A tool for the preliminary trajectory design of interplanetary missions exploiting gravity assists and low-thrust propulsion

In this paper a new approach to the preliminary design of either low-thrust gravity assist or multiple swing-by ballistic trajectories with deep space manoeuvres, is presented. Such goal is achieved combining a novel shape-based approach for the description of low-thrust arcs with a global optimisation search, based on a hybridization of an evolutionary step with a deterministic branching step. An inverse, or shape-based, method is proposed in order to reduce the computational effort related to the design of low-thrust arcs. The trajectory is described in terms of a parameterised 3D evolution of the equinoctial elements, and the acceleration necessary to follow the given trajectory is found just by algebraic computation. Through such an approach it is possible to easily compute a low-thrust arc connecting two points in phase-space, with a very low computational time since numerical propagation is avoided. The proposed methodology together with the optimisation method is implemented in an automatic preliminary design tool capable of either deeply or quickly investigate interplanetary trajectories. Various test cases, considering pure gravitational multiple swing-by transfers or low-thrust direct transfers and/or a combination of them, are presented and the effectiveness of the proposed method in combination with the particular hybrid optimisation technique proved.