Optimal trajectory generation for spacecraft formation flying reconfiguration

Multiple spacecraft formation flying has been identified as an enabling technology for many future space missions. In this paper, the generation of fuel optimal trajectories for spacecraft formation reconfiguration with finite thrust was investigated. The relative dynamics model of spacecraft in the formation was developed by employing the famous Clohessy-Wiltshire equations. Based on these equations, the generation of transferring trajectory for spacecraft was modeled as an optimal control problem with defined transferring time and known initial and terminal relative states. A direct method was applied to convert the trajectory optimization problem to a nonlinear programming problem. By this approach, the whole transferring trajectory was separated into several finite thrust arcs and non-thrust arcs. Then a collocation method based on Hermite interpolation was used to produce the constraints of thrust arcs, and the state transition matrix based on Clohessy-Wiltshire equations was applied to produce the constraints of non-thrust arcs. Meanwhile, to avoid collisions among spacecrafts during formation reconfiguration, an imaginary 3-dimensional sphere surrounding each spacecraft was introduced as a constraint of collision avoidance. The nonlinear programming problem was solved by sequential quadratic programming method. A numerical simulation of a formation reconfiguration with three spacecrafts was performed. The results showed that the present nonlinear programming method can be used to generate optimal trajectories for spacecrafts in the formation and the total fuel consumption of the whole formation is guaranteed.