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“ It seems clear that a wide range of industrially useful feedstocks can be economically provided for LEO and lunar utilization, using materials delivered first from low Earth orbit, later from the Moon, and ultimately from asteroidal and other resources. Sufficient knowledge of lunar materials exists to permit development and implementation of a variety of processing options; similar technology definition for asteroidal materials awaits more detailed information on specific bodies or the development of more generalized processing schemes appropriate to the space environment. ”

“ In particular, thread rolling is an adaptation of the rolling process that may be ideally suited to high-vacuum manufacturing environments. Conventional die-cutting methods for threaded fasteners produce cutting chips. In space, these chips could contact-weld and foul other equipment if released as isolated fragments. Thread rolling overcomes both problems. Because threads are impressed, no fragments are produced, thus obviating chip vacuum welding. This cold-forming process has long been used in the fastener industry to produce precision threads at high production rates. ”

“ In conventional lathing a metal tool is used to fabricate metal stock; hence, cold welding of the tool and stock becomes a serious potential problem. Basalt stock possibly could be turned, or basalt tools designed, to help alleviate this difficulty. Cutting fluids of the conventional type are unsuitable for space and lunar applications due to vacuum sublimation and the need for fluid reconstitution. The production energy, required consumables, and machine productivity ratio for turning are equivalent to those for mechanical milling, as are the required transportation costs. ”

“ Most glues are carbon-based. The relative scarcity of this element in space suggests that carbon-based glues should be used only where they cannot be replaced by other materials. Boron and carbon, the two most common substances used in composites on Earth, are both rare in space: aluminum and iron fibers may replace them in nonterrestrial fabrication of composites. Energy for fabrication and glue curing is quite small compared with requirements for welding, and production of iron and aluminum fibers for epoxies should consume less energy than forming solid metal pieces. ”

“ The major energy expenditure for glues is transportation from Earth. Careful studies are needed to determine tradeoffs between using glues as bonding materials or in composites, and welding or metal-forming requirements.
Space utilization of glues and composites imposes several restrictions yet also offers several advantages. Zero-gravity has little impact - the absence of atmosphere is much more significant. Many resins and glues used on Earth are fairly volatile and deteriorate under vacuum ”

“ Metal fasteners are "consumed" during the process of fastening, but since they can be fashioned primarily from abundant lunar iron and aluminum the need for consumables and energy is about the same as that required to fabricate parts from these metals. The machines to manufacture and apply metal fasteners on Earth are serviceable in space applications if modified for zero-g and vacuum-compatibility. ”

“ Casting, powder metallurgy, and plastics. Casting is a process in which melted fluid is introduced into a mold, allowed to cool to produce a solid product, and then this product is ejected. The primary limitation in terms of potential space utilization is the gravity required for all casting processes except permanent mold, centrifugal, die, and continuous casting. However, terrestrial gravity and atmosphere also create most of the major difficulties associated with these techniques on Earth. ”

“ However, gravity is necessary to feed fluid into the mold. It is not easy to replace gravity feed because expendable mold castings tend to be fragile; any type of pressure feed will likely damage the mold and ruin the final product. Another problem is that expendable molds draw heavily on inputs comparatively difficult to supply nonterrestrially. Some materials for temporary molds, such as sand in sand casting, can be recycled, but processes such as investment casting may require significant Earth inputs to remain viable space manufacturing alternatives. ”

“ Many conventional techniques are rejected because they do not meet these unique requirements for space manufacturing. For instance, most standard machining operations are unsuitable due to the cold weld effect which occurs in a vacuum environment. Many joining techniques require prohibitively large quantities of imported consumables, and thus are inappropriate for a self-sustaining space industrial complex. Some casting and molding practices must be rejected since they require gravitational forces. ”

“ Joining processes of some sort are universally required for manufacturing. Materials joining techniques include welding, brazing, soldering, adhesive bonding, metal fastening, stitching, shrink fitting, and press fitting. Sintering, the joining process associated with powder metallurgy, has already been discussed. Methods for joining plastics are not covered because these materials are inappropriate in the context of early space manufacturing ”

“ Clay metal and ceramic technologies suggest a number of theoretical and experimental projects or demonstrations related to both near- and long-term terrestrial and nonterrestrial operations. Experiments on grain size distribution, dimensional changes, compositions of metals and ceramics, and choices of binders with regard to porosity, new molding and forming techniques which might be employed in space, and the general area of automatic production, inspection, and robot handling are all appropriate research topics. ”

“ Thus, primary energy systems (e.g., solar mirrors) can be very low in mass per unit of output and reasonably simple to fabricate. Grains of powder created, stored, and manipulated in a very hard vacuum should have minimal surface contamination and therefore will be susceptible to useful contact welding. Good internal bonding of powders thus may occur through grain contact, sintering, and melting. Lack of gas bubbles in a vacuum-manufacturing environment will also aid the production of well characterized parts. ”

“ There exists a great commonality between forming techniques in powder processes and in plastics. In addition, powder techniques are capable of making most, if not all, of the equipment necessary for plastics forming. ”

“ Contact welding occurs almost too easily in the vacuum environment of space. Prevention of undesired cold welding is probably a more challenging problem than weld creation during manufacturing. Friction welding may be combined with vacuum welding to facilitate removal of protective coatings from workpieces as well as to enhance bonding.
Electronic welding techniques (electron beam, laser beam, and induction/high-frequency resistance welding) all appear feasible for space applications. ”

“ Cold welding should not occur during grinding unless very fine abrasive grit is employed. However, tool life (e.g., of abrasive wheels) is likely to be short if grinding techniques are used exclusively to shape and mill in the same manner as mechanical milling and turning. Production energy, consumables, and mass/production ratio again are about the same as for mechanical milling. Grinding equipment transportation costs are relatively high, partly because of the massive machines involved that are often larger than milling equipment. ”

“ Sintering, an increased adhesion between particles resulting 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 smoothen pore walls.
Most, if not all, metals can be sintered. ”

“ Brazing and soldering differ from welding in that a molten filler metal joins the workpieces at a lower temperature than is required to melt the workpieces themselves. Of the 15 brazing and soldering techniques identified in table 4.17, only vacuum (fluxless) brazing displays exceptional compatibility with the space environment. Compared with vacuum welding, vacuum brazing requires some heat to melt filler material but can bond a greater variety of materials - refractory and reactive bare metals, ceramics, graphites, and composites (Schwartz, 1979) . ”

“ Research on aluminum-coated cast basalt and shell reinforcement by spun basalt is of critical importance in determining the feasibility of the electroforming manufacturing option. Automated processing also should be investigated, particularly with regard to monitoring electrical current densities as a function of metal deposition rate and techniques of mandrel-shell separation (while keeping electrolyte losses to a minimum) .
Machining. Machining processes, for the most part, suffer several limitations as manufacturing methods in automated lunar, asteroidal, or orbital factories. ”

“ More than 40 manufacturing techniques were found appropriate for a near-term evolutionary SMF. The logical limit of this analysis is to determine whether or not there are technological subsets which could be embodied in compact systems to produce most of the mass of subsequent generations of machines of production. These bootstrapping systems or "starting kits" should take advantage of local available materials and be compatible with the use of automation and robotics. ”

“ A very large range of products is possible. Virtually any item which can be manufactured by forging, extruding or casting can be duplicated either directly or with appropriate reworking. In addition, special articles such as high-strength or highly refractory composites, filaments, linings for friction brakes, metal glasses, heat shields, electrical contacts, magnets, ferrites, filters, and many other specialized products can be made. Very complicated parts composed of metal and refractory components are directly producible. ”

“ Interlace fasteners on Earth are made of organic threads of various sizes and compositions and are used mostly for joining fabrics. A major space-related use of interlace fasteners is in the manufacture of fabrics, primarily for space suits. Threads, strings, and ropes have been fabricated from nonvolatile inorganic materials having superior tensile strength and flexibility. There is little need for consumables except for bonding agents in the making of ropes. ”

“ Thread- and wire-production machines can be used in space with no specific modifications, and stitching-, rope-, and cable-making devices require only simple alterations to take best advantage of zero-g conditions. Even in applications where the fabric must hold pressure, metal and basalt fibers should prove adequate with minor design changes. ”

“ Although electroforming is well-suited to the production of thin-walled vessels it also requires an electrolytic working fluid, which downgrades it to a lower priority than magnetic pulse forming for space manufacturing. ”

“ Robotics and teleoperated processes could be implemented using current automation technology in laser cutting, measuring, and flaw detection because sophisticated computer vision is not required. Laser operations such as case hardening, shaping, and impurity detection require more sophisticated machine intelligence technology than is presently available. Most LBM techniques today involve a certain degree of teleoperation, which suggests a potential compatibility with broader automation. ”

“ Iron, aluminum, and titanium are abundant on the Moon; such nonterrestrial resource candidates will likely receive early attention. This suggests a favorable Tukey ratio for fasteners. The manufacture of iron and titanium units from lunar or simulated lunar material is a worthwhile early materials-processing experiment. The space environment enables metal fasteners to replace welds in many applications because the loads are generally lower in zero-g. Vacuum welding may strengthen bonds meant to be permanent. ”

“ Mechanical milling of metals in a high vacuum environment is exceedingly difficult with current technology because of the cold-welding effect. The machine mass/production ratio, required consumables, production energy requirements, and mass-multiplication or Tukey ratio are not favorable. ”

“ The lack of atmosphere and gravity in space are not serious impediments to the use of LBM; in fact, the absence of air may make lasers slightly more efficient in orbit or on the Moon. The only difficulty arising from the lack of atmosphere is plasma removal. ”

“ In space, and to a lesser extent on the Moon, gravity effects can be so drastically reduced that uncompacted porosities of less than 1-3% may be possible. As an added benefit, in space individual parts can be gently transported to heating or pressure modules without the danger of fragmentation by gravity or rough handling. ”

“ Metal fasteners may be grouped into two categories those producing a semipermanent bond and those requiring either a releasable bond or a sliding bond. Screws, nuts, bolts, rivets, brads, retaining rings, staples and clamps are used for semipermanent fastening of objects when stress bonds or environmental conditions preclude gluing, do not require welding, or where the bond is intended for an indefinite service life. ”

“ Stitching is the process of joining parts by interweaving a piece of material through holes in the items to be coupled. The bond is frictional if the linked pieces are not rigid or tension-produced if they are. ”

“ Rates of expansion, heating and cooling of the workpiece (which presumably can be well controlled over long periods of time in space using solar energy) , gravity gradients, rotation and handling limitations during the formation phase must all be considered. It may be that the largest objects must be formed in very high orbits so that continuous sunlight is available during critical periods and gravitational tidal effects remain small. ”