where V is the volume per inhabitant, N is the crew size for the reference (zero g) case, and N*/N is the crew size ratio discussed earlier. Its surface area is 4ttR2 with a mass of 500 g/cm2 = 5000 kg/m2. Thus, The annual transportation cost savings attributable to the large habitat is simply where/is the fraction of the reference cost that is saved, e.g., if crew stay time is increased from 90 days to 2 years,/ = 7/8. Therefore, the affordable cost per unit mass of the large habitat structure and shielding, is where P is the write-off period in years. Simplifying, Figures 2 and 3 present the size and mass of the habitat as a function of references case work force, and Fig. 4 presents the affordable cost of structure and shielding with the product fP set equal to 10. The write-off period here is about 12 years. Write-off practice varies widely, and is influenced by tax law. Industry generally uses as short a write-off period as is allowed because depreciation generates cash flow. For government projects, the OMB has tended to use a levelized annual cost of capital investment roughly equivalent to a write-off period of ten years. The Skylab space station provided about 100 m3/person for its crew of three. This was remarkably generous for its time; the Apollo module provided about 4 m3/person. The passenger module depicted earlier in this paper provides about 2.5 m3/person. Contemporary zero-g habitat designs for SPS crews provide about 45 m3/person . . . this is “hotel” space and does not include work area volume except for those workers concerned with habitat maintenance and operations. By comparison, the space colony studies have provided quite large volumes; the original O'Neill Model I was about 3000 m3/person; the MIT study provided about 10,000 m3/person; the Stanford Torus allowed about 2500 m3/person. It is evident from Fig. 4 that the allowable cost for structure and shielding is in the range of expected Earth transportation costs to geosyncyronous oribt ($60/kg) only in the case of large crews or relatively small (Skylab-ish) volume per person. Another factor that must be considered is the spin rate required to establish a given dynamical gravity field. The defining equation is
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