Appendix 5C: LMF Paving Robot Subsystem
The Platform of the Lunar Manufacturing Facility (LMF) described in section 5.3.4 serves as the physical foundation for both the original deployed Seed and the growing and mature LMF manufacturing complexes. According to Nichols (1976), "pavement is a surfacing for traveled areas, which is intended to provide a long-lasting, smooth, clean, supporting surface; to spread loads sufficiently so that base material can support them; and to protect the base against damage by traffic...." These factors are almost as important on the Moon as in terrestrial applications - a simple graded surface would require frequent maintenance, lack cleanliness, and provide no firm foundation base to anchor SRS factory machines. A small crew of platform-building or paving robots is probably necessary for any fully automated lunar factory.
5C.1 Basic LMF Platform Design
The best material for construction of the platform ideally should be plentiful, easy to work, and most suitable for the job in terms of structural strength. Native lunar basalt appears to satisfy all three requirements adequately (Rowley and Neudecker, 1980).
Green (1980a, unpublished Summer Study document) has discussed the properties of lunar basalt at length. Raw lunar soil may be fused at about 1550 K, then allowed to cool and solidify into a very, hard, exceptionally strong material. If cooling is virtually immediate - minutes or tens of minutes - the liquid basalt is quickly quenched and becomes a polymeric glassy substance. The material is very strong but also moderately brittle, permitting cracks to propagate rather easily. Using this option, it is necessary to divide the platform into small square-meter-size slabs to help isolate fracture failures and to permit relatively easy maintenance and repair. If the liquid basalt is permitted to cool more slowly - allowing perhaps several hours for the melt to pass from full liquidity at 1570 K to hard solid below about 1370 K - the material anneals into a crystalline form. This method of platform construction takes much longer and requires more energy, but would produce a far less brittle foundation. Such a basalt crystal platform could be prepared as one continuous surface, whereas the glassy basalt platform must be made in slab-sized sections.
Green has also pointed out that Moon soil has characteristics necessary to make an excellent basalt casting due to the uncontaminated, unweathered nature of the lunar material and an extraordinarily low viscosity which is necessary for superior basalt castings. Dunning (1980, unpublished Summer Study document) considered the mechanical properties of cast basalt and found them comparable to those of cast iron and many fine steels, and superior to aluminum, brass, bronze, and copper both in compression and shear strengths. Compression strength is important in many construction applications, and shear strength is a necessary requirement for all foundation materials (U.S. Department of the Interior, 1952). A list of the properties of cast basalt is collected and modified from Anderson (1977), Baumeister and Marks (1967), and several other sources in table 5.9.
Having chosen the foundation material, the team next considered the physical configuration. According to Nichols, concrete pavements for highways are generally about 15-25-cm thick, 30 cm and higher for airport runways. Adjusting for the 0.17-g lunar gravity and the attendant reduced forces to be sustained, the equivalent load bearing strength on the Moon would require a thickness of perhaps 2.6-4.3 cm for highways. Both highways and encounter heavier use than the LMF platform is expected to receive in normal use, so a choice near the lower end of this range appears justified especially since basalt appears to be stronger than concrete in compression and shear (Baumeister and Marks, 1967; Zwikker, 1954). Consequently, a thickness of 3 cm (Green, 1980b, private communication) was tentatively selected. The square-meter size of individual slabs represents a compromise between limiting possible structural damage caused by fracture propagation and the minimum reasonable size from a practical construction standpoint.
Individual slabs comprising the platform should be formed with a 5-cm margin around the edge (slab separation 0.1 m). Rather than a second sintering pass by the paving robots, slabs are placed close enough so that overheating beyond the nominal square-meter target area for a brief period during each production cycle is sufficient to sinter neighboring blocks. (Some backfilling may be required as about 1-cm horizontal shrinkage is anticipated upon cooling.) A simple diagram of the slab pattern is shown in figure 5.34. Calculations suggest that the baseline design for paving robots should permit each device to prepare about six slabs per day in continuous operation.
