Chapter 10 Human Life Support for Advanced Space Exploration

Chapter 10

HUMAN LIFE SUPPORT FOR ADVANCED SPACE EXPLORATION Steven H. Schwartzkopf I . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232 I1. Human Life Support Requirements . . . . . . . . . . . . . . . . . . . . . . . 232 111. Life Support Functions and Technology Selection . . . . . . . . . . . . . . . 233 A . Atmosphere Regeneration . . . . . . . . . . . . . . . . . . . . . . . . . 234 B . Water Purification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235 C . Waste Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235 D. FoodProduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 236 E. Food Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237 IV. System Design of a Controlled Ecological Life Support System . . . . . . . . 238 A . American Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . 238 B . Russian Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . 238 C. Concept for a Controlled Ecological Life Support System . . . . . . . . 239 V. Conceptual Design o f a Life Support System for a Lunar Base . . . . . . . . 240 A . FoodProduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 242 B . FoodProcessing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245

Advances in Space Biology and Medicine Volume 6. pages 231-253 Copyright 0 1997 by JAI Press Inc All rights of reproduction in any form reserved ISBN:0-7623-0147-3

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C. Atmospheric Regeneration . . . . . . . . . . . . . . . . . . . . . . . . D. WaterPurification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . E. Waste Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VI. In Situ Resource Utilization . . . . . . . . . . . . . . . . . . . . . . . . . . . VII. Design Characteristics and Breakeven Analysis . . . . . . . . . . . . . . . VIII. Conclusions and Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

.245 ,245 .246 246 . .248 . 2 51 .252

1. INTRODUCTION Whether the human race will choose to remain an exclusively terrestrial species or expand permanently into space by establishing colonies on the Moon, Mars or elsewhere, will ultimately be influenced by a multitude of factors. One of the most important of these factors will be the availability of technology to economically support human life in extraterrestrial environments.On prior space missions, U.S. astronauts have been sustained almost exclusively by open-loop, non-recycling systems utilizing physicochemical life support technologies. In these systems, consumables such as food, drinking water, and oxygen were stored in the spacecraft, while waste materials were simplyjettisoned overboard or stored for disposal upon the return to Earth. The simplicity, effectiveness, and reliability of this approach successfully met the challenge of demonstrating the feasibility of human short term spaceflight. For activities such as the establishment of permanent human settlements on the Moon and Mars, the open-loop, non-recycling systems used previously will not be adequate. Designers of life support systems for such endeavors are faced with a much different challenge. They must take into account factors such as the necessity of providing a familiar, Earth-like living environment to promote human productivity and psychological well-being, and the need to increase the self-sufficiency and decrease the operating costs of the life support system. To meet these design challenges,a new approach to the support ofhuman life must be implemented. This paper describes such an approach.

11. HUMAN LIFE SUPPORT REQUIREMENTS Human beings require substantial amounts of consumable materials to maintain life (Table 1). Without recycling, over 8,000 pounds of oxygen, food, and water (for food preparation, showers, personal hygiene, and clothes washing) are needed to sustain a person for one year. These estimates are nominal amounts for a person with a body mass of approximately 155 pounds (70 kg). The actual amounts required can increase substantially with higher body mass or with physiological changes such as an elevation in the level of physical exertion.' In addition, these mass estimates would increase even firther when the packaging materials and the

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Table 1. Nominal Life Support Consumables Required for Humans Consumable Food Oxygen Water, food preparation Water, drinking Water, hand/face wash Water, shower Total

Mass in kgyr 408 304 263 676 662 1325

3638

structural support(s) required to contain the consumablesduring a launch into space are included. At the current cost of launching a Space Shuttle into low Earth orbit, estimated to be $1 1,000 per pound,* the average annual cost of supporting a single crew member in low Earth orbit would be over $88 million. Naturally, due to the greater distances involved, the cost of supplying life support materials to a human crew at a base on the Moon or Mars will be even higher. As a consequence,the development of technologies which can produce the necessary life support consumables in the spacecraft or at a base site is economically essential if advanced manned space missions are to become reality. Production of consumables on site can only be accomplished by regenerative life support system technologies which recycle organic wastes or by technologies which use available in situ resources (such as lunar or Martian regolith) as raw input materials.

I l l . LIFE SUPPORT FUNCTIONS AND TECHNOLOGY SELECTlON The primary functions required for the direct support of human life include: atmosphere regeneration, water purification, waste processing, food production, and food processing. There are two types of technology available to provide these functions: physicochemical and bioregenerative. Physicochemical life support technologies include a broad range of physical and chemical reactors, devices such as incinerators, electrolyzers, distillation devices, and selective absorbent filters. Bioregenerative technologies utilize living organisms (such as bacteria, algae, higher plants, or animals) as the “reactors” to provide life support functions. Each primary life support function is described below, accompanied by a discussion of the types of technology available to provide the required life support.

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A. Atmosphere Regeneration

The major operations performed in atmosphere regeneration are the removal of CO,, water vapor, and trace contaminants (both gaseous and particulate), and the generation of 0,. Traditionally, physicochemical technologies have been used to provide these function^.^ The most common method of removing CO, on U.S. space missions, for example, has been by absorbing it onto lithium hydroxide (LiOH). The adsorption reaction is irreversible, however, and requires periodic replacement of the LiOH with fresh material to maintain operation. For short duration missions, this replacement is feasible, but as mission durations lengthen, the cost of resupplying LiOH becomes prohibitive. It was for this reason that the longer duration Spacelab mission employed a system in which CO, was removed by a regenerable molecular sieve.4This absorbent traps CO, in microscopic pores on its surface, and it can be regenerated by exposure to vacuum and heat. For long duration missions, once the CO, has been removed, it can be further processed by reduction to recover the 0,.3This can be accomplished by reacting the CO, with H, at high temperature in the presence of a catalyst (e.g., Bosch or Sabatier reactors) or by electrochemical separation. Alternatively, the CO, can be supplied to plants, which through the photosynthetic process will convert it to 0,. Water vapor is emitted into the spacecraft atmosphere through crew respiration and perspiration, as well as through activities such as eating, food preparation, showering, and hand and face washing. On long duration missions, water vapor will also be emitted by washing and drying of clothes. This water is generally removed from the atmosphereby a condensingheat exchanger and liquid collection system. In some applications, such as space suits or short duration missions, regenerable absorbents have been used for water vapor removal. Once the water vapor has been collected and returned to liquid form, it must be processed by a water purification system (discussed below) before it is recycled. This purification step is necessary because many water-soluble atmospheric trace contaminantswill dissolve in the water as it condenses. In the closed atmosphere of a spacecraft or habitat, gaseous or particulate trace contaminants emitted by materials, machinery, and the crew or any other living organisms can accumulate to levels which may be health-threatening. For the International Space Station, particulate filters are combined with a gaseous trace contaminant control system which uses activated carbon and catalytic oxidation to remove contaminants. The particulate filters in this system also remove any airborne bacteria from the spacecraft atmosphere. On prior U.S. space missions, oxygen for the crew has been supplied in pressurized cylinders stored aboard the spacecraft. For longer duration missions, this method is inadequate, and a means of producing 0, must be provided. Physicochemical technologies for the production of 0, include water electrolysis, water vapor electrolysis, and the reduction of CO,. As noted above, plants can also be used to provide 0, to the crew through photosynthesis.

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B. Water Purification

For most U.S. space missions, potable water has been provided from storage tanks on the spacecraft. Both because water is heavy and because a significant amount of water is required to support human life, onboard storage is not feasible for long-duration missions. For this reason, the Space Shuttle provides potable water as a by-product from its fuel cells, which combine H, and 0, to produce electrical power and H,O.For vehicles which do not use fuel cells, water purification must be provided to enable recycling. The water availablefor recycling comes from three primary sources: atmospheric humidity condensate, wash water (from food preparation, hand and face washing, showering, and tooth brushing), and urine. A variety of physicochemical technologies have been developed for water recycling. These technologies include simple distillation, filtration (e.g., reverse osmosis, multifiltration) and phase change processes (e.g., Vapor Compression Distillation’ or freeze crystallization). Higher plants can also supply essentially pure water through the process of transpiration. Typically, plants transpire between 200 and 1000 liters of water per kilogram of dry biomass per year.6 Water processing with higher plants involves supplying the water to the plants through irrigation or in the form of hydroponic nutrient solution. This water is absorbed by the plants and transpired as water vapor, which is collected on a condensing heat exchanger and then purified for human use through exposure to ultraviolet radiation (UV) or ozone to remove bacteria and trace organic contaminants. C. Waste Processing

Historically, the development of waste processing technology has had low priority due to the short duration of previous missions. On most flights, feces and solid wastes are simply stored for return to Earth, while urine is often just vented overboard. Several physicochemical technologies have been investigated for processing and recycling solid wastes. They include dry oxidation (incineration), wet oxidation, and supercritical wet oxidation. These high energy methods generally convert organic waste materials into inorganic salts, water, and gases. Bioregenerative technologies for waste processing include bacterial reactors and combinationsof higher plant and bacterial systems.Bacterial reactors, both aerobic and anaerobic, have an extensive history of application in domestic sewage treatment plants. Aerobic systems typically require higher energy inputs to maintain oxygenation (e.g., aerating pumps, mixers). Anaerobic systems require very little energy, but have very slow process rates, and the anaerobic bacteria are more susceptibleto changes in environmental condition^.^ Combining higher plants with anaerobic bacterial systems provides several distinct advantages. The most significant of these is the capability for increasing the removal of ammonia and nitrate nitrogen at a higher rate than that obtained with bacterial systems without plant^.^

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However, such systems are less efficient in removing carbonaceous compounds than plant-free bacterial systems. For longer missions, all solid and liquid wastes (e.g., feces, urine, food preparation waste) must be processed and converted into a usable product. One ofthe most promising recycling technologies, low pressure wet oxidation, is typically carried out at conditions below 230°C and below 3460 kPa (500 psi). The process breaks down organic material through a combination of hydrolysis and oxidation. Since low molecular weight compounds such as acetic acid tend to be refractory to the process, low power wet oxidation processes lead to lower oxidation efficiency. The result is a breakdown of solids and reduced oxidation demand, with a product liquor containing a mixture of inorganic salts and soluble low molecular weight organics.* D. Food Production

Previous missions have supplied astronauts with food through on-board storage. For long duration missions, however, it will be necessary to produce food on board the spacecraft. The food must be produced either by growing edible organisms (microbes,plants, animals) or by direct conversion of waste or in situ materials into food. Physicochemical methods of synthesizing carbohydrate, fat, and protein have been developed and tested.'.'' Unfortunately, foods produced in this fashion are typically not well-accepted by humans. Consumption of such synthetic foodstuffs has produced a number of undesirable side effects including nausea and diarrhea.' An additional problem with this method of food production is that many of the chemical syntheses involved require the use of substrates of such high purity that they would be difficult or impossible to obtain from life support system wastes. As a consequence, if physicochemical food production were used in space, it would require resupply shipments of the high purity substrates from Earth. The earliest research on bioregenerative food production focused on the use of unicellular algae (e.g., ChZoreZlu; see chapter 11 by Wolf in this volume) and small vascular plants of the family Lemnaceae.' In both instances the biomass produced was physiologicallyunacceptable as a human foodstuff. Since the late 1970s, attention has been focused on the inclusion of crop plants in life support systems. Crop plants provide a nearly ideal solution to the problem of designing a food production system for space use. Human crews are accustomed to consuming these materials on Earth, and therefore no physiological or psychological barriers to their use as foodstuffs exist. Current estimates of the amount of growing area required to feed one person range from 20 to 30 square meters, depending on the species of plants grown. For example, Hoff et al.,13 proposed a mixture of ten species (soybean, peanut, wheat, rice, potato, carrot, chard, cabbage, lettuce, and tomato) which satisfy all human nutritional requirements and require a growing area of 24 m2. Animals can also play a part in food production systems for advanced missions. Previously, the primary objection to their use has been the ecological inefficiency

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Table 2. Efficiency Characteristics of Various Animal SDecies2’ Animalffroduct Beef Swine Lamb

Rabbit Broiler Chicken

Eggs

Feed Conversion Efficiency

Harvest Index

Production Efficiency

17 40 25 33 50 36

49 45 23 47 59 90 100 56 45 60 60 60

8.3 18 5.8 16 30 32

Milk

33

Shrimp

40 50 67 67 67

Prawns Catfish Carp Tilapia

33

22 22 40 40 40

Notes: All figures in percent. Feed Conversion Efficiency = (kg Biomass Gain / kg Feed) x 100. Harvest Index = (Edible Biomass/Total Biomass) x 100. Production Efficiency = (kg Edible Biomass/ kg Feed) x 100.

which occurs between trophic levels. As an example, only about 10% of the food provided to cattle is converted into meat (Table 2). Thus, from an energetics perspective, it would make more sense to feed plant materials to a human crew directly rather than feeding them to an animal and subsequently using parts of the animal as human food. This concept neglects the idea of feeding animals with the plant parts that humans would not normally consume. Bacteria, molds and yeasts, as well as several species of vertebrates (e.g., fish, chickens) can use such materials as foodstuffs. In addition, Table 2 shows that several animal species have conversion efficiencies considerably above the 17% value typical of beefcavle. Thus, with proper speciesselection,animals can potentially play a substantial role in the overall food production system.

E. Food Processing Food processing technologies would make the biologically-produced materiaIs suitable for human consumption. These technologies may be grouped into two general categories: (1) processing of materials normally edible by humans, and (2) processing of normally inedible materials to convert them into an edible form. Edible materials may be eaten directly after washing, cooked for immediate consumptionor for storageand later consumption,or processed to remove a specific component, either to enhance digestibility or to obtain a component for specific uses. Materials which are normally inedible, would be extracted or treated to

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prepare them for human consumption or processed to produce feedstocksfor animal consumption.

IV. SYSTEM DESIGN OF A CONTROLLED ECOLOGICAL LIFE SUPPORT SYSTEM A. American Experiments

Early in the American space program, research was conducted to evaluate these physicochemical and bioregenerative technologies for human life support applications. Bioregenerative technology was viewed as being able to provide weight savings in several of the basic functional areas, with the most notable being atmosphere regeneration. Experiments were done to evaluate the effectiveness of both algal ( e g , Chlorella; see chapter 11 by Wolf in this volume) find bacterial (e.g., Hydrogenomonas) reactors for atmosphere regeneration. Studies which applied these reactors to regenerate the atmosphere of animals held in sealed chambers showed that the concept was partially feasible. However, these studies also showed that significant problems existed with respect to balancing respiratory gas (0, and CO,) production and utilization, as well as in the development of control systems. In two of these experiments,for example, atmospheric CO, concentrationsreached over while in another experiment, 0, concentration reached over 60%.16 Neither of these results would have been acceptable for human life support applications. U.S. research into bioregenerative technology was largely terminated in the late 196Os, because planned manned space flight activities were all of short duration and near-Earth. As a consequence, it was decided that life support for these missions could be easily handled by a combination of on-board storage,dumping overboard, and the physicochemical regeneration processes described above. In the late 1970s, as the possibility of establishing a permanent human presence in space was again considered, the American space program began to reevaluate bioregenerative technologies. B. Russian Experiments

In the Soviet space program, regenerative life support technologies had been studied without significant interruption through a set of experiments conducted with a series of closed, manned Bios chambers beginning in the early 1960s. The first Bios was only 12 m3in volume. However,the final chamber (Bios 3) had grown to a volume of 315 m3, and was composed of four equally-sized compartments, including two phytotrons (where crop plants were grown hydroponically),an algal culture room, and quarters for a crew of up to three. The earliest Bios chambers had a total plant growing area of only 13 m2 per occupant. This growing area was sufficient to provide each occupant with all of the oxygen for breathing and about

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40% ofthe food he or she required. In the earliest Bios chambers, research was also conducted on the use of a single-celled alga (Chlorellu) as an atmospheric gas regenerator and food source. Although the algae worked well as a CO, scrubber and 0, producer, it proved to be a poor source of human food. Life support experiments were conducted in Bios 3 as recently as 1983,” when a long duration experiment was conducted to simulate a round trip from Earth to Mars with a crew of two. In Bios 3, the plant growing area had been enlarged to 30 m2 per person. The plants were grown hydroponically, and included wheat, chufa, peas, dill, kohlrabi and many other vegetables. Although the later experimentswere completed successfully, with full atmospheric and water recycling, they demonstrated only partial recycling and closure of the life support system. Even in Bios 3, the occupants had to rely on a portion of their food (20%) from on-board storage. C. Concept for a Controlled Ecological Life Support System

Conceptually, it is possible to design a life support system based exclusively on either physicochemical or bioregenerative technology. However, both kinds of technology have characteristicadvantages and disadvantages.For example, physicochemical technologies tend to be fast-acting, but they usually perform only one specific function, and frequently require large amounts of power. Bioregenerative technologies are often characterized by slow reaction rates and larger volumetric requirements, but are frequently multi-functional (i.e., plants can produce food, recycle water, and help process waste all at the same time) and often operate with very little electrical power. Thus, by carefully selectingand combiningtechnologies with offsetting advantages and disadvantages it is possible to develop a hybrid life support system design which can offer significant improvement of performance over systems which are purely based upon physicochemical or bioregenerative technologies. An important method of integrating the two kinds of life support technology is through the design and development of a Controlled Ecological Life Support System, further abbreviated as CELSS. This system combinesbiological functions, such as photosynthesis (primarily for food and oxygen production, and CO, removal) with physicochemical functions (such as gas separation,and water vapor condensation), and attempts to mimic some of the basic behaviors of the Earth’s biosphere. The fundamental idea behind this kind ofhybrid design is that the system incorporatesthe process control and recycling capabilities found in natural ecosystems to provide an increased stability in the entire life support system. A generalized top-level schematic of a CELSS is presented in Figure 1. This figure illustrates the fundamental flows of life support materials through the system. In this example, crop plants produce food for the crew. As additional benefits of the food production subsystem, the plants take up CO, produced by the crew, produce 0, for crew respiration and for use in oxidizing waste materials, and produce water vapor which can be condensed and collected to supply the crew’s

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figure 1. Generic diagram of a CELSS illustrating primary mass flows between subsystems.

drinking and hygiene water. In the food processing subsystem, the products of the crop plants are processed into forms which are palatable to the crew. The crew’s metabolic waste materials (urine and feces), miscellaneous solid wastes (tissues, wipes, writing paper, and so on), and inedible plant biomass from the food production and processing subsystems are all supplied to the waste processing subsystem. These wastes are oxidized, and used as a supply of inorganic nutrients and CO, for the crop plants Pure water produced by the waste and waste water processors is resupplied to the crew for use, or recirculated through the waste processing subsystem.

V. C O N C E P T U A L D E S I G N OF A LIFE SUPPORT SYSTEM FOR A L U N A R BASE In late 1989,Lockheed initiated a NASA-funded case study to develop a conceptual design for a lunar base life support system which could accommodate a crew that would grow from an initial size of 4 to 100 people at base maturity. The lunar base was chosen as a focal point because prevailing opinion supported the idea that the Moon, and not Mars, would be our next home. During the initial portion of the Lunar Base CELSS study, several different design concepts were evaluated. Based upon a series of detailed analyses and trade-off studies,one ofthese design concepts (Figure 2) was selected for the development of a detailed conceptual design.18The five major subsystems of this design are described below.

figure 2. Functional block diagram of the Lunar Base CELSS conceptual design.

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Figure 3. Cross sectional diagrams and design parameters of the Lunar Base CELSS plant growth unit designs.

A. Food Production

For the Lunar Base CELSS project, the design rapidly focused on the selection and development of the food production system, as it turned out to be both the largest and the highest power consumer. Two systems were selected to produce food: a crop growth unit and an aquaculture unit for the freshwater fish nlapia. Based on an analysis of human nutritional requirements, the plant growth unit was designed to include wheat, soybean, peanut, lettuce, tomato, and carrot. The use of nlapia was incorporated as a means of producing a small amount of animal protein for crew consumption. With this minimum set of plant species, supplemented by about 50 gm per person per day of nlapia meat and some multiple vitamins, a nutritionally adequate diet can be produced. To accommodate an increase in crew size from 4 to 100, a series of three plant growth unit designs was developed. These three designs span a range from a small system which is ready to run upon landing (tumkey system), to a large system which requires installation,inflation,and construction before it can begin operation.Cross sections and fundamental design parameters of these three units are presented in Figure 3. All three designs incorporate hydroponic plant growth techniques. The first design uses a metal pressure hull based on the initial design and dimensions developed for a Space Station module, and provides about 100 m2of growing area. An artist’s concept of this unit is provided in Figure 4. The artist’s concept illustrates both an artificial lighting system and a sunlight collection and distribution system to supply photosynthetically active radiation.

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Figure 4. Artist's concept of a Space Station module-based plant growth unit on the lunar surface.

The second design is a hybrid which includes an inflatable shell attached to a rigid backbone, providing about 224 m2 of growing area. This design employs an aluminum backbone and airlock, incorporates premounted utilities, The shell consists of a tough, polyurethane-coated nylon material. This second design combines a moderate degree of assembly with partial turnkey operation, and thus occupies an intermediateposition between the turnkey concept presented in Figure 4, and the full assembly version described below. This third design is a large inflatable unit with approximately 528 m2 of plant growing area (Figure 5). The shell, excluding the airlocks,would be fabricated fiom the same polyurethane-coated nylon material used in the second design. To begin operations, both inflatable concepts require the addition of atmosphere after they are positioned on the moon. Plants growing in these units would make use of direct sunlight during the lunar day, either through a sunlight collection system like the one pictured in Figure 4, or by direct illumination through the wall. In this way these concepts decrease the total power requirement of the life support system. This type of inflatable technology provides a significantreduction in launch mass (over a hard shell), and is feasible using advanced materials and materials technolo-

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Figure 5. Artist’s concept of an inflatable plant growth unit on the lunar surface.

gies available today. In addition, although the concept of an inflatable greenhouse on the surface of the Moon may seem unsafe, this is not the case. Another study has shown that the dangers posed to plants from galactic cosmic radiation, solar flares, and meteorite strikes are statistically very low, even over an assumed 20-year lifetime of the fa~i1ity.I~ The primary drawback of this concept is that the lunar night lasts two Earth weeks. As a consequence, it is necessary to provide artificial lighting for the plants during the lunar night in order to maintain optimal growth and productivity. To handle the growth in crew size, different combinations of the three plant growth unit designs would have to be installed as the base evolves. For a crew of four, one of the Space Station-based modules would be sufficient. As the crew grows to 30, another module and three ofthe hybrid designs would have to be added. Finally, to meet the life support needs of a crew of 100,addition of three of the large inflatable units would be required. One of the primary drawbacks to the use of higher plants in food production involves the need for lighting to support photosynthesis. The minimum acceptable photosynthetically active radiation level is 300 to 600 pmol/m*/sec, when metal

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halide or high pressure lamps are used. If artificial lighting is used for the plants, 0.5 to 1.O kW of electrical energy per square meter of growing area will be required to produce this level. However, the electrical power required for illumination can be reduced by using natural sunlight during the lunar day and providing low-intensity artificial lighting (200 to 300 pmol/m2/sec) supplemented by higher atmospheric CO, concentrations during the lunar night. The Lunar Base CELSS study has identified two feasible methods for using sunlight, one using translucent, greenhouse-like structures for plant growth areas and the other using sunlight collectors and light conduits as distributors in opaque-walled plant growth units. B. Food Processing

In the Lunar Base CELSS study, food processing hardware is minimized to decrease launch mass. The selected food processing hardware would support grinding of grains and beans to produce flours as well as general grinding of plant biomass. Manual operations are assumed to be used for preparation of grain for milling or fish meat for cooking.Human-inedibleplant material would be processed by feeding it to the EZapia, either directly or after drying and grinding it into smaller pieces. C. Atmospheric Regeneration

The atmospheric revitalization subsystem in the Lunar Base CELSS design utilizes higher plants for all CO, reduction and 0, production. The atmospheres of the crew, plant, and animal chambers are isolated from one another by separate physicochemical CO, and 0, removal systems (liquid-based scrubber/concentrator). This atmospheric isolation provides for independent control of the respiratory gas concentrations in the different chambers and helps prevent the spread of contaminantsand microorganisms between the chambers. Temperatureand humidity control are handled by standard condensing heat exchangers. Trace contaminant removal is performed by a modified Space Station trace contaminant control system design, which uses activated carbon adsorbent and a catalytic oxidizer. The activated carbon beds adsorb most of the contaminants, and the catalytic oxidizer oxidizes any remaining contaminants at high temperature. The trace contaminant control system adsorbent must be regenerated periodically by applying heat and vacuum to the adsorbent beds. The effluent material can be captured and stored as waste, or it can be degraded by the waste processing system and recycled. D. Water Purification

Water reclamation by higher plants has been chosen as the primary method of purification for the Lunar Base CELSS design. Water for drinking and food preparation is obtained by treating condensate, collected directly from the crew cabin, to remove trace contaminants. However, the volume of condensate water is

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not suficient to fill the crew’s need for drinking and food preparation. Therefore, the Lunar Base CELSS design proposes to make up for this deficit by recovering condensate from the plant growth chamber and purifying it with the same systems. Water for hygiene and clothes washing is taken directly from the plant condensate collection and treated by UV light to remove bacteria and to degrade trace organic compounds.The remainder of the condensate from the plant chamber and aquaculture unit is recycled by returning it to the hydroponic nutrient solution, or by adding it to the aquaculture system to make up for evaporative losses. E. Waste Processing

The waste processor selected for the Lunar Base CELSS design is a low-pressure wet oxidation system. This system is selected because it provides the most complete processing at the lowest energy expenditure. The wet oxidation unit receives all solid waste materials not fed to the aquaculture unit. These solid wastes includes metabolic wastes produced by crew, animals and plants, and the non-metabolic waste materials derived from packaging materials, daily activities,and so forth. The wet oxidation unit will degrade these materials and then supply the effluent to the plant growth chamber for addition to the hydroponic nutrient solution. The effluent materials will then be further processed by the plants and bacteria.

VI. //V S/TU RESOURCE UTILIZATION Utilization of material from the Moon could significantly affect the ultimate design and operation of a lunar base life support system. A significant design advantage in establishing a base on the lunar surface rather than an orbiting station is the availability of in situ resources. Table 3 summarizes the elemental composition of the loose surface material (regolith) of the Moon, determined in samples obtained from the Apollo and Lunar missions.Table 4 summarizesthe elemental composition by percent of a typical plant (corn) and the human body. It appears that over 95% of plant biomass and over 87% of human biomass is composed of oxygen, carbon, hydrogen and nitrogen. Thus, on a mass basis these four elements are the most important for Lunar Base CELSS implementation. Of the four elements, only oxygen is present in Lunar regolith in large amounts. As a consequence, from a life support perspective the extraction of oxygen from regolith must be the initial target for the technological development of in situ resource utilization as well as the primary focus for interfacing with the Lunar Base CELSS. Although trace amounts of carbon, hydrogen and nitrogen might be extracted from lunar regolith, these elements will have to be supplied from Earth, at least initially. However, as the capability for resource extraction develops, there will be less need to rely on supplying even these elements from the Earth. The Lunar Base CELSS design includes two methods by which oxygen can be added to the support system. First, oxygen can be directly added to the crew

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Table 3. Elemental Composition of Lunar Regolith22

Element A1

Ca Cr Fe K Mg Mn

Na 0 P S Si Ti

Mare High Ti A-11

Mare High Ti A-17

7.29 8.66 0.21 12.2 0.1 2 4.93 0.1 6 0.33 41.6 0.05 0.1 2 19.8 4.60

5.80 7.59 0.31 13.6 0.06 5.80 0.19 0.26 39.7 0.03 0.13 18.6 5.65

Mare Low Ti A-12

7.25 7.54 0.24 12.0 0.22 5.98 0.1 7 0.36 42.3 0.14 0.10 21.6 1.84

Mare LowTi A-15

5.46 6.96 0.36 15.3 0.08 6.81 0.19 0.23 41.3 0.05 0.06 21.5 2.11

Mare Low Ti L-16

8.21 8.63 0.20 12.7 0.08 5.30 0.1 6 0.27 41.6 0.06 0.21 20.5 2.11

Basin Ejecta A-14

9.21 7.71 0.15 10.3 0.46 5.71 0.11 0.52 43.8 0.22 0.08 22.4 1.02

Table 4. Elemental Composition of Plant and Human Tissues23 Element

0 C H N

Si K

Ca P Mg S

CI

Al Fe Mn

Na Zn Rb

Plant fZea mais)

Human

44.44 43.57 6.24 1.46 1.17 0.92 0.23 0.20 0.1 8 0.1 7 0.1 4 0.1 1 0.08 0.04

14.62 55.99 7.46 9.33 ,005 1.09 4.67 3.11 0.16 0.78 0.47

-

-

0.01 2 0.47 0.01 0.005

Basin Ejecta A-15

9.28 6.27 0.19 9.00 0.1 4 6.28 0.1 2 0.31 43.8 0.07 0.08 21.7 0.79

Basin Ejecta A-17

10.9 9.1 9 0.18 6.68 0.1 3 6.21 0.08 0.30 42.2 0.06 0.06 21 .o 0.97

248

STEVEN H. SCHWARTZKOPF

atmosphere on an as required basis. Second, oxygen from in situ resource utilization could be added to the oxygen storage buffer included in the atmospheric control subsystem. The conceptual design assumes that in worst case the oxygen could be isolated by the same type of installation that is to be used for the isolation of oxygen from the plant growth unit. At best, the oxygen stream from the in situ resource utilization plant would be filtered to remove particulates and then added to the crew chamber or buffer. Thus, it appears that both interfaces are simple and direct, and neither requires any unique or specific hardware. Carbon, hydrogen, and nitrogen are also available in regolith, but at much lower concentrations. Accordingly, the implementation of in situ resource utilization technology for their extraction is a lower priority than that of oxygen. The addition of nitrogen to the Lunar Base CELSS would be as straightforward as the addition of oxygen, and should not require any unique hardware. Carbon and hydrogen addition would be easiest in the form of CO, and water, respectively. Specific hardware would be required to oxidize both elements prior to their addition to the support system, but the actual addition of their oxidized forms will present no problems, since storage buffers for both H,O and CO, exist in the conceptual design. Still another form of in situ resource utilization would involve the recovery of plant macro- and micro-nutrient elements from regolith. At this point, the interfacing requirements for this type of technology are difficult to establish, as the chemical form in which the elements would be extracted determines the method of addition to the support system.

VII. DESIGN CHARACTERISTICS AND BREAKEVEN ANALY S IS The estimated masses of the Lunar Base CELSS conceptual design to support the three crew sizes of 4, 30, and 100 persons are presented in Table 5. It is clear that the plant growth unit constitutes by far the largest subsystem in all three cases. In the 4-person crew, the plant growth unit accounts for 82% of the total life support system mass, while for the 30- and 100-person crew sizes, these units account for 79% and 74%, respectively, of the total mass. This percentual decrease is due to the addition of the larger, but lighter, plant growth unit designs as the base nears maturity. The second largest subsystem is the aquaculture unit, which comprises 9-1 2% of the total life support system mass. The food and oxygen reserves were calculated for different time intervals. For the food production system it was assumed that it might take up to one full crop cycle, expected to last 60-90 days, to return to equilibrium. Thus the food reserve was set to last for a 90-day period. Interruption of the food production cycle will also stop the biological oxygen production. However, it was assumed that an adequate oxygen production would already resume about 30 days after starting a new crop.

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Table 5. Mass Estimates of Lunar Base CELSS Components for Different Crew Estimated Mass by Crew Size

Subs ystem/Component

4

30

Plant Growth Unit(s) Solid Waste Processing Atmosphere Regeneration Water Purification Aquaculture (Tdapia) Food Processing Inflation Gas 90-Day Food Reserve 30-Day Oxygen Reserve

12,322 63 271 31 1,366 26 0 565

Total

15,038

-3%

100

78,641 273 1,169 233 10.1 693 52 1,446 4,239

209,081 808 3,016 778 3,695 122 12,014 14,130

2352

9.840

99,174

283,484

Note: All figures in kilograms.

Estimates of the electrical power required to operate the life support system for these three crew sizes are shown in Table 6. The maximum power values are required only during the lunar night, when the entire photosynthetic active radiation required by the plants must be provided by artificial light. The minimum operating power values, applicable during the lunar day when this radiation can be supplied by natural sunlight, are also presented for comparison purposes. It is clear that the use of sunlight will dramatically decrease the total power requirement for the life support system. One of the most important questions addressed by the Lunar Base CELSS study was to estimate the economic feasibility of such a system. Myers2’ has developed a method to do this by graphically determining the mission length at which a regenerativelife support system begins to pay off economically.In this “breakeven” method the mass of life supportconsumablesis added to the mass of the life support hardware required to maintain one person. Figure 6 presents the results of such an Table 6, Power Estimates of Lunar Base CELSS for Different Crew Sizes Power Requirement (kW) ~~

Crew Size 4 30 100

~~

lunar Night (maximum)

lunar Day (minimum)

72 61 7 1,700

12 94 226

STEVEN H. SCHWARTZKOPF

250 RESUPPLY

FOOD PRODUCTION

...”.,..

WITH WASTE PROCESSING

\

,,..**’.

....

,...I..

. .. ......‘

...a

MASS PER PERSON

...a

/,

.

._. .. .’ , ..

_,

\

”

ATMOSPHERE REGENERATION

0 0

MISSION DURATION

Figure 6. Graphical method for determining breakeven points (after Myers2’).

analysis, where the mass to be launched per crew member is plotted as a function of the mission duration. In this figure the line labeled “RESUPPLY” represents a scenario in which all consumables must be resupplied and no recycling life support equipment is used. The y-intercept of this line is the launch mass at the time of first launch, which is set to zero. The slope of this line is equal to the resupply mass required to support one person per time unit expressed as a fraction of the mission duration. The addition of regenerative equipment increases the initial launch mass, as given by the y-intercept. However, the need for resupply of consumables decreases, as expressed by a decreased slope of the line. In the scenario labeled “WATER RECYCLING the mission launch mass reflects the addition of the water recycling hardware. The breakeven point comes relatively soon at point 1. Thereafter, the resupply mass required to sustain one person decreases to 35% of that required without recycling of water in this hypothetical case. This reduction equates directly to an economic savings, as the launch cost of each kilogram of material is approximately constant for a specific launch vehicle. When additionallyhardware for atmospheric regeneration is included, the breakeven point comes later (point 2), but the resupply mass decreases to 6.3% in this case, as indicated by the line labeled “ATMOSPHERE REGENERATION.” When in addition hardware for production of food and recycling of solid waste is incorporated, the breakeven point comes still later (point 3), but the resupply mass falls to nearly zero, as indicated by the line labeled “FOOD PRODUCTION WITH WASTE PROCESSING.” It is clear that increasing the regenerative capability lengthens the time required to reach the breakeven point, but greatly decreases the required resupply of consumables. Full recycling pays off only for longer missions.

Human Life Support

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O F0

1 2 3 MISSION DURATION (yr)

4

Figure 7. Cumulative launch mass of open loop, partially closed ( P K ) and closedloop life support systems (CELSS) for a 4-person lunar base as a function of mission duration.

By means of such a breakeven analysis we have calculated the potential mass savings produced by a Lunar Base CELSS designed as described in this chapter. It has been performed by using the estimated life support mass required for a 4-person crew. Figure 7 illustratesthe cumulative mass of life support materials required by a 4-person crew as a function of time, if no recycling is used (OPEN LOOP). If equipment to recycle air and water is added, the initial launch mass increases, but the breakeven point relative to 100% resupply is already reached after 1 month, and the cumulative amount of life-sustaining materials launched during the mission is greatly reduced (P/C). When a complete recycling system for atmosphere, water, food, and waste is transported to the Moon, the breakeven point relative to 100% resupply comes after about 5 months and after 2.5 years relative to air and water recycling only. Thus, in terms of cumulative launch mass savings, a complete recycling Lunar Base CELSS makes sense only for a mission of 2.5 years duration or longer.

VIII. CONCLUSIONS AND SUMMARY The requirementsfor a human life support system for long-duration space missions are reviewed. The system design of a controlled ecological life support system is briefly described, followed by a more detailed account of the study of the conceptual design of a Lunar Base CELSS. The latter is to provide a safe, reliable, recycling lunar base life support system based on a hybrid physicochemicalhiological regenerative technology. The most important conclusion reached by this study is that implementation of a completely recycling CELSS approach for a lunar base is not only feasible, but eminently practical. On a cumulative launch mass basis, a 4-person Lunar Base CELSS would pay for itself in approximately 2.6 years relative to a physicochemi-

STEVEN H. SCHWARTZKOPF

252

cal aidwater recycling system with resupply of food from the Earth. For crew sizes of 30 and 100, the breakevenpoint would come even sooner,after 2.1 and 1.7 years, respectively, due to the increased mass savings that can be realized with the larger plant growth units. Two other conclusions are particularly important with regard to the orientation of hture research and technology development. First, the mass estimates of the Lunar Base CELSS indicate that a primary design objective in implementing this kind of system must be to minimize the mass and power requirement of the food productionplant growth units, which greatly surpass those of the other air and water recycling systems. Consequently,substantial research must be directed at identifying ways to produce food more efficiently. On the other hand, detailed studies to identify the best technology options for the other subsystemsshould not be expected to produce dramatic reductions in either mass or power requirement of a Lunar Base CELSS. The most crucial evaluation criterion must, therefore, be the capability for functional integration of these technologies into the ultimate design of the system. Secondly, this study illustrates that existing or near-term technologies are adequate to implement a Lunar Base CELSS. There are no apparent “show-stoppers” which require the development of new technologies. However, there are several areas in which new materials and technologies could be used for a more efficient implementation of the system, e.g., by decreasing mass or power requirement and increasing recycling efficiency. These areas must be further addressed through research and development. Finally, although this study focused on the development of a Lunar Base CELSS, the same technologiesand a nearly identical design would be appropriatefor a Mars base. Actually, except for the distance of transportation, the implementation of a CELSS on Mars would even be easier than it would be on the Moon. The presence of atmospheric CO, on Mars, although in low concentration, coupled with the fact that the dayhight cycle on Mars is very similar to that on Earth, makes the use of light-weight, greenhouse-like structuresfor growing food plants even more feasible than on the Moon. There are some environmental problems, which would have to be dealt with, like dust storms and the large amount of ultraviolet radiation incident on the planet’s surface. However, the materials and methods are largely available today to develop such a life support system for a Mars base.

REFERENCES I . Calloway, D.H. Basic data for planning life-support systems. In: Foundations ofspace Biology and Medicine (Calvin, M. and Gazenko, O.G., Eds.), pp. 3-2 1. Vol. 111. National Aeronautics and Space Administration, Washington, D.C., 1975. 2. Bozich, W.F. Presentation at TMSA Space Requirements Conference. March, 1991, Los Angeles, CA. 3. Gary P. Noyes, Carbon Dioxide Reduction Processes for Spacecraft ECLSS: A Comprehensive Review. Proceedings 18th Intersociety Confirence on Environmental Systems, July 1988, San Francisco, paper 88 1042.

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4. Wieland, P.O. Designingfor Human Presence in Space: An Introduction to Environmental Control and Life Support Systems. NASA-Marshall Spaceflight Center, NASA Reference Publication no. 1324, NASA, Washington, D.C., 1994. 5. Friedman, M.A., Schwartzkopf, S.H., Styczynski, T.E., Tleimat, B., Tleimat, M. Gray Water Recycling with a Unique Vapor Compression Distillation (VCD) Design. Proceedings 22nd International Conference on Environmental Systems, July, 1992, Seattle, WA, paper no. 9213 18. 6. Martin, J.H., Leonard, W.H., Stamp, D.L. Principles of Field Crop Production. Macmillan Publishing Co., New York, 1976. 7. Wolverton, B.C., McDonald, R.C., Duffer, W.R. Microrganisms and higher plants for waste water treatment. Journal of Environmental Quality. 12(2):23&244, 1983. 8. Lamparter, R.A. Personal Communication, 1991. 9. Berman, G.. Murashige, K. (Eds.) Synthetic Carbohydrate:An Aidto Nutrition in the Future. Final Report of the Stanford-Ames NASNASEE Summer Faculty Systems Design Workshop, NASA Contract NGR-05-020-409, January, 1973. 10. Shapira, J. Design and evaluation of chemically synthesized food for long space missions. In: The Closed Life-Support System, NASA-Ames Research Center, NASA SP- 134. pp. 1 7 5 1 87. NASA, Washington, D.C., 1967. 11. Miller, R.L., Ward, C.H. Algal bioregenerative systems. In: Atmosphere in Space Cabins and Closed Environments ( K . Kammermeyer, Ed.), pp. 186-222. Appleton. New York, 1966. 12. Ward,C.H., Wilks, S.S.,Craft H.L. Useofalgaeandotherplantsin thedevelopmentoflifesupport systems. American Biology Teacher 2 5 5 12-524, 1963. 13. Hoff, J.E., Howe, J.M., Mitchell, C.A. Nutritional and CulturalAspectsofPlant Species Selection f o r a Controlled Ecological Life Support System. NASAContractor Report#166324,NASA-Ames Research Center, Moffett Field, CA, 1982. 14. London, S.A., West, A. Gaseous Exchange in a Closed Ecological Life Support System. Report from Aerospace Medicine Research Lab, Wright-Patterson A.F.B., Ohio, 1962. 15. Bowman, R.O., Thomae, F.W. An algae life support system. Aerospace Engineering, 19(12): 2-2, 1960. 16. Bowman, R.O., Thomae, F.W. Long-term non-toxic life support of animal life with algae. Science, 134:55--56, 1961. 17. Ivanov, B., Zubareva, 0.To Mars and back again on board bios. Soviet Life. April 1985, pp. 22-25. 18. Lockheed Missiles & Space Co., Inc. Lunar Base Controlled Ecological Life Support System (LCELSS). Preliminary Conceptual Design Study: Final Report. LMSCIF280196, April 30,199 1. 19. Schwartzkopf, S. Hazard and risk assessment for surface components of a lunar base controlled ecological life support system. Proceedings 22nd International Conference on Environmental Systems, July 1992, paper 921285. 20. Myers, J. Space biology: ecological aspects-introductory remarks. American Biology Teacher 25:40W12, 1963. 21. Phillips, J.M., Harlan, A.D., Krumhar, K.C., Caldwell, M.S., Crowlie, C.M., Ramsbacher, L., Meyer, B.S. Studies of Potential Biological Components of Closed Life Support Systemsfor Large Space Habitats: Research and Technology Development Requirements, Costs, Priorities and Terrestrial Impacts. Final Report, Grant NSG2309. NASA-Ames Research Center, 1978. 22. Phinney, W.C., Criswell, D., Drexler, E., Garmirian, J. Lunar Resources and Their Utilization. In: Space-Based Manufacturingfrom Non-terrestrialMaterials (G. O’Neill, Ed.), pp. xx-xx. Princeton University Press, Princeton, NJ, 1977. 23. Epstein, E. Mineral Nutrition ofPlants: Principles and Perspectives. John Wiley and Sons, New York, N.Y., 1972.