6. 39 meter/sec. The ES group felt that this made a mechanical link along the rim unlikely. The lower relative velocity near the spin axis suggested connecting hull and shield near the tips of the endcaps; this location would also provide the longest moment arm for the precessive torque. Figures 6.17 and 6.18 present the active closed bearing design considered. Figure 6.17 shows a wheel bearing between a projection of the hull and the shield,inside the shield's light beam or cargo cutout (the figure merely illustrates the concept--an actual bearing would be more complicated). The hull projection would spin within an intermediate ring. This ring would counterspin with the shield. Between the shield and the ring, a set of variable-length couplings (e.g., hydraulic pistons) would serve both as measurement devices and as force applicators. Thus the intermediate ring, while spinning with the shield, would be concentric with the hull projection and would wander around with it relative to the shield. To apply forces to the hull and shield, the appropriate variable-length couplings would be extended or contracted. These could have two effects. Forces in the same direction at each end of the colony would translate the hull relative to the shield. Forces in opposite directions at each end of the colony would precess hull and shield at the same rate in the same direction. Precessing the hull relative to the shield would not be possible by the use of the couplings since they could only apply equal and opposite torques to hull and shield. The operation of the active closed bearing system is schematized in Figure 6.18. To determine the position of the colony's axis relative to the Sun, data from astronomical equipment would be processed by computer. A colony axis guidance and control algorithm would calculate the torque parameters (magnitude and direction) required to precess the colony and operate the appropriate variable-length couplings in the bearings to apply those torques . To calculate the position of the hull relative to the shield and the time derivative of that position, the variable-length
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