6.74 larger torques, allowing a faster up-spin rate and a greatly simplified up-spin scenario since hull and shield now spin together. Most important, the new hull design eliminates the problem of keeping spinning hull and stationary shield apart. The configuration no longer needs critical bearings (open or closed) or complicated guidance and control systems. The new concept also allows electrical pathways from the inner hull to the outer hull and easy access to the shield. Hull and shield operations can now be powered from one source. Because the shielding in the compartments is surrounded by air rather than vacuum, the thermal conductivity of the shield is higher than in the earlier configuration. Should heat pipes still be necessary, inspection and repair can be done without exiting the hull. The bulkheads also serve as metallic thermal pathways. The hull configuration change increases the angular momentum of the spinning section of the colony by an order of magnitude. This reduces the unwanted precession and nutation. A given torque applied now causes a precession rate smaller by an order of magnitude. And the asymmetry due to a given unbalanced mass is now smaller (relative to the total mass) by an order of magnitude as well. Of course, the new hull and shield design also introduces problems of its own. As stated in Section VI.4.2, the idea of a shield integral with the hull had been rejected because it was felt that this would require thicker hull sections and thus more refined material and more sophisticated assembly techniques. Indeed, substitution of a double-hull capable of supporting loads due to hull, agriculture, building, and shielding masses,and each hull able to take the atmospheric load, did increase the requirement for refined metal. However, the SDA group had designed the single-walled hull on a leak-before-break criterion, meaning that a crack in a hull plate should grow through it and leak air before reaching critical size and propagating unstably across the whole plate. This led to a hull thickness larger than that required by considerations of static stress alone and a sizable refined material requirement. Preliminary cost studies had also indicated that once the mining, transportation, refining and fabrication systems for the construction
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