6.33 Strictly speaking, the shield's motion would require occasional corrections since it would not be expected to be completely exempt from unbalanced forces (meteorite impacts and solar wind would affect it, for example). However, the ES group anticipated that these corrections would be minor and that the shield axis would, therefore, be an adequate Sun-pointing reference. The job of the open bearing system would be to line up the hull axis (within tolerances) with the axis of symmetry of the shield. To this end, it would have to apply precessive torque to the hull and correct for the effects of nutation and unwanted precession. The ES group devised an active open bearing system to fill these needs. Its operation is schematized in Figure 6.16. First, the system needed the ability to discern, rapidly and accurately, the position of the hull's axis of symmetry relative to the shield. The ES group proposed a set of light sensors (e.g., lasers) mounted on the hull reflecting their light off the inner surface of the shield to measure the hull-shield spacing~ .This information would be processed by computer to calculate the position of the hull's axis of symmetry and the . time derivative of that position. The computer could then determine if an emergency situation were present or in the making. Such an emergency (the hull approaching the shield or the hull axis changing position rapidly) would trigger emergency measures using external forces on the hull. These will be covered later. In the absence of an emergency, the computer would use a guidance and control algorithm. This would not interfere unless the hull axis should stray outside a "safe zone". To counter the effects of nutation and unwanted precession, the ES group proposed a set of internal controls. For example, nutation due to mass concentrations could be reduced by a set of trim tanks. By pumping water from some tanks into others, the computer could create corrective mass concentrations, symmetrical about the axis of symmetry to the offending ones. Similarly, a set of masses accelerated on tracks could resist unwanted precession by applying remedial torques to the hull.
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