Passive Shield
Passive shielding is known to work. The Earth's atmosphere supplies about 10 t/m² 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/m² 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 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.
TABLE 4-1 — PARAMETERS OF POSSIBLE HABITATS, 1 RPM
(a) Single component
| Parameter | Single torus | Single sphere | Single cylinder | Single dumbbell | | :--- | :--- | :--- | :--- | :--- | | Projected area ($10^6$ m²) | 0.67 | 0.67 | 0.67 | 0.67 | | Radius of rotation (m) | 830 | 895 | 895 | 895 | | Radius of shell (m) | 65 | 895 | 895 | 326.5 | | Length of cylinder (m) | — | — | 119 | — | | Structural mass (kt) | 156 | 3520 | 3520 | 380 | | Atmospheric mass (kt) | 45 | 16,000 | 16,000 | 1400 | | Shielding mass (Mt) | 9.9 | 11.4 | 11.4 | 1.3 | | Total mass (Mt) | 10.1 | 31.0 | 31.0 | 3.1 | | Internal volume ($10^6$ m³) | 6.9 | 3000 | 3000 | 145 | | Openness (m³/m²) | 10 | 4500 | 4500 | 216 |
TABLE 4-1 — Concluded
(b) Multiple components
| Parameter | Banded torus | Beaded torus | Multiple dumbbell | | :--- | :--- | :--- | :--- | | Number of components | 10 | 50 | 25 | | Projected area ($10^6$ m²) | 0.67 | 0.67 | 0.67 | | Radius of rotation (m) | 830 | 830 | 895 | | Radius of shell (m) | 20.5 | 65 | 65 | | Length of cylinder (m) | — | — | — | | Structural mass (kt) | 49 | 156 | 72 | | Atmospheric mass (kt) | 4.5 | 45 | 14 | | Shielding mass (Mt) | 3.1 | 9.9 | 4.5 | | Total mass (Mt) | 3.2 | 10.1 | 4.6 | | Internal volume ($10^6$ m³) | 0.69 | 6.9 | 2.2 | | Openness (m³/m²) | 1 | 10 | 3.3 |
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 m² per person to 35 m², 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 $0.95 \pm 0.05$ g and 1 rpm, the design allows $0.85 \pm 0.15$ g and 1.9 rpm some interesting possibilities emerge. Under these new conditions, parameters for the same geometries discussed earlier are summarized in table 4-2. 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 technology 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, and other equipment can be made at the construction site. Only the relatively light-weight parts such as rollers, dies, and electronic controls would be brought from Earth.
REFERENCES
- O'Neill, G. K.: The Colonization of Space, Physics Today, vol. 27, Sept. 1974, pp. 32-40.
- von Braun, W.: Crossing the Last Frontier, Colliers, March 22, 1952.
- Clarke, A. C.: Islands in the Sky, John C. Winston, 1952.
- Clarke, A. C.: Rendezvous with Rama, Harcourt Brace Jovanovich, 1973.
- Flügge, W.: Stresses in Shells, Springer-Verlag, New York, 1967.
- Timoshenko, S.: Theory of Plates and Shells, McGraw-Hill, New York, 1959.
- Hill, P. R.: Experience with Rotating Environmental Systems, presented at the International Symposium on the Role of the Vestibular Organs in the Exploration of Space, Pensacola, Florida, Jan. 1965.
- Gray, D. Z.: The Vivarium, 1974 (private communication).
- Tsiolkovsky, K. E.: The Rocket into Cosmic Space, Na-ootchnoye Obozreniye, Science Survey, Moscow, 1903.
- Hannah, E. C.: Radiation Protection for Space Colonies, Princeton University, 1975 (to be published).
- Levy, R. H., and French, F. W.: Plasma Radiation Shield: Concept and Applications to Space Vehicles, Journal of Spacecraft, vol. 5, no. 5, May 1968, pp. 570-577.
- Bernal, J. D.: The World, the Flesh and the Devil, Methuen & Co., Ltd., London, 1929.
TABLE 4-2.— PARAMETERS OF POSSIBLE HABITATS WITH OTHER CRITERIA (EXTERNAL AGRICULTURE)
(a) Single components
casting beds and other equipment can be made at the space colony rather than have to be brought from Earth.
Building the Habitat Shell
Figure 4-7 — Schematic of a Bernal sphere configuration.
Image
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.
TABLE 4-2 — Concluded
(b) Multiple components
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 light-weight, 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 assembling 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.