6.70 the basic hull shape or dimensions, certain persistent problems involving safety, structural integrity, and ease of fabrication were suggesting that the study group take a second look at the general design. Figure 6.29 presents a set of colony cross-sections highlighting the design evolution and its problems. Part of these problems stemmed from the single-walled hull concept. Section VI.7.6 described the effect of a major leak allowing the air to escape to vacuum. The immediate safety measures required would disrupt colony operations. The FT group also foresaw a difficult repair scenario since the cracked plate would have a pressure difference across it and exterior operations would require moving between hull and shield with the shield speeding by 5 meters away (Figure 6.29(a)). The possibility that water-corrosion might help cause such hull leaks under the agricultural areas worried the SDA group. This suggested separating the cropsoil from the hull by raising it on decks (Figure 6.29(b)). The single-walled hull was a very large thin-shell structure. The SDA group anticipated oscillations (bending, torsion, bulging) in the hull possibly leading to cyclic stresses and fatigue in the plates. The proposed alternatives was a compartmented double-hull (Figure 6.29(c)), whose box-girder-like construction would greatly improve the hull dynamic properties. The ES group also wrestled with a set of problems inherent to the separate shield. Most worrisome was the critical orientation requirement of the hull within the shield. As described in Section VI.5.5, the active open bearing system was not fail-safe and there were considerable practical problems with the alternative closed bearing system (see Section VI.5.6). In either case, the safety of the configuration called for the design of complicated and critical attitude control systems. The ES group also anticipated a delicate up-spin scenario to keep hull and shield apart while the hull's spin rate was brought up to 2-3 RPM. The ES group also worried about the pointing requirement of the colony axis at the Sun. The hull's needed precessive torque would apply stresses and expend fuel, and require accurate navigation and guidance systems.
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