Table of Contents

4. Choosing Among Alternatives

A few years after people move into the first colony, the system should settle down and operate as described in chapter 1. But why is the colony shaped as a torus and located at L5 with ore supplies from the Moon? Why is it not a sphere out at the asteroids or near a moon of Mars, or a cylinder in geosynchronous orbit around the Earth, or some other combination of alternatives? What are these alternatives, and why were they rejected? The purpose of this chapter is to answer these questions by evaluating reasonable alternatives in terms of the goals of the design study (ch. 1) and the criteria laid out in chapters 2 and 3.

A successful systems design combines subsystems satisfying various conflicting criteria to produce a unified working entity. The parts of the space colony- transportation, mining, the habitat, manufacturing, agriculture, and so on - must interact and interrelate in such a way that the demands of each for energy, raw materials, manpower, transport, and waste removal can be met by the overall system. In turn this system must satisfy the physiological, cultural, architectural, and physical criteria necessary to maintain a permanent human community in space using near-term technology and at a minimum cost.

In 10 weeks the study group was able to assemble only one reasonably consistent picture of life in space; there was no time to go back through the system and attempt to find optimal combinations of the subsystems. Moreover, again because time was short, many of the comparisons among alternative subsystems were more qualitative than study group members would have liked.

Effort devoted to alternatives depended upon the particular subject. A great deal of time was spent considering different forms for the habitat, how to handle the shielding and how to process lunar material. Less time was given to considering alternative patterns of siting the colony and its parts, of different ways to achieve life support, or of various possible transportation systems. In some cases much effort was expended but few alternatives were generated; an example is the system for moving large amounts of matter cheaply from the Moon to the colony. No alternative at all was found to the manufacture of solar satellite power plants as the major commercial enterprise of the colony.

It is important to realize that the alternatives described in this chapter constitute a major resource for improving the proposed design and for constructing new designs that meet other criteria. Rejection of any concept for the current "baseline system" does not mean that concept is fundamentally flawed. Some alternatives were rejected because they failed to meet the criteria, which were deliberately chosen conservatively and might well be changed on the basis of future experience or under different assumptions. Others were rejected simply because information about them was incomplete. Yet others were not chosen because their virtues were recognized too late in the study to incorporate them into a unified overall picture.

The alternatives might also be useful for designing systems with other goals than permanent human settlement in space; for example, space factories with temporary crews, or laboratories in space. Alternatively, new knowledge or advances in technology, such as the advent of laser propulsion or active shielding against ionizing radiation, might make rejected subsystems very desirable.

THE SHAPE OF THE HABITAT

What shape is most suitable to house this colony of 10,000 people? The question is particularly interesting for several reasons. The appearance and arrangement of the habitat are most obvious and understandable by everyone, being the most direct exhibition of the reality of the idea of the colony - seeing the form is believing - and the habitat natutrally attracts a great deal of attention although it is only one part of a much larger system. Moreover, the reader may already be aware of one or more possibilities: the rotating cylinders proposed by O'Neill (ref. 1), the torus of Von Braun (ref. 2), and their corresponding entities in the science fiction of Arthur C. Clarke (refs. 3, 4). The subject is also one particularly suited for systematic treatment and can serve as an excellent example of the methodology of systems design.

Some General Considerations

Because it is expedient, although not entirely justified, to treat the shielding which protects against the dangerous radiations of space separately from the choice of the geometry of the habitat's structure, that problem is left to a subsequent section. Subject to possible effects of the shielding, the choice of habitat geometry is determined by meeting the criteria of the previous chapter at minimum cost. In considering how different configurations may supply enough living space (670,000 m^2) and how they meet the physiological and psychological needs of people in space, the following discussion uses the properties of materials outlined in (refs. 5, 6). The possible "smooth" shapes are the ones generated from the curves in figure 4-1.

Four fundamental configurations arise:

  1. A sphere-by rotating curve 1 about either axis
  2. A cylinder-by rotating curve 2 about the z axis
  3. A torus-by rotating curve 3 about the r axis
  4. A dumbbellby rotating curve 3 about the z axis

    Neglecting secondary effects from variations in pseudogravity and localized bending stresses from discontinuities in deformations, the study group concluded that all possible membrane shapes, that is, any possible habitat, must be one of the four simple forms described above or some composite of them as shown in figure 4-2.

    The desire to keep structural mass small favors small radii of curvature. As figure 4-3 shows, the wall thickness to contain a given pressure drops quickly with decreasing. Of course, structural mass can also be reduced by lowering the pressure of the gas. Both possibilities turn out to be useful.

    A Rotating System With 1 g at Less Than 1 rpm

    Rotation is the only feasible way to provide artificial gravity in space. Pseudogravity depends upon both rotation rate and radius of rotation, and figure 4-4 shows the lines of constant pseudogravity as functions of these two variables (ref. 7). On the graph are shown a number of rotating systems: C-l through C4 are the rotating cylinders proposed earlier (ref. l) by O'Neill; T-1 is a torus and S-1 is a sphere described later in this chapter; Arthur C. Clarke's Rama (ref. 4) is shown, as are space stations of Gray (The Vivarium) (ref. 8), Von Braun (ref. 2) , and Tsiolkovsky (ref. 9). Obviously only systems with radii of rotation greater than 895 m can lie on the line g= 1 below 1 rpm.

    An aluminum cylinder like C-3 would weigh about 42,300 kt and have a projected area of 55 X I0^6 m^2, enough to hold 800,000 people - rather than the 10,000 people of the design criteria. Similarly a sphere of radius 895 m would hold 75,000 people and weigh more than 3500 kt if made of aluminum. A dumbbell shape has the advantage that the radius of curvature of the part holding the atmosphere can be made small while the radius of rotation remains large. However, in this configuration people could only live on the cross section of the spheres, and to hold 10,000 people with 670,000 m^2 of projected area the spheres would have to be 326.5 m in radius. Together they would weigh about 380 kt.

    A torus also permits control of the radius that contains the atmosphere separately from the radius of rotation. Moreover, the torus can distribute its habitable area in a large ring. Consequently, the radius needed to enclose the 670,000 m^2 of projected area can be quite small, with a correspondingly small mass-about l50 kt for a torus of major radius 830 m and minor radius 65 m (where the mass of internal structure is neglected). The advantages of the torus compared to the sphere and cylinder are discussed further in appendices B and C which define some criteria and parameters useful for such comparisons. The important point is that for a given radius of rotation about four times more mass is required to provide a unit of projected area in a cylinder or a sphere than in a torus of small aspect ratio. Thus, among the simple, basic shapes the torus is clearly superior in economy of structural mass.

    If minimum structural mass were the only concern, composite structures would be the choice. Twenty-five pairs of dumbbells would supply 670,000 m^2 with spheres 65 m in radius and a total mass of 72 kt. The spheres could be made smaller still and formed into a ring to make a beaded torus. Alternatively, the toruses themselves could be made with quite small minor radii and either stacked and connected together to form a kind of banded torus, or built separately to form a group of small, independent habitats.

    However, as pointed out in the previous chapter, it is desirable to compensate for the artificial and crowded nature of the habitat by designing it to give a sense of spaciousness. Composite structures are rejected largely on architectural criteria of environmental perception. Not only would they be more difficult to build than the simpler shapes, but also their short lines of sight, little free volume and internal arrays of closely-spaced cables and supporting members would produce an oppressive ambience.

    If the colony were composed of a number of small structures, there would be problems of communication and transport between them as well as the drawbacks of small scale. Nevertheless, as table 4-1 shows, multiple structures (and composite ones too) offer substantial savings in mass, and it might well be that some of their undesirable aspects could be reduced by clever design. It would be an attractive option to be able to build up a colony gradually out of smaller units rather than to start off with an initial large scale structure. The subject of multiple and composite structures is worthy of more consideration. The various properties of possible configurations are summarized in table 4-1. The parameters show the mass requirements and indicate the degree of openness of the different structures. The single torus, although not the best design in many respects, seems to give the most desirable balance of qualities. Relative to the sphere and cylinder it is economical in its requirements for structural and atmospheric mass; relative to the composite structures it offers better esthetic and architectural properties. Because of its good habitability properties, large volume, a variety of possible internal arrangements, the possibility of incremental construction, a clear circulation pattern, access to zero gravity docks and recreation at the hub, agriculture as an integral part of the living area, and a clear visual horizon for orientation, the torus is adopted as the basic form of the habitat. The dimensions of this single torus are given in the first column of table 4-1.

    TABLE 4-1 (a) (gif format)

    TABLE 4-1.- PARAMETERS OF POSSIBLE HABITATS, 1 RPM

    (a) Single component
    Single torus,
    R maj=830 m,
    R min=65 m
    Cylinder with
    spherical endcaps,
    R = 895 m,
    L = 8950 m
    Sphere,
    R = 895 m
    Dumbbell,
    R = 895 m,
    R sphere =316 m
    Number of components 1 1 1 1
    Structural mass at 1/2 atm, kt 150 42,300 3545 380
    Projected area, m^2 6.8 X 10^5 550 X 10^5 50.3 X 10^5 6.3 X 10^5
    Surface area, m^2 2.1 X 10^6 60.3 X 10^6 10.1 X 10^6 2.5 X 10^6
    Shielding mass, Mt 9.9 23.3 46.7 33.5
    Volume, m^3 6.9 X 10^7 2265 X 10^7 300 X 10^7 13.2 X 10^7
    Mass of atmosphere, kt 44 14,612 1930 85
    Segmentation Easy, optional Difficult Difficult Difficult
    (Vistas) Longest line of sight, m 640 10740 1790 732
    (Vistas) Solid angle of 50 percent
    sight line, sr
    0.5 0.09 4.2 4.2
    (Vistas) Fraction of habitat
    hidden from view
    0.70 0 0 0.5
    (Communication),
    Longest distance of
    surface travel, m
    2600 11,800 2800 1800
    (Communication), Fraction viewable by
    internal line of sight
    from one place
    0.3 1 1 0.5
    Interior: Openess Good Good Good Good
    Interior
    Population capacity at
    67 m^2/person
    10,000 820,000 75,000 10,000


    SHIELDING

    The need to shield humans adequately from the ionizing radiations of space imposed some significant design decisions. An ideal shield would bring the radiation dosage below 0.5 rem/yr cheaply and without impairing the contact of the colonists with their environment. However, after considering active shields which electromagnetically trap, repel or deflect the incident particles, and a passive shield which simply absorbs the particles in a thick layer of matter, the study group chose the passive shield for their design.

    Active Shields

    When a charged particle passes through a magnetic field, its path curves. Thus, as figure 4-5 shows, the proper configuration of magnetic field lines can form a shielded region which particles cannot enter.
    Since for a given magnetic field the curvature of the path of a particle is inversely proportional to its momentum, the region is shielded only against particles below a certain cutoff momentum or cutoff energy. Particles above this cutoff energy can still penetrate (ref. 10).

    The problems of magnetic shielding become apparent when the cutoff energy has to be chosen. Protection against heavy ion cosmic rays, the so-called high-Z primaries (i.e. the iron nuclei and others mentioned in chapter 2) and most solar flares would be achieved with a cutoff of 0.5 GeV/nucleon. The difficulty is that most secondary particles are created from the primary flux above 2 GeV/nucleon which can penetrate the shield and generate secondaries in the mass of the shield itself. As a consequence a magnetic field around the torus with a cutoff of 0.5 GeV/nucleon and a structural mass of about 10 kt, corresponding to a thickness of matter of 0.5 t/m^2, would actually increase the exposure to about 20 rem/yr. Only the addition of shielding to an extent of 1.3 t/m^2 could reduce the dosage to a level equivalent to there being no secondary particle generation by shielding, that is, about 8 rem/yr. Furthermore, even then a specially heavily shielded shelter would be required as protection against secondaries produced by the strongest solar flares. The consequences of the production of secondary particles are shown in figure 4-6.

    A cutoff of 10 or 15 GeV/nucleon would eliminate so many of the high energy particles that even with secondary production the dose would not be above 0.5 rem/yr. A shield of this capability would also protect against the effects of the strongest solar flares, and no shelter would be needed. The difficulty is that the structural mass required to resist the magnetic forces between superconducting coils precludes this design even for the most favorable geometry, namely, a torus.

    Similarly, electric shielding by a static charge seems infeasible since a 10-billion-volt potential would be required for even moderate shielding. On the other hand, a charged plasma which sustains high electrical potential in the vicinity of the habitat is a more promising approach (ref. 11). However, means to develop such a plasma requires extensive research and technical development before a charged plasma might be considered for design. Some further details of this approach are given in appendix D.

    Passive Shield

    Passive shielding is known to work. The Earth's atmosphere supplies about 10 t/m^2 of mass shielding and is very effective. Only half this much is needed to bring the dosage level of cosmic rays down to 0.5 rem/yr. In fact when calculations are made in the context of particular geometries, it is found that because many of the incident particles pass through walls at slanting angles a thickness of shield of 4.5 t/m^2 is sufficient. Consequently it was decided to surround the habitat with this much mass even though it requires that many millions of tonnes of matter have to be mined and shipped to the colony.

    TABLE 4-1 (b) (gif format)

    TABLE 4-1.- PARAMETERS OF POSSIBLE HABITATS, 1 RPM

    (b) Multiple components
    Multiple dumbbells,
    R sphere = 65 m,
    R = 875 m
    Multiple torus,
    R maj =880 m,
    R min = 15 m
    Banded torus,
    R major = 880 m,
    R min = 15 m
    Number of components 25 4 1 (7 bands)
    Structural mass at 1/2 atm, kt 75 100 112
    Projected area, m^2 6.6 X 10^5 6.6 X 10^5 6.6 X 10^5
    Surface area, m^2 2.7 X 10^6 2.1 X 10^6 1.7 X 10^7
    Shielding mass, Mt 9.9 9.7 7.0
    Volume, m^3 5.8 X 10^7 1.6 X 10^7 2.1 X 10^7
    Mass of atmosphere, kt 37 10.4 13.2
    Segmentation Unavoidable Unavoidable Easy
    (Vistas) Longest line of sight, m 130 45 161
    (Vistas) Solid angle of 50 percent
    sight line, sr
    4.2 0.11 0.11
    (Vistas) Fraction of habitat
    hidden from view
    0.98 0.94 0.82 to 0.94
    (Communication),
    Longest distance of
    surface travel, m
    1800 2600 2600
    (Communication), Fraction viewable by
    internal line of sight
    from one place
    0.02 0.06 0.06
    Interior: Openess Poor Poor Poor
    Interior
    Population capacity at
    67 m^2/person
    10,000 10,000 10,000

    Table 4-1 shows the shielding masses required for different configurations; the single torus requires 9.9 Mt of shield. This much mass cannot be rotated at the same angular velocity as the habitat because the resultant structural stresses would exceed the strength of the materials from which the shield is to be built. Consequently the shield must be separate from the habitat itself and either rotated with an angular velocity much less than 1 rpm or not rotated. To minimize the mass required, the shield would be built as close to the tube of the torus as possible, and therefore the rotating tube would be moving at 87 m/s (194 mph) past the inner surface of the shield from which it is separated by only a meter or two. The consensus of the study group was that the engineering necessary to assure and maintain a stable alignment between the moving torus and its shield would not, in principle, be difficult. However, no attention in detail was given to this problem.

    WHAT IF THE CRITERIA CHANGE?

    The conservative design criteria presently adopted for permanent life in space are derived from research on Earth and in space, especially Skylab missions, that gives very little indication of the actual effects of living in space for many years. In the time leading up to the colonization of space more information will become available, and it may lead to substantial changes in the configuration proposed in this study.

    Higher Population Density

    A very simple change would be to reduce the amount of area available per person. Under these circumstances several of the structures described in table 4-1 would be made less massive. By placing the agriculture outside the shielded area and by reducing the remaining projected area available from 47 m^2 per person to 35 m^2, substantial savings could be made in both structural and shielding mass (table 4-2).

    This 25 percent increase in crowding may not be so drastic as it appears, since use can be made of the three dimensionality of space in a way more effective than is done on Earth. With sufficiently large overhead spaces between levels, several levels could be included in a habitat while maintaining an impression of openness. This approach would be particularly advantageous if the gravity criteria were relaxed as well.

    Lower Simulated Gravity and Higher Rotation Rates

    It is particularly interesting to examine the consequences of simultaneously relaxing the requirements of pseudogravity and rotation rate. If instead of 0.95 +/- 0.05 g and 1 rpm, the design allows 0.85 +/- 0.15 g and 1.9 rpm some interesting possibilities emerge. Under these new conditions, parameters for the same geometries discussed earlier are summarized in table table 4-2

    TABLE 4-2 (a) (gif format)

    TABLE 4-2.- PARAMETERS OF POSSIBLE HABITATS WITH OTHER CRITERIA (EXTERNAL AGRICULTURAL)

    (a) Single Components
    Single torus,
    R maj=209m,
    R min=27 m
    Cylinder with
    spherical endcaps,
    R = 236 m,
    L = 2360 m
    Sphere,
    R = 236 m
    Dumbbell,
    R = 236 m,
    R sphere = 33.3 m
    Number of components 1 1 1 1
    Structural mass at 1/2 atm, kt 4.6 775 64.6 0.4
    Projected area, m^2 0.7 X 10^5 38.5 X 10^5 3.5 X 10^5 0.07 X 10^5
    Surface area, m^2 2.2 X 10^5 42.0 X 10^5 7.0 X 10^5 0.07 X 10^5
    Shielding mass, Mt 1.0 19.4 3.3 1.4
    Volume, m^3 0.3 X 10^7 46.8 X 10^7 5.5 X 10^7 0.031 X 10^7
    Mass of atmosphere, kt 1.9 299 35.2 0.20
    Segmentation Unavoidable Difficult Difficult Unavoidable
    (Vistas) Longest line of sight, m 206 2800 470 67
    (Vistas) Solid angle of 50 percent
    sight line, sr
    0.9 0.09 4.2 4.2
    (Vistas) Fraction of habitat
    hidden from view
    0.65 0 0 0.5
    (Communication),
    Longest distance of
    surface travel, m
    720 3100 740 540
    (Communication), Fraction viewable by
    internal line of sight
    from one place
    0.35 1.0 1.0 0.5
    Interior: Openess Fair Good Good Fair
    Interior
    Population capacity at
    35 m^2/person
    2000 110,000 10,000 2,000


    TABLE 4-2 (b) (gif format)

    TABLE 4-2.- PARAMETERS OF POSSIBLE HABITATS WITH OTHER CRITERIA (EXTERNAL AGRICULTURAL)

    (b) Multiple components
    Multiple dumbbells,
    R = 236 m
    R sphere = 33.3 m
    Multiple torus
    R maj = 209 m,
    R min = 27 m
    Banded torus,
    R maj =209 m,
    R min = 27 m
    Number of components 50 5 1 (8 bands)
    Structural mass at 1/2 atm, kt 20 23.2 26
    Projected area, m^2 3.5 X 10^5 3.6 X 10^5 3.6 X 10^5
    Surface area, m^2 13.9 X 10^5 11.2 X 10^5 8.2 X 10^5
    Shielding mass, Mt 7.2 5.2 3.6
    Volume, m^3 1.5 X 10^7 1.5 X 10^7 1.8 X 10^7
    Mass of atmosphere, kt 9.9 9.5 11.3
    Segmentation Unavoidable Unavoidable Easy
    (Vistas) Longest line of sight, m 67 100 100
    (Vistas) Solid angle of 50 percent
    sight line, sr
    4.2 0.9 0.9
    (Vistas) Fraction of habitat
    hidden from view
    0.99 0.93 0.9
    (Communication),
    Longest distance of
    surface travel, m
    540 720 720
    (Communication), Fraction viewable by
    internal line of sight
    from one place
    0.01 0.07 0.1
    Interior:Openess Good Fair Poor
    Interior
    Population capacity at
    35 m^2/person
    10,000 10,000 10,000

    A major consequence is that the radius of rotation now becomes 236 m as figure 4-4 confirms.

    With this new radius of rotation neither a single torus nor a single dumbbell can supply sufficient space for a colony of 10,000. A cylinder, as before, supplies far too much. The sphere, on the other hand, supplies exactly the right amount and becomes an attractive possibility for a habitat. As the table shows, however, multiple and composite structures would still be contenders although they would be even more deficient in the desirable architectural and organizational features.

    To be more specific, figure 4-7 illustrates a possible spherical design with the agriculture placed in thin toruses outside the shielded sphere.

    This configuration has been named the Bernal sphere in honor of J. D. Bernal (ref. 12). When the Bernal sphere is compared with its nearest competitor, the banded torus, it is seen to be particularly efficient in its shielding requirements, needing 300,000 t less than the banded torus and millions of tonnes less than any other configuration. The Bernal sphere, however, requires from 3 to 4 times as much atmospheric mass as the other possible forms, and from 2 to 4 times as much structural mass.

    Higher Radiation Exposures

    As more is learned about the effects of ionizing radiation, it is possible that larger exposures to radiation might be found to be acceptable. Such a change in this criterion would make active magnetic shielding an interesting possibility and might also favor the development of a plasma shield. Of course, if higher levels of radiation became acceptable, a smaller amount of passive shielding would be needed so that the mass of shielding might become less significant in determining habitat design.

    Any of these changes might shift the favored emphasis from one geometry to another. A choice of a particular form would again have to balance aesthetic against economic requirements, and it is certain that more investigation of this problem will be necessary. A particularly important question is the relative cost of shielding mass, structural mass, and atmospheric mass. Knowledge of these costs is basic to deciding which geometric alternative to select.

    FABRICATION TECHNIQUES

    Although the construction of large structures in space places strong emphasis on fabrication techniques, relatively little attention was devoted to the subject by the summer study group. The few alternatives considered did not seem to be mutually exclusive, but instead mutually supportive. Only a brief description of these alternatives is given.

    Initial Construction Facilities

    Fabrication facilities needed to build the habitat and supporting factories and power plants were described at a Princeton Conference, May 1975, on metal forming in space by C. Driggers.This proposal has been adopted. Standard tecnnology for hot and cold working metals is sufficient to form the sheet, wire and structural members needed. An extensive machine shop must be provided so that many of the heavy components of a rolling mill, extrusion presses, casting beds and other equipment can be made at the space colony rather than have to be brought from Earth.

    Building the Habitat Shell

    Assembly of the habitat from aluminum plate and ribs proceeds first from the spherical hub (including docking facilities) outward through the spokes to start the torus shell. Both the spokes and shell are suitable for construction by a "space tunneling" concept in which movable end caps are gradually advanced along the tube as construction proceeds. This allows "shirt-sleeve" conditions for workmen as they position prefabricated pieces brought through the spokes and make the necessary connection. Large pieces of shield are placed around the completed portions as the slag material becomes available from the processing plant. Internal structures are built when convenient. However, every effort must be made to complete the basic shell and the first layer of shielding as quickly as possible so that spin-up can begin, gravity can be simulated, and the construction crew and additional colonists can move in to initiate life support functions within the habitat. A critical path analysis will reveal the best sequencing of mirror, power plant, shield, and internal construction.

    An alternative technology for fabrication in space, which deserves more investigation, is the making of structures by metal-vapor molecular beams. This is discussed in more detail in appendix E. If proved out in vacuum chamber experiments, this technique may cut the labor and capital costs of converting raw alloys into structures by directly using the vacuum and solar heat available in space. Its simplest application lies in the fabrication of seamless stressed-skin hulls for colony structures, but it appears adaptable to the fabrication of hulls with extrusive window areas and ribs, as well as to rigid sheet-like elements for zero-gravity structures such as mirrors and solar panels.

    A simple system might consist of a solar furnace providing heat to an evaporation gun, which directs a conical molecular beam at a balloon-like form. The form is rotated under the beam to gradually build up metal plate of the desired strength and thickness. While depositing aluminum, the form must be held at roughly room temperature to ensure the proper quality of the deposit.

    Structures Inside the Habitat

    To fulfill the criteria set forth in chapter 2, a lightweight, modular building system must be developed to serve as an enclosing means for the various spatial needs of the colony.

    Modular building systems developed on Earth can be categorized into three general types: that is, box systems using room-size modules; bearing-panel systems; and structural-frame systems. A box system entails asembling either complete shells or fully completed packages with integrated mechanical subsystems. Bearing-panel systems use load-bearing wall elements with mechanical subsystems installed during erection. Structural-frame systems use modularized framing elements in combination with nonload-bearing wall panels and mechanical subsystems which are normally installed during erection. Other systems which have seen limited application on Earth but would be appropriate in the colony include: cable supported framing systems with nonload-bearing fabric and panel space dividers, and pneumatic air structures using aluminum foil and fiberglass fabrics with rigid, aluminum floor elements.

    In selecting a baseline configuration, box systems were rejected because they normally involve the duplication of walls and floors and tend to be overly heavy. If metal vapor deposition is developed as a forming technique however, this type of system would become highly desirable. Bearing-panel systems were likewise rejected since they do not allow integration of mechanical subsystems except during erection, and since walls are heavy because they are load bearing. Cable and pneumatic systems were rejected due to their inability to span short distances without special provisions. However, they might be highly desirable because of their flexibility and lightness if a lower gravity environment proves acceptable in the colony.

    The system that appears most suitable for use in the colony might involve a light, tubular structural frame (composed of modular column and beams) in combination with walls that are nonload bearing and with prepackaged, integrated mechanical subsystems (such as bathrooms) where needed. This system provides lightweight modularity to a high degree, good spanning capabilities, easily obtainable structural rigidity, and short assembly time since all labor intensive mechanical systems are prefabricated. A schematic (ref. 13) of some possible components of such a system is shown in figure 4-8. Applications of such a system to the colony are many and could be applied to all necessary enclosures with proper adaptation to the various specialized needs of life in space.

    Some of the possible materials and components investigated as especially suitable for building in space are illustrated in appendix F. Elements that are light and strong and could be made from materials available in space are favored. The exterior and interior walls and the floor components are built from these materials. The floor components are based on extremely light yet strong elements designed for Skylab.

    THE PEOPLE IN THE COLONY

    It is not usual to think of human population as something to be designed. Nevertheless the numbers, composition, age and sex distribution, and productivity of the colonists bear importantly on the success of the project and on the creation of a suitable design. The study had to consider who should be the colonists, how many there should be, what skills they must have, and how they should organize and govern themselves. The alternatives are numerous and the grounds for choosing between them not as definite as for the more concrete problems of engineering, but it was possible to make what seem to be reasonable choices based on the goals of having in space permanent communities of sufficient productivity to sustain themselves economically.

    Size and Suitability of Population

    It is possible in principle to specify a productive task, for example, the manufacture of solar power satellites, and then calculate the number of people necessary to perform it, the number needed to support the primary workers, and the number of dependents. The sum of such numbers does not accurately define the population needed to found a colony since the calculation is complex. Even a casual consideration of what is necessary for a truly closed society would suggest that a colony population be far in excess of any reasonable first effort in space.

    A similar approach would bypass the calculation just described and simply copy the population size and distribution of a major productive urban center on Earth. The difficulty, however, is that such communities are quite large, on the order of some hundreds of thousands of people. Moreover, close inspection reveals that human communities on Earth are less productive by labor force measurement standards than what would be needed in at least the early stages of space colonization.

    One way to have a colony more productive than Earth communities would be to make the colony a factory, populated only by workers. The colony would then be only a space station, with crews of workers rotated in and out, much as is done on the Alaska pipeline project. Aside from the serious problems of transportation, such an approach does not meet the goal of establishing permanent human communities in space.

    In the face of these difficulties a rather arbitrary decision is made to design for a colony of 10,000 with an attempt to bias the population in directions that favored high productivity but does not compromise too badly the goal of setting up a community in which families live and develop in a normal human way. It is also assumed that the completed colony is not an isolated single undertaking, but is a first step in a rapidly developing program to establish many colonies in space.

    Ethnic and National Composition

    The possible variations in nationality or ethnic composition are in principle very great. The actual composition will depend largely on who sponsors and pays for the colonization. If colonization were undertaken as a joint international project, the composition of the population would surely reflect that fact. On balance, however, it seems reasonable for the purposes of this design to assume that the first space colony will be settled by persons from Western industrialized nations.

    Age and Sex Distributions

    The initial population of the first colony is projected to grow from a pool of some 2000 construction workers who, in turn, bring immediate family members numbering an additional one to three persons per worker. Selective hiring of construction crew members tends to bias this population toward certain highly desirable skills, and toward the younger ages. In anticipation of the labor needs of the colony and the need to avoid the kinds of burdens represented by large dependent populations, a population is planned with a smaller proportion of old people, children and females than the typical U.S. population. It is a close analog of earlier frontier populations on Earth.

    The proposed population is conveniently described in terms of differences from the population of the United States as described in the 1970 Census (ref. 14). These changes are illustrated in figure 4-9 which compares the colony with the composition of a similar sized community on Earth. The sex ratio is about 10 percent higher in favor of males, reflecting both the tendency of construction workers to be male and the expectation that by the time construction begins in space an appreciable fraction of terrestrial construction workers are female. Partly for this last reason and partly because of the anticipated need for labor in the colony, sizable increase in the proportion of married women in the labor force is assumed. Most striking is the substantial shift of the population out of the more dependent ages - from under 20 and over 45 into the 21 to 44 age class.

    Export Workers

    Productivity of any community is importantly influenced not only by the size of the labor force but also by the share of worker output going for export. Numbers on modern U.S. communities (see, e.g., appendix G) indicate that in our complex society the percentage engaged in export activity is generally less in the larger cities than in the smaller towns. The maximum activity for export seems to be about 70 percent. Without taking into account the peculiarities of life in space, the study group assumes that 61 percent of the workforce, or nearly 44 percent of the population of the initial colony would be producing for export (see fig. 4-9). This percentage declines as the colony grows. Conversely at an early stage in its development when the population is about 4300, the workforce is about 3200, with 2000 producing for export. Appendix G provides the data from which these assumptions are derived.

    Social Organization and Governance

    The form and development of governance depend strongly on the cultural and political backgrounds of the first colonists. The subject is rich with possibilities ranging from speculative utopian innovations to pragmatic copies of institutions existing on Earth. Among the alternatives easily envisioned are quasimilitary, authoritarian hierarchies, communal organizations like kubbutzim, self-organized popular democracies operating by town meetings, technocratic centralized control, or bureaucratic management similar to that of contemporary large corporations.

    It seems most likely that government for the initial colony would be based on types of management familiar in government and industry today. There would be elements of representative democracy, but the organization would surely be bureaucratic, especially as long as there is need for close dependency on Earth. But whatever the forms initially, they must evolve as the colonists develop a sense of community, and it is easy to imagine at least two stages of this evolution.

    First there is the start of colonization by some Earth-based corporate or governmental organization. Later, as continued development leads to more and more settlement, the colonists form associations and create governance bodies which reflect rising degrees of community identity, integration and separation of decision making powers from organizations on Earth. These changes evolve first within a single habitat and then cooperative and governmental relations develop when neighboring habitats and a larger community grow. The rate at which this evolution occurs is uncertain.

    LIFE SUPPORT

    What do the colonists eat and how do they obtain this food? What do they breathe? How do they deal with the industrial and organic wastes of a human community in space? These questions pose the basic problems to be solved by life support systems. Richness of life and survival from unforeseen catastrophes are enhanced by diversification and redundance of food supplies, energy sources, and systems for environmental control, as well as by variety of architecture, transportation and living arrangements, and these considerations are as important in choosing among alternatives for life support as in making choices among other subsystems.

    Food

    Food supplies can be obtained from Earth or grown in space or both. Total supply from the Earth has the advantage that the colony would then have no need to build farms and food processing facilities or to devote any of its scarce labor to agriculture. However, for a population of 10,000 the transport costs of resupply from Earth at 1.67 t/yr per person is about $7 billion/yr. The preferred choice is nearly complete production of food in space.

    Whatever the mode of production, it must be unusually efficient, thereby requiring advanced agricultural technologies (ref. 15). Direct synthesis of necessary nutrients is one possibility, but such biosynthesis is not yet economically feasible (J. Billingham, NASA/Ames, personal communication), 1. Also, algae culture and consumption have long been envisioned as appropriate for life in space, but upon close inspection seem undesirable because algae are not outstandingly productive plants nor are they attractive to humans (ref. 16). The best choice seems to be a terrestrial type of agriculture based on plants and meat-bearing animals (ref. 17).

    This form of agriculture has the advantage of depending on a large variety of plant and animal species with the accompanying improvement in stability of the ecosystem that such diversity contributes (ref. 18). Moreover, plants and animals can be chosen to supply a diet familiar to the prospective colonists, that is, a diet appropriate to a population of North Americans biased in favor of using those plant and animal species with high food yields. Photosynthetic agriculture has a further advantage in that it serves as an important element in regeneration of the habitat's atmosphere by conversion of carbon dioxide and generation of oxygen. It also provides a source of pure water from condensation of humidity produced by transpiration (ref. 19).

    Choices of food sources within the general realm of terrestrial agriculture become a compromise between preference and diversity on the one hand and efficiency on the other. For the colony, efficient use of area (even at expense of efficiency measured in other terms, i.e., as energy) is a critical factor to be balanced against a varied and interesting diet. For example, the almost exclusive use of rabbits and goats for animal protein previously proposed (ref. 15) for space colonies is rejected as being unnecessarily restrictive and seriously lacking in variety.

    Recycling Wastes

    High costs of transportation place great emphasis on recycling all the wastes of the colony. Because in the near future Earth appears to be the only practical source of elements fundamental to agriculture - carbon, nitrogen, and hydrogen - they must initially be imported from Earth. To avoid having to continually import these elements, all wastes and chemicals are recycled with as small a loss as possible.

    Waste water can be treated biologically as in most terrestrial communities, physiochemically, by dry incineration, or by some more advanced technique such as electrodialysis, electrolysis, vapor distillation or reverse osmosis (ref. 20). Each of these alternatives is ruled out for various reasons. Biological treatment provides only incomplete oxidation and produces a residual sludge which must then be disposed of with attendant risks of biological contamination. Physiochemical treatment has no organic conversion, and is chemically a difficult process. Dry incineration requires an external energy source to maintain combustion and it produces atmospheric pollutants. All the advanced processes are incomplete in that the resulting concentrates require further treatment.

    Wet oxidation (Zimmerman process) has none of the foregoing defects. Operating at a pressure of 10^7 MPa (1500 lb/in.^2) and a temperature of 260 degrees C, wet oxidation with a total process time of 1-1/2 hr produces a reactor effluent gas free of nitrogen, sulfur and phosphorous oxides; a high quality water containing a finely divided phosphate ash and ammonia. Both the reactor gas and the water are sterile (refs. 21, 22). At solids concentrations greater than 1.8 percent the process operates exothermally with an increase in the temperature of the waste water by 56 degrees C (personal communication from P. Knopp, Vice-President, Zimpro Processing, Rothschild, Wisconsin). These definite advantages lead to the choice of this process as the basic technique for purification and reprocessing within the space colony.

    Composition and Control of the Atmosphere

    The desired composition of the atmosphere is arrived at as the minimum pressure needed to meet the criteria for atmospheric safety stated in chapter 2. This results in the atmospheric composition detailed in table 4-3.

    TABLE 4-3 - HABITAT ATMOSPHERE


    T = 20 +/- 5 DEGREES C

    Relative humidity = 50 +/- 10 percent
    GAS (kPa) (mmHg)
    O2 22.7 170
    N2 26.6 200
    CO2 <0.4 <3
    Total pressure 50.8 380
    Water vapor 1.0 7.5

    1 standard atmosphere = 101 kPa

    Its outstanding features are: normal terrestrial, partial pressure of oxygen, partial pressure of carbon dioxide somewhat higher than on Earth to enhance agricultural productivity, and a partial pressure of nitrogen about half of that at sea level on Earth. Nitrogen is included to provide an inert gaseous buffer against combustion and to prevent certain respiratory problems. Because nitrogen must come from the Earth, its inclusion in the habitat's atmosphere means there is a substantial expense in supplying it. This fact, in turn, suggests that it is desirable to hold down the volume of atmosphere in the habitat, a factor taken into consideration in the discussion of the habitat geometry given earlier. The total atmospheric pressure is thus about half that at sea level on Earth.

    Atmospheric oxygen regeneration and carbon dioxide removal are by photosynthesis using the agricultural parts of the life support system. Humidity control is achieved by cooling the air below the dewpoint, condensing the moisture and separating it. Separation of condensate water in zero gravity areas (such as the manufacturing area and hub) by hydrophobic and hydrophilic materials offers the advantage of a low pressure drop and lack of moving parts (ref. 23) and is the preferred subsystem.

    Trace contamination monitoring and control technology is highly developed due primarily to research done in submarine environments. The habitat environment is monitored with gas chromatograph mass spectrometer instruments (ref. 24). Trace contamination control can be effectively accomplished by absorbtion (e.g., on activated charcoal), catalytic oxidation, and various inert filtering techniques.


    SATELLITE SOLAR POWER STATIONS: NO ALTERNATIVES

    An important goal for the design for space colonization is that it be commercially productive to an extent that it can attract capital. It is rather striking then that the study group has been able to envision only one major economic enterprise sufficiently grand to meet that goal. No alternative to the manufacture of solar power satellites was conceived, and although their manufacture is likely to be extremely valuable and attractive to investors on Earth, it is a definite weakness of the design to depend entirely on this one particular enterprise. A number of valuable smaller scale manufactures has already been mentioned in chapter 2 and, of course, new colonies will be built, but these do not promise to generate the income necessary to sustain a growing space community.

    There is some choice among possible satellite solar power stations (SSPS). Two major design studies have been made, one by Peter Glaser of Arthur D. Little, Inc. (ref. 25), and the other by Gordon Woodcock of the Boeing Aircraft Corporation (ref. 26). Conceptually they are very similar, differing chiefly in the means of converting solar power to electricity in space. Woodcock proposes to do this with conventional turbogenerators operating on a Brayton cycle with helium as the working fluid; Glaser would use very large arrays of photovoltaic cells to make the conversion directly.

    There is not a great deal to argue for the choice of one system rather than the other, except perhaps that the turbogenerator technology proposed by Woodcock is current, while Glaser relies on projections of present day photovoltaic technology for his designs. In the spirit of relying on current technology, the Woodcock design seems preferable, but a definite choice between the two is not necessary at this time. A more detailed description of the SSPS alternatives with a discussion of microwave transmission and its possible environmental impact is given in appendix H.

    WHERE THE COLONY SHOULD BE LOCATED

    Chapter 2 surveyed space and described what is there and how space is shaped in terms of distance, propulsive effort and gravitational attraction. These aspects of space together with the location of needed resources are important to choosing a site for the habitat. The community should be located for convenience with respect to its resources-sunlight, weightlessness, and minerals - and also with access to and from its principal market, Earth. The site should be chosen by balancing the needs of production against the needs of marketing the product.

    Near to but not on the Moon

    The minerals of space are to be found in the distant outer planets, the asteroids, the nearer and more accessible planets like Mars, the moons of other planets, or our own Moon. Of course the Earth is a primary source of mineral wealth too. It seems reasonable to place the colony near one of these sources. For reasons explained in the next section, the Moon is chosen as the principal extraterrestrial source of minerals, hence the habitat should be near the Moon.

    But where should the habitat be placed in the vicinity of the Moon? At first glance the Moon's surface seems a good choice, but any part of that surface receives the full force of the Sun's radiation only a small fraction of the time. Moreover, on the Moon there is no choice of gravity; it is one-sixth that of Earth and can only be increased with difficulty and never reduced. Space offers both full sunshine and zero gravity or any other value of simulated gravity one might choose to generate. An additional difficulty with a lunar location is related to the major product of the colonies, SSPS's. Transporting them from the Moon to geosynchronous orbit is not economically viable. For ease of exploitation of the properties of space, the habitat should be located in free space.

    In Free Space at L5

    Although there is no stable location at a fixed point in space in the Earth-Moon system, the colony could be located in any one of a number of orbits in free space. These orbits can be around the Earth, or the Moon, or both the Earth and the Moon. Those near either the Earth or the Moon are rejected because of the frequency and duration of solar eclipses which deprive the colony of its light and energy. Large orbits around the Earth make it difficult to deliver the large mass of material needed from the Moon, while large orbits around the Moon become orbits in the Earth-Moon system about which little is known at the present time. These last two options, while not chosen, present interesting alternatives which should be examined more closely.

    There remain the orbits about the five libration points. Three of these, L1, L2, and L3, are known to be unstable, and to maintain orbits around any of these three points for long periods of time requires appreciable expenditures of mass and energy for station keeping.

    There do exist, however, large orbits around both of the remaining Vibration points, L4 and L5. These have been shown to be stable (refs. 27, 28). A colony in either of these orbits would be reasonably accessible from both Earth and Moon. One of these libration points, L5, is chosen for the location of the first space colony. This choice is somewhat arbitrary for the differences between L4 and L5 are very slight.

    MINING, TRANSPORT, AND PROCESSING IN SPACE

    From where will come 10 million tonnes of matter needed to build a colony? And where and how will it be processed, refined and shaped into the metals, glass and other necessary structural material? The topography of space shapes the answer to the first question; human ingenuity offers answers to the second. A major problem only partly solved is how to transport large quantities of matter from mines on the Moon to space. Some possible solutions to that problem are suggested.

    Sources

    As noted previously, lunar materials have been chosen to supply the great bulk of mass necessary for the first colony, including the shell and internal structure, passive shield, soil, and oxygen. As indicated in figure 4-10, only a small percentage of the mass, including initial structures, machinery, special equipment, atmospheric gases other than oxygen, biomass, and hydrogen for water, comes from Earth.

    This decision has been made for a variety of reasons. Of the bodies in the solar system which might supply materials, the other planets are eliminated by the expense of transportation from their surfaces, and the moons of the outer planets by transport times of years and by costs. This leaves the asteroids, comets, and the moons of Mars.

    While the composition of the moons of Mars is unknown, both the comets and asteroids are apparently abundant sources of organic materials in addition to rock and possibly nitrogen and free metals as well. For immediate future applications, however, the Moon's position makes it attractive and, compared to the asteroids, the Moon has advantages of known properties, a distance suitable for easy communication, and it allows perhaps simpler overall logistics.

    However, when the space colonization program is begun, technical and economic imperatives seem likely to drive it quickly toward exploitation of asteroidal rather than lunar materials and toward much less dependence on Earth. Long before the results of mining activity on the Moon became visible from the Earth, the colony program would be obtaining its materials from the asteroids. Given that source, the "limits of growth" are practically limitless: the total quantity of materials within only a few known large asteroids is enough to permit building space colonies with a total land area many thousands of times that of the Earth.

    Processing: Where?

    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.

    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.

    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. 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 FeTi03, 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 (SiO2), the basic ingredient in glassmaking, is found in abundance on the Moon. However, another basic constituent, sodium oxide (Na20), 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 Na2O 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 Na20 from the Earth (personal communication, J. Blummer, Vice-President for Research, Libbey OwensFord 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 SiO2), 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.

    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 super conducting 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 draft 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 l0^-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.

    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.

    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-l flyback" is discussed in appendix C of chapter 6.

    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.


    Chapter 5

    Table of Contents