# Advanced Automation for Space Missions/Chapter 4.3

4.3 Initial LEO "Starting Kit" Facilities
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.
Approximately 10 man-years of research effort already have been devoted to lunar materials processing alternatives (Billingham et al., 1979; Criswell, 1978, 1979; Waldron et al., 1979) on the Moon and in space. The assembly of large structures in space from pre-formed parts has also received much study. Most of this work is reviewed in the MIT (Miller and Smith, 1979) and General Dynamics (Beck, 1979) studies on the manufacture of components for satellite solar power stations using lunar and terrestrial materials processed in factories deployed wholly from Earth.
Options available for manufacturing a wide range of machines or systems of production in space or on the Moon from locally available industrial feedstocks have received far less study. Virtually no effort has been directed toward answering the following questions: (1) What mass fraction of available and foreseeable machines of production can be produced in space from available materials, and (2) how might a hierarchy of production technologies be "grown" in space to create an ever-increasing variety of product and production options? Thus, the growth of industrial capacity can be partially or totally decoupled from terrestrial export of key processing resources.
A broad survey and analysis of a number of basic terrestrial manufacturing processes for their potential nonterrestrial applicability suggests several alternative starting kit scenarios, as described in section 4.3.1. Special attention is then given to "starting kits" in section 4.3.2. A "starting kit" is an initial space manufacturing unit of minimal mass and complexity which, given a supply of feedstock material, can produce second-generation tools (and some products) with which production capability may be gradually expanded further.
4.3.1 Survey of Terrestrial Manufacturing Processes
A survey of basic terrestrial manufacturing processes was accomplished by examining a representative sample of reviews of the field (Amstead et al., 1979; Bolt, 1974; Campbell, 1961; DeGarmo, 1979; Lindberg, 1977; Moore and Kibbey, 1965; Schey, 1977; Yankee, 1979) and then generating from this "review of reviews" the taxonomy of approximately 220 manufacturing processes in table 4.17. A listing created in this manner is reasonably comprehensive, though probably not complete. Four major categories emerged: (1) casting and molding (powder metallurgy), (2) deformation (forming and shearing), (3) machining (milling, drilling, and lathing), and (4) joining.
Table 4.17.- Taxonomy Of Manufacturing Processes :
| Casting and molding Casting Sand Plastic mold Shell mold Investment (lost wax, precision) Permanent mold Centrifugal Die Slush or slurry Full mold Low pressure Continuous Molding Molding Powered metal Compaction plus sintering Plastics Injection Compression Transfer Extrusion Blow Rotational Thermoforming Laminating Expandable bead Foam Rotomolding Vacuum plug assist Pressure plug assist Matched mold Deformation (forming and shearing) Forming Forging Smith Hammer Drop Press Impact (see also extrusion) Upset No draft High-energy-rate Cored Incremental Powder Rolling Shape Ring Transverse Orbital Cross-rolling Thread Stretching (expanding) Drawing (shrinking) of wire bar or tube Embossing Coining Stamping Sizing Redrawing Bulging Necking Nosing Ironing Deep drawing Swaging Extrusion Spinning Bending Miscellaneous other Peening Guerin process Wheelon process Magnetic pulse Explosive Electroforming Staking Seaming Flanging Straightening Shearing Line shearing (slitting) Blanking Piercing or punching Follow-up on #2 and #3 Trimming Shaving Notching Perforating Nibling Dinking Lancing Cutoff Machining (material removal) Milling Peripheral (slab) Face Chemical Turning Facing Boring Spinning (flow turning) Knurling Cutoff (parting) Drilling Reaming Countersinking Tapping Sawing Filing Broaching Shaping Horizontal Vertical Special purpose Planning Double housing Open-side Edge or plate Pit-type Grinding (abrasive machining) Abrasive jet machining Honing Lapping Superfinishing Barrel finishing Vibratory finishing Spindle finishing Abrasive belt Polishing Buffing Burnishing Grit- or shot-blasting Tumbling Wire brushing Electropolishing Electro-chemical grinding Routing Hobbing (hubbing) Ultrasonic Electrical discharge Electron beam Electrochemical Chemical Photochemical Laserbeam | Joining Welding Arc Shielded metal Gas metal Pulsed Short circuit Electrogas Spray transfer Gas tungsten Flux-cored Submerged Plasma arc Carbon arc Stud Electroslag Atomic hydrogen Plasma-MIG (metal inert gas) Impregnated tape Oxyfuel gas Oxyacetylene gas Methylacetylene propadiene (MAPP) Air-acetylene Oxyhydrogen Pressure gas CO$_2$ Resistance Spot Projection Seam Flash butt (flash) Upset(butt) Percussion Solid state Ultrasonic Explosive Diffusion Hot press Isostatic hot gas Vacuum furnace Friction Inertia Forge Cold Roll Electron beam Laser beam Solid-state Axial-flow gas Cross-flow gas Thermit Induction Low frequency (50-450 Hz) High frequency (induction resistance; 200-450 kHz) High frequency resistance Electromagnetic Flow Brazing Torch Induction Furnace Dip Resistance Infrared Vacuum Soldering Iron Resistance Hot plate Oven Induction Dip Wave Ultrasonic Sintering (of powdered metals) Adhesive bonding (incomplete) Thermo-setting and thermoplastic Epoxy Modified epoxy Phenolics Polyurethane Adhesive alloys Miscellaneous other powders, liquids, solids, and tapes Metal fasteners Screws Nuts and bolts Rivets Pins Cotter Groove Tapered Roll Retaining rings Quick-release Stitching Stapling Shrink fitting Press fitting Plastic Hot-air-welding Friction Heated metal plate Solvent Dielectric Magnetic Ultrasonic Radio frequency welding |
The remainder of this section consists of reviews and analyses of the processes in each of the four major categories that are potentially useful in space. All methods have been closely scrutinized with respect to a substantial fraction of the criteria listed in table 4.18. 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. Many deformation techniques are eliminated because of their tendency to produce inconvenient waste debris.
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 development (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.)
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. For example, liquid metals have a lower kinematic viscosity than water, and develop significant velocity by falling only a few centimeters. This condition creates turbulence, erosion of mold materials, and entrapment of air and mold gases. Manipulation of molten materials under controlled, low-gravity conditions and in vacuum may provide significant advantages (Adams, 1977).
There are two basic approaches to casting. The first, expendable mold casting, is the simplest process and the least likely to go wrong. 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.
Nonexpendable mold casting, on the other hand, relies less on the conditions of gravity and pressurized atmosphere. The molds tend to last for a greater number of runs. The main disadvantages are that (1) production devices tend to be large, on the order of tons, and (2) the processes are more complicated than for expendable mold casting. A more complete review of both methods from the standpoint of space applications may be found in appendix 4B.
The key problem appears to be mold/pattern preparation, the heart of the casting process. This problem provides an excellent focus for future artificial intelligence and robotics technology development efforts: A robot which can produce a mold/pattern to close tolerances is required (appendix 5F). Such manipulation might be initially performed via teleoperation, followed by a gradual evolution toward complete automation. Mold/pattern design is a fine art for which some type of expert system may be required for near-autonomous operation. The development of more precise robots with enhanced feedback and access to an expert system for casting technology should alleviate the mold production problem.
Casting processes have some definite advantages with respect to space applications. For instance, expendable mold casting is simple and nonexpendable mold casting requires no gravity. A potential solution to the gravity problem for expendable molds might be the generation of artificial gravity via centrifuge. Centrifuges are capable of applying great pressures, although force gradients inevitably will be present even in large rotating systems. Research is needed to identify and circumvent the difficulties of mold/ pattern production in space.
Another casting/molding manufacturing technique is powder metallurgy. In this process, primary material is powdered and then placed in a suitable mold or extruded through a die to produce a weakly cohesive part. High pressures and temperatures then are applied to fuse powder particle contact points until a sufficient flow of material closes all pore spaces. Powder metallurgy can be conducted in a minimum facility able to produce an everwidening range of increasingly complex parts and tools (Jones, 1960). A considerable theoretical and applications knowledge base already exists to help extend powder technologies into space(Bradbury. 1979).
Any material which can be melted can be powdered. Reformation does not necessarily require complete liquefaction, so the usual "phase rules" of melting may be ignored. The formation process thus has much greater flexibility than casting, extrusion forming, or forging. Controllable characteristics of products include mechanical, magnetic. porosity, aggregation, and alloying properties of metals and nonmetals. Many useful production options are 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 flesh 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 following 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 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.
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. Many nonmetallic 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 operations in space.
Several techniques have been developed for the powdering 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 streams of metal, liquids such as water, or gases; or electrified. Solar thermal energy may be used in any of these processes, which represent the major energy-intensive step in powder metallurgical manufacturing.
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.
The "flow" nature of powder metallurgical techniques is amenable to automation and remote control at all stages from design through production and inspection. The virtually complete separation of the major energy input stages from the design embodiment stage permits the early use of precise but low-force-level devices for near-final shaping. Powder metallurgy can use lunar iron and aluminum, is appropriate for vacuum manufacturing, is insensitive to particle or photon radiation, and can take advantage of zero- and reduced-gravity conditions. It is worth noting that vapor deposition of materials can also be considered as an alternative or supplemental process to powder metallurgy in some applications - such as the production of sheets or large areas of metals. An extended discussion of powder metallurgy appears in appendix 4C.
Plastics are mostly hydrocarbon-based. Raw materials necessary for their preparation are relatively rare in lunar soil. Hence, they must be extracted from bulk materials of carbonaceous chondritic asteroids or eventually from the atmospheres of other planets, their moons, or the solar wind, or else be brought up from Earth. Except for special uses in critical cases, it does not make sense to plan the extensive utilization of plastics in the early phases of space industrialization. These substances may be replaced by sintered or pressure-formed metals or by ceramic parts in many applications. A critical new research area is the possibility of replacing plastics in resin and composite applications with materials derived primarily from inorganic elements found in lunar soil in greater abundance (Lee, 1979).
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. Thus, if supplies of hydrocarbons ever should become more easily available (see section 4.4.2), the machinery and automation support already would be in place or readily adaptable to this purpose.
Deformation. Deformation includes ten major operations in forming and four in shearing, each of which may be further subdivided as indicated in table 4.17.Major aspects of these processes related to current industrial robot applications and possible automated space manufacturing options are provided in appendix 4D. Highlights of forming processes especially suitable for extraterrestrial utilization are given below. All shearing processes may involve cold welding, and can be performed best by laser beam or other techniques. The team noted that many space structures (such as photovoltaic cells) will be very thin, and thus are more appropriate for laser or E-beam cutting than the comparatively thicker members of typical terrestrial structures.
Regarding forming processes in space, low-weight electromagnetically driven forges may be optimal in view of the special technology created for the electromagnetic mass launcher (Kolm, 1977). At present, "mass-driver" forges are not used on Earth, although magnetic impact welding is being explored industrially at Maxwell Laboratories in San Diego, California.
Powder forging, inasmuch as it would apply to metal- and basalt-sintering options, deserves special consideration for research and nonterrestrial deployment. Powder forging is a relatively new technique able to produce more accurate parts at a lower cost than alternative methods. Unlike other processes, 1600-mesh basalt or lunar "soil" (plus plasticizer) pre-forms could possibly be forged in one operation by a single blow from a set of preheated closed dies. (For terrestrial basalts the temperature would be in the range of 1495-1515 K.) The terrestrial coining process to increase part density by reducing voids may be unnecessary in space, since vibratory or electrostatic quenching techniques may serve the same purpose to optimize forces in powders. Prior to forging, pre-forms are usually coated with graphite to prevent oxidation and provide lubrication. It is not presently known if graphite is required in the vacuum of space, since oxidation versus lubrication tradeoffs have not yet been quantified.
Rolling processes are well-suited to lunar operations, particularly when combined with the ribbon aluminum production line detailed by Miller and Smith (1979; see appendix 4D). 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. Other applications have been recently devised, including forming small gear teeth, splines, and knurl patterns. It is possible that backing pieces for the moving and stationary dies needed for thread rolling could be made of cast basalt.
Extrusion has high potential for space manufacturing, as suggested previously in connection with powder metallurgy. Conventional fabrication methods may be modified to produce lunar spun basalt using advanced automation techniques. An argument for pressurized lunar/space factories can be made if basaltic fiber manufacture is planned, since micron-diameter fibers exhibit vaporization losses under high vacuum (Mackenzie and Claridge, 1979).
A considerable amount of research and development is needed in all phases of vacuum metal extrusion operations. Little is known of dissimilar feedstock/die material cold welding effects, or of enhanced ductility. For basalt melt extrusion, studies are required to determine whether a spun product can be made from low-viscosity lunar basalt either by mechanical drawing or centrifugal spinning (see appendix 4D). Research on the following engineering variables would be useful: (1) Viscosity control; (2) speed of the winding drum; (3) duration of preload remelt; (4) chemistry of raw feedstock; (5) surface tension of melt; (6) temperature coefficient of viscosity; and (7) alternate cooling techniques (other than water). Favorability criteria driving this research include availability of basalt, availability and suitability of electrical energy on the Moon or in space for basalt processing, amenability of robots to high temperature components handling, and usefulness of the product in lunar and cis-lunar systems.
Four of the ten miscellaneous forming methods listed in table 4.17 deserve particular attention because they may be applicable to lunar or asteroid surface operations: shot-peen forming, vapor deposition, magnetic pulse forming, and electroforming. 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. (Vapor deposition and electroforming accomplish similar functions.)
Vapor deposition of both polycrystalline and amorphous silicon has been chosen by Miller and Smith (1979) as part of their design for a space manufacturing facility. Their study found deposition rates of 0.5-0.4 um/min to be a reasonable output for an energy input of 6 kW. Scaling up such procedures could result in the production of single crystal parts such as rivets or other more complex items; hence, vapor deposition provides a possible alternative to powder metallurgy. Hybrid structures, in which thin layers of vapor-deposited structures (such as mirrors) are later stiffened with basalt or basalt composites, are yet another possibility. Vapor deposition also is ideal for gossamer structures. Among the most significant products of this type which could be constructed might be solar sails (Drexler, 1980), devices in the shape of 10-ton spheres 100 nm thick and 3 km diam (see section 4.4.4).
Shot-peen forming is the method of choice for manufacturing airfoil sections with compound curves, where it is desired to form the metal leaving little residual stress. A computer-controlled shot-peen former is currently in use by Wheelabrator-Frye, Inc. of Gardena, California.
Magnetic-pulse forming could draw upon the magnetic accelerator technology now under development for lunar ore transport, as reported in the 1979 Princeton Conference on Space Manufacturing (Grey and Krop, 1979). Forming is accomplished using very intense pulsating magnetic field forces lasting only a few microseconds. Electrical energy stored in capacitors is discharged rapidly through a forming coil. (The capacitor bank currently used in the Princeton mass accelerator research program can supply 4X10$^6$ W.) In magnetic pulse forming, high-intensity magnetic fields behave much like compressed gases. The metallic workpiece can be uniformly impressed with pressures of up to 340 MN. Three basic methods of magnetic pulse forming are shown in figure 4.12.
Combined with a magnetic driving foil, magnetic pulse forming may be particularly amenable to shaping nonmagnetic superplastic metals (Mock, 1980). A new ternary eutectic of aluminum, zinc, and calcium (Alloy 08050) has been developed by the Alcan Aluminum Corporation which could possibly be pulse-formed into complex shapes. Products currently manufactured using magnetic-pulse forming technology include steering gears, drive shafts, ball joints, shock absorbers, and the assembly of vial caps, potentiometers, instrument bellows, coaxial cables and electric meters.
Electroforming is a modification of electroplating in which metal parts are deposited onto an accurately machined mandrel having the inverse contour, dimensions, and surface finish required of the finished part (fig. 4.13). Thin-wailed structures (less than 16 mm) can be fabricated using this technique, with dimensional tolerances to 2.5 um and 0.5 um surface finishes (DeGarmo, 1979). Metals most commonly deposited by electroforming include nickel, iron, copper, and silver. Mandrels may be made of aluminum, glasses, ceramics, plastics, or other materials, although if nonmetals are used the form must be rendered electrically conductive. Plating temperatures and current densities must be carefully controlled to minimize internal stresses in the formed product. The final part must be carefully removed from the mandrel if the latter is to be reused. The electroforming process is suitable for automated techniques because few moving parts are involved and the operations are relatively simple.
Electroforming is considered a promising option for lunar and other nonterrestrial applications. Extremely thin-walled products can be manufactured, and mandrels may be prepared from aluminum and sintered/cast basalt. The need for an electrolyte-plating solution requires the electroforming unit to be pressurized and, possibly, operated only in an accelerated frame. The anode plate is consumed during the forming process, but iron and titanium are widely available for this purpose. The electrolyte is recycled (except when leakages occur), and energy constraints appear minimal.
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. The major limitation is the sensitivity of these techniques to the atmospheric configuration. Production efficiency, consumable requirements, and the ratio of machine mass to machine productivity further limit the utility of machining methods (table 4.19). The most promising options currently available are grinding and laser beam machining, techniques which appear to be both useful and adaptable to the space environment.
Table 4.19.- Comparison Of Basic Machining Processes :
| Technique | Production energy,$^a$ J/kg | Consumables required,$^b$ kg used/kg produced | Machine mass/productivity,$^c$ kg/(kg/hr) |
| Mechanical milling | 2-21X10$^6$ | 1.1-3.0 | 10-1000 |
| Chemical milling | (3.1X10$^5$)$^d$ | 1.01-1.5 | 0.5-10 |
| Ion milling | 1-10X10$^7$ | 1.0-1.1 | 1000 |
| Turning (lathing) | 3.1X10$^7$ | 1-2 | 100-1000 |
| Drilling | 10$^4$-10$^5$ | 1.01-1.1 | 10-100 |
| Grinding | 10$^6$-10$^7$ | 1-3 | 100-10,000 |
$^a$Production energy = energy required/mass of product. $^b$Consumables required = mass of starting materials/mass of product. $^c$Machine mass/productivity = machine mass/(mass of product/hr). $^d$HF milling solution (concentrate) calculated from heat of formation.
Milling can be divided into three basic categories - mechanical, chemical, and ion. 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. Chemical milling is feasible only if reagents are produced from nonterrestrial materials; if not, the mass-multiplication ratio is prohibitive. Also, the efficiency and adaptability of chemical milling in high vacuum are low. Ion milling is also energetically inefficient.
Cold welding also is an inherent problem in turning operations under hard vacuum. 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.
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. Offsetting this disadvantage is the widespread availability of abrasives such as spinel (Al$_2$O$_3$) in lunar soil.
Laser beam machining (LBM), first demonstrated in 1960, may prove an extremely useful machining technique in future space manufacturing applications. On Earth, LBM already has attained "production machine" status. There are four types of laser processes theoretically available (solid-state, gas, liquid, and semiconductor), but only solid-state and gas systems are currently used in industrial machining.
Solid-state lasers employ a ruby, yttrium-aluminum-garnet (YAG), or neodymium-doped glass (Nd-glass) crystal rod that converts incoherent light from a krypton tire or tungsten-aluminum flash lamp to coherent optical radiation at a discrete wavelength. Solid-state devices are somewhat wavelength-limited (0.69-1.06 um; Yankee. 1979) at the present time, and hence are of limited utility as generalized machining tools because the material to be worked must be wavelength-compatible with the laser. Solid-state systems can be employed effectively in some metal processing applications, although efficiency is lower than t`or gas lasers (Way, 1975) and only pulsating-mode operation is possible.
Gas lasers (fig. 4.14) have discharge and zig-zag tubes filled with argon or carbon dioxide (CO$_2$) which convert incoherent optical flash lamp radiation to coherent light with a wavelength of about 10.6 um. Gas lasers are employed in continuous mode for nonmetal machining and in pulsed mode for metal machining. Since metallic substances are highly reflective at the CO$_2$ wavelength a pulsed beam (10$^{-9}$-10$^{-6}$ sec bursts; Cross, personal communication, 1980) is needed to penetrate the surface and vaporize the metal (which causes a drop in reflectivity, and enhanced energy absorption). The efficiency of metal machining with gas lasers also is not high.
Laser beam machining has a wide variety of applications in manufacturing. Indeed, some tasks can only or best be accomplished by utilization of laser techniques, such as internal welding, high-accuracy dynamic balancing, case hardening, photoetching, flash trimming, insulation and coating stripping, drilling, measurement and testing to accuracies of +/-0.2 um (Yankee. 1979). flaw detection, and impurity removal (e.e.. black carbon inclusion removal in diamonds). Still, LBM remains a micromachining technique and cannot reasonably be expected to replace bulk machining tools such as surface grinders or mills. Lasers are inherently inefficient; LBM requires a great deal of energy to machine comparatively minute amounts of material (Product Engineering. 1970; Way, 1975; Yankee, 1979). The energy of production, required consumables, and machine productivity ratios are unfavorable for bulk mass-fabrication at the present state of the art. Laser research projects funded by DOD and various military agencies have developed tunable helium-neon and xenon-fluoride lasers with relatively high (30%) conversion efficiency. The predicted peak efficiency with minor redesign, according to the developers, should approach 50% (Robinson and Klass, 1980). This is far in advance of contemporary machine shop LBM technology, which offers only 0.1-5% efficiency for solid-state lasers and 10% efficiency for CO$_2$ gas devices (Belforte, 1979). The advantage of tunable lasers is their ability to match lasing wavelength to the optimal absorption wavelength of the workpiece material.
LBM is very well suited to automated operation. Automatic laser beam machining of plastic flash already has been accomplished (Belforte, 1979; Product Engineering, 1970; Yankee, 1979), and a certain degree of automation is employed in laser welding. 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.
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 terrestrial LBM a gas jet removes vaporized material (plasma) from the workpiece. The gas jet technique is less feasible in space because it is difficult to generate gases without a great deal of energy. Fortunately, an electrostatic field probably could be utilized to carry away the highly ionized plasma, perhaps using a coil as a kind of "plasma vacuum cleaner."
The major limitation of LBM involves the production of its component parts. A solid-state laser requires a garnet, ruby, or Nd-glass crystal and a halogen, krypton, or xenon flash lamp; a gas laser requires CO$_2$ or neon gas. These materials are not easily produced in a near-term SMF. For example, 10-100 tons of lunar soil must be processed to produce enough carbon (by sublimation upon heating) for the CO$_2$ in one laser tube (Criswell, 1980; Williams and Jadwick, 1980; see also appendix 5F). Halogens, xenon, and krypton are not present in sufficient abundance on the Moon to easily produce the flash lamps (Williams and Jadwick, 1980) - at the pulse rates normally employed in solid-state lasers, flash lamp life is between 10 hr and 1 week under continuous operation. Garnet, ruby, and neodymium are not known to be present on the Moon or in space, although spinel (available on the lunar surface) might possibly be used instead of garnet. All these components must be produced in space if the SMF ultimately is to expand in a self-sufficient manner.
Joining techniques. 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; besides exhibiting poor mass-multiplication ratios due to their hydrocarbon composition, most plastics are volatile and degrade quickly when irradiated by strong ultraviolet light. Many joining techniques used on Earth, and all which appear feasible in space, are readily automatable. A detailed analysis of welding, brazing, and soldering techniques may be found in appendix 4E. A review of adhesives, fasteners and fitting technologies and their possible applicability in SMF operations appears in appendix 4F.
Welding leads to the permanent joining of materials, usually metals, through the application of some suitable combination of temperatures and pressures (DeGarmo, 1979). Approximately 40 different welding techniques have been utilized on Earth (Lindberg. 1977), the majority of which fall into one of five major categories: electric arc welding, oxyfuel gas welding, resistance welding, solid-state welding, and "electronic welding."
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. NASA has already made considerable effort to investigate these processes, including successful experiments with E-beam and laser beam welding in space (Schwartz, 1979). E-beams and laser beams are extremely versatile technologies. For example, lasers can drill, cut, vapor deposit, heat treat, and alloy, as well as weld an incredible variety of materials. High-frequency resistance and induction methods can also weld many materials with greater efficiency (60% vs 10%; Schwartz, 1979) than lasers can, though lasers and E-beam welders are capable of more precise work.
E-beam devices probably are the easiest of the electronic welders to construct in space. Major requirements include a vacuum, an electron-emitting filament or filament-plus-cathode, deflection plates, and a high-voltage power supply. Filament consumption rates range from 2-1000 hr/filament. Lasers, on the other hand, require precision-ground mirrors, flash lamp and rod (or gas and heat exchanger), etc. These parts are more numerous, more complex, and demand far greater precision of manufacture than those of an E-beam welder. As indicated in the previous section, gases needed for flash lamps in solid-state and gas lasers appear to be in short supply on the Moon, suggesting a poorer mass-multiplication or Tukey ratio. Likewise, neodymium-doped yttrium-aluminum-garnet (Nd:YAG) rods for solid-state lasers are difficult to produce from lunar resources. Both E-beam and laser-beam welders may draw tens of kilowatts of electrical energy in normal operation.
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).
Under the general classification of "adhesives" are glues, epoxies. and various plastic agents that bond either by solvent evaporation or by bonding agent curing under heat, pressure, or with time. The recent introduction of powerful agents such as "super-glues" that self-cure permits adhesive bonds with strengths approaching those of the bonded materials. Epoxies are combined with metallic and nonmetallic fibers to form composites. Use of such materials, whose strength-to-weight ratios equal or exceed those of many metals, will perhaps constitute the primary application of adhesives in space.
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; however, some of them, once cured, are vacuum compatible. The planned early use of composite beams for space construction requires that such compatible bonding agents be available. (Actual use of these agents may need to be under atmosphere.) Many hydrocarbon-based glues weaken under the influence of radiation, and more research is required to develop radiation-resistant adhesives and bonding agents. The unsatisfactory Tukey ratio for current carbon-based adhesives is one of the major hindrances to their use in the long run. Manufacture of composite structural parts from nonterrestrial materials and the possibility of silicon-based bonding agents offer the promise of dramatic increases in mass-multiplication for nonmetallic bonding agents.
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. They are semipermanent in that they may be undone for some purpose such as repair. Nonpermanent fasteners include quick-release clips and clamps meant to come off at a specified time, and pins which allow relative movement of fastened parts. Pins are used where movements are not as rigidly constrained, as with bearings.
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.
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. Surface poisoning or the use of incompatible metals would be required for breakable bonds.
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. 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. Ultrafine threads can be produced in space because the zero-g conditions enhance controllability of the extrusion pull rate.
The possibilities offered by metal and basalt threads (see section 4.2.2) and the comparatively unsophisticated character of fabric-stitching, rope-, and cable-making equipment promise exceedingly low Tukey ratios for these processes. The high-radiation and vacuum environment of space precludes the use of many terrestrial thread materials because of volatility and susceptibility to radiation deterioration. Basalts and metals appear capable of filling this applications gap. Lunar iron can be used to manufacture threads, strings, ropes and cables; Moon-like basalts already have been spun into 0.2-4.0 um fibers (an established commercial process). 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. The Space Activity Suit (Annis and Webb, 1971), for instance, maintains pressure by tension rather than by retaining a cushion of air.
Shrink fitting is accomplished by heating one piece so that a hole in it expands to accept (usually under pressure) another piece within that hole. Contraction with cooling then locks the two together. Press fitting is a related process requiring higher pressures but no heat. These two techniques are prime candidates for space assembly operations. Because no additional materials are employed, only power is consumed. Both processes are far more energy- and material-efficient than welding, and produce strong bonds. Beams made from rigid materials and many parts can be joined this way. (For example, gears are routinely attached to shafts by shrink fitting.) No bonding agents are required, and the parts materials (metals) are abundant in space. Zero-g permits lower-energy/lower-strength bonds. Shrink or press fitting is preferable to welding for light bonding; however, vacuum welding may provide added strength. Metals and other conductors may be heated by induction techniques, making possible an extremely high mass multiplication .
4.3.2 Summary of Analysis of Production Options for Space
The survey in section 4.3.1 provided necessary background information for selection of processes which are especially appropriate for nonterrestrial materials utilization, summarized in table 4.20. All major manufacturing categories (casting, molding, deformation, and joining) are represented by at least five techniques. Containerless processing, with many potential applications for space, is an entirely new category possible only under zero-g conditions.
Table 4.20.- Manufacturing Processes Applicable To Space :
| Based on terrestrial experience ||
| Preferable | Usable with recycling or adaptation |
| Casting ||
| Permanent Centrifugal Die Full-mold Low-pressure Continuous | Sand Shell |
| Molding ||
| Powder metals and ceramics |
| Deformation ||
| Thread rolling Magnetic pulse forming Electroforming (basalt electrolyte) Rolling - reversing mill | Forging (with electrical drives) Lead-in mill Extrusion (basalts) Spinning (glass and basalt) |
| Machining$^a$ ||
| Laser Electron beam | Turning (basalts) Drilling (basalts) Grinding (recycle binder, using Al$_2$O$_3$-grit) |
| Joining ||
| Cold/friction welding (metals) Laser-beam welding Electron-beam welding Induction/HF-resistance welding Fluxless/vacuum brazing Focused solar energy Metal fasteners (permanent) Stitching (metal or inorganic threads) Staples Shrink and press fitting | Metal fasteners (need fusion preventers) Glues(need carbon) |
| Containerless ||
| Surface tension Fields: E&M, Centrifugal, Gravity gradients Direct solar heating (differential) Vapor deposition | Metal and/or ceramic clays (binder loss) |
| Contaminants ||
| Powder/slab - cold welding Foaming (metals/ceramics) | Metal and/or ceramic clays (binder recycling and loss) |
$^a$In a vacuum environment most machine techniques will require a pressurized container to prevent cold-welding effects.
As previously noted, these techniques were chosen because of their advantages with respect to the selection criteria given in table 4.18. It is anticipated that the R&D necessary to adapt the techniques to useful productive tasks in space will be significantly less than that associated with processes where development must await investigations of a fundamental nature or more extensive space operations (either unmanned or manned). It should be possible to incorporate the consequences of the earliest possible applications of these techniques in space to the planning of space operations in the mid-1980s and beyond.
Table 4.21 summarizes 12 generic functional components required for space production of devices or products which could be manufactured by the techniques listed in table 4.13 using lunar-derived materials. (A brief discussion of these components appears in section 4.4). All functional elements except #9 (glasses) and #12 (lasing media) can be made directly by adaptations of powder metallurgy-based "starting kits." These two items would require the creation of derivative or second-generation production systems.
Table 4.21.- Functional Components Required In Nonterrestrial Manufacturing And Available Materials :
| Functional components | Materials |
| Structures | Metals (Fe, Al, Ti, Mg) Ceramics/glasses/basalts Reinforced materials |
| Refractories: molds, orifices | Major lunar minerals Chromia, titania, titanium silicide, glasses |
| Dies | Steels (C, Si, Ni, Co) Silica carbides |
| Heaters: direct solar, electric$^a$ | Mirrors (Al and/or inorganic shaped materials) Si (and others) solar cells |
| Insulation (electric and thermal) (glass fiber mattes) | Basalts, ceramics, inorganic fibers, glasses Soil, wools, foams - inorganic |
| Magnetic material (motors, separators) | Iron and alloys Magnetic ceramics |
| Electrical conductors (motors, electromagnets, control circuits$^a$) | Al, Fe, Ca - low temperature |
| Grinders | Spinel in glass matte/Ca wheels |
| Glasses (optics)$^a$ | Si, SiO$_2$ (+ mixes of major and minor elements) |
| Adhesives and coatings | Metals, ceramics |
| Lubricants and fluids$^b$ | Sulfides, SO$_2$ (trace CO$_2$, H$_2$O, and compounds of K, O, N, Na, H) |
| Lasing media$^{a,b}$ | CO$_2$ |
$^a$These specific products require second-generation or higher-generation production hierarchies. $^b$This component is a major problem because it requires chemical elements which are rare on the Moon.
The team did not reject the use of the nearly 200 manufacturing procedures listed in table 4.10 for eventual use in space. However, most of these options require special support (e.g., supplies from Earth, special atmospheric conditions) or generally are low-ranked by the criteria in table 4.18. Flexible techniques such as provided by a terrestrial machine shop may be feasible and even necessary during future development of growing space industrial operations, but appear less fruitful to implement in the near-term.
In any event, a number of manufacturing options apparently exist that are sufficiently adaptable to the SMF mission, and a growing hierarchy of materials processing and manufacturing systems, in principle, is possible. Section 4.3.3 considers a subset of the general hierarchy in table 4.20 which appears to offer virtually a one-step method for manufacturing most of the devices of production (and other products) from both native-lunar and refined-terrestrial feedstocks. Section 4.4.1 examines near- and mid-term development of an expanding manufacturing complex in LEO.
4.3.3 Starting Kits
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. Most likely many such kits can be created, their designs strongly influenced by the materials available locally for manipulation.
The present effort focused on the handling of metals and ceramics known to be available from lunar or asteroidal materials, or potentially importable from Earth at low unit cost. No attempt was made to produce conceptual systems able to operate in the hydrocarbon-helium atmospheres of the outer planets and their moons, or in the sulfur-rich atmosphere of Venus or surface of Io. One major approach to starting kits suitable for near-term space manufacturing useful on the Moon involves powder metallurgy. This case was examined in some detail to help clarify the concept. Another approach using large blocks of metal was also briefly considered.
General comments on powder metallurgy and space. An extensive discussion of the development of powder metallurgy appears in appendix 4C. Powder metallurgy appears to offer several basic advantages for space manufacturing. Virtually all the energy for powdering metals, glasses, and possibly ceramics, can be provided by direct solar thermal power. 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.
It should be possible to achieve 90% or better of the ultimate powder density in "green" compact parts prior to final forming, if made under low-g conditions. This is because, in the zero-g operating environment of the SMF, very fine grains of the appropriate size and shape distributions could be placed in the void spaces between larger grains. On Earth this cannot be done reliably, since gravity causes smaller grains to settle toward the bottom of the green compact, producing parts of irregular density, composition, and strength (proportional to final density).
On Earth, large presses, sometimes also operating at high temperatures, are required to squeeze the parts to 99% or more of final density from original densities of 70-90%. Major changes in physical dimensions may occur. It is conceivable that the need for such pressing operations can be eliminated almost entirely for many products and the changes in physical dimensions between green compacts and final product largely avoided by using either direct sunlight or electric heating in space for forming final parts. If very dense green compacts of near net-shape can be prepared then final parts should require minimal cutting or trimming which makes the use of laser or electron-beam devices in final shaping conceivable. Such devices are presently relatively inefficient for materials removal but are capable of very fine-tolerance operations.
Much terrestrial experience is available on powder technologies applicable to both metallic and nonmetallurgical materials. Many of the experiments necessary to adapt this technology to space could be performed in early Spacelab missions. In addition, there can be strong interaction among designers in the planning of parts derived from powders (e.g., overdesign size of parts for additional strength) and the evolution of in-space production techniques.
Impact molder system for production from powders. Figure 4.15 illustrates the impact molder powder process starting kit which consists of a powder/liquid injector (7) and a two-dimensional die (2) enclosed in a scatter shield (3) The shield prevents grains which are misaimed or which do not stick to the working face from drifting out of the production area. Wasted grains can be removed and eventually recycled. The injector directs particles (8) sequentially across that portion of the working face (1) of a part which needs building up, continuously adding thickness as desired at any particular point. Insertable shields can be used to create voids and produce internal patterns (not shown). Metal grains are cold-welded at the instant of impact and coalesce by cooling. Size-distribution management of injected metal powder particles should make possible parts of minimum porosity (i.e., no greater than 3-5%). Vapor-deposition techniques might be useful in decreasing the porosity still further.
The developing workpiece is actively inspected by scanning electron microscopes or optical sensors (5) which guide the beam to areas where the surface is rough, appears too porous, or has not adequately been filled. Beam crosssection is fixed by the interior shape of the ceramic die. This die can be made by a casting process or by cutting out blank disks. Rollers or other grippers (4) slowly extract the workpiece from the die as it is formed. A starting surface (6) must be provided upon which powder forming can begin and to which extraction devices may be attached.
After formation, parts move to an inspection station for final trimming by a high-energy laser (which exerts no force on the workpiece) or other cutting device. If necessary, pieces are sliced perpendicular to the formation plane to produce more complex parts than can be manufactured directly from the die. It should be possible for a precision, low-mass robot to hold pieces for final trimming. Final choice of finishing tool depends on the tolerances achievable in parts formation as well as tool efficiency.
The impact-molder system produces rodlike components in the first operation of the procedure. It should be possible to build more complex parts by repositioning rod components perpendicular to the die (2) and using the side of the finished part as the starting point for appendages. The process can be repeated as often as necessary so long as access to the die mouth is possible.
Throughput varies depending on the velocity of scanning beam material, number density of particles, mass of individual particles, and cooling rates obtained at the casting die when powders are used. Parts which can tolerate large porosity prior to sintering possibly may be produced at the rate of 1-10 kg (of machinery)/kg-hr, Parts demanding low initial porosity (less than 5%) and very high tolerances must be composed of a wide range of grain sizes, and smaller grains must be placed most precisely by the ejector. The anticipated production rate of these parts is 0.01 kg/kg-hr or less.
Several different injection systems may be used depending on the velocity and mass of the grains to be accelerated. More massive particles must be emplaced by mechanical ejectors, perhaps to be operated by electric motors. Smaller particles (less than or about 1 um) may be propelled by precision electrostatic systems. Deposition rate M (kg/hr) is of the order M = fpvA, where f = filling factor of the beam, p = density of input metal (taken as 5000 kg/m$^3$), v = injection velocity, and A = injection nozzle area (assumed 1 mm$^2$). If the reasonable values f = 0.1 and v = 100 m/sec can be obtained, then M = 180 kg/hr. Specific input power P (W/kg) is given by P = 1/2 pfAv$^3$ = Mv$^2$ hence P = 500 kW/(ton/hr) in the above example. Equipment mass is dominated by the ejector electrical supply (at v = 100 m/sec), suggesting a total system productivity of about 5 ton machinery/(t/hr product) and assuming a solar array with specific power rating 10 ton/MW. Note that M scales with v whereas P scales with v$^3$ - at early stages of production it may be advantageous to operate at low ejection velocities and accept the implied lower throughputs. These estimates are significantly lower than those for mechanical milling - about 2 MW/(ton/hr) and more than 10$^4$ ton/(ton/hr) given in table 4.19.
Most of the energy required for the powder-making process can be supplied as direct focused sunlight by systems with intrinsic power of 300 MW/ton. Thus, the solar input subsystem represents a small contribution to the total mass of the powder processor. Little material should be consumed in the production process, with die wear dominating losses.
One major disadvantage of this approach is its primary applicability to production of metal parts or metal-coated ceramic parts. Most other materials must be passively restrained during the sintering process. Parts appropriate to the preparation of ceramics or fused basalts or other nonmetallic materials require the creation of a subsequent set of tools for the construction of ceramics and basalt manufacturing facilities.
There are several areas for applications of robotics and advanced automation techniques in production, process monitoring and parts handling. Process monitoring is required in powder preparation, sorting, storage, and recombination. Very high speed monitoring is necessary at the impact surface of the part under production, especially if a wide range of grain sizes is needed to reduce porosity. Many options for such monitoring that will include active means (e.g., scanning electron beams, sonar interior scanning, radiation transmission measurements) and passive means (e.g., optical examination, temperature) must be examined. In effect, machine intelligence is applied at the microscopic level of the materials handling process. Very detailed analysis of macro-handling of parts is necessary, including such operations as extraction, moving parts in physical space without impacting adjacent objects, parts repositioning for trimming, cutting, or sintering, and monitoring the effects of these operations. Finally, parts are passed to assembly robots or automated lines. Many of the procedures are extensions of present technologies of automatic transfer in terrestrial practice. However, there will be far more emphasis on reliability, scheduling, flexibility, and repairability.
Metal- and ceramic-clay-based starting kit. According to Jones (1960), the concept of manufacturing metal objects from powders formed into clays using spinning or sculpting techniques is a very attractive one. This is true especially if it is possible to avoid drying out periods and obtain high densities with relatively brief sintering times. Binders are feasible for Earth applications - polystyrene and polythene in particular, each of which is recoverable and nonreactive with the more common metals, and both are suitable for the production of clay-like metal masses. While such recyclable organic binders may be useful in space and on the Moon, certainly it would be more advantageous to obtain binders from local sources. Desired characteristics include the following:
* The binder should impart a stiff clay-like quality to the metal or ceramic mass and permit easy manipulation, have a sufficiently low volatility under the desired working conditions to allow a reasonable working period, and leave no residue following the completion of sintering.
* The binder should not require removal prior to placing formed clay into the sintering oven, but should not disrupt the molding during volatilization.
* The rigidity of the molding should be maintained during the early phase of sintering.
* The binder and its solvent (if needed) should not react chemically with the powder either at working or elevated temperatures, nor should they attack furnace components or elements of the recovery system.
* Binder and solvent should be nontoxic under the working conditions in which they are used.
Table 4.22 identifies several binders appropriate for use on Earth. The last compound listed is preferred on the basis of slow evaporation rate, high boiling point, and high flash point. Thermoplastic binders such as polybutene dissolved in xylene with a hydrocarbon wax, or ethyl silicate, are other possibilities. These are introduced into molding furnaces at moderate (430 K) temperatures and have permitted the successful molding and sintering of small objects. Unfortunately, workpiece rigidity is insufficient for terrestrial manufactures bigger than 5 cm; larger items tend to slowly collapse at room temperatures. Clearly, bigger parts could be made on the Moon, and there is no serious limit on the size of objects which could be sculpted in space.
Table 4.22.- Metal/Clay Binders Appropriate For Terrestrial Use :
| Binders | Boiling range, °C | Flash point, °C | Evaporation rate$^a$ |
| Methyl amyl acetate | 143-150 | 110 | 47 |
| Ethylene glycol diacetate | 186-195 | 205 | 2 |
| 2-ethylhexyl acetate | 195-205 | 190 | 3 |
| 2-methoxyethyl acetate | 137-152 | 140 | 31 |
| Ethyl benzene | 134-137 | 85 | 91 |
| Carbitol acetate | 213-223 | 230 | <1 |
| Decahydronaphthalene | 190-200 | 160 | 10 |
| Tetrahydronaphthalene | 203-220 | 185 | 1 |
$^a$H-butyl acetate = 100
Binders in space may be able to function in two additional ways. First. the compounds may be selected to inhibit contact welding between grains to facilitate the greatest packing of voids by filler grains. Second, initial binder evaporation could expose surfaces to permit preliminary contact welding prior to full sintering of the part. An extensive literature search should be conducted to determine whether or not such compounds can be derived from lunar and asteroidal materials. Lee (1979) has suggested several liquid silicon-based and Ca-O-Al compounds that could be derived predominantly from lunar materials. Perhaps such fluids (for which recovery is not as critical) could be adopted for vacuum forming.
The powder metallurgy approach to manufacturing has considerable potential in nonterrestrial low- or zero-g applications. There is virtually a complete separation of the three basic stages of production: (1) creation of working materials (high energy), (2) embodiment of a design into a mass of clay to form a part, and (3) hardening of the part by contact welding and sintering. Very complicated designs can be produced by machines able only to apply relatively small forces, allowing considerable quantities of mass to be formed for very little energy but potentially with high precision.
Figure 4.16 illustrates three techniques for pattern impression. One possibility is to inject the clay into a mold. This mold may be very intricate provided it is sacrificed after sintering, a modest penalty because of the low initial temperatures. Second, clay could be packed around "melt forms" (recoverable from the vapor) to make pipes, conduits, and other structures with internal passages. Third, parts could be sculpted directly from masses of clay. These masses could be initially amorphous or might be preshaped to some extent by molds or spinning techniques as in the manufacture of pottery on Earth.
Advanced automated pottery techniques are not limited to the production of metal parts because sintering is used in the final stage. For instance, metal and ceramic parts could be interleaved in the clay stage to produce, say, electrical machinery. In such applications the porosity of the different ceramic and metal powders in the various portions of the respective clays is carefully controlled so that differential expansions and contractions during the formation process do not ruin the part. In addition, hollow metal grains would permit local metal volumes to decrease under planned stresses as necessary during the sintering process. Conceivably, this could allow very complicated metal paths to be melted directly into the body of a ceramic material having a much higher melting point and also to produce exceedingly complex composites.
It is interesting to speculate on the ultimate limits of the above techniques with respect to the size and complexity of the final object. 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. Perhaps. in the ultimate limit. major mass fractions of spacecraft, space stations or habitations could be manufactured in monolithic units by this process.
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. Indeed. one of the most important characteristics of starting kits is the easy automatability of the tools involved.
In the basic kit, forming and shaping functions of the fabrication robot are farthest from deployable state of the art. But tools and techniques have been chosen that can generate a wide variety of products of differing complexity using relatively few simple modes of operation. These starting kits could be deployed in the near-term as part of a fault-tolerant, easily reprogrammable prototype SMF.
Macro-blocks and contact welding. It is conceivable that many useful tools and products, especially very large parts, could be quickly manufactured from metal blocks of various sizes. The same or similar metal blocks with clean surfaces will cold-weld when pressed together with sufficient force. One problem with this approach is that pressures in excess of 10$^7$ Pa may be required even for blocks with extremely smooth surfaces, making large powerful presses impractical in the early phases of an incremental space industrialization program. One possible solution is to manufacture a very fine "dust" of hollow particles of the same metal as the pieces to be joined. Dust particles should have approximately the same radius as the asperities of the large blocks. This "dust" is then evenly distributed over the contact surface of one of the pieces to which it would adhere by cold welding and the second piece is pressed upon it. Joining pressure need only be sufficient to flatten the hollow spheres, permitting them to flow into and fill voids between the two macrosurfaces. Electrical current passing across the gap between the blocks could heat the dust and further promote joining.
This approach to construction would allow the use of a small number of furnaces and molds to produce standard sets of blocks from appropriate sources of metals. The blocks could then be contact-welded to manufacture a wide range of structures. While such blocks would not allow detailed flexibility of design as might be permitted by the two powder metallurgy systems described earlier, the throughput of the system for the construction of large repetitive objects would likely be significantly higher. A major potential difficulty requiring far more study is the degree of smoothness necessary prior to joining and the precise size distributions of hollow powders used to fill the gaps between the blocks. This may limit the maximum size of blocks which can be joined with minimal preworking.
Starting kit technology development. Sufficient knowledge exists with respect to powder metallurgy, space operations in LEO and on the lunar surface, and about lunar materials near the Apollo landing sites for development of starting kits to begin. Naturally, the relevant concepts should be fully reviewed by experts in the respective fields. These reviewers might also define key experiments and tests necessary for convincing near-term demonstrations (see section 5.6 for a useful relevant methodology). For instance, it would be useful to demonstrate (perhaps in low-g aircraft or sounding-rocket flights) the sintering of multisized powders which are well-mixed prior to sintering. Detailed consideration should also be given to the design of subsequent components by conceivable starting kits.
Demonstration of the full capabilities of contact welding may not be possible from Shuttle-supported facilities in LEO without incorporating a molecular shield into the mission and performing the key tests beyond the immediate vicinity of the Shuttle. Even at LEO there is sufficient ambient gas (e.g., highly reactive atomic oxygen) that surface contamination may be significant. However, LEO experiments should be able to show the full potential of powder techniques with respect to powder forming using solar energy, zero-g, and green mold densification, final product sintering or fusing using solar energy, and working with metallic/ceramic clays in space including binder recovery techniques.
The powder approach possibly may be useful on the lunar surface. Fine-grained (1-10 um) metallic iron is present in lunar soils to 0.1% by weight. This metal can be extracted magnetically and separated from adhering glass and minerals by direct heating. Such iron may be used as a structural, electrical, or magnetic engineering material. Various other lunar soil components can be used for structural and insulating purposes. Hence, it appears possible to effectively utilize native iron using little more than a thermal processing technology capability. If so, then the "starting kit" approach can be employed to create much larger iron-processing facilities on the Moon over a period of time by "bootstrapping" what is essentially a very simple system.
Chapter 5 of this report explores the initial deployment of "starting-kit-like" devices capable of self-replication as well as growth.