Abstract
This paper discusses on the minimum fuel trajectories of interplanetary spacecraft for sample-return mission. The spacecraft is propelled by a solar electric propulsion system. Both going and returning trajectories with four boundary conditions are optimized simultaneously. Because available thrust of solar electric propulsion is very low, the propulsion system is firing for a long duration. The available thrust depends on the distance between the spacecraft and the sun. The problem is described as an optimal control problem to find the amplitude and direction of thrust. The optimal amplitude of thrust is maximum or minimum value according to optimal control theory. In order to obtain highly accurate solution, the equations of motion are transformed to a new description in which the amplitude of thrust is treated as a constant value. Two problems are discussed in this paper. The first problem is that the staying period at the target planet is not specified. In this problem, it is assumed that the spacecraft departs to and returns from the target planet at the optimal time. The optimal trajectories and the optimal staying period are shown in the results. The results show how the performance of thruster affects to the fuel consumption and the whole flight time. Higher thrust shortens the whole flight time and higher specific impulse improves the fuel consumption. In the returning problem, the staying period is specified. The staying period is required to be longer or shorter than the optimal period shown in the previous problem. The results show that the going trajectory changes in the case of longer staying period and the returning trajectory changes in the case of shorter staying period. The reason is discussed.
