Low thrust gravity assist trajectory optimization based on hybrid method

The application and advantage of low thrust propulsion system has been validated in practical mission launch even though the trajectory design difficulty is increased. The trajectory optimization problem brought by the use of low thrust propulsion in conjunction with gravity assist is researched. This paper proposes a hybrid technique for solving the strong nonlinear and multiple constraints optimal control problem using a hybrid technique. First, a nonlinear programming problem is proposed by abandoning the terminal constraints brought by Pontryagin maximum principle and treating costates as optimization variables; Then an adjoint control transformation is used for converting costate variables to other variables partially which possess actual bounds and physical meaning; Finally, differential evolution algorithm is employed for searching the initial values of nonlinear programming problem for local accurate optimization. Related low thrust gravity assist trajectories to outer planets by various gravity assists are designed for demonstrating the effectiveness of the proposed hybrid technique.