A couple of minutes walk brings a view of tiers of fields and ponds and cascading water (fig. 5-11). The upper level where you enter is surrounded by a number of ponds holding about 90,000 fish. There are similar ponds in the other two farms. From the ponds the water flows down to lower levels where it irrigates fields of corn, sorghum, soy beans, rice, alfalfa, and vegetables, and provides water for livestock. The multiple tiers triple the area of cropland (fig. 5-12).
On the second tier down a farmer shows you around. The wheat growing on this tier, he tells you, will be ready for harvesting next week. Each of the three agricultural areas in the colony grows essentially the same crops; however, harvests are staggered to provide a continuous supply. On another tier enormous tomatoes grow in a special control zone with elevated levels of carbon dioxide, temperature, and humidity. On one of the lower levels, the farmer impresses you with the fact that this farm, like the others contains some 20,000 chickens, 10,000 rabbits, and 500 cattle. The lowest level is enclosed and kept at very low humidity to permit rapid drying of crops to hasten produce flow from harvest to consumption. Because of its high productivity the colony's agriculture feeds 10,000 people on the produce of 61 ha (151 acres). You marvel that so fruitful a garden spot is actually in barren space, thousands of miles from any planet.
Figure 5-11 — View of the agricultural areas. (See also frontispiece to ch. 3.)
Image
Figure 5-12 — Cross-section of agricultural region.
<!-- image -->The agricultural system supplies an average person of 60 kg with 2450 cal (470 g of carbohydrates and fats and 100 g of protein) and almost 2 l of water in food and drink each day (ref. 2). Plants and animals are chosen for their nutritional and psychological importance (ref. 3) (e.g., fresh fruits, vegetables, and beef). The principal crop plants and animals and the areas devoted to each are given in tables 5-4 and 5-5. Fruit is not included in these tabulations. The trees are grown in residential areas and parks where they provide beauty as well as fruit.
The crops are grown in a lunar soil (ref. 4) about 0.3 m deep. This soil is made into a lightweight growth matrix by foaming melted rock. The yields are greater than those achieved on Earth because of improved growing conditions and the ability to grow crops on a year-round basis. The higher levels of carbon dioxide, improved lighting, and temperature and humidity control increase productivity to approximately 10 times that of the typical American farm. Terrestrial experiments (ref. 5) have produced fivefold increase in yield for production of vegetables in controlled greenhouses. (For more details on the agricultural system, see appendix C.)
TABLE 5-4 — PLANT AREAS
| Crop | Area, $m^2$/person | Total Area, ha | | :--- | :--- | :--- | | Grains | 20 | 20 | | Soybeans | 15 | 15 | | Alfalfa | 15 | 15 | | Vegetables | 10 | 10 | | Total | 60 | 60 |
Notes: Fruit in colony provides 250 g/person/day. Grains and soybeans — dry weights. Sugar is obtained from sorghum, perhaps from honey. Cattle use part of the plant roughage.
Life Support Systems
Next stop on your tour is the waste processing facility at the bottom of the agricultural area. It is an important part of the life support system because it maintains a delicate balance between the two opposing processes of agricultural production and waste reduction. A sanitation technician explains the operation of the facility.
He points out that on Earth production and waste reduction are balanced, at least partly, by natural processes. Water is extracted from the atmosphere by precipitation as rain; biodegradable materials are reduced by bacterial action. In space neither of these processes is fast nor reliable enough. The colony, lacking oceans and an extensive atmosphere in which to hold wastes, is limited in its capacity for biomass and cannot duplicate Earth's natural recycling processes. Instead, it uses mechanical condensation of atmospheric moisture and chemical oxidation of wastes to reduce the recycling time to 1-1/2 hr. This approach minimizes the extra inventory of plants and animals necessary to sustain life and to provide a buffer against breakdowns in the system.
Agriculture uses sunlight, carbon dioxide, and chemical nutrients to produce vegetation and from that, to raise animals. Oxygen and water vapor released as byproducts regenerate the atmosphere and raise its humidity. A considerable amount of vegetable and animal waste is produced along with human wastes of various kinds — sewage, exhaled carbon dioxide, and industrial byproducts — and all these have to be recycled.
Waste processing restores to the atmosphere the carbon dioxide used up by the plants, reclaims plant and animal nutrients from the waste materials, and extracts water vapor from the atmosphere to control the humidity of the entire habitat and to obtain water for drinking, irrigation, and waste processing. He tells you that balancing waste generation and waste reduction is a major accomplishment of the designers of the colony, for it eliminates any need to remove excess wastes from the habitat thereby avoiding having to replace them with expensive new material from Earth.
TABLE 5-5 — ANIMAL AREAS
| Animal | Number/person | Area, $m^2$/person | Total Area, ha | | :--- | :--- | :--- | :--- | | Fish | 27 | 0.1 | 0.1 | | Chickens | 6 | 0.1 | 0.1 | | Rabbits | 3 | 0.1 | 0.1 | | Cattle | 0.15 | 0.7 | 0.7 | | Total | — | 1.0 | 1.0 |
Notes: Sources for areas required per animal. Fish: Bardach, J. E.; Ryther, J. H.; McLarney, W. O.: Aquaculture: The Farming and Husbandry of Freshwater & Marine Organisms. © 1972 (Wiley-Interscience: New York). Chickens: Dugan, G. L.; Golueke, C. G.; Oswald, W. J.; and Risford, C. E.: Photosynthesis Reclamation of Agricultural Solids and Liquid Wastes, SERL Report No. 70-1, University of California, Berkeley, 1970. Rabbits: Henson, H. K., and Henson, C. M.: Closed Ecosystems of High Agricultural Yield, Princeton Conference on Space Manufacturing Facilities, May, 1975. Cattle: Kissner, Wm.: Dept. of Civil Engineering, University of Wisconsin — Platteville: Personal Communications.
The technician explains that water is processed at two points in the system. Potable water for humans and animals is obtained by condensation from the air. Because evapotranspiration from plants accounts for 95 percent of the atmospheric moisture, most dehumidifiers are located in the agricultural areas. Because of the rapidity with which plants replace the extracted water it is important that the dehumidification system be reliable. Otherwise the air would quickly saturate, leading to condensation on cool surfaces, the growth of molds and fungi, and an extremely uncomfortable environment. Several sub-units are used for dehumidification.
The dehumidifiers work in conjunction with the heat exchange system which carries excess heat from the habitat to the radiator at the hub. For water condensation in the torus' gravitational field, normal condensation techniques are used. Figure 5-13 shows schematically (ref. 6) how water is removed in zero-g areas such as the hub. The humidity is controlled by varying the temperature of the coolant and the rate at which air is passed through the unit. To cool and dehumidify it, the atmosphere must be passed through a thermal processor several times per day.
Water is also a byproduct of the continuous wet oxidation process (ref. 7) shown schematically in figure 5-14.
Figure 5-13 — Water removal.
Image
Figure 5-14 — Continuous wet oxidation process.
Image
Figure 5-15 — Dual water supply, kg/person/day.
<!-- image -->The complete water supply illustrated in figure 5-15 provides 25 times the potable water needed to satisfy the metabolic requirements of the colonists and their animals. (Figure 5-16 considers only metabolic requirements and does not include water for waste transport.) In addition, some 250 kg of recycled water per person per day is used for waste transport. In spite of this extensive dilution, the total per capita water use in the colony is only 75 percent of U.S. domestic water usage. Consumption is limited by use of recirculating showers, low volume lavatories, and efficient use of water in food preparation and waste disposal. Any increase in water for waste transport reduces the amount of condensed atmospheric water which can be used for irrigation, and increases the recycle water. In addition, 200 kg/person of water is set aside for emergencies and fire protection.
Water condensed from the air is heated to 16° C and fed into the fish ponds. After flowing through the ponds, the water is
Figure 5-20 — The mass catcher.
Image
THE MASS CATCHER AT L2
The problem of collecting the stream of material launched by the mass-driver is solved by a kind of automated "catcher's mitt," the mass catcher, located at L2. Although the catchers are fully automated there is a 2-person space station at L2 for maintenance personnel. This station is adequately shielded against possible hits by stray payloads.
Because it would be dangerous to navigate in the vicinity of the catcher while the launcher is operating, you are not able to visit the catcher personally. Instead you learn about it from an operator who is at the Moon base on recreation leave.
He tells you that the mass catcher is an active device to capture payloads of lunar material shot by the mass launcher. The payloads are solid blocks 0.20 m in diameter, made of compacted and sintered lunar soil. Each payload has a mass of 10 kg and arrives at L2 with a speed of 200 m/s.
The catcher is in the form of a thin, light net, 10 m² in area, which is manipulated by three cables to position the net anywhere within an equilateral triangle. The cables are wound on reels which move on three closed loop tracks. Each side of the equilateral triangle is 1 km, thus providing a 0.43 X 10⁶ m² catch area. The total mass of the catcher is 220 t.
Station-Keeping With the Rotary Pellet Launcher
You learn that the mass catcher uses an unusual propulsion device — a rotary pellet launcher — to position the catcher so that it is always facing the incoming stream of payloads. Furthermore, this device provides a counterthrust to the force of some 2000 N imparted to the catcher by the stream.
Further details of the mass catcher are provided in appendix G, and because of their importance the trajectories from the Moon to L2 and their relation to stationkeeping are described in appendix H. The mass catcher is illustrated in figure 5-20.
Figure 5-21 — The rotary pellet launcher
Image
The rotary pellet launcher is a heavy tube rapidly rotating to accelerate and eject small pellets of rock. (See fig. 5-21.) Velocities as high as 4000 m/s may be attained, equal to the exhaust velocities of the best chemical rockets. The pellets themselves are sintered or cast directly from lunar rock, with no chemical processing required. The launcher uses 5 percent of the mass received as propellant. (For further analysis see appendix I.)
This rotary pellet launcher is mechanically driven by an onboard nuclear power system rated at 20 MW. The power plant radiator is situated in such a manner as to radiate freely to space while being shielded from impacts of stray masses. The inner surface of the radiator is insulated and made highly reflective, so as to avoid heating the catcher.
The transport of lunar material from the catcher to the colony is accomplished using a space ore-carrier. The trip from L2 to L5 requires some 2 months. The rotary pellet launcher is the primary propulsion system since a thrust of only several thousand newtons must be obtained over a period of weeks to perform the mission. Of course, the rotary launcher cannot be used in the vicinity of either the colony or the mass-catcher, because of the danger from its exhaust of high-velocity pellets. For low-velocity maneuvers in these vicinities chemical rockets can be used.