This essay describes a concept for a greenhouse suitable for incorporation into a lunar settlement. The greenhouse is modular and easy to assemble, and successive modules may easily be connected to form a larger system as the settlement grows.
The acreage required to support a lunar base may be estimated by taking the food required and dividing this into the estimate crop yield. The list of Human Daily Needs and Effluents in Artemis Data Book section 4.3.5, Life Support Systems, gives 3.9 lb (1.77 kg) of food per day per person (food plus water in food). Adding a factor of 1.5 to account for the fact that crop yields are raw and not prepared, we arrive at 5.85 lb (2.6 kg) produce per day, or approximately 2100 lb (950 kg) of produce per person per year.
The above assumes a primarily plant-based diet. I also assume that the site, and thus the greenhouse, will be unoccupied for several months between visits, at least initially, so that food could be grown and stored between human visits to the lunar exploration base. As desired, and as payload constraints permit, meat, eggs, and dairy products could easily be brought along in freeze dried or frozen form. In addition, I recommend that the greenhouse be initially dedicated to those crops which provide the maximum edible yield for minimum room; this probably means that vegetables, high-density fruits (e.g. tomatoes, melons, gourd, and cucumbers), and herbs be grown. Legumes and grains will probably be imported until the colony has grown enough to afford the acreage required.
An estimate for the crop yield may be arrived at by interpolating from yields claimed for existing greenhouse structures. An article on Hydroponics in the Middle of the Desert from Mayhill Press gives an estimate of 100,000 lb (45,000 kg) of food per acre, or 2.3 lb per square foot (11 kg/square meter), per year, for vegetables in the field. This is a reasonable estimate to use for a lunar greenhouse.
So, one person could be supported continuously, or 3 people intermittently, by approximately 1,000 square feet (85 square meters) of growing area. This is a first level approximation and may vary considerably as further research is done on the diet require for lunar workers and the yields which may be expected from lunar greenhouses.
Editor's Note: The Mayhill Press article cited above actually says "The average yield for vegetables in the field is about 5 tons for each acre used ... Yet in the greenhouse at Riyadh, the American company gets more than 200 tons for each acre ..." So the numbers shown in this essay maybe be exaggerated by a factor of 40 depending on what assumptions we make about the efficiency of hydroponics. We'll get this analysis updated as soon as we can.
Please excuse the interruption. We now return you to your regularly scheduled Artemis Data Book.
At least for initial missions, it will be necessary to care for and harvest crops robotically, using a semiautonomous robot/telefactor system, during the times in which the site is unoccupied and to then store the produce for the next crew. Fortunately, many crops can safely kept in a cool environment ("root cellar") for extended periods of time - and a lunar base should not have many problems with rodents or other pests. In addition, a small blanching and freezing system may be required for otherwise perishable crops.
A small hive of bees will probably be required for plant fertilization. It would be necessary to feed them artificially for a while until the crops are ready, with sugar water or honey, but otherwise they should require relatively little care. Caring for the bees would certainly be less effort than trying to artificially inseminate the entire crop base, and their honey produced would be a welcome addition to the site's diet.
The exact nature of the ecological cycle is yet to be established and there is continuing debate as to whether the crops should be grown hydroponically, in a non-organic medium with added nutrient (e.g. sand farming), or in soil. In addition, the inedible biomass and crew waste products may be recycled through composting - which would produce methane for use in fuel cells - or by sterile incineration ("Pyrolysis") in a solar furnace. The proposed greenhouse design is flexible enough to accommodate all of the above with minimal design changes.
Figure 1: End View of Greenhouse Tube
The proposed greenhouse is based on a modular tube, 4 meters in diameter, 50m long. Figure 1 shows a cross section of the tube. Each module provides 200 square meters of growing space in a series of 4 trays, each 1 meter wide, with a 1 meter wide walkway between the trays. The trays may be hydroponic, sterile soil/nutrient solution, or soil, as required.
The floor of the tube is filled with lunar soil to provide a level surface (also reducing the air volume required). The tube is also supported by berms bulldozed on either side. The tube itself is impermeable plastic and rib-stop nylon with a silvered exterior surface, for heat retention, and a matte-white interior surface, for maximum light diffusion. Additional insulation may or may not be required.
Suspended from the ceiling are twin tracks which support the "Robocrop" (credit to Vik Olliver for this) semi-autonomous agricultural robot/telefactor. These tracks are in the form of aluminum tubing, suspended by eye hangers from guy cables. For ease of assembly, the eye hangers slip into slots punched into the tubing - no bolts required. The guys attach with clips.
The upper row of agricultural trays and the tracks are all supported with guy wires from aluminum hoops spaced at 2-meter intervals in the tube. These hoops comprise 4 quarter-sections, all identical, with a "W" corrugated cross section - all of the quarters will stack together very closely for shipping. The quarters use a tab-slot arrangement for ease of assembly, with no fasteners required. To assemble the greenhouse, the structure is inflated, then the hoops are assembled, in the shirt-sleeve environment, and slid into place, held in place by Velcro fasteners or snaps. In addition to providing support during greenhouse stocking and structural support for the trays, the hoops also provide support to keep the inflated structure from collapsing in case of drastic air loss.
An alternative design for the above would utilize inflatable hoop rings which would be made as an integral part of the greenhouse structure, located at 2-meter intervals as before. The rings would be inflated separately from the greenhouse interior and then filled with rigid epoxy foam to provide structural support. However, the hoops would not be strong enough to support the agricultural trays and Robocrop, so these would have to be supported by rectangular metal frames.
Along the edge of the upper trays are cable runs which hold power and communications cables, fiber optic cables, and water hoses. The water hoses have misting/sprinkler heads every meter or so.
The fiber optic cables provide light for the greenhouse, either from solar concentrators during the day, or - during the lunar night - from high-efficiency, high-intensity light sources. A "light fountain" effect is achieved by simply terminating a fiber, holding the end at an angle with a simple wire brace, and aiming the end at the ceiling. The interior of the tube is covered with a matte white finish to maximize the illumination back to the plants (a silvered or mirrored finish is not desirable, since it would give "hot spots" and dark areas). The "light fountains" are located at 1 meter intervals.
Given that the tube is 50 meters long and that the light sources are centrally located, 25 fibers start out in each direction from the source, in either direction, one fiber terminating every meter. This central lighting concept is further described in a separate technical report, Fiber Optic Lighting Systems.
Figure 2 shows an 800 square meter greenhouse system based on these modules, which could grow enough food - along with utility and decorative plants - to supply 6 to 8 people continuously. For the initial missions, where the base is periodically occupied, one module should be sufficient, assuming food can be stored between missions.
As shown, each module has two airlocks on either side of the center; the airlocks can function alone, or can be locked together to provide passage from one module to another. The outermost airlocks of the first and last modules serve as entrances to the entire assembly, the remainder serve as intermodule passages and as safety bulkheads in case loss of integrity in one or more modules. Other configurations are possible. For instance, if the airlocks are located at the ends of the modules then they could be assembled in star configurations around a central hub.
Alternatively, the modules could be toroidally shaped, instead of cylindrical, with airlocks at 90 degrees (for a square array) or 60 degrees (for a hexagonal array) around the circumference. In this case, the solar concentrators for each module could be located in the hole in the middle.
In conclusion: this article describes a preliminary concept for a modular, inflatable greenhouse capable of supplying a majority of the nutrition required by the inhabitants of a lunar base. The greenhouse is light, compact when stowed for transport, easily assembled on site, rugged, and easily maintained. In addition, the total growing area may be easily increased through the addition of greenhouse modules at any time. Furthermore, the concept uses an advanced lighting concept which should provide maximum light for minimum energy, with a high degree of reliability and maintainability.
Related Documents in the Artemis Data Book
Human Daily Needs and Effluents
NASA news release on Bioregenerative Life Support
Fiber Optic Lighting Systems