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April 10, 2009

Progress Toward Reconfigurable Spacecraft Via Magnetic Flux Pinning




There is progress working in the lab and in theory how to create reconfigurable spacecraft. The dynamics and control problems have been worked on in theory, math and simulation and the joints have been mocked and tested on an air table. Success would transform space operations and systems and enable large particle based solar sails, large optical telescope arrays and large lens for focusing lasers and networks of satellites and flexible modules for space stations. Shoer and Peck and others at their lab are doing great work in this area.

Magnetic reconfiguration would allow for unique structures in space. An example is below. The speed and flexibility of the reconfiguration is especially powerful if the magnetic joints and components are more compact.


A modular spacecraft could be constructed with interfaces consisting of combinations of magnets and Type II superconductors, establishing a non-contacting interaction between modules via magnetic flux pinning. This stable field interaction allows fractionated or modular spacecraft to fix their relative positions and orientations in a passive, virtual structure without mechanical connection, active control, or power expenditure. We report an experiment that investigates the mechanical properties of this magnetostatic interaction by finding the linear restoring forces and torques on a flux-pinned magnet and superconductor simultaneously in 6DOF for small displacements from a static equilibrium. Our results indicate that flux pinning is promising for modular spacecraft assembly and station-keeping applications, providing mechanical stiffnesses over 200 N/m at small (5 mm) magnet-superconductor separations and potentially useful nonzero stiffnesses at larger (over 3 cm) separations, with significant structural damping. We find that increasing the magnetic flux density at the superconductor surface strengthens the flux-pinning forces, suggesting that higher stiffness can be obtained over larger distances by increasing or focusing the pinned magnetic field. Still larger separations will be possible with other superconductors, particularly single-domain superconductors, which show evidence of flux pinning up to a separation of 7 cm or more. Related experiments may be used in the future for performance verification of flux-pinning spacecraft systems.










Reconfiguration of fractionated spacecraft is a challenging dynamics and control problem. It is possible that these challenges can be partly or fully addressed by instead treating the problem of spacecraft reconfiguration as a kinematic one. Selection of the appropriate kinematic constraints adds determinism and robustness to modular systems. The extensive theory of multibody kinematics and kinematics of machines can then apply to spacecraft reconfiguration applications. The need for active control and actuation during reconfiguration maneuvers decreases if the system kinematics are prescribed in such a way.

We view the flux-pinned interface as an enabling technology for such reconfigurable kinematic systems. FPIs are capable of locking and freeing joints between spacecraft modules (altering the spacecraft Jacobian), as well as latching onto and releasing the modules entirely (changing the incidence matrix of the multibody system). We have described two simple ways in which FPIs enable the formation of mechanisms to reconfigure a modular space system. In addition, we have demonstrated a simple kinematic mechanism incorporating an FPI on an air table. Future work in this area will concentrate on the development of suitable flux-pinned interfaces for the formation of kinematic mechanisms and on maneuver strategies for such mechanisms in spacecraft reconfiguration. However, the prospect of treating reconfigurable, modular spacecraft systems as kinematic mechanisms has more general application than to systems mated with flux pinning.


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