roughly 40% of the total base resupply. The economic leverage attainable through onboard food production is quite small compared to that of reducing crew exchange trips. At the expected prevailing cost of freight shipment to GEO of $60/kg, the available savings are on the order of $50,000 per man-year for the former, compared with about $1 million per man-year for the latter. One may list advantages and disadvantages of onboard food: Advantages Reduced resupply freight “Greening” of the onboard environment Fresh fruits and vegetables Sharing of the atmosphere revitalization workload Disadvantages Light input of —400 W/m2 required for effective growth Problems of agricultural waste disposal Labor required (although gardening has recreational value to some people) Land area required Some very crude initial calculations indicate that this will be a nonlinear trade-off: to the extent that the advantages stated can be secured without increasing habitat cost or crew size, onboard food production will be beneficial. Further, even cruder calculations suggest that the break point is likely to be reached with onboard production of fresh fruits and vegetables. Dried (and perhaps frozen) products can be shipped by electric orbit transfer vehicle with an associated trip time of roughly six months. Refrigeration in the form of cryogenic argon is available on the EOTV. Fresh products such as dairy and fresh meats can be shipped by high-thrust orbit transfer vehicle at somewhat higher cost. The high-thrust vehicle trip time is about 5 days (Earth-GEO including payload transfer operations in low Earth orbit). A supply arrival every two weeks is likely. One may imagine the food requirements to be roughly met by 25% fresh = 17.5 tons per shipment (420 tons/yr) 35% dried/frozen = 590 tons/yr 40% onboard = 670 tons/yr The associated food transport cost is on the order of 0.2% of the total cost of SPS construction and maintenance. Although space settlement advocates have expressed enthusiasm for complete onboard food self-sufficiency, the economic motivation for it, in the case of the GEO construction base, is negligible. In other habitat applications, complete onboard self-sufficiency may be important for safety reasons. Evaluation Using a relatively crude analysis, we have found that the cost/benefit ratio for a large “settlement” type habitat is one of the order of unity, considering its potential for reduction of crew transportation cost. If a large part of the habitat's shielding mass could be obtained from the Moon at low transportation cost, the ratio might be
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