It will be assumed that the output accumulates in a stock which continuously generates revenue according to it magnitude. Solar power satellites are one example of such an output. As in the case of capital inputs, wear can be accounted for with expendable replacement parts or reduced revenues per unit output. Again, the output and related expendables may be made from either terrestrial or extraterrestrial resources or both. Sales of manufactured goods could be accommodated by modifying the model. A flow diagram is shown in Fig. 1. The capital stock K is supplied in annual increments AKE from Earth and AKS from space. To keep the capital functioning, expendables AX must be imported (AXE) or produced locally (AXS) each year. The Space Manufacturing Facility (SMF) produce ^QS, which, together with some material brought up from Earth (AQE), makes up the final product AQ. Finally, for the output to bring in revenue, it must be supplied with some expendables each year (AXq). The mass flow can always be reduced to this form, no matter how many specific inputs, outputs, or process steps. However, it may sometimes be advantageous to consider some of these factors separately. The lunar mass flow through this system is limited by the capacity of the machinery to process it. Assuming full employment of all equipment, this constraint can be expressed mathematically as follows: where all bold terms are functions of time and p is the productivity of capital in mass output per (year-mass of equipment). The scaling functions A, B, C, D, and E are included for generality. They will all be unity provided that the other functions are appropriately defined. However, it is desirable to retain the scaling terms so that waste, overhead, and the like can be treated independently. Define the proportions, respectively, of process expendables (m), capital (n), output (/), and output-related expendables (mJ which are produced from extraterrestrial material over time. Averaging these and the other related ratios over sufficient periods that their time dependencies can be ignored, we have
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