Abstract: The reusable rocket recovery landing in the atmosphere uses aerodynamic force and thrust to complete a high-precision pinpoint vertical soft landing. In the powered descent phase, there are various disturbances such as thrust deviation, aerodynamic model deviation and wind disturbance, which will reduce the terminal landing precision and lose the performance index, so the guidance system faces the difficulty of disturbance rejection. To solve this issue, a combined disturbance compensation guidance method is proposed, which divides the disturbances into modelable and unmodelable disturbances according to whether they are modelable to be described or not, and treats them sepa-rately. The modelable disturbances are those that can be described by a model and are considered for optimal guid-ance to improve the performance index; while the unmodelable disturbances are those that are difficult to be de-scribed by a model and are only considered to compensate their adverse effects on the terminal constraints in real time. In the combined disturbance compensation guidance framework, first, a disturbance estimator is designed to estimate the two types of disturbances in real time. Then, a neighboring optimal disturbance compensation guidance algorithm is designed which corrects and compensates the optimal guidance command in real time using the estimat-ed values of modelable disturbances, to improve the performance index using modelable disturbances while ensuring the terminal constraints. Finally, a terminal invariance disturbance compensation guidance algorithm is designed to ensure terminal invariance by calculating the terminal constraint perturbations caused by unmodelable disturbances and compensating the adverse effects of unmodelable disturbances on terminal constraints in real time. Simulation results show that the proposed combined disturbance compensation guidance method can improve the optimality of performance index while ensuring the terminal landing precision, and has strong robustness to various disturbances.

Key words: reusable rockets, powered descent, disturbance compensation, neighboring optimal guidance, terminal invariance