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 TABLE 4.18. SELECTION CRITERIA FOR SPACE MANUFACTURING OPTIONS Make other options: Can this process be used to manufacture other basic process equipment?

Productivity: Is the production rate adequate for the intended purpose? Production rate should be high relative to machine mass. Required consumables: What materials are consumed by the process (e.g., gasoline and oil for internal combustion engines)? Note that electrical power is not considered a “‘consumable” in this analysis. Production energy: How much electrical power, fuels, and other energy resources are required to operate the process? (Some figures in these analyses may be underestimates by a factor of 2-4, as they indicate power input to or output from a final stage rather than the total power required by the system.)

Preparation steps: What is involved in making the process machine(s) and in preparing materials for processing by such machines? Production environment: What special environmental characteristics are necessary in order to allow the process to operate effectively? Of particular concern are atmospheric pressure (can the process operate in a vacuum, or is some form of atmosphere required?) and gravity (can the process operate in zero-g, or low lunar gravity, or is terrestrial gravity necessary or desirable?).

Automation/teleoperation potential: Is it feasible to consider automating the process, or at least operating it manually from a remote location?

People roles: What roles must people play, if any, either on Earth, the Moon, or in space? R&D required: Does the process appear to have a good potential for nonterrestrial use, and what research and develop- ment (R&D) steps may be necessary to enhance the viability of the process in such a setting? (Techniques to be used for production in the early phases of space manufacturing should be testable on Earth or in early LEO systems.)

Tukey ratio: What fraction of the amount of materials required to utilize a process can be obtained from nonterrestrial sources as opposed to terrestrial sources? (Inverse of mass multiplication ratio.) possible through powder metallurgy. For instance, cold welding and porosity control are two aspects which can more easily be manipulated in space than on Earth. Cold welding first was recognized in the 1940s as a widespread effect between like metals. If two flat, clean surfaces of metal are brought into contact, they join at the molecular level and the interface disappears. Cold welding is, strongly inhibited by surface flaws such as oxide layers, especially in those which are softer than the parent metal. Such films do not form quickly on fresh metallic surfaces of grains manufactured in the hard vacuum of space, as they do on Earth. Thus, metal powders will naturally form very cohesive structures upon contact or slight compression. On Earth it is difficult to achieve porosities of less than 10% in uncompressed or lightly compressed powder forms. Significant changes in dimensions of parts may occur fol- lowing a sintering or pressing operation. Theoretically, it should be possible to achieve arbitrarily low porosities by combining grains of many different sizes. However, this is not practical on Earth due to gravitational separation effects, In space, and to a lesser extent on the Moon, gravity effects can be so drastically reduced that uncom- pacted porosities of less than 1-3% may be possible. As an added benefit, in space individual parts can be gently trans- ported to heating or pressure modules without the danger of fragmentation by gravity or rough handling. Sintering, an increased adhesion between particles result- ing from moderate heating, is widely used in the finishing of powder parts. In most cases the density of a collection of particles increases as materials flow into grain voids, and cause an overall size decrease in the final product. Mass movements permit porosity reduction first by repacking, then by evaporation, condensation, and diffusion, There are also shift movements along crystal boundaries to the walls of internal pores, which redistribute internal mass and smoathen pore walls. Most, if not all, metals can be sintered. Many nonmetal- lic materials also sinter, including glass, alumina, silica, magnesia, lime, beryllia, ferric oxide, and various organic polymers. A great range of materials properties can be obtained by sintering and subsequent reworking. It is even possible to combine metals and nonmetals in one process. Solar energy may be used extensively for sintering opera- tions in space. Several techniques have been developed for the powder- ing of metals, Streams of metal can be atomized with or without gases; thrown against rotating surfaces and sprayed out; thrown off high-speed rotating wheels (especially those being melted as source material); projected against other cae