smaller unit cost ($/kg) than equivalent materials can be supplied from the Earth, the Moon is presumably the best source for the bulk of the materials needed. Although I agree with the conclusion, I want to point out that comparison between terrestrial demandite and lunar soil composition is not a good basis for consideration of the availability of the needed materials on the Moon, because demandite for a space economy will differ drastically from demandite for a terrestrial economy. Even for construction for a given purpose, such as a solar power unit, we can expect that a major difference will exist between the two, because the design of construction in space will be based on little or no gravity. The effects of this difference are so fundamental that the demandite composition for terrestrial construction will bear little relation to that for space. On Earth, almost 90% of nonfuel demandite makes up materials such as brick, concrete, and tile that are used in compressive loading — columns, walls, foundations, roads, etc. Criswell (3) has emphasized the huge reduction (—99.8%) in the mass of material needed for a given function in space from the mass needed for the same function on Earth by comparing, for example, the Grand Coulee Dam (or a terrestrial solar power unit) and an SPS having equivalent energy output. However, the major part of the mass of either of the terrestrial installations is concrete, in compressive loading due to gravity, whereas the bulk of the space construction will have no such gravitational stresses, and hence will not require much material having a high compressive strength. Thus, much of the discussion that has gone on of the possible technology of melting and casting blocks of fused lunar materials may have relatively little pertinence for most applications in a SPS, though such blocks may well be valuable for constructing radiation and micrometeorite shielding around lunar base buildings. Fused Basalt The currently available technology of melting and casting of silicates can provide a wide range of useful products, in addition to simple structural block, but very little attention has been paid to it in the discussions of materials for space. Thus, fused mullite (3Al2O3-2SiO2) is an excellent refractory for some special duty installations involving high temperatures and abrasive or corrosive environments. However, far more important, perhaps, are the possibilities of manufacture of many more mundane products from simple fused lunar soil. Fused and cast basalt is relatively simple to produce, and has many desirable properties such as a very high resistance to abrasion and corrosion, and a surprising mechanical strength, even when hot. Many different commercial products are currently being made from cast slag and basalts of various compositions in many countries in Europe. The technology of cast basalt has been explored in detail in Czechoslovakia. Thus, Kopecky and Voldan (17) reported on extensive studies of the viscosity, liquidus temperatures, melting range, and phase assemblages obtained for various basalts and additives, and the grain size and texture obtained at various cooling rates, as these features are important in controlling the properties of the products. They found that spherulitic crystallization of pyroxene yields products having maximum abrasion resistance. Extensive studies of silicate crystal morphology under various conditions of crystallization (e.g., 19) reveal that control of such morphology no longer need be empirical. Commercial companies in Europe currently market fused basalt [1] tiles and slabs in a variety of size and shapes for lining chutes, bins and hoppers, [2] straight and curved pipes, [3j reducers, [4] complex T’s and Y’s, and [5] centrifugal slurry pumps; they even market a fused basalt cyclone separator (Fig. 2). The piping is of particular
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