low lunar orbit (A V = 4200 m/sec) might be refueled with lunar oxygen at the refining site (-y = .857) and with liquid hydrogen at LEO. Since the payload (crew) is carried both ways, using equation (C6) with the assumption of ISPc = 470 seconds and ec = .1 results in a value of n = 31.98. For a bulk cargo carrier on the same route, without a payload back from the refining site to LEO, and the assumption of eB = .05, it can be found that /x = 3.407. While analytical expressions exist for the propellant/payload ratio along a single route, there are 12 routes in the system transport map. Propellants for one path may need to be carried as payload on connecting routes. The same route may have different outbound fueling ratios (y), depending on the type of propulsion system under consideration. For this reason, rather than having a single monolithic program which attempts to route propellants under all circumstances, the routing problem was addressed by a heuristic approach: each case under consideration (setup, chemical with lunar oxygen, electromagnetic propulsion, etc.) has a “transport map overlay”. This feature calculates the necessary propellant flows to allow the operation of the desired system, and adds these propellant flows to the corresponding payloads. Designing these overlays is quite straightforward after the selection of refueling points, and it will not be further detailed here. Once the payloads are updated with propellant transfers, the costing procedure can begin. In order to define non-recurring costs, the system must be sized, which allows the estimation of costs occurred in setup. From machine productivity, the machinery mass of site i can be found to be Al Each member of the crew is assumed to work an 8 hour shift five days a week, or 2000 hr/year. The crew mass and bulk cargo during setup are found in the same manner as during the operational phase. The initial system cost (non-recurring) is the R&D cost plus initial earth launch cost plus setup crew wages and training, plus an additional $10,000 per kg on the manufacturing machinery. This results in a development cost of (subscript S defines start up) One more element remains before operational costs can be calculated: earth labor costs. If a fraction of the SPS is brought from earth at the manufacturing stage, for example, that material must be mined and refined on earth, with corresponding labor costs. Earth surface sites can be defined, equivalent to the corresponding sites in space. This system is shown in Fig. Cl. Productivity in space and on earth is assumed to be the same; however, it is assumed that earth workers are paid a flat wage of $40,000 per year, with no training costs. (This assumes that the greatest majority of training is involved with coping with the space environment.) The mass passing through each earth site is Since all assembly is done in space, there is no earth site 4. The size of the earth crew at each site is found from equation (C3), substituting MESl for MNl. The yearly operational cost is the sum of earth launch costs and labor costs in space and on the ground, and is expressed by Since the size of the crews varies due to learning curve effects, the entire costing algorithm needs to be rerun for each year of operation when summing yearly costs to get the cumulative cost. However, there is also the cost of financing to be considered since capital tied up in a development program is no longer available for investment and both capital and expected returns are lost. A 10% interest rate is usually assessed on aerospace projects to determine the “real” costs of a program. Assuming /?years to maturity, the net future value of the development cost is
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