7. 15 attainable in a space-based manufacturing facility be accurately determined so that an efficient structural design may be formulated. VII.3.3: Fabrication Techniques: Having chosen the primary structural material, the fabrication method remains to be specified. The qualities desired in the completed structure are: a straightforward and reliable method of analysis; efficient and economic means of inspection and repair; and tolerance of the probable modes of damage that may be inflicted upon the hull. In current aerospace design two basic fabrication methods are used. These are the Fail Safe and Slow Crack Growth (SCG) techniques. A Fail Safe design tends to be lighter than the corresponding SCG design and is characterized by an ability to sustain the failure of one or several components while retaining overall structural integrity. SCG design ' is continuous in nature and demands that the total accumulated damage in the structure be below a critical level. A more complete discussion of the nature of these basic design methods may be found in Section VII.5. Three specialized fabrication techniques were proposed and are briefly examined here. The first was the use of vapor deposition (7.9). This method vaporizes the metal and in its gaseous state the metal is projected against a thin target. This target could be a mylar balloon formed in the shape of the colony's hull. The depositing head continually passes over the target's surface until the required thickness of material has been deposited into a continuous sheet hull. This method is technically interesting, but has several drawbacks. First, it is not a state of the art technique. The authors of this paper were not familiar with all the implications of this method and could not justify any limited analysis that we might perform. Secondly, how to deposit this material with anything but basic low carbon steel properties is presently unknown. Work hardening, precipitation hardening, and heat treatment do not seem possible at this time for the vapor deposition technique. Third, this type of structure implies the use of the slow crack growth criterion which was eventually discarded as a means of guaranteeing the 30-year lifetime of the colony. Finally , it would be difficult to attach any interior structures to this hull
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