Microgravity Demonstrations of Flux Pinning for Station-Keeping and Reconfiguration of CubeSat-Sized Spacecraft

Abstract

M AGNETIC flux pinning, a noncontacting interaction between Type II superconductors andmagneticfields, has been studied at length by the scientific community for its applications to levitating objects in a 1g environment [1–3]. However, due to the unpowered passive stability that flux pinning can provide, it also has many potential applications for the assembly and reconfiguration of modular space structures [4] and spacecraft formations [5]. Current approaches to autonomous docking of space vehicles [6,7], aswell as spacecraft reconfiguration and formation flying [8–10], rely heavily on active controllers. However, a permanent magnet flux-pinned to a superconductor experiences a passive restoring force that attracts it to the position and orientation it held when the superconductor first cooled below its critical temperature. Previous work and laboratory experiments have suggested that this passively stable effect provides sufficient stiffness and damping to bind modular spacecraft together over separation distances up to about 10 cm [11]. Several possible means for actuation of a flux-pinning interface may be superimposed on the passive stability of this interaction, which requires no power to the superconductor except that required for cooling. In addition to actuation by time-varyingmagnetic fields such as those from electromagnet coils [12], aflux-pinned space systemcan exploit symmetries in the pinned magnetic field to form a noncontacting kinematic mechanism in which the modular components do not touch one another but have some specified kinematic degrees of freedom (DOF) [13]. A simple noncontacting mechanism consisting of a single revolute joint on an air-table testbed has been demonstrated in a laboratory setting [14]. This note reports the results of two demonstrations of magnetic flux-pinning technologies implemented onCubeSat-sized spacecraft during microgravity flights as part of the NASA Glenn Research Center Facilitated Access to the Space Environment for Technology Development and Training (FAST) program in August 2009. In the first experiment, a CubeSat mockup was flux pinned to a CubeSatscale vehicle carrying superconductors and was expected to demonstrate low-stiffness, noncontacting, passive station-keeping in 6 degrees of freedom (6 DOF). The second experiment studied the reconfiguration of two CubeSat mockups between equilibrium configurations via a revolute joint formed by a flux-pinned noncontacting kinematic mechanism. It was expected that the spacecraft would move about an axis defined by the flux-pinned interface rather than their respective centers of mass. These microgravity flight results highlight the role magnetic flux pinning might play in future small satellite operations. Each experiment was performed on a microgravity aircraft with two free-floating modules: one containing an array of magnets appropriate to the experiment, and the other containing superconductors in a Dewar of liquid nitrogen. Three experimenters participated in each flight, two equipment managers to monitor the position of the free-floating modules at all times, and one data collector who operated the motion-capture camera. Figure 1 is a diagram of the test setup.