Catching lunar material at L2 means that transport must then be provided to L5. It appears most practical to use mechanical pellet ejectors powered by an onboard nuclear system of 25 MW. This same system is used to offset the momentum brought to the catcher by the payloads arriving at up to 200 m/s.
THE TRANSPORT SYSTEM
The transportation requirements of a colony are much more extensive than merely getting material cheaply from the Moon to the factories of the colony. There must be a capability for launching about 1 million tonnes from the Earth over a total period of 6 to 10 years. There must be vehicles capable of traversing the large distances from Earth to L5 and to the Moon. There must be spacecraft that can land equipment and people on the Moon and supply the mining base there. Fortunately, this is a subject to which NASA and the aerospace industry have given considerable thought; the study group relied heavily on this work. A schematic representation of the baseline transportation system is shown in figure 4-13.
Figure 4-13 — Baseline transportation system.
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Figure 4-15 — SSME powered modular tug.
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From Earth’s Surface to Low Orbit
The space shuttle is to be the principal U.S. launch vehicle for the 1980s. However for space colonization applications, the shuttle has low payload per launch and requires too many flights with excessive launch costs per kg. At the other end of the launch vehicle spectrum, a number of advanced concepts have been studied. These include a large winged "Super-Shuttle," fully-reusable ballistic transporters resembling giant Mercury capsules, and even use of a laser rocket with a remote energy source. Such concepts are not considered in this primary study because of uncertain technologies, excessive development costs, and long leadtimes. However, one concept for the "F-1 flyback" is discussed in appendix C of chapter 6.
Figure 4-16 — Paths through space for space colonization.
<!-- image -->The colony has to rely on lift vehicles derived from and, therefore, dependent on the shuttle and other already-developed boosters. Studies have been made on shuttle-derived heavy lift launch vehicles with two and with four solid boosters (fig. 4-14). In these, the manned shuttle vehicle is replaced with a simple vehicle having automated avionics and increased freight capability. The four-booster configuration has a payload of 150 t at under $20 million per launch.
A discussion of the environmental impact on the ozone layer of Earth by launch vehicles is given in appendix N.
Transport Beyond Low Earth Orbit
For routine transport of people and freight, the system uses single-engine vehicles employing space-storable, liquid-gas propellants in modular tankage. The NERVA nuclear rocket is rejected in favor of the space shuttle main engine (SSME). NERVA offers high performance but represents a new development, and involves the safety considerations associated with nuclear systems. The SSME represents an available, well-understood engine. Moreover, with oxygen for refueling available at L5 from processing of lunar ores in industrial operations, the SSME vehicle performance would approach that of NERVA. Consequently the SSME as shown in figure 4-15 has been selected. Details are given in appendix M.
For passenger transport, the launch vehicle cargo fairing accommodates a passenger cabin holding 200 people. A single SSME could also be used to land over 900 t of cargo on the lunar surface.
For transport of major systems involving their own large power plants, electric propulsion is feasible. Such systems include the L5 construction shack with its 300 MW power plant, and the solar-power satellites to be built at the colony for delivery to geosynchronous orbit. Candidate propulsion systems include ion rockets, resistojets, and mechanical pellet accelerators. In particular, for the baseline system, large numbers of standard ion thrusters are clustered, thus permitting application of current electric-propulsion technology. It is possible in the future that a Kaufman electrostatic thruster could be developed with oxygen as propellant. As described in the next chapter, a rotary pellet launcher is proposed to power the tug which brings the lunar ore from L2 to the processing plant at L5.
SUMMARY
Thus the system described in chapter 1 is arrived at. It carries 10,000 colonists in a toroidal habitat positioned at L5 orbiting the Sun in fixed relation to the Earth and Moon and exploiting the paths through space in figure 4-16. Mining the Moon for oxygen, aluminum, silica, and the undifferentiated matter necessary for shielding, the colonists ship a million tonnes per year by electromagnetic mass launcher to L2. There, with the active catcher, the material is gathered and transshipped to L5 to be refined and processed. With small amounts of special materials, plastics, and organics from Earth, the colonists build and assemble solar power stations which they deliver to geosynchronous orbit. The colonists also raise their own food and work on the construction of the next colony. The following chapter gives a more detailed picture of how the various parts work together.
APPENDIX A: MATERIAL PROPERTIES FOR DESIGN
To estimate the masses of components for alternate configurations, nominal design values for a variety of physical properties must be assumed for the materials involved. Only a few "standard" metal alloys are shown in table 4-4, chosen to give good weldability, corrosion resistance, and forming properties. A more careful specification of specific alloy percentages for structural components in a final design is expected to reduce the actual structural mass somewhat from that derived from this conservative approach. For particular applications where, for example, cyclic stress reversal may induce fatigue, high temperatures cause creep, or low temperatures cause brittleness, special materials must be used.
TABLE 4-4 — MATERIAL PROPERTIES OF SOME METAL ALLOYS
| Material | Density, $kg/m^3$ | Yield Stress, MPa | Working Stress, MPa | Modulus of Elasticity, GPa | | :--- | :--- | :--- | :--- | :--- | | Aluminum (6061-T6) | 2700 | 240 | 160 | 70 | | Titanium (6Al-4V) | 4500 | 830 | 550 | 110 | | Steel (High Strength) | 7800 | 1200 | 800 | 210 |
Figure 4-17 — Projected areas of basic shapes.
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Figure 4-18 — Habitable volume of a torus.
<!-- image -->A safety factor of 1.5 is applied to the yield stress to give safe working values. This corresponds to the standard for large civil engineering structures combining dead- and live-load factors. In all large structures the material is proof-tested before use, so that, in reality, the end result of processing and fabrication is not a dubious variable but can be a controlled parameter. At least one good machine for static and dynamic testing of the strength of materials must be included in the laboratory equipment brought from Earth to the colony site.
The strength properties of ceramic-type materials and "soil" (table 4-5) are low, and experimental results from any pilot study for processing lunar ores should help define them more precisely. While it should also be possible to grow long glass fibers having great structural strength, such materials are not assumed for construction of the first colony.
TABLE 4-5 — MATERIAL PROPERTIES OF SOME CERAMICS AND SOILS
| Material | Density, $kg/m^3$ | Compressive Strength, MPa | Tensile Strength, MPa | | :--- | :--- | :--- | :--- | | Glass (Fused Silica) | 2200 | 1100 | 50 | | Lunar Soil (Compacted) | 1500-2000 | 0.1-1.0 | 0 | | Fused Lunar Rock | 2500-3000 | 100-500 | 10-50 |
APPENDIX B: PARAMETERS OF HABITABILITY
In a rotating habitat in space three factors affect the area and volume available for residence. Due to physiological considerations living and sleeping are confined for a large portion of the day to a volume where the change in pseudogravity, g, is less than some amount, $\Delta g$, which experience must determine. The habitable volume is that volume where $\Delta g/g$ is less than or equal to some number which the study group calls the habitability parameter, $\epsilon = \Delta g/g$. It is not inconceivable, for example, that ways may be found to live safely and comfortably through the entire range from 0 to 1 g. In that case $\epsilon$ would equal 1, and the entire volume of the space colony is habitable.
Projected Area
City planners and architects design human habitation in terms of the surface area on which buildings may be constructed. In the kinds of habitats discussed in this study, the curvature of surfaces on which colonists might live is often pronounced. It seems reasonable to define available surface area as the projection of area onto a plane perpendicular to the direction of the pseudogravity. In a torus the projected area is a strip through the diameter of the tube of the torus (see fig. 4-17). If the minor radius is r and the major radius is R, the projected area for a torus is $A_{pt} = 4\pi rR$.
This is just $1/\pi$ times the total skin area of a torus. Note that if the torus is spun so that there is 1 g of pseudogravity at the outermost surface, and if the aspect ratio, $\alpha = r/R$, is greater than the habitability parameter, $\epsilon$, the plane of projected area is outside the habitable volume. For all the cases considered, $\alpha < \epsilon$, and the above formula is sufficient.
For a rotating sphere the projected plane of usable area is the surface of a cylinder inscribed in the sphere (see fig. 4-18). The surface of this cylinder should not be more than $\epsilon R$ above the surface of the sphere. The projected area then is $A_{pr} = 4\pi R^2 \sqrt{\epsilon(2-\epsilon)}$.
At $\epsilon = 0.29$ this expression has a maximum $A_{pr} = 2\pi R^2$. Consequently for $\epsilon \ge 0.29$ the expression for the maximum can be used. (Alternatively, for smaller $\epsilon$, the habitat might be spun to produce 1 g at $R/\sqrt{2}$ to maximize the available area).
For a cylinder of radius R and length L the projected area is just the surface area $2\pi RL$. Table 4-6 summarizes the expressions for projected area in different geometries.
Habitable Volume
Although projected area represents an important concept in conventional architectural thinking, the available volume in the habitat may be more relevant in specifying the apparent population density and the quality of life. Habitable volume is defined as that volume in which the pseudogravity does not vary more than the specified amount, $\Delta g$, from the nominal value of g. Consequently, habitable volume depends on $\Delta g/g$.
For a cylinder of length L and radius of rotation R, the habitable volume is the annulus between R and $(1 - \epsilon)R$. In a sphere with a pseudogravity no greater than 1 g on its surface, habitable volume is the figure of revolution of the shaded area (as for the sphere in fig. 4-18).
In a torus with 1 g at its outermost circumference, habitable volume is the shaded area of the tube revolved around the axis of rotation. The formulas for these volumes are given in table 4-6.
TABLE 4-6 — PROJECTED AREAS AND HABITABLE VOLUMES
| Geometry | Projected Area | Habitable Volume | | :--- | :--- | :--- | | Torus | $4\pi rR$ | $2\pi^2 r^2 R [1 - (1-\epsilon)^2]$ | | Sphere | $4\pi R^2 \sqrt{\epsilon(2-\epsilon)}$ | $\frac{4}{3}\pi R^3 [1 - (1-\epsilon)^3]$ | | Cylinder | $2\pi RL$ | $\pi L R^2 [1 - (1-\epsilon)^2]$ |
$\epsilon = \Delta g/g$ r = minor radius of torus R = radius of rotation L = length of cylinder
Area and Volume Requirements
The study group determined that a reasonable standard of projected area is 67 $m^2$/person. Also, a detailed inventory of structures and facilities required for individual and community life suggests that habitable volume should be about 1740 $m^3$/person. Consequently, a habitat, or a collection of habitats, suitable for a given population of 10,000 people, must provide an area of 670,000 $m^2$ and a volume of 17,400,000 $m^3$. These numbers determine the geometry in a fundamental way.