Low-Thrust Earth–Moon Trajectory Design Using Successive Convex Programming

The Earth–Moon cargo spacecraft is essential for the sustainable development and utilization of lunar resources. The application of a high-specific-impulse, low-thrust propulsion system can substantially improve payload efficiency. However, the perturbed dynamics of the Earth–Moon 3-body system pose marked convergence challenges in low-thrust trajectory optimization. This paper proposes an enhanced sequential convex programming (SCP) method for optimizing low-thrust transfer trajectories between the primary and secondary bodies in arbitrary 3-body systems, which is also applicable to the Earth–Moon scenario. The proposed approach enhances algorithmic convergence by introducing an improved initial profile. First, an approach for constructing the initial profile is presented, in which the primary-body to secondary-body trajectory is divided into 2 phases: escape from one body and capture by the other body. Initial trajectories are designed using the indirect method under the 2-body model for each phase, and the 2 segments are concatenated to form the improved profile. Next, the SCP approach utilizing the enhanced profile is presented, replacing the traditional linear interpolation profile with the more feasible improved profile, thereby enhancing the algorithm’s convergence. Finally, the method is applied to design fuel-optimal transfer trajectories from either highly elliptical Earth orbits or inclined geosynchronous orbits to the lunar polar orbit. The results show that when the SCP algorithm based on a linear interpolation profile fails to identify a feasible solution, the proposed method successfully achieves convergence to one. By applying thrust-magnitude homotopy, the method transitions the fuel-optimal solution toward a near-time-optimal one. The proposed method offers substantial potential for trajectory design in future transportation missions between 2 celestial bodies, particularly for the Earth–Moon scenario.