7.9 VII.3: CONFIGURATION AND MATERIALS-ALTERNATIVES VII.3.1: General Remarks: A project of this magnitude which is subjected to tight cost restrictions and a rigorous operating envelope must have all the apparent alternatives for materials and alloys, assembly techniques, and structural layout thoroughly evaluated. This report cannot evaluate all the alternatives in detail (i.e., doing complete designs for each alternative and then doing comparisons based on cost and structural integrity). However, we do attempt to outline several alternatives which presented themselves to us, do a terse comparison, and allow the readers to carry out more intensive comparisons themselves. VII.3.2: Materials: The selection of the primary structural material is constrained by several factors, the first of which is cost. There are two basic costs involved, that of transporting the material from its source to the construction site, and that of the processing necessary to turn out useful components. Since all basic material is to be supplied from the Moon, the material used in the structure should be present in lunar rock in sufficient quantities to make the processing stage cost effective. If the Transport Linear Accelerator (TLA) and Orbital Transport Vehicle (OTV) (see Section IX.l) are assumed operational, the transportation costs involve the overhead for these systems as well as the transportation of any required alloying elements from the Earth. As is discussed in Chapter IX, the high cost of Earth to LS transportation would argue for a material requiring the least mass of alloying elements. The second major constraint involves the mechanical properties of the material. The discussion of Section VII.5 indicates the importance of fracture mechanics considerations in the structural design. Thus, the material fatigue properties, particularly fracture toughness (K 1 c), are as important as the yield or ultimate strength. A specific desigh criteria which influences the material(s) selection is the leak-before-break requirement. Briefly, this criteria demands that a flaw originating either on the surface of a plate or in its interior must not reach critical length until some
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