw angle hypothesis, the arbitrary-yaw iterative guidance law is formulated to deal with large yaw guidance problem caused by initial deviations and long-time flight. To compensate for the guidance errors due to the ignorance of terminal position constraint in the baseline guidance, a geometry updater is developed to update the target based on analytical geometry relationship. Furthermore, considering that the terminal target continuously moves following the earth, the time updater is designed to determine the candidate virtual orbit according to the estimated time-to-go. Simulations under various nominal flight trajectories and different initial deviations as well as the Monte Carlo simulation are carried out. Results illustrate that the proposed guidance algorithm performs well, showing high precision, strong adaptability and robustness.