Processing: Where?
The decision as to whether to process at the colony or on the Moon is dictated by various factors. The lunar site has the advantage of being close to the ore source and having a gravity which might be used in some chemical processing. Lunar processing might be expected to decrease the amount of material to be shipped to the colony. However, closer examination reveals that the colony's shielding requirements exceed the slag production of the processing plant; hence, no transportation is saved by processing at a lunar site. Moreover, lunar processing also possesses certain definite disadvantages when compared to processing at the site of the colony. Plant facilities shipped from the Earth to the Moon require much greater transportation expense than for shipment to the colony site. In addition, solar furnaces and power plants are limited to a 50 percent duty cycle on the Moon. Without power storage this would curtail operations at a lunar processing site. Radiators for process cooling are less efficient and, therefore, larger when placed on the Moon, because they have a view of the Sun or of the hot lunar surface. Finally, even at only 1/6 of Earth's gravity, components of the plant have significant weight. On the Moon this requires support structure and cranes and hoists during assembly. But these are not needed if processing is done at the colony site. Based on these considerations, it appears that major processing should take place at the colony site.
A variety of alternatives exist for the processing of lunar ores to yield materials for the colony. These involve various combinations of processing site, materials to be produced, and chemistry. Optimization requires a detailed analysis of manifold possibilities. The study limited itself to choosing a plan which seems achievable and advantageous based on reasonable extrapolations of current technology.
Processing: What and How?
The colony requires various materials which are obtainable from the lunar soil. Silica is needed for windows and solar cells. Oxygen is the major component of the colony atmosphere and is required for manufacturing water. It is also a rocket propellant. Silica and oxygen are essential to the success of the colony and therefore must be extracted from lunar ore.
Figure 4-10 — “Sources of Materials” pie chart of resources and their locations relative to L5.
<!-- image -->However, there is some latitude for choice and optimization among the variety of metals available. Aluminum, titanium, magnesium and iron are all potential construction materials. Although aluminum is chosen as our basic structural material, a decision to refine titanium might have some special advantages. On the Moon, titanium is in the form of a magnetic mineral (ilmenite) which can, in theory, be easily separated from the bulk of the lunar ore. In addition, use of titanium for structure would result in significant savings in the total amount of refined material because, although more difficult to form and fabricate, its strength-to-mass ratio is greater than that of the other metals available. Since ilmenite is basically $FeTiO_3$, significant amounts of iron and oxygen can be extracted as byproducts.
These facts support a recommendation that the alternative of titanium refining should be studied in detail. Possible methods for refining titanium are presented in figure 4-11 and discussed in appendix I.
Most of the remaining metal oxides in the ore must be separated from one another by rather complex techniques before further refining of the metals. Aluminum is the only other metal which justifies detailed consideration. In addition to excellent structural properties and workability it has good thermal and electrical properties (see appendix A). It is chosen as the principal structural material only because information concerning titanium processing is somewhat less definite and, in particular, the magnetic separation technique for lunar ilmenite has not yet been demonstrated.
The various methods by which aluminum might be refined from lunar anorthosite are shown schematically in figure 4-12. The system chosen is melt-quench-leach production of alumina followed by high temperature electro-winning of aluminum from aluminum chloride. Alternative paths are discussed in appendix I.
To provide window areas for the space structure, glass is to be manufactured from lunar materials. Silica ($SiO_2$), the basic ingredient in glassmaking, is found in abundance on the Moon. However, another basic constituent, sodium oxide ($Na_2O$), which is used in the most common flat plate and sheet glass industrially produced, is found in only small percentages in the lunar soil. Glass processing on Earth uses $Na_2O$ primarily to lower the melting temperature that has to be generated by the furnace (refs. 29, 30). Since the solar furnace to be provided for processing the lunar material will be capable of generating temperatures considerably higher than those which could possibly be needed for this process, it appears unnecessary to supply additional $Na_2O$ from the Earth (personal communication, J. Blumer, Vice-President for Research, Libbey-Owens-Ford Company, Toledo, Ohio, Aug. 1975).
To date, glasses made from lunar soil samples returned by the Apollo missions have been dark in color. The techniques necessary to manufacture glass from lunar materials which possesses the properties needed for efficient transmission of sunlight into a space habitat have not been demonstrated (personal communication, Pittsburgh Plate Glass Company, Pennsylvania, Aug. 1975). However, it is believed that additional materials research will permit glass of adequate quality for a space facility to be processed from the lunar soil with a minimum of additives (if any) brought from the Earth (personal communication, D. R. Ulrich, Air Force Office of Scientific Research, Washington, D. C., Aug. 1975).
A possible technique which may prove feasible in space for large scale production is the removing of almost all nonsilicate ingredients by leaching with acid. Again, the availability of high furnace temperatures is a prerequisite to meet the melting temperature of silica, and the manufacturing process will have to be shown to be manageable in space. The resulting glass, of almost pure silica (> 95 percent $SiO_2$), possesses the desirable properties of low thermal expansion, high service temperature, good chemical, electrical, and dielectric resistance, and transparency to a wide range of wavelengths in the electromagnetic spectrum.
Requirements for volume, mass, and energy of a glass-processing unit, a description of a sample process, and an elaboration of lunar soil constituents are given in appendix J.
Transport of Lunar Material
The construction of the colony depends critically on the capability of transporting great quantities of lunar material from the Moon to the colony without large expenditures of propellant. There are three parts to this problem: launching the material from the Moon, collecting it in space, and moving it to the colony. Two principal ways to launch have been devised, along with some variations.
Figure 4-11 — Summary of processes by which titanium can be extracted from ilmenite. Heavy line indicates the preferred process.
<!-- image -->Figure 4-12 — Processes by which aluminum can be extracted from anorthosite. Heavy line indicates the preferred process.
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One method is to launch large payloads, of about 60 t, by firing them from a large gas gun. The gun is operated by using nuclear power to compress hydrogen gas and then permitting the gas to expand the length of the launch tube. Because hydrogen must be obtained from Earth, its replacement is expensive, and consequently after each launch the gas is recovered through perforations in the end section of the launch tube which is encased in an enclosed tube. Further details are given in appendix K.
The system is of interest because of its conceptual simplicity and light weight. But the principal drawback of the gas gun system is the difficulty of collecting the payloads once they have been launched because their dispersion is large. Collection needs a fleet of automated interceptor rockets. The propellant requirement for interception is about 1 percent of the total mass launched. In terms of technology that may be available in the near future, these interceptor rockets have to use chemical propulsion with hydrogen as fuel. The second drawback is that the gas gun requires the development of sliding seals able to withstand high pressures and yet move at high velocities and still maintain acceptable leakage rates. Despite the uncertainties about precision of aim, the difficulties of automated rendezvous and interception, and the associated propulsion requirements, the concept appears fundamentally feasible and worthy of more study. However, the uncertainties are sufficient to make another alternative more attractive at this time.
The alternative method, which is the one chosen for this design, involves an electromagnetic mass accelerator. Small payloads are accelerated in a special bucket containing superconducting coil magnets. Buckets containing tens of kilograms of compacted lunar material are magnetically levitated and accelerated at 30 g by a linear, synchronous electric motor. Each load is precisely directed by damping the vibrations of the bucket with dashpot shock absorbers, by passing the bucket along an accurately aligned section of the track and by making magnetic corrections based on measurements using a laser to track the bucket with great precision during a final drift period. Alignment and precision are the great problems of this design since in order to make efficient collection possible, the final velocity must be controlled to better than $10^{-3}$ m/s. Moreover, the system must launch from 1 to 5 buckets per second at a steady rate over long periods of time, so the requirements for reliability are great. This system is considerably more massive than the gas gun. More details about it are given in the next chapter.
The problem of catching the material launched by the electromagnetic mass driver is also difficult. Three possible ways to intercept and gather the stream of material were devised. Two so-called passive catchers (described in more detail in appendix L), involve stationary targets which intercept and hold the incoming material. The other is an active device which tracks the incoming material with radar and moves to catch it. The momentum conveyed to the catcher by the incident stream of matter is also balanced out by ejecting a small fraction of the collected material in the same direction as, but faster than, the oncoming stream.
An arrangement of catching nets tied to cables running through motor-driven wheels permits rapid placement of the catcher anywhere within a square kilometer. By using a perimeter acquisition radar system, the active catcher tracks and moves to intercept payloads over a considerably larger area than the passive catchers. Unfortunately this concept, described in more detail in the next chapter, has the defects of great mechanical complexity. Nevertheless, although many questions of detail remain unanswered and the design problems appear substantial, the active catcher is chosen as the principal means of collecting the material from the mass launcher on the Moon.
Despite possible advantages it seems desirable not to place the catcher at the site of the colony at L5. For three reasons L2 is chosen as the point to which material is launched from the Moon.
First, the stream of payloads present an obvious hazard to navigation, posing the danger of damage if any of the payloads strike a colony or a spacecraft. This danger is particularly acute in view of the extensive spacecraft traffic to be expected in the vicinity of the colony. The payloads, like meteoroids, may well be difficult to detect. Hence, it appears desirable to direct the stream of payloads to a target located far from the colony.
Second, L2 is one-seventh the distance of L5, permitting use of either a smaller catcher or a less-accurate mass-driver.
Third, to shoot to L5 requires that the mass-driver be on the lunar farside. For launch to L2, the mass-driver must be on the nearside. By contrast, a nearside location for the mass-driver permits use of our knowledge of Moon rocks brought back in Apollo flights, and there are a number of smooth plains suitable for a mass-launcher. The nearside also permits line-of-sight communications to Earth.