life-support systems
Life-Support Systems
in space flight, the complex of measures directed at maintaining the life processes of the crew of a spacecraft during flight.
The upper strata of the earth’s atmosphere and, even more so, space and the surface conditions on planets of the solar system are unsuitable to support the life of highly organized beings, including man. However, the life and activities of man in space can be maintained by creating living environments close to the optimal range of life on earth (in earth’s biosphere) in spaceships, man-made satellites, or planetary stations; this applies equally to the air (the artificial atmosphere of the spaceship) and to those elements of the environment (in the broad sense of the word) that are necessary for nutrition and the maintenance of the water balance of the human body.
Human existence is based on a continuous exchange of matter and energy with the environment. The function of the life-support system is to make this possible. Thus, a life-support system is a complex of devices and assemblies and material reserves that provide the necessary conditions for the life processes of the crew throughout the course of flight. Partial systems, or subsystems, of this complex provide for particular aspects of the body’s life processes (metabolism): nutrition, water exchange, gas exchange, heat exchange (thermoregulation), the performance of natural functions, and so on. This is the typical structure of a life-support system in the frequently used, narrow meaning of the term. A life-support system may be collective (for example, that of a spaceship or planetary station) or individual (for example, autonomous systems used together with spacesuits).
In a broader sense, the term is sometimes taken to include, in addition, all other devices and objects that provide for the hygienic, everyday, cultural, and aesthetic requirements of the crew. The necessity for the more complete satisfaction of these requirements increases substantially with longer duration of the crew’s sojourn in space, when these aspects of human activity may acquire vital significance. Partial life-support systems are divided into nonregenerative systems, which provide for the creation of on-board reserves of food, water, and oxygen, and regenerative systems on the regeneration of these substances from the products of human life processes or those of other inhabitants of spaceships and satellites.
The theoretical possibility of regenerating all substances necessary for human life processes is based on the fact that the body eliminates, in the composition of the products of its life processes, all those chemical elements it has obtained in the form of food and water or absorbed while breathing oxygen. Thus, in a practical sense, a closed cycle of necessary substances is created. The regeneration of food substances (from the carbon of carbon dioxide gas and from the water and mineral elements of urine and feces) may be accomplished, in principle, by using autotrophic organisms capable of photosynthesis or chemosynthesis. Exploratory research is also being conducted on the artificial synthesis of nutritional carbohydrates from water and gaseous carbon dioxide.
Calculations for life-support systems proceed from man’s requirements for food, water, and oxygen and from the quantity of the discharged products of the vital processes, which together constitute the material balance of metabolism in the human body (see Table 1). In addition, the amount of water
| Table 1. Approximate material balance of human metabolism | |||
|---|---|---|---|
| Consumption, in g per man-day | Elimination, in g per man-day | ||
| Food................. | 500 | Carbon dioxide................. | 930 |
| Oxygen................. | 800 | Water vapors................. | 840 |
| Water................. | 2,200 | Urine................. | 1,500 |
| Feces................. | 230 | ||
| Total................. | 3,500 | Total................. | 3,500 |
needed for personal toilet is calculated; with nonregenerative systems and short flights, this comes to about 100 g per man-day, on long flights increasing to 2–2.5 kg per man-day. Water, depending on the amount used for personal toilet, makes up 60–80 percent of the mass of stored substances. For this reason, regenerative water-supply systems make the weight balance of the life-support system lower than do non-regenerative systems in proportion to the number of crew members and the duration of flight. (Proceeding from this, the material balance in life-support calculations is measured in man-days.)
Systems of oxygen-supply are distinguished by a diversity of theoretical approaches and solutions (see Table 2). The methods of oxygen regeneration shown in Table 2 are only those that have been most fully elaborated; they do not exhaust the possible technological principles of regeneration. The methodology and apparatus for regenerating oxygen by the electrolysis of water provides for human gas exchange by means of an apparatus that weighs approximately 30 kg and has an electric capacity of 10 watts for each liter of oxygen. The biological regeneration of oxygen may be accomplished by photosynthetic unicellular algae, of which Chlorella has
| Table 2. Basic technological principles of oxygen-regenerating systems | |||
|---|---|---|---|
| Nonregenerative systems | |||
| Physical | Physicochemical | Chemical | |
| Forms of stored oxygen | Molecular oxygen: gaseous, liquid | Chemically bound as water | Chemically bound in peroxides, superperoxides, and ozonides of alkali metals, perchlorates and hydrogen peroxide |
| Methods of mobilizing the reserve | Stepwise reduction of a high-pressure gas; evaporation of a liquefied gas and reduction | Electrolysis of water (free or bound by phosphoric anhydride) | Chemical decomposition of oxygen compounds of metals while absorbing water and carbonic acid; catalytic decomposition of hydrogen peroxide |
| Energy sources | Internal energy of a compressed or liquefied gas | External energy sources | Exothermic reaction energy |
| Regenerative systems | |||
| Physicochemical | Biological | ||
| Oxygen sources | Carbon dioxide gas and water excreted by man as products of the oxidation of foods | Carbon dioxide gas and water excreted by man as products of the oxidation of foods | |
| Methods of regeneration | Electrolysis of water; direct reduction of carbon dioxide gas by hydrogen to carbon and water with subsequent electrolysis of water; reduction of carbon dioxide gas by hydrogen to methane (or carbon monoxide) and water with subsequent electrolysis of water | Photosynthesis of green plants, chemosynthesis of autotrophic bacteria (for example, hydrogen oxidizing bacteria) | |
| Form of consumed energy | Thermal, electrical | For photosynthesis, light; for chemosynthesis, electrical (to obtain hydrogen) | |
been most studied. In laboratory experiments the possibility of providing for human gas exchange for up to 60 days has been shown, with a volume of algal culture on the order of 20–30 liters per person and a loss of mineral salts of about 50 g per man-day. Such a system simultaneously provides for absorption of the gaseous carbon dioxide eliminated by man. In more complex variations of the photosynthetic regenerative system, the loss of mineral salts may be decreased several times by using the mineral elements of urine. This system simultaneously provides for the most power-consuming stage of the regeneration of water from urine, evaporation. Moreover, part of the biomass of algae may be used in the dietary ration (up to 20 percent of the protein portion of the ration). The use of chemosynthetic gas exchangers on the basis of hydrogen-oxidizing bacteria is expedient in the presence of an electrolytic system where the hydrogen obtained in the system is not used for the hydrogenation of gaseous carbon dioxide, carbon monoxide, or methane in the ongoing physicochemical processes. In order to maintain the composition of the spaceship’s atmosphere, it is necessary not only to compensate for oxygen loss, but also to remove excess carbon dioxide gas and water vapors. Carbon dioxide may be removed by physical methods (freezing, condensation) and by the use of alkaline chemical absorbents. It is more economical to use regenerated sorbents (zeolites, carbonates). The alternate operation of two cartridges of zeolite in a “sorption-desorption” regime ensures absorption of the carbon dioxide gas eliminated by two crew members; the apparatus weighs about 40 kg.
Excess water vapors may be removed from the air chemically by means of nonregenerable chemical absorbents and regenerable sorbents (zeolites), or physically by methods such as freezing and condensation. In today’s spaceships some of the water vapors are condensed on the cold surfaces of the air-liquid heat exchangers that are part of the thermo-regulation system of the living quarters.
The partial life-support systems—the regeneration of oxygen and the removal of gaseous carbon dioxide and water— are part of a single complex for maintaining the composition of the spaceship’s atmosphere. Sometimes the system of thermoregulation and the filters for cleansing the air of harmful impurities are included in this complex. The functions of these systems may be performed by certain independent devices. Thus, specifically, the problem of the life-support system of the atmosphere in the American Mercury, Gemini, and Apollo spacecraft was solved by using oxygen reserves and nonregenerative absorbents of carbon dioxide gas and water vapors. Chemical systems provide for the conjunction of the processes in question within a single system. Precisely such a solution was used in the Soviet spacecraft Vostok, Voskhod, and Soyuz, where a nonregenerative system with the superperoxide of an alkali metal was used. The release of oxygen by the regenerative substance is associated with the fully determined amounts of absorbed water and carbon dioxide gas (see Figure 1).
The water-supply system is based on water reserves. In the spaceship Apollo, drinking water was also manufactured from the reserves of oxygen and hydrogen “burned” in electrochemical generators (fuel cells) to obtain electrical energy. Various physicochemical methods have been worked out for the regeneration of water from condensates of urine and atmospheric humidity. A condensate of atmospheric vapors is quite effectively cleansed of the inevitable organic impurities by catalytic oxidation and by means of ion-exchange resins and coals. In the most well-developed methods of regenerating water from urine, evaporation schedules are used at various pressures and temperatures and with subsequent catalytic oxidation of the contaminants in the vapor stage and purification of the resultant condensate with sorbents. These methods allow for the regeneration of a large portion of the used water, with further improvement they will make possible a practically closed cycle of water regeneration.
In contrast to the above systems, food provision holds no imminent prospects for a transition to regenerative systems. Food reserves in a spacecraft consist of produce and prepared dishes that are either preserved in their natural state or
dehydrated. The regeneration of food substances is possible through the use of photosynthetic green plants. Inasmuch as the problem of the absorption of gaseous carbon dioxide and the regeneration of water would also be solved in such a system it is possible to create a life-support system in the style of a closed ecological system based on a closed biological cycle of a limited amount of matter. Substances needed by man are continuously being reconstituted in such a system, owing to the vital processes of plants, animals, and microorganisms. In order to do this it is necessary to arrange a complex of the necessary organisms into the sort of functional closed chain (including man) in which the “output” characteristics of the preceding link correspond to the parameters of the “input” of the succeeding one. As a result of this type of organization of matter-energy relations between the elements of the system, a new quality emerges—an integral system of a higher order, possessing the properties of a closed thermodynamic system. Such a system is theoretically capable of autonomous existence (without the entry of matter from outside), to the extent that this is made possible by the correlation of the input and output characteristics of neighboring links in the system. With this, a situation would arise for the first time in which the existence of the system itself would become dependent on the life processes of man as one of its functional elements. That dependence is so great that the usual notion of a life-support system as something external to man loses its basis, insofar as man would here be an object of provision to the same extent that he would himself be necessary as a component part of the system as a whole. This points to the arbitrary nature of the term “life-support system” in relation to closed ecological systems that include man.
REFERENCES
Problemy kosmicheskoi biologii, vols. 5–7. Leningrad-Moscow, 1967.Kosmicheskaia biologiia i meditsina. Moscow, 1966.
O. G. GAZENKO