Advanced Automation for Space Missions/Chapter 4.2.2
4.2.2 Extraction and Materials Processing Alternatives
The development of material processing techniques suited to nonterrestrial conditions is absolutely essential if the proposed SMF growth scenario is ever to take place. Studies have been conducted on the gathering of lunar materials for use in situ and elsewhere (Criswell (see Carrier), 1980; Fields and Weathers, 1967). Ultimately, SMF output must be fabricated from feedstock derived from lunar, asteroidal, or other space materials. The production of such diverse components as lubricants, coils, semiconductor chips and structural components requires a versatile and efficient raw material processing capability. Furthermore, this processing system must be fully automation-compatible. Mass multiplication is one key consideration in a growing space-processing facility. Every effort should be made to minimize both the quantity of processing materials brought from Earth per unit of nonterrestrial products, and the mass of the capital equipment (both terrestrial and nonterrestrial) per unit of output per unit of time. It is desirable for the fraction of all such terrestrial material supplied per unit of output product, called the "Tukey Ratio" (Heer, unpublished draft notes of the Proceedings of the Pajaro Dunes Goal-Setting Workshop, June 1980), to approach zero as deployment and growth proceed - or, alternatively, for the mass multiplication (referenced to Earth-originating materials) to approach infinity .
Another important aspect of SMF design is the ability of the primary processing equipment to accept a wide range of input materials, thus minimizing the need for intensive and extended exploration and characterization of source materials. It appears that this approach already may be possible for the explored regions of the Moon due in part to the limited variety of lunar materials and glasses (Waldron et al., 1979). Additionally, mass multiplication factors in excess of 100 can be anticipated for one or more proposed lunar materials processing schemes (Criswell, 1978, 1979). As on Earth, a continuing tradeoff between availability of primary materials, processing options, and substitution of materials can be expected. Systems designed for the Moon might not be appropriate for Mars, an iron asteroid, or Titan. Still, most of this section describes silicate minerals processing as these are the dominant components of lunar soil and seem likely to be representative of the composition of many asteroids, Mercury, and the moons of Mars. Since the Solar System offers a much wider range of compositions and conditions, many alternative types of manufacturing facilities may be expected to evolve, many of which may eventually prove useful on Earth.
Chemical extraction techniques. The first most important component of the SMF is the chemical processing facility. The ultimate success of the space manufacturing venture hinges upon the ability to process nonterrestrial materials without importation of terrestrial reagents. This task is further complicated by the additional requirement that the processing capability grow at a rate equal to or greater than the overall growth rate of the SMF. The applicability of a number of established chemical engineering technologies to the processing of low-latitude lunar materials, including (1) carbothermic reduction, (2) carbochlorination, (3) electrolysis, (4) NaOH treatment, and (5) HF acid leaching, has been suggested (Waldron et al., 1979).
In carbothermic reduction anorthite is broken down and refined. The aluminum oxide reacts with carbon to produce useful metallic aluminum and carbon monoxide (Phinney et al., 1977). The thermodynamics of this process requires that the processing vessel be maintained at 2400 K. High-temperature condensates such as SiC, Al4C3, and Al4O4C are present, along with the gases Al2O, SiO, Al, and Si. These are likely to prevent the key reactions from achieving equilibrium (Waldron et al., 1979).
In the carbochlorination process, titanium, iron, and aluminum are refined from anorthite and ilmenite by reaction with carbon and chlorine (Rao et al., 1979). This process does not require high reaction temperatures. However, chlorine recycling involves very massive equipment (Waldron et al., 1979).
Electrowinning of aluminum from anorthite powder dissolved in a mixture of alkaline earth chlorides at 75 K has been considered (Criswell (Das et al.), 1980). This approach requires only a moderate amount of energy.
Iron and titanium can be refined from ilmenite by treatment in molten NaOH (Rao et al., 1979). TiO2 is soluble in NaOH, unlike Fe2O23, and thus the two compounds can be separated and refined. High temperatures (1000-1300 K) are necessary for this process.
Lunar soil may be broken down into its elemental constituents by the HF leaching technique (Waldron et al., 1979). This process begins with the dissolution of lunar soil in a heated HF solution, followed by a series of steps including ammonium salts fusion, silicon hydrolysis, metal oxide production, acid recovery, fluoride hydrolysis, ion exchange and platable-metals separations, precipitation and crystallization, and metal oxide reduction.
Most of the reagents used in the above processes are rare on the Moon compared to the known average lunar composition. Thus, recycling and leakage must be regarded as critical problems. Thermal dissipation is another major problem because many techniques involve exothermic reactions which generate heat that is difficult to dispose of due to the unavailability of direct conductive cooling in space. HF acid leaching appears to be the most promising for interim processing and short-term growth of the SMF. More (valuable) elements can be extracted in this way than any other process studied to date. However, while the HF process appears quite efficient there are several potential pitfalls associated with the deployment of an acid leach system. HF usually is stored in polymer containers because it dissolves most metals and all silicates. Such polymers cannot easily be derived from lunar soil. Containerless reaction technology cannot be employed because of the sublimation problem. Possibly etch-resistant solid silane containers could be developed, but these would have to be maintained at 75 K or colder, resulting in prohibitively sluggish reaction rates. Yet another potential problem is leakage. The numerous steps involved in the HF acid technique significantly increase the likelihood of accidental loss of vital process fluids.
It is important that the reagents, plumbing, and containment vessels for the chemical processing plant eventually be produced from nonterrestrial materials - importation of these commodities is not feasible if the long-term growth rate is to be exponential. As to the first of these necessities, calculations by Freitas (1980b), based on an HF leach factory module capable of processing roughly 4000 t/yr of lunar soil, indicate that sufficient hydrogen and fluorine can be produced to allow replication of the required reagents. The calculations assumed 95% recovery of hydrogen and 50% recovery of fluorine due to leakage, which may be too optimistic. On the other hand, these limitations may be offset by discoveries of richer sources of hydrogen (e.g., Arnold, 1980) and fluorine on the Moon or by changes in physical-to-chemical processing ratios. It appears that at least short-term growth of SMF capability is possible with the use of HF acid leach extraction. The remaining problems of producing plumbing and containment vessels from nonterrestrial materials appear insoluble at present; however, importation of polymeric plumbing and make-up reagents is feasible for short-term growth.
The methods discussed above are well-suited to short-term nonexponential SMF growth. Table 4.12 summarizes the recommendations of a recent workshop on silicate and other lunar-like minerals processing (Criswell, personal communication, 1980). New processing methods which do not require aqueous solutions or reagents composed of rare nonterrestrial elements might help to achieve a long-term self-sufficient, exponentially growing SMF (Grodzka, 1977). Possible new avenues of research may include silicon- and oxygen-based processes, advanced zone refining or fractionation techniques, induced immiscibility in melts, and rapid controlled-crystal-nucleation methods.
Electrophoretic processing. An important initial step in the generation of new processing options for dry, granular materials found on the Moon is the development of an effective mineral separation or primary beneficiation process. If the primary materials of interest for a particular refined product.(such as lunar anorthite plagioclase for aluminum and silica) can be isolated, then the problem of developing a self-sufficient chemical beneficiation process is far less difficult (Rao et al., 1979).
Every chemical processing option for beneficiating lunar soil suggested to date requires chemicals that are relatively scarce on the Moon. Some of these options may demand high levels of automation not presently available. It is therefore desirable to develop new processing options that can be expanded with little or no importation of terrestrial materials and that are either self-automated or automation-compatible. A promising new primary beneficiation technology opportunity appears to be electrophoretic separation, a one-step, self-automatable technique (Dunning and Snyder, 1981).
Electrophoresis is defined as the transport of electrically charged particles in a direct current electric field (Pier, 1973). The movement occurs as a result of the electrostatic potential between the layer of ions adsorbed from the suspension medium onto the surface of particles and the bulk suspension medium. The layer of adsorbed ions is called the "Helmholtz double layer" or the "electrical double layer." It consists of the potential determining layer (the surface of the particulate material), the Stern layer (the layer of adsorbed ions from the atmosphere), and the Guoy layer (the bulk fluid) (Pier, 1973; Jungerman, 1970). The electrophoretic potential is defined as the electrostatic potential between the Stem layer and the bulk fluid. If the electrophoretic potential is positive or negative, a particle moves towards one of the electrodes in the system. The direction of movement depends on the relative charge signs of the particle and the electrode, and the velocity is a function of the magnitude of the electrophoretic potential. If the potential of a particle is zero (the isoelectric point), particles remain stationary and suspended. Electrophoretic separation depends on differential migration rates for particles in the bulk suspension medium (although electrode-reaction electrophoresis is employed for electroplating). The major requirement for successful beneficiation is that the particulate matter be sufficiently fine-grained to remain suspended in the bulk medium. The ideal grain size for geologic materials is 25-60 um (Westwood, 1974).
Electrophoresis has been used by physiologists and biologists since the 1930's as a tool for separation and identification of enzymes, proteins, lipids and blood cells. Tests were performed on blood cells during the Skylab and Apollo-Soyuz experiments with good success (Henderson and Vickery, 1976; Schoen et al., 1977), and the electrophoretic phenomenon has been utilized as a terrestrial separation technique for clays and limestones.
Of the numerous electrophoresis technologies only a few are suitable for geologic materials. One technique - high-voltage zone electrophoresis -- is particularly well-suited to lunar soil separation because it is a one-step, self-automated separation method. Typically, a tank is filled with suspension medium into which two electrodes are inserted. Filter paper is mounted on both electrodes. When an electric field is applied, mineral particles move toward the filter paper and are trapped in various positions along its length. Each mineral phase migrates to a discrete area depending on the magnitude and sign of the electrophoretic mobility. These phases then may be removed in a single, simple automated step.
Lunar soil is ideally suited to electrophoretic separation. Average grain size is 40 um (Williams and Jadwick, 1980), well within the optimal range cited earlier for geologic materials. This grain size distribution is also very poorly suited to conventional mineral separation techniques involving electrostatic or electromagnetic (cf. Inculet and Criswell, 1979), flotation, or density characteristics. The low gravity of the Moon and the absence of gravity in space should be extremely beneficial to the electrophoresis process because settling is either minimal or nonexistent (Henderson and Vickery, 1976; McCreight, 1977; Saville and Ostrach, 1978; Vanherhoff and Micale, 1976; Weiss et al., 1979). Electrophoretic separation of minerals is only moderately temperature-dependent, thus eliminating another source of potential difficulty (Bier, 1978). Finally, the isoelectric points of lunar minerals have enough variation to ensure extremely efficient separation. A few of these values are tabulated in table 4.13.
|Mineral||Aqueous isoelectric point (pH)||Source|
|Spinel (MgAl2O4)||9.1||Bloom and Gutmann, 1977|
|Hydroxyapatite (Ca5(PO4)3OH)||7||Bloom and Gutmann, 1977|
|Fluorapatite (Ca5(PO4)3(FOH))||6||Bloom and Gutmann, 1977|
|Rutile (TiO2)||5.8||Bloom and Gutmann, 1977|
|Fayalite (Fe2SiO4)||5.7||Feuistenav and Raghaven, 1978|
|Olivine ((Mg FE)2SiO4)||5.7||Feurstenav and Raghaven, 1978|
|Hematite (Fe2O3)||4.8||Bloom and Gutmann, 1977|
|Forsterite (Mg2SiO4)||4.1||Feurstenav and Raghaven, 1978|
|Clinopyroxene (Ca, Na, Mg, Fe2, Mn, Fe3, Al, Ti(SiAl)2O6)||2.7||Feurstenav and Raghaven, 1978|
|Anorthite (Ca(AlSi3O8))||2.4||Feurstenav and Raghaven, 1978|
|Albite (Na(AlSi3O8))||1.9||Feurstenav and Raghaven, 1978|
Suspension media options are a major research area in the development of lunar electrophoretic separation technology. Aqueous solutions commonly are used for bulk suspension due to the availability and ionization potential of water. For this reason, isoelectric points customarily are defined in terms of aqueous pH. Carbon tetrachloride also has been used as a high-voltage zone electrophoresis medium. Aqueous and carbon tetrachloride suspensions may be impractical for lunar separation facilities because of the relative scarcity of carbon, hydrogen, and chlorine on the Moon. Further, leaks in the system would be devastating if all major reagents must be imported. Some means must be found to thoroughly dry the output stream and to return these fluids to the bath. Alternative bulk media derived wholly from lunar materials might possibly be devised; for instance, silane or low-temperature basalt slag suspension fluids. The problem is hardly trivial, though it appears to present no fundamental insurmountable technological barriers.
Using high-voltage zone electrophoresis, only one medium is needed for a wide range of minerals. Other techniques require the ionic concentration of the operating fluid to be varied to match the isoelectric point (expressed in activity or concentration of a particular ion analogous to aqueous pH) of the desired mineral for each electrophoresis cell. This seems an unnecessary complication.
Other problem areas include fused mineral grains and iron coatings. Fused mineral grains, which are relatively common.in lunar soil (10-20% by volume, Criswell, personal communication, 1980), are not amenable to electrophoretic separation because the isoelectric points are ill-defined. This may actually be beneficial since only pure mineral grains will be separated, thus eliminating the need for additional more complicated separation techniques. Iron coatings on mineral grains caused by "sputtering" also may be present in lunar soil. If coatings are thicker than about 30 nm, efficiency of the electrophoretic process decreases. Fortunately, the very existence of these coatings is open to some question, and there is no evidence at present that they are thicker than 10 nm. Also, if the coatings do not entirely cover the grain surfaces the problem of lessened electrophoretic activity is significantly reduced.
Electrophoretic separation appears highly adaptable to automation. The process itself is largely self-regulating and the collection of separated minerals appears to be a trivial robotics task. An automated biological electrophoresis system already has been designed and is under construction (Bartels and Bier, 1977).
An automated high-voltage zone electrophoretic separation system for lunar materials might require a large tank with two electrodes and filter paper (perhaps comprised of spun basalt fibers) suspended between them. The tank would be filled with some liquid medium closely matching the isoelectric point of a particular mineral of interest. After insertion of lunar soil a direct current electric field is applied to initiate separation. Grains of the mineral whose isoelectric point has been selected plate out near the center of the paper, the other minerals in discrete bands nearby. Individual mineral species are then extracted by robot scoops as the filter paper rolls continuously through the tank.
The proposed automated mineral separator consists of an input port, a suspension tank, two electrode cells, a bond of basalt fiber filter paper, a spectral scanner calibration unit, robot extraction scoops, and repository bins. These components are illustrated in figure 4.1. The sequence of automated operations, as suggested by figure 4.2, is roughly as follows:
- Lunar soil is introduced via the input port into the suspension tank.
- Lunar soil goes into suspension and begins to separate and move towards the electrodes.
- Individual mineral species move towards the electrodes along paths and with velocities which are a function of their electrophoretic potential.
- Various mineral species are trapped and plated onto a bond of filter paper continuously rolled through the suspension tank. The paper is connected to both electrode cells. Each mineral phase plates out in a unique area which is a function of the electrophoretic potential of that phase, resulting in discrete bands of pure minerals arranged across the filter paper.
- The paper is rolled through the extraction module where the width and composition of each band of trapped grains are measured and verified by a spectral scanner and vision module. Robot scrapers remove individual mineral phases and deposit them in receptacles.
The suspension, mobility, separation, and plating or entrapment steps in this process are self-regulating. The only steps requiring new automation are input, calibration, and extraction. The separator most probably can be scaled up to the requisite size for any given throughput rate, as present-day electrophoresis cells vary a great deal in capacity. The ratio of the volume of suspension medium to the volume of. suspended soil can be as high as 1:1 (Micromoretics, Inc., personal communication, 1980).
Metallurgy of basalt. The occurrence of large quantities of tholeiitic (olivine-poor) basalt on the Moon has focused attention on its "metallurgy" (Kopecky and Voldan, 1965; Kopecky, 1971) and on its possible uses as a material for SMF construction. Early work in France involved substituting melted basalt for glass and was not directed toward improving.the product over the raw material. German researchers advanced another step by evolving a technology for recrystallizing the melt and casting it into simple shapes. The Soviet Union began experimentation with basalt in the 1930s at the Moscow Rock Foundry Works. Processed basalt currently is being manufactured either on a pilot or factory scale in Czechoslovakia, Poland, Sweden, Italy, and the United States. Czechoslovak Ceramics distributes its products mostly to Sweden and England (see fig. 4.3). Many basic patents are held by Mr. H. L. Watson of the now-dissolved Compagnie Cenerale du Basalte in France.
From laboratory studies and operational experience, raw feedstock basalt should contain pyroxene ((Ca,Mg,Fe)SiO3) in excess of 60%, as it imparts desirable qualities (such as resistance to abrasion, mechanical strength, and chemical resistivity) to the recrystallized mass. Magnetite (Fe3O4) and olivine ((Mg,Fe)2SiO4) also are important because they induce crystallization, but their concentration should not exceed 10%. Higher fractions would reduce the SiO2 content, leading to the formation of larger crystals that promotes bursting on annealing. (Also olivine, which has a high melting point and thus is difficult to melt, would not dissolve in the short time available for fusion, especially if present as large crystals.) Feldspars ((Ca,Na)Al2Si2O8) influence the viscosity and regulate the rate of crystallization. Nepheline (NaAlSiO4) and plagioclase feldspars should be present within the ratios 1:1 to 1:3,with a total content of about 20%. Other rock types such as melaphyres (alkali feldspars) and tephroites (Mn2SiO4) have been investigated (Kopecky and Voldan, 1965), but technological difficulties prevent their exploitation at present.
In addition, the material must be fine-grained, homogeneous, unweathered, nonporphyritic, and uncontaminated. A melting temperature range of 1500-1600 K must be associated with a relatively low viscosity (100-1000 poises) in order to cast well. The casts should recrystallize easily in a fine-grained state and not crack after cooling. Favorable factors for lunar basalt include the uncontaminated, unweathered nature of the material as well as an extraordinarily low viscosity.
However, little work has been done to assess certain other factors which might affect lunar basalt casting. For instance, in the manufacture of cast and sintered basalt different successions of minerals crystallize out depending upon the rate of cooling of the melt. By slow cooling and annealing of the casts the following succession is observed: magnetite, olivine, monoclinic pyroxene, plagioclase, then monoclinic amphibole. With rapid chilling, involved in the sintering process, the succession is: magnetite, pyroxenes, amphibole, olivine, and finally plagioclase. Inasmuch as crystallization of the castings depends on melt viscosity, control of that viscosity determines the quality of the final product. Turbulent flow arising from very low viscosity enhances the production of crystals of unequal size and creates swirls in the finished coating, so silica may have to be added to increase the viscosity of thin lunar basaltic melts. On the other hand, excessively high viscosities produce an undesirable laminar structure. The optimum is defined by a Reynolds number of about 1000. On the Moon, reduced gravity should slightly improve the casting process by reducing the onset of turbulence for a given crystal size. Stokes' equation would apply to a higher value for the terminal velocity of particles, therefore, laminar flow on the Moon would persist at higher terminal velocities than on Earth. Perhaps the effect of gravitational separation of mineral phases often seen during melting, and the inhomogeneities produced in casting, would also be less apparent in lunar cast basalt.
The results of laboratory gradient melting studies by Kopecky and Voldan were applied to the manufacture of cast basalt. The low crystallization speed of plagioclase (3-10 min) prohibits the crystallization of this mineral and it persists as a residual glass phase. Other newly formed crystalline phases of the pilot plant closely resemble the gradient furnace products, except that the cast basalt minerals are more skeletal and dendritic. The most apparent feature in cast basalt is the zonality of the product, which is a function of the cooling rate.
In commercial manufacturing operations in Czechoslovakia, the raw material (8-15 mesh basalt) is melted at 1575-1625 K in vertical gas-fired Lehr furnaces, a process similar to open-hearth steel production. The molten material then is conducted into a homogenizer drum where, at carefully controlled and slightly reduced temperatures, the melt begins to crystallize. The subsequent casting is similar to conventional metallurgical techniques except for differences imposed by the greater viscosity and cooling rates. Static casting in the sand molds originally employed produced a product having rough surfaces and poor tolerances. Metal molds (fig. 4.4) have now replaced sand molds and currently are used in the production of tiles, plates, and fittings. Recently, centrifugal casting methods (fig. 4.5) have resulted in a product of superior quality. Annealing furnaces (fig. 4.6) are used to cool the castings from 1100 K to room temperature over a 24-hour period. Careful control of temperature reduction is required to prevent bursting and other imperfections on annealing.
Most of the castings weigh 3-80 kg. The largest, representing the limits of present-day equipment, weighs 300 kg; the smallest is 0.34 kg, a 60-mm diameter ball. Tiles usually are made in thicknesses of 25-40 mm; pipe walls typically are 15-20 mm thick, with a maximum of 50 mm. The lower limit of thickness is determined by the rate of heat loss and the danger of vitreous solidification. Research is needed on the effects of reduced gravity and on the maximum mass of various castings.
The sintering process is similar to that employed in powder metallurgy (see sec. 4.3.1). The basalt frit made from molten metal is finely ground (1600 mesh), impregnated with a plasticizer, shaped under a pressure of 1000 kg/cm2, then sintered in electric furnaces at 1395-1415 K. Sintered basalt is valuable in the manufacture of small articles such as nozzles, wire-drawing dies, spheres. and other special fabrications.
Basalt fibers for industrial and commercial applications also currently are produced overseas. Basalt fiber research programs and demonstration units have been implemented at Washington State University (Subramanian et al., 1975, 1976, 1977, 1978, 1979) and at the University of California at Los Angeles (Mackenzie and Claridge, 1979). Production methods for spinning basalt include: (1) continuous fiber simple extrusion and reeling, similar to standard glass fiber production (Andreevskaya and Plisko, 1963), and (2) staple fiber extrusion augmented by air or steam jets including centrifugal spinning methods (Dubovkaya and Kosmina, 1968). Both methods warrant further research for robotics applications and automated manufacturing (Kato et al., 1978) in lunar environments. The typical composition of spun basalt (in wt %) is represented by SiO2 (50%), Al2O3 (15%), TiO2 (3%), FeO (11%), Fe2O3 (2%), MnO (0.2%), CaO (9%), MgO (5%), K2O (1%), Na2O (3%), and P2O5 (1%). The fibers are brown in color because of their iron content. Table 4.14 provides a list of compositions of raw feedstock and other fiber characteristics. Tensile strengths are comparable to those of E-glass.
|Sample number, rock flow, source||X-6, Lolo Flow, Whitman County, WA||RC-11. Imnaha, Rocky Canyon, ID||K-9068, Elephant Mt., Saddle Mt., WA||K-9064, Middle Yakima, Saddle Mt., WA||O-2, Sweet Home, OR||K9048, Pomona, Saddle Mt., WA||RC-3, Imnaha, Rocky Canyon, ID||K-9017, Lower Yakima, Saddle Mt., WA||BCR-P, Yakima Flow||E-glass Owens-Corning|
|Temperature of drawing, °C||1325||1300a||1250||1250||1250||1250||1250||1250||1360a||1250a|
|Speed of drum, rpm||515||250||370||515||370||250||370||515||250||250|
|Number of samples||40||29||30||18||25||20||20||20||31||23|
|Average diameter of fiber, u||13.0||12.2||13.5||9.0||11.4||11.8||11.3||10.2||12.1||12.2|
|Average tensile strength,|
aAir jet used.
(Subramanian et al, 1976)
Both continuous and staple fibers can be made from basalt. Continuous fibers are produced using standard glass fiber production equipment. After the feedstock is fused in an electric furnace, the melt is fed to electrically heated platinum.rhodium bushings containing 200-300 perforations. As shown in figure 4.7 (Subramanian et al., 1975), a drum winding pulls the fibers from the platinum.rhodium bushing perforations. Fiber diameter is a function of melt temperature and drum or centrifugal nozzle speed. Temperatures range from 1525-1675 K; thread diameters usually are in the 10-15 um range, although superfine fibers 0.2-0.4 um thick reportedly have been manufactured in Russia.
Staple fibers are produced using melting tank furnaces that feed electrically heated centrifugally spun platinum-rhodium bushings. Jets of air or steam moving parallel to a fiber extruded from the centrifugally spun nozzles tear it into short lengths (about 30 mm) which fall onto a porous drum under vacuum. Either continuous or centrifugal spinning staple methods may be applicable for lunar fiber production.
Silanes (organosilicon compounds) have been evaluated as coating materials on basalt fibers to permit adhesion of the fibers to epoxy composites (Subramanian et al., 1976, 1979). The results Showed that silane coupling agents are effective in improving interfacial bond strength in basalt fiber-polymer systems and that basalt fiber has excellent Potential as a reinforcing fiber for polymer composites. The tensile Strength and tensile elastic moduli of epoxy composites of silane-treated basalt fibers are presented in figures 4.8 and 4.9, respectively, as a function of volume fraction Vf.
Processed or machined basalt has been suggested as a logical construction material with which to produce the component parts Of large space and lunar structures. The strength of this basalt and of other construction materials must be compared. In table 4.15 the proportional limit, ultimate strength, and modulus of elasticity of sintered basalt are measured against those of carbon steel, cast iron, malleable cast iron, wrought iron, cast aluminum, aluminum alloy 17ST, rolled brass, cast bronze, and drawn copper.
|Material||Proportional limit, MN/m2||Ultimate Strength, MN/m2||Modulus of elasticity, MN/m2||Source|
|0.2% Carbon steel (hot rolled)||238||241||142||408||248||306||2.0×105||1961 AISC Manual|
|0.2% Carbon steel (cold rolled)||408||414||244||544||416||408||2.0×105|
|0.8% Carbon steel (hot rolled)||476||491||286||816||503||714||2.0×105|
|Malleable cast iron||414||428||156||367||261||326||1.6×105|
|Aluminum alloy 17ST||217||223||143||381||242||218||4.1×104|
|Sintered basalt||36||550||-||36||550||-||1.1×105||Kopecky and Voldan, 1965|
The physical properties of basalt compare quite favorably with those of conventional construction materials. The compressional strength and elastic modulus are quite high; that is, basalt as a construction material is far more rigid than other substances listed, a quality of some importance in large space structures. One drawback is tensile strength, roughly an order Of magnitude lower for basalt than other typical construction materials. This problem can be overcome either by designing structures such that basalt components are not exposed to high tensile or extensional stress states or by producing a compound basalt reinforced with fibers. The first alternative is impractical, as large structures contrived to reduce tensile stresses on basalt components would be difficult to design and of limited utility. Compound basalts could be prepared by sintering basalt-sodium flux materials and imbedding the melt with a cross-hatched pattern of basalt filaments to increase tensile and shear strength without sacrificing rigidity. (The sodium flux reduces the fusion point of the mixture so that the basalt filaments do not themselves melt.) Finally, the low thermal expansion coefficient (7.7×10-6/K around room temperature) and thermal conductivity of sintered basalt (8×10-4 J/m2sK) are very suitable for lunar application, enhancing the structural rigidity of sintered basalt.
One last potential problem is machinability. Cast basalt has a rather irregular surface, a property inappropriate for some construction components, and needs some surface and internal grinding. Also, the hardness of cast and sintered basalt is high, 8.5 on the Moh's scale. A grinding compound of higher hardness is therefore needed, preferably some substances found on the lunar surface. A logical choice is spinel (Moh's value 9.0), which probably can be extracted from lunar soil by an electrophoretic technique.
A summary of possible methods and applications of processed lunar basalt is presented in table 4.16.
|Cast basalt||Sintered basalt||Spun basalt (fibers)|
Machine base supports (lathes, milling machines)
Furnace lining for resources extraction
Large tool beds
Pipes and conduits
Conveyor material (pneumatic, hydraulic, sliding)
Linings for ball, tube or pug mills, flue ducts, ventilators, cyclers, drains, mixers, tanks, electrolyzers, and mineral dressing equipment
Tiles and bricks
Expendable ablative hull material (possibly composited with spun basalt)
Heavy duty containers for "agricultural" use
Radar dish or mirror frames
Thermal rods or heat pipes housings
Supports and backing for solar collectors
Low torque fasteners
Furniture and utensils
Low load axles
Scientific equipment, frames and yokes
Light duty containers and flasks for laboratory use
Cloth and bedding
Resilient shock absorbing pads
Insulator for prevention of cold welding of metals
Filler in sintered "soil" cement
Strainers or filters for industrial or agricultural use
Ropes for cables (with coatings)
4.2.3 Transport to Low Earth Orbit
In the near term two sources of raw materials may be tapped to supply a space processing center in LEO - the Earth itself and the Moon (see fig. 4.10). Earth may provide material, primarily feedstock, by way of the Shuttle and derived vehicles. The possibility of using a land-based electromagnetic accelerator for ground-to-LEO transport offers the tantalizing promise of greatly reduced supply costs for feedstock payloads able to withstand the 104-105 m/sec2 accelerative loads required for direct launch from Earth (Mongeau et al., 1981).
Bock et al. (1979) have studied the retrieval of lunar materials to various points in space, using chemical rockets burning lunar LOX and aluminum powder or terrestrial H2. The objective is to transport from the Moon to cislunar orbital space many times more mass than could be supplied from Earth at equal cost. A particularly appealing proposal for near-term acquisition of lunar resources using chemical propulsion has been suggested by Waldron et al. (1979). The potential fuel is lunar silicon and terrestrial hydrogen combined to form silanes, which then are burned as rocket fuel with lunar oxygen. Even if mass drivers supplant this use of lunar-derived propellants for bulk transport, the silane/LOX system, if feasible, would still be useful in trajectory correction (RCS), stationkeeping, and related specialized applications.
The costs and mechanics of STS launch and operations are treated extensively in the literature and will not be reviewed here. Two relatively new proposals - the lunar silane/LOX propellant scenario and the Earth-based electromagnetic catapult - are treated in more detail below. Calculations are presented for the total and net lunar mass that could be delivered to LEO in terms of multiples of the hydrogen needed from Earth.
Lunar supply of a LEO station. To demonstrate early net growth in space the team considered the problem of supplying a LEO station with bulk materials from the Moon. There will be only moderate initial supply from Earth and very limited resupply thereafter. A LEO facility able to accept raw lunar stock and a very small factory able to extract oxygen from and load lunar soil into arriving spacecraft for Moon-to-LEO transport are assumed already to exist. The initial supply base will likely be located at a previously visited Apollo site. A more sophisticated version of the lunar base produces both oxygen and silane (from lunar silicon and Earth-supplied hydrogen). The overall plan requires an Orbital Transfer Vehicle (OTV), a Lander, and a supply of hydrogen from Earth. OTV and Lander are fueled by terrestrial-supplied hydrogen and lunar-derived oxygen or by silane and lunar-derived oxygen. Lander is loaded with lunar soil to be processed and delivers it to the OTV. The OTV returns to the manufacturing facility in low Earth orbit. There, at the SMF, part of the soil is used to produce oxygen (or oxygen and silane) to refuel the OTV and Lander. The remainder is available as raw material for the manufacture of useful output. Either the H2-O2 or the SiH4-O2 combination allows significant multiplication of resource mass beyond that supplied from Earth.
This scenario could be accomplished according to the following sequence:
- The OTV carrying Lander and the required hydrogen leaves LEO with impulse δV1 m/sec.
- OTV passes low over the lunar surface (50 km altitude) and releases Lander, then returns to LEO on a free-return trajectory using aerobraking. No propulsion is assumed for any of these maneuvers.
- Lander burns fuel (δV2 m/sec) to enter an elliptical lunar orbit with apolune at the point of separation and perilune at the surface of the Moon.
- Lander burns fuel (δV3 m/sec) to land and rendezvous with the already emplaced lunar soil processor. Lander arrives carrying only the hydrogen required for a return to LEO.
- The lunar processor supplies Lander with native oxygen. If the silane alternative is used, the processor also takes Lander's hydrogen and converts it to silanes (predominantly SiH4) using lunar silicon.
- Lander is loaded with a cargo of lunar soil destined for the LEO manufacturing facility.
- Lander lifts off from the Moon (δV4 m/sec) and returns via aerobraking to LEO to rendezvous with the orbiting manufacturing facility.
- Lander and OTV are refueled for a return trip to the Moon.
The above procedure has been worked out parametrically without specifying the masses of OTV and Lander. The same fuel and oxidizer are used at each bum. It is desired to determine the incremental cost of one kilogram of lunar payload delivered to LEO which is not needed for fuel in terms of incremental mass lifted to LEO from the Earth. The full mathematical analysis is presented in appendix 4A - only the results are given here.
Let a be the tankage fraction needed to carry the payload from the Moon, B the propellant tankage fraction, and BH the fraction of the total fuel-plus-oxidizer combination that is hydrogen. If X is as given in equation (2) of appendix 4A, and P is the mass of the payload not needed for propellant replenishment, then the mass of extra hydrogen that must be lifted from Earth to deliver 1 kg of extra lunar payload to LEO (dMHlift/dP) is given by equation (3) of appendix 4A. The following values are given for H2-O2 propellants:
- c = 4.414 km/sec (Isp = 450 sec)
- BH = 1/9
- a = B = 0.038
- δV1 = 3.2244 km/sec
- δV2 = 0.84303 km/sec
- δV3 = 1.69147 km/sec
- δV4 = 2.51872 km/sec
- X = 0.39718
- dMHlift/dP = 0.2075,
so the multiplier is (0.2075)-1 = 4.82 kg of extra payload gained for every kilogram lifted to LEO from Earth. For SiH4-O2 propellants:
- c = 3.463 km/sec (Isp = 353 sec) BH = 1/24
- X = 0.49420
- dMHlift/dP = 0.12921,
so the multiplier is 7.739 kg/kg.
If the OTV is eliminated and Lander alone leaves LEO and returns, then for H2-O2:
- X = 0.39799
- dMHlift/dP = 0.20335, so the multiplier is 4.92;
and for SiH4-O2:
- X = 0.47696
- dMHlift/dP = 0.12395, so the multiplier is 8.067.
The team concludes that significant multiplication of resources at LEO is attainable if part of the propellant required to run the system is drawn from the Moon. Lunar oxygen production allows 4.82 kg of raw material to be brought to LEO from the Moon for every kilogram of hydrogen lifted from Earth. If the OTV is removed, this multiplier factor rises to 4.92. Production of silanes as well as oxygen may allow 7.74 kg of raw material to be brought to LEO from the Moon for every kilogram of Earth-supplied hydrogen. If no OTV is used, this figure rises to 8.07. (Allowing Lander to complete the round trip without an orbital transfer vehicle increases performance slightly if the fuel for the first propulsive burn is stored in the space allotted to the payload on the return trip.) The foregoing parametric analysis indicates the advisability of continuing with this line of research. A very small initial plant on the Moon could permit the utilization of lunar materials in LEO early in space manufacturing experimentation.
Earth impulse launch supply of a LEO station. The use of launchers to propel material from the lunar surface has been a key element in space manufacturing and colony-building scenarios for many years (Grey, 1977). Even more revolutionary is the concept of an impulse launcher to lift cargo off the surface of the Earth (Mongeau et al., 1981). If payloads are of sufficient size and are projected almost vertically, atmospheric resistance reduces velocity by only about 15% (see Kolm in Grey, 1977). Since the launch must be nearly perpendicular to minimize atmospheric drag, it is not feasible to supply a LEO station directly. (About 7 km/sec of horizontal velocity would have to be added after launch, so there would be no advantage in using an impulse launcher.) But if payloads are lofted to geostationary altitude (GEO), a burn there of only 1.5 km/sec puts the cargo in an orbit tangential to the Earth's atmosphere. Aerobraking then lowers the apogee until a final burn circularizes the orbit and allows rendezvous with the LEO facility.
Although modern rockets are very thermally efficient, only about 0.5-1.0% of the energy originally available in the propellant tanks is finally delivered to the payload; the rest is expended accelerating propellants and vehicle mass. The impulse launcher is vastly more efficient, allowing all but about 3% of the energy required to reach LEO to be imparted to the payload while it is on the ground. The 3% expenditure is made by a booster fired at apogee to raise perigee to the upper levels of Earth's atmosphere.
Two methods of impulse launch have been proposed. The first is a simple version of the rail gun as shown in figure 4.11. It suffers from major inefficiencies (I2R losses) but illustrates the principle. In this system, current flow through a plasma causes magnetic pressure to be exerted by the are on the projected base. The second type of impulse launcher uses superconducting coils as suggested by von Tiesenhausen (personal communication, 1980) and Kolm (in Grey, 1979). For a given acceleration and final velocity, the second (induction motor) launcher is 2-3 times longer than the first, since payloads are hurled forward in a bucket and the bucket eventually must be decelerated. The projectile is a 1000 kg mass in the form of an ogive 1.1 m diam and 6.3 m long. The launcher operates at 300 kW average impact power and launches the payload at 11.05 km/sec.
If 80% efficiency and power storage in homopolar generators between launches is assumed, one shot can take place every 3.5 min. The firing tube is about 1.5 km long for a 5000-g launch, or 2.5 km including bucket-slowing if the linear induction motor impulse device is employed. At 80% efficiency a launch requires 7.63×1010 J or 21,200 kW-hr of energy. Electricity costs about $0.05/kW-hr, therefore the equivalent cost of an impulse launch in terms of power requirements is $1060.
The projectile slows to 10.22 km/sec by 100 km altitude, the limit of the sensible atmosphere. Ten percent of the launch mass and 16.9% of the launch energy have been lost by this point due to ablation. When the projectile reaches GEO altitude it orients itself horizontally and the solid booster fires, providing a delta-V of about 1500 m/sec. This places the payload on an atmosphere-grazing trajectory allowing aerobraking and orbital circularization. If the solid booster (Isp = 300) has an inert mass of 100 kg and the aerobraking shield is 25 kg, then the net mass delivered to LEO is:
- (1000-100)exp(-1500/9.8X300) - (100 + 25) = 415 kg
This represents a power cost of just $2.55/kg. Even if the upper stage motor costs as much as $100,000, the total expense to LEO is $304/kg. If the cargo is launched around the Moon to obtain the requisite horizontal velocity by a gravitational assistance maneuver, the mass to LEO is (1000-100) - 25 = 875 kg and the cost of launch energy rises to about $1100, or $1.25/kg. Even if guidance and personnel requirements raise this figure by an order of magnitude it still is only 2% of the most optimistic estimate of expected Shuttle costs. The major savings for impulse launching occur because the usual need of accelerating large masses of propellants in addition to cargo is avoided.