6.81 is available from the sizable percentage of that element in lunar soil (see Section VIII.2.1). Current silicon solar cells have a mass-produced efficiency of 10.7% (the best cells do better) (6.10). From this, the ES group extrapolated an array efficiency (supports and gaps included) of 10%. The solar cell array area is: Asolar cells = [ (240) 2 - (178) 2] (rr) = 8. 14xl04 m2 and their electrical power production is: Pinputelectrical max (.1) (8.14xl0 4 ) (1.38xl0 3 watts/m 2 ) = l.12xl0 7 watts The colony's electrical power needs, as described in Section VI.10.5, are: pinputelectrical 9.3Sxl0 6 watts The larger figure includes a 20% allowance in array area for degradation of the solar cells' abilities over 30 years (6.10). VI.9.4: Docking System: Since the hull spins at 2 to 3 RPM, it is not possible to dock anywhere but on the spin axis. The docking system consists of three major items: a docking port accessing the colony's cargo airlock; a transfer pod to move the cargo; and a free-floating docking facility. Figure 6.34 shows the docking port and transfer pod. The docking system is sized to handle a full Shuttle payload: a cylinder 5 meters in diameter and 18 meters long (6.11). Table 6.2 lists the dimensions of the docking port and transfer pod components. The 7-meter length clearance is for the opening of doors and for handling equipment if required. Docking to the colony is a three-step process. The transfer pod first aligns itself with the colony axis and spins up to match the colony's spin. This can be done with thrusters or with fly-wheels
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