Robust Trajectory Optimization Against Missed Thrust Events Using Sequential Convex Programming

Low-thrust propulsion systems play a crucial role in deep space exploration. However, the extended operation time required for such systems increases the likelihood of missed thrust events (MTEs), in which scheduled thrust operations cannot be executed as planned. Conventional robust trajectory design typically guarantees sufficient margin throughout the entire trajectory, which often results in overly conservative or even infeasible solutions for missions with stringent propellant budgets. We propose an MTE-probabilityaware low-thrust trajectory optimization method based on adaptivemesh sequential convex programming (SCP). By embedding a predefined distribution of MTE scenarios into the SCP framework, it simultaneously optimizes the nominal and recovery trajectories to minimize the expected propellant consumption rather than enforcing a uniform worst-case margin. The framework incorporates a fully adaptive time mesh to capture structural changes in thrust arcs and introduces a tailored step acceptance and trust-region update strategy to maintain feasibility even when these changes appear. We validate the approach in the Earth-Moon circular restricted 3-body problem (CR3BP) using a case study of an under-development secondary payload mission inserted into a direct lunar transfer trajectory. The results demonstrate that the proposed method robustifies the nominal trajectory against multiple predefined MTE scenarios with only a slight increase in propellant consumption. Despite this modest $\Delta V$ increase, the structure of thrusting arcs changes significantly: the number of arcs is reduced, and both the length and direction of the first post-separation arc, one of the most critical operations for secondary payloads, are altered. These findings highlight that probability-aware robust optimization can yield trajectories that are both propellant-efficient and operationally realistic, providing valuable initial solutions for subsequent mission design phases targeting deep space escape after a lunar re-encounter.