Comparative analysis of the docking systems for the space tug project 'STRONG'

This paper discloses the trade-off process that led to the selection of a mechanism for the space tug of the STRONG mission. The objective of the space tug is to dock and transport space payloads from intermediate orbits into great orbits. For this purpose five mechanism concepts were considered: a mechanism based on the Stewart-Gough platform controlled in position, a Stewart-Gough platform with impedance control, a central mechanism with passive shock absorbers, a central active mechanism and a mechanism with articulated arms carrying grippers. Several trade-off parameters were taken into consideration: mass, complexity of the mechatronics system, control complexity, adaptability, reliability and functional confidence. For each parameter a weight was selected in order to give more or less relevance to some of them. In order to evaluate each criterion, different tools were used. CAD models of the mechanisms were created to evaluate masses and working volumes; numbers of servo-actuators and sensors for each concept were taken into account; various control logics during the soft and hard docking phases were investigated; functional adaptability to different spacecrafts masses was analyzed; custom simulation software were written to evaluate dynamic performance. The outcome of the decision-making procedure was the central mechanism with passive shock absorbers, mainly for its simplicity and reliability. This concept may be integrated during development and design with some active features to improve performance and adaptability. In the paper potential improvements are discussed