Abstract: Considering the parametric uncertainties during the landing process of the vertical takeoff and vertical landing launch vehicle, an uncertainty optimization method of sequential optimization and reliability assessment based on nonintrusive polynomial chaos expansion for return trajectory is proposed. First, an optimization model of the return multi-flight-phase trajectory under the nominal conditions is established. Then, considering both robustness and reliability of the trajectory, an optimization model for the return trajectory under uncertainties is established, which consists of the robust optimization objective function, reliability-based path constraints and robust equality constraints. Finally, the robust optimization objective function and robust equality constraints are quantified based on the nonintrusive polynomial chaos expansion method, by which the original stochastic robust optimization problem is transformed into the equivalent deterministic optimization problem in the high dimensional state space. Meanwhile, to improve the evaluation efficiency of path constraints, the most probable point method is improved by using nonintrusive polynomial chaos expansion, and the sequence optimization and reliability assessment strategy are further developed. Numerical results show that the proposed method has good robustness and can meet the requirements of reliability in engineering, and also has high accuracy and efficiency.

Key words: vertical takeoff and vertical landing launch vehicle, return trajectory, parametric uncertainty, nonintrusive polynomial chaos expansion, stochastic robust optimization, sequential optimization and reliability assessment