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Mining robots deliver raw lunar soil strip-mined from the pit to large input hoppers along the edge of the entry corridors into the chemical processing sector. The primary responsibility of the materials-processing subsystems is to accept faranoute, extract from it the necessary elemental and chemical substances required for system growth, replication, and production, and then return any wastes, unused materials, or slag to an output hopper to be transported back to the surrounding annular pit by mining robots for use as landfill.

It is possible to achieve qualitative materials closure (see sec. 5.3.6) - complete material self-sufficiency within the Lunar Manufacturing Facility (LMF) - by making certain that chemical processing machines are able to produce all of the 84 elements commonly used in industry in the United States and the global economy (Freitas, 1980). However, such a complete processing capability implies unacceptably long replication times T (on the order of 100-1000 years), because many of the elements are so rare in the lunar or asteroidal substrate that a vast quantity of raw soil must be processed to obtain even small amounts of them. By eliminating the need for many of these exotic elements in the SRS design, replication times can be cut by as much as three orders of magnitude with current or foreseeable materials processing technologies.

Hence, it is desirable to determine the minimum number of elements and process chemicals and to fix the lowest extraction ratio R (kg input material/kg useful output material, see sec. 5.3.6) which can still maintain closure of the system, thus minimizing the replication time T.

5E.1 Minimum LMF Requirements: Elements and Process Chemicals

The elemental and chemical requirements of the expanding LMF fall into a fairly small number of broad categories summarized in table 5.11. Note that these are the minimum (or very nearly so) requirements for LMF qualitative materials closure - an "adult" LMF entering production phase may need additional chemical processing capabilities which may be programmed into the factory's operational software. Table 5.11, however, lists only those minimum requirements necessary to achieve closure for a seed during the growth phase.

Demonstration of materials closure plausibility. The components in table 5.11 were obtained first by taking a very basic list of necessary elements (the first four categories) for the entire LMF and adding to these any additional substances necessary to chemically produce the original items. This resulted in an increase in the number of items, therefore, all newly added items themselves had then to be similarly checked to ensure that each of them could be produced from the materials already at hand. This procedure was iterated until closure apparently was achieved.