Reconfigurable Spacecraft as Kinematic Mechanisms Based on Flux-Pinning Interactions

T HE challenges of the space environment require that spacecraft exhibit a high degree of mission assurance, which often takes the form of autonomous fault tolerance [1,2]. However, the technology for spacecraft repair and reconfigurationmissions has not yet matured to the point where autonomous operations are also robust. Many completed and envisioned spacecraft reconfiguration or repair techniques involve substantial human-in-the-loop activity, including Advanced X-Ray Astrophysics Facility servicing activities and Hubble Space Telescope repair and expansion missions [3]. Another example is the construction of the International Space Station, which astronauts have assembled and reconfigured piecemeal during many hours of extravehicular activity. Fully autonomous repair and reconfiguration tasks have been achieved only recently, such as on the Orbital Express mission [4], with extensive sensing and active control solutions [5]. We propose to depart significantly from traditional approaches to reconfiguration by treating modular, reconfigurable spacecraft as kinematic mechanisms. This proposal addresses the need for robust reconfiguration techniques in space without treating the problem of reconfiguration as one of docking or formation flight. In so doing, its approach incorporates passively stable physics, involving little to no active control at the level of the interface between modules and focusing on architectural control of the system start and end states. This methodology blurs the distinction between deployments and reconfigurations, where “reconfiguration” refers to changes in spacecraft structure, and possibly function, at the system level. There are many reasons why reconfigurable spacecraft architectures are desirable: this capability may allow advantageous changes to a spacecraft’s mass properties, adapt the spacecraft to changing mission requirements, or enable autonomous self-repair. The reconfiguration of modular or swarm spacecraft is currently studied in this context [6,7]. Traditional approaches view these operations as problems in active control [8], particularly as extensions of rendezvous and docking tasks [7,9]. However, we suggest that spacecraft achieve reconfiguration without detaching from one another, by forming appropriate kinematicmechanisms. The degrees of freedom of these mechanisms lead to deterministic reconfiguration behaviors in the sense that the spacecraft can be engineered such that the possiblemotions and configurations of the system are limited and a desired subset of possible configurations becomes a stable reconfiguration sequence. This mitigates the risk of instability from active control, as actively controlled equilibria may not be trivial to define, create, identify, or exploit, and the stable subset of configurations may not be as susceptible to unmodeled dynamics or interactions between the controller and system dynamics. These systems also offer the potential advantage of low power and fuel consumption, saving spacecraft mass. One enabling technology that provides an opportunity for spacecraft reconfiguration via this kinematic-mechanism paradigm is the flux-pinned interface (FPI) [10,11]. This interface links spacecraft modules with high stiffness and damping. It does so through an action-at-a-distance force provided by interactions between magnetic fields and type II superconductors such as yttrium barium copper oxide (YBCO). The flux-pinning interaction is passively stable and acts in multiple degrees of freedom (DOF). Two features of an FPI are central to the formation of flux-pinned mechanisms. First, FPIs can allow certain DOF to remain unconstrained by pinning forces [12], enabling the formation of fluxpinned joints. Second, the operation of an FPI depends on the applied field and the thermal state of the superconductor; manipulating the temperature of the YBCO (for instance, by extending or retracting a sunshade) or temporarily superimposing another magnetic field therefore toggles the interface on and off. Concurrent work is in progress to characterize and design flux-pinned interface systems [13,14]. Section II of this Note identifies some mathematical tools for treating reconfigurable spacecraft systems as kinematicmechanisms. This Note develops its treatment in the particular context of a reconfigurable system using FPI technology, but the kinematicmechanism concept may be extended to other architectures as well. Section III then provides some examples of spacecraft reconfiguration through kinematic-mechanism techniques. The examples include detailed descriptions of two reconfiguration sequences for changing the order and relative orientation of a line of modules, as well as an air-table demonstration of a simple FPI-based reconfiguration.