OTV will deliver an effective average jet velocity of about 4550 m/s. With the payload of 46,150 kg, the propellant required for each leg of the trip is 84,000 kg. It is assumed that refueling propellant is delivered to geosynchronous orbit by electric orbit transfer vehicle (otherwise the personnel OTV must deliver its own return propellant at much higher cost). These assumptions and calculations allow estimation of the transportation cost per man-year in geosynchronous orbit as about $1.1 million as detailed in Table 2. Simplified Cost Parameters for Large Habitats The large habitat will be a rotating vessel, heavily shielded, and will house the work crews and their families. The large habitat must provide for a greater number of people than the zero-g GEO base in view of accompanied tours. The ratio, N*/N, however, is likely to be in the vicinity of 1.2:family members excepting small children are assumed employable. Added institutional overhead will be minimized to the degree practicable . . . this is not a “normal” community, but organized like a military base. There might be a commissary and a club, but not a multitude of diverse shops and restaurants. Roughly comparable services are assumed in the crew complement of the “conventional” large space stations. The large rotating habitat design will encompass a number of cost advantages and disadvantages relative to the zero-g station. These are summarized in Table 3. For this highly preliminary analysis, it is assumed that these factors are roughly a push. The dominant delta cost factor for the large habitat is attributed to the added pressure vessel structure and the shielding mass. In the size range of likely interest, the sum of structure and shield mass is dictated by the shielding requirement, since structure acts as shielding. (For very large pressure vessels, the structurally-required wall thickness may exceed the shielding requirements.) If, as a first-order approximation, we assume a spherical vessel,* then its radius is *A sphere is not necessarily the best shape. It provides the least shield mass for a given enclosed volume, but some of the enclosed volume is not usable. See Ref. 9 for a thorough review of configuration factors. TABLE 2 REFERENCE TRANSPORTATION COST
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