We can express the cost of the lunar base as the annual amortization cost of investing in the base, plus the annual cost of operating the base. This can be expressed as, Where CB is the cost of the base, a is an annual amortization factor (about 0.12 for 10% discount), b2 is the burden factor for lunar transport, m is resupply mass per manyear, NB is the base crew size, b'2 is the burden factor for round-trip transportation of a crew cab, mc is the mass of the crew cab, Nc is the capacity of the crew cab (number of people), and S is the average crew stay time at the base in years. The cost of the base may be further expressed as Now for some typical values: Characteristically, Ch has been about equal to CT. This is probably more than coincidence ... the optimum level of sophistication in hardware lightweighting and reliability is probably near the point where these costs are equal. From Woodcock (1986), the mass of base facilities to support crew is about 50T/4 people or 12.5 T/person. Representative production facility figures for oxygen production were given in Table I; including a mass driver and nuclear power supplies, a value of 350% for lOOOT/year production is about right. The payoff factor is approximately 10, for direct launch to LEO, or about 6 for an L2 staging point. Note that because the payoff factor enters into the breakeven equation as 1-1 /PF, returns diminish rapidly for payoff factors above about 5. Values for b2 and th are approximately 5 and 10, respectively. Values for ms are about 1.8 T/manyear without lunar food growth and about 0.7 T/manyear with it. For this example case, we assume that a 20-person base can handle 1000T per year production. The crew cab mass is taken as 6.5T/10 crew, and staytimes from 0.5 to 5 years are considered. Fig. 2 shows results of substituting these typical values into the break-even equation for the case of lunar oxygen delivered to low Earth orbit. In this case the factor b{ is equal to one.
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