6.75 materials had been set up, the actual amount of material these were used on was not a major cost driver. Thus, the increase in this refined material requirement due to the configuration change was not prohibitive after all. As for the fabrication techniques, the new design poses fewer fabrication problems, needing only two concentric shells, rigidly connected together, rather than the three shells required earlier. As stated in Section VI.4.5, the ES group had attached the parabolic mirror to the stationary shield to keep its structure at a minimum. The new hull and shield configuration requires that the mirror spin at 2 to 3 RPM and have sufficient supporting structure to stay together. However, the relative position and orientation of the parabolic mirror, the secondary mirror, and the window in the hull are now fixed by solid connections. While the order-of-magnitude increase in angular momentum improves the unwanted precession and nutation problems, it also raises the precessive torque required to keep the colony axis pointed at the Sun,and the corrective torques to offset nutation and unwanted precession,by the same order of magnitude. Thus, the thrusters would either use ten times as much propellant or be mounted ten times further outward from the hull's center of mass. The study group felt that both these alternatives were unfortunate and decided to change the colony orientation instead. Figure 6.31 shows the new overall orientation. Rather than pointing the colony axis directly at the Sun, it is pointed at right angles to it along the q-axis introduced in Section VI.5.1. No precessive torques are applied to the colony,which thus maintains this orientation as it travels around the Sun. An elliptical flat mirror, positioned outward from the parabolic mirror and at 45° to the q-axis and the sunward direction, reflects the sunlight into the parabolic mirror. Because the parabolic mirror no longer serves as a direct screen between the sunlight and the exterior of the hull, the new configuration requires a shadow reflector to shield the colony from the warming solar radiation. This reflector should be far from the colony
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