6.42 couplings (possibly supplemented by light sensors as described in Section VI.5.5) would provide their lengths to a computer. The system would not interfere if the hull axis should remain within a safe zone. Should nutation and unwanted precession lead this axis astray, a hull-relative-to-shield guidance and control algorithm would activate internal controls and the variable-length couplings. The internal controls would be the same as those described for the open bearing system in Section VI.5.5. These controls could precess the hull relative to the shield should the hull and shield axes not remain parallel. The couplings could translate the hull relative to the shield. Should an emergency arise (or be in the making), the computer would have access to emergency thrusters to provide external forces to the hull. These thrusters would be the same as those in the open bearing design (see Section VI.5.5). The ES group noted a number of advantages to this active closed bearing system. This design would be fail-safe; should the precession torque mechanism fail, both hull and shield would stop their rotations about the q-axis. Provided the hull-relative-to-shield guidance and control remained operational, the first major problem would be the loss of the pointing accuracy at the Sun. The Cassegrain two-mirror arrangement would no longer operate and agricultural areas would lose sunlight. But this problem would take far longer to become serious than the two days in the open bearing design. To remedy the problem would require either resumption of the torque between hull and shield or applying equal and opposite external torques to hull and shield (the emergency thrusters could torque the hull). There would be a safety feature in the spun shield. Should a fragment break loose from the shield, it would drift away from the colony. In the open bearing design, loose pieces would stay in the vicinity. Unlike the open bearing system, this design would require propellant only for emergencies. Also, its up-spin could be handled by reacting hull and shield against each other. This propellant economy suggested that the design could be used on later colonies and should therefore be on the prototype.
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