DOCKING MECHANISM CONCEPTS FOR THE STRONG MISSION

The main topic of this work is the design of a docking mechanism for the STRONG mission. This foresees the transport of a standard space platform carrying payloads from Low Earth Orbit into greater orbits by means of a space tug. The mission periodically also envisions the refueling of the space tug by a dedicated docking mechanism with an orbital tank. On the base of the mission described above, the task of this mechanism is to mechanically couple the space tug with LEO located space vehicles. The docking system has to recover the maximum position and attitude misalignments, due to the GNC performances, between the two spacecraft during the docking maneuver. To do this several concepts have been defined aiming to provide different solutions to the docking problem. Among these concepts the optimal solution may be found by defining some trade-off criteria and applying them to a selection procedure. Draft design tools have been created for each concept in order to evaluate their characteristic parameters involved in the trade off procedure. Four mechanical concepts were individuated: a mechanism based on the Stewart-Gough platform, a passive central mechanism (probe-drogue), a mechanism formed by articulated arms transporting grippers to grasp the mounting ring of the expendable launch system Vega and a central active mechanism. The first mechanism is a 6SPS parallel manipulator that may operate following two different control approaches for the soft docking phase. The first control logic is characterized by a position control loop with an optical feedback. The second one instead rules the impedance of the platform. The second concept is a “classical” probe-drogue mechanism similar to the one present in the Russian Progress, Soyuz and the European ATV spacecrafts. This mechanism consists of a rod mounted on the chaser that has to be introduced inside a conical seat on the target side. The third mechanism consists of one active degree of freedom group of articulated arms equipped with grippers able to grasp the mounting ring present in all the payloads launched with the Vega rocket. Finally, the last mechanism is a three degrees of freedom central mechanism. This mechanism is at first controlled in position with an optical sensor. This phase ends when the rod of the mechanism is inserted inside a small conical seat. Subsequently, spring loaded elements perform the soft docking followed by an impedance control phase and then the hard docking phase concludes the task. The paper shows the analysis carried out to provide a preliminary design of the mentioned concepts that will be the base of a trade off procedure to select the optimal one.