i.e., if space settlements can provide a cheaper labor force, this will influence the trade-off of terrestrial versus extraterrestrial resources. TRANSPORTATION COSTS AND HABITAT ECONOMICS If we can provide a crew habitat at geosynchronous orbit (or, anywhere else in space that work crews are needed), and thereby provide an acceptable domicile for an extended period, i.e., several years, then this habitat can reduce crew rotation and resupply costs. One may then trade off the costs of long-term habitation amenities against savings in crew transport costs. The value of transport costs is a critical parameter. Other important parameters are the number of people to be supported, the duration of their stay, and the required level of radiation protection. The reference SPS construction and maintenance scenario (8) presumes transport of crews to and from geosynchronous orbit every 90 days. A modified Shuttle with a passenger capacity of 75 serves the Earth-LEO leg and a hydrogen/oxygen OTV with the same capacity serves the LEO-GEO leg. The crews are housed at GEO in a conventional (albeit large) zero-g space station, the “construction base.” The personnel orbit transfer vehicle (POTV) provides a total shielding of 5 g/cm2 against Van Allen radiation. The GEO station provides about the same through judicious location of structure, meteoroid protection, and subsystems and equipment, and also provides a “storm shelter” area of heavy (20-40 g/cm2) shielding against solar flare protons. The comparison case provides for infrequent transport of crews; the average GEO stay time might be two or more years. Long stay times are accommodated by a large habitat with artificial gravity (0.5 to 1 g), and crews are permitted accompanied (with family) tours. Accordingly, heavy shielding of the habitat is provided to reduce radiation dose to 0.5 rem/year. The space manufacturing studies (e.g., Ref. 5) have estimated that 500 g/cm2 will suffice. Both options assume that the transportation costs to low Earth orbit (LEO) can be provided at $33/kg of gross pay load by HLLV and that cargo transportation to GEO can be provided at $65/kg by HLLV and electric orbit transfer vehicle (EOTV). Crew flights to low Earth orbit are assumed to cost $75,000 per crewman, equivalent to a $5.6 million shuttle flight with 75 persons per flight.+ tThis may seem artificially low in cost. However, a reusable shuttle booster coupled with operational learning might approach it. We wish to avoid any possibility of biasing this trade-off in favor of large habitats. Fig. I. Orbit transfer passenger module. (Note: Configuration illustrated is for 80 passengers. Configuration analyzed for this paper had one fewer row of seats.)
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