Docking mechanism for the STRONG mission: Design, mathematical modeling, and experimental testing

The objective of the STRONG mission is the deployment of payloads from Low Earth Orbit into greater orbits. The orbit raising is accomplished using a space tug with electric propulsion. The mating between the tug and the payload is achieved using a custom designed docking mechanism. The scope of this article is to describe this docking mechanism, outline its design process, introduce the mathematical models used for the preliminary verifications, and present the designed experimental tests. The docking mechanism is composed of an active part and a passive one. The active part is equipped with a retractable probe mounted on a universal joint. The base of this joint may translate along two orthogonal directions using controlled actuators. This feature increases the flexibility of the mechanism making it more adaptable to different payloads. The passive part is a conical frustum (drogue) suitable for the probe reception. The design process was conducted undertaking several steps. Firstly, loads and power requirements were estimated. These requirements led to the selection of appropriate commercial actuators. Subsequently, the ECSS standards were used to verify the suitability of the actuators. Secondly, a simplified 2D model was implemented. The objective of this model was to test the dynamic response of the system avoiding to commit too early to an actuator model. Thirdly, the implementation and results of a complete 3D model further proved the correct behavior of the system. Finally, the experimental tests were designed so as to demonstrate the technical readiness of the mechanism up to level 4. The tests rig is based on a robotic serial manipulator fitted to a support. The passive part of the mechanism will be attached to the manipulator while the interface of the active part will be fixed to the support. The manipulator will follow several trajectories simulating the relative movements between the spacecraft.