Shape Memory Alloy Based Hard Docking Mechanisms for two-stage CubeSat Docking

On-orbit docking methods already in use in larger spacecraft, such as the ISS, have a long history and a high Technology Readiness Level (TRL); however, on-orbit docking systems for small satellites have yet to be evaluated in a space environment. Currently, no docking mechanisms involve an airtight seal for transferring materials from one small spacecraft to another to allow applications such as on-orbit servicing and in-space assembly of small sats into larger modular spacecraft. Instead, most proposed docking techniques for CubeSats use a single phase involving magnetic force (soft capture). Using magnets for docking is undesirable, as they cause Electromagnetic Interference (EMI) when one CubeSat’s electronics moves in the presence of a docking magnetic field. In addition, the electromagnets used for docking consume substantial amounts of power to maintain a docked configuration and require complex control algorithms to perform a successful dock. The electromagnetic force between the two CubeSats alone may not be enough to support an airtight seal for material transfer, hence calling for a mechanical latching system (hard capture) as a second stage in docking. This paper discusses using geometric pairs of docking adapters with a second stage involving Shape Memory Alloy (SMA) spring-loaded latches. Based on 3D simulation results and analytical calculations, prototypes have been developed for testing in simulated conditions in the laboratory. The current adapter prototypes are PLA 3D-printed models. In their present state, the prototypes can complete the first and second stages of docking. Ground-based systems such as 6-DOF robotic arms mimicking the ADC systems of the spacecraft and air tables to simulate the frictionless environment of space will be used to validate the designs. We aim to demonstrate a successful docking scenario between two CubeSats equipped with metal 3D-printed prototypes of our docking adapters. In addition, we aim to perform experiments to measure the precision, repeatability, and reliability of the SMA springs used in the adapters. The experiments consist of loading the springs and applying current to actuate the springs. The SMA springs will be characterized by analyzing the resulting deflection, force, response time, current, and temperature data. Through this characterization, we will arrive at requirements for space qualification of the mechanism. We further discuss the power consumption and optimization of the mechanism’s mass, power, and volume.