Available information on the important environmental elements in manned space vehicles and their effect on human occupants is formulated here as a synthesis of data collected from a large number of sources in the literatures of aviation medicine, submarine, and deep sea-diving medicine, and human heat regulation.1-34
1 Adolph, E. F., and Associates, Physiology of Man in the Desert, Interscience Publishers, Inc., New York, 1947.
2 Armstrong, H. G., Principles and Practice of Aviation Medicine, 3d ed., the Williams & Wilkins Co., Baltimore, 1952,
3 Berry, C. A., The Environment of Space in Human Flight, Institute of the Aeronautical Sciences, New York, Preprint No. 796, January 1958.
4 Buettner, K., Man and His Thermal Environment, Mechanical Engineering, November 1957.
5 Burnight, T. R., Physics and Medicine of the Upper Atmosphere, ch. 13, Ultraviolet Radiation and X-rays of Solar Origin, University of New Mexico Press, Albuquerque, 1952.
6 Burton, A. C., and O. G. Edholm, Man in a Cold Environment, Physiological an Pathological Effect of Exposure to Low Temperatures. Edward Arnold (Publishers) Ltd., London 1955.
7 Carlson, L. D., Man in Cold Environment, a Study in Physiology, Alaskan Air Command, Ladd Air Force Base, Fairbanks, Alaska, August 1954.
8 Code, C. F., et al., The Limiting Effect of Centripetal Acceleration on Mans Ability To Move, Journal of Aeronautical Science, 14 :117, 1947.
9 Cranmore, Doris, Behavior, Mortality, and Gross Pathology of Rats Under Accelerative Stress, Aviation Medicine, April 1956.
10 Dill, D. B., Life, Heat, and Altitude, Physiological Effects of Hot Climates and Great Heights. Harvard University Press Cambridge, Mass., 1938.
11 Flight Surgeons Manual, Air Force Manual 160-5, Department of the Air Force, U. S. Government Printing Office, Washington, D. C., 1954.
12 Gagge, A. P., Man's Response to Temperature Extremes, Proceedings of the Fifth AGARD General Assembly, Ag 20/P10, June 1955, pp. 82-91.
13 Gagge, A. P., et al., The Influence of Clothing on the Physiological Reactions of the Human Body to Varying Environmental Temperatures, American Journal of Physiology, 124, 1938, pp. 31-50.
14 German Aviation Medicine World War II, vols. I and II, prepared under the auspices of the Surgeon General, USAF, Department of the Air Force, U. S. Government Printing Office, Washington, D. C. 1950.
15 Haldane, J. B. S., What Is Life? Boni and Gaer, New York, 1947.
16 Harrington, L. P., A Survey Report on Human Factors in Undersea Warfare, ch. 13: Temperature and Humidity In Relation to the Thermal Interchange Between the Human Body and the Environment, Committee on Undersea Warfare, National Research Council, Washington, D. C., 1949.
17 Krieger, F. J., A Casebook on Soviet Astronautics, pt. II, The RAND Corp., Research Memorandum RM-1922, June 21, 1957.
18 Lee, D. H. K., Human Climatology and Tropical Settlement, University of Queensland, Brisbane 1947.
19 Lewis, C., USAF School Simulates Living in Space, Aviation Week, January 27, 1958.
20 Ley, Willy, and W. von Braun, The Exploration of Mars, The Viking Press, New York, 1956.
21 Mayo, A. M., Survival Aspects of Space Travel, Aviation Medicine, October 1957.
22 McFarland, Human Factors in Air Transportation, McGraw-Hlll Book Co., New York, 1956.
23 Simons and Parks, Climatization of Animal Capsules in Upper Stratosphere Balloon Flights, Jet Propulsion, July 1956, p. 565.
24 Stapp, J. P., Collected Papers on Aviation Medicine, AGARD-ograph No. 6, ch. 14: Tolerance to Abrupt Deceleration, Butterworth Scientific Publications, London, 1955.
25 Stauffer, F R, Acceleration Problems of Naval Training. 1. Normal Variations In Tolerance to Positive Radial Acceleration, Aviation Medicine, June, 1953, p. 167.
26 Stewart, W. K., Some Observations on the Effect of Centrifugal Force in Man, Journal of Neurological Psychiatry, 8: 24, 1945.
27 Stewart, W. K., Lectures on the Scientific Basis of Medicine, II, The Physiological Effects of Gravity, London, 1952-53, pp. 334-343.
28 Stoll, A. M., Human Tolerance to Positive G as Determined by the Physiological End Point, Aviation Medicine, August 1956, p. 356.
29 Submarine Medicine Practice, Bureau of Medicine and Surgery, Department of the Navy, NAVMED-P 5054, U. S. Government Printing Office, Washington, D. C., 1956.
30 Webster, A. P., Acceleration Limits of the Human Body, Aviation Age, March 1956, p. 26.
31 Zuidema, G. D., et al., Human Tolerance to Prolonged Acceleration, Wright Air Development Center, WADC TR 56-406, October 1956.
32 Nadel, A. B., Human Factors Requirements of a Manned Space Vehicle, General Electric Technical Military Planning Operation, Rept. No. RM 58TMP-10, April 10, 1958.
33 Air University Quarterly, summer issue, 1958.
34 Hullinghorst, Col. R L., Meeting the Physiological Challenge of Man in Space, Army Information Digest, vol. 13, No. 10, October 1958, p. 38.
In order to function properly, any system must be maintained within certain physical and chemical limits. Man is no exception. The primary elements of the environment that have an immediate bearing on the health and well-being of the man are the following:
In general, if one plots any limiting stress or departure from some preferred environment against time, a curve results which separates the figure into two general areas, as in figure 1. The area below the curve represents conditions under which man can operate more or less normally and efficiently. The area above the curve represents conditions which the man cannot tolerate and under which he cannot operate. Because of the severity of the stress in this area, he is either incapable of performing, or is unconscious, or is dead. Separating these two regions is a broad transition band representing (for any given individual) a gradual loss of efficiency and loss of ability to recover promptly when the stress is removed. The deeper we penetrate into the transition band, the greater the physiological strain.
The following general observations are significant:
First, the effect of the gaseous environment and respiratory requirements (fig. 2). This figure represents the region of human tolerance to variations in the partial pressure of oxygen in the inspired air. The normal sea-level value is 149 millimeters of mercury. The lower band in the figure represents the minimum oxygen partial pressure that can be tolerated in the air entering the lungs. Man's reaction depends, of course, both upon the level of inspired oxygen pressure and the rate of penetration into the intolerable region.
For gradual penetration into the region of too little oxygen, the usual symptoms are: Sleepiness, headache, lassitude, altered respiration, psychologic impairment, inability to perform even simple tasks, and eventual loss of consciousness.
For a sudden transition into excessively low oxygen pressures, the intermediate symptoms are bypassed and the man rapidly loses consciousness, goes into spasms or convulsions. If the low oxygen pressure is accompanied by low total pressure, the sudden decompression is characterized by pains in the chest and in the joints due to bubble formation (or bends) and by confusion, delirium, and collapse.
Too much oxygen can also be lethal, as indicated by the upper curve, which delimits the oxygen toxicity region, the symptoms again depending upon rate of onset. Prolonged exposure to one atmosphere of pure oxygen, for example, eventually produces inflammation of the lungs, respiratory disturbances (coughing, gasping, and pulmonary congestion), various heart symptoms, numbness of the fingers and toes, and nausea.
Exposure to still higher partial pressures of oxygen produces nervousness, discomfort, irritation of the eyes and virtual blindness, nausea, loss of consciousness, and convulsions. In general, for long exposure times, inspired oxygen partial pressures should be kept well within the extremes of 80 and 425 millimeters of mercury to avoid undesirable effects. It should be mentioned also that one's tolerance to other stresses (e. g, acceleration ) is impaired by a low oxygen pressure.
It is perhaps easier to appreciate the respiratory requirements of men if they are expressed in terms of the total pressure and atmospheric composition (fig. 3). Here total pressure in pounds per square inch is plotted against the volume percentage of oxygen in the ambient "air." Normal sea-level conditions are 14.7 pounds per square inch absolute (p. s. i. a.) and 21 percent oxygen. Proceeding from this point, we may reach the critical low-oxygen region by several different routes. For example, if we maintain the total pressure at 14.7 p. s. i. a. and reduce the concentration of oxygen, a critical point for the unacclimatized person is reached when the oxygen concentration has fallen to about 11 percent. Or, if we keep the composition of the air constant at 21 percent oxygen and reduce the pressure (which is what happens when we ascend to higher altitudes in our atmosphere), a critical point is reached when the ambient pressure has fallen to about 8.4 p. s. l. a. This corresponds to air at about 15,000 feet. An equivalent condition for a man breathing an atmosphere of pure oxygen would be reached when the ambient pressure was about 2.5 p. s. i. a., which corresponds to the ambient pressure at about 42,000 feet.
Deleterious effects of the inert diluent do not become apparent until the total pressure reaches several atmospheres (40-50 p. s. i. a.). If the inert diluent is nitrogen, above this pressure nitrogen narcosis is encountered, the effects of which have been compared to alcohol intoxication: confusion, disorientation, loss of judgment, and unconsciousness. However, if helium is the inert diluent, much higher total pressures may be tolerated without any adverse effects, up to possibly 20 atmospheres total pressure.
Since man normally produces carbon dioxide at nearly the same rate as he consumes oxygen, this is an important environmental consideration (fig. 4).
Above 15 to 20 millimeters of mercury partial pressure of carbon dioxide in the inspired air (equivalent to 2 to 3 percent carbon dioxide at 1 atmosphere), the subacute effects are a noticeable increase in the breathing rate, and distention of the air sacs of the lungs, with impairment of the normal gas exchange in the lungs. Partial pressures above about 35 millimeters of mercury (equivalent to 5 percent carbon dioxide at 1 atmosphere) can be tolerated for a period of only a few minutes. Acute symptoms are heavy panting, marked respiratory distress, fatigue, stupefaction, narcotic effects, unconsciousness, and eventual death. For exposure for long periods of time most authorities recommend that the inspired carbon dioxide pressure be kept below 4 to 7 millimeters of mercury (0.5 to 1 percent at 1 atmosphere pressure) .
The relative position or orientation of the subject is of prime importance in determining tolerable levels of gravitational or acceleration force, or "g force.' As the g force is gradually increased, certain effects are observed (table 1) .
TABLE 1.-Gross effects of acceleration forces
|Earth normal (32.2 feet/second')||1|
|Hands and feet heavy; walking and climbing difficult||2|
|Walking and climbing impossible; crawling difficult; soft tissues sag||3|
|Movement only with great effort; crawling almost impossible||4|
|Only slight movements of arms and head possible||5|
Positive longitudinal g's, short duration
(blood forced from head toward feet):
|Visual symptoms appear||
2.5 - 7.0
3.5 - 8.0
|Confusion, loss of consciousness||
4.0 - 8.5
|Structural damage, especially to spine||
> 18 - 23
Transverse g's, short duration (head
|No visual symptoms or loss of consciousness||
0 - 17
28 - 30
|Structural damage may occur||
> 30 - 45
Figure 5 shows the time-tolerance relationships for positive longitudinal forces and for transverse forces (either prone or supine, prone being the position of lying face down and supine being the position of lying on one's back).
For the transverse position, human subjects in Germany during World War II were subjected to 17 g's for as long as 4 minutes reportedly with no harmful effects and no loss of consciousness. The curves indicated for very long periods of time are extrapolations and are speculative, since no data are available on long-term effects. Col. John Stapp, Air Force Missile Development Center, has investigated extreme g loadings, up to 45 g's, sustained for fractions of a second; These are the kind of accelerations or decelerations that would be experienced in crash landings. For these brief high g loadings, the rate of change of g exceeds 500 g's per second.
As a matter of interest, the beaded line on the figure indicates the approximate accelerations that would be experienced by a man in a vehicle designed to reach escape velocity with three stages of chemical burning, each stage having a similar load-factor-time pattern. This curve enters the critical region for positive g's. Most individuals would probably black out and some would become unconscious. However, for individuals in the transverse position, this acceleration could be tolerated and the individual would not lose consciousness.
It is important to note that there is no compelling reason for adopting elaborate measures either in equipment design or in flight crew selection to achieve extraordinarily high g force tolerances. Virtually all rocket engines are capable of being throttled to reduce thrust, and thereby vehicle acceleration; in this way g forces can be limited to any reasonable value (say, 4 or 5 g) .
High g forces are more likely to be an important factor in the reentry of ''capsules '' with no lifting surfaces. Glide vehicles can limit reentry loads to very modest levels.
There is only a very limited body of information available concerning the effects of complete absence of g force. The longest periods of weightlessness so far experienced by human beings have been of the order of 40 to 50 seconds in aircraft on zero g flight trajectories. As with other forms of stress, different people react in different ways to weightlessness. Some individuals find it unpleasant and some seem to enjoy it, but the durations so far have all been short. At present it is believed that at least some individuals will be able to adapt to a weightless condition for long periods of time. In any event, there are ways of avoiding weightlessness through vehicle rotation, if it is found to be a generally undesirable condition.
The longest space flight to date by an animal was that of the dog in Sputnik II. The vehicle was under zero g, but the acceleration force actually experienced by the animal was dependent upon the rotation rate of the satellite and the location of the dog relative to the axis of rotation-and these factors are not known with certainty at present.
Temperature is a more familiar variable element in man's environment-also a more complicated one to depict in simple terms (fig. 6). In the high-temperature region of the chart, for example, relative humidity is as important as temperature in its effect on human tolerance. It is well known that much higher temperatures can be tolerated if the humidity is low. Other important factors are thermal radiation, wind velocity, acclimatization, and the amount of activity being engaged in. The curves presented here assume that adequate drinking water is being supplied to make up for water lost in perspiration, which can amount to very sizable quantities at high temperatures.
The general symptoms of excessively high temperature, depending upon degree and length of exposure, are: lassitude, loss of efficiency, weakness, headaches, inability to concentrate, apathy, increase in cardiac work, nausea, visual disturbances, increased oxygen uptake, increased body temperature, heat stroke, and convulsions.
In the low-temperature region, the situation is also highly complicated and the position of the critical region is affected by the amount of activity, thermal radiation, wind velocity, acclimatization, and the insulating effectiveness of gloves and boots. The curves of figure 6 suggest a limitation for a man wearing the warmest clothing, that he can wear and still retain ability to move freely (clothing about 1 inch thick). The usual hazards from low temperature are cold injury to extremities, frostbite, exhaustion, local freezing, depression of respiratory and circulatory functions, general hypothermia, paralysis, and death.
As indicated above, temperature has a profound effect on the quantity of water required per man per day. It has a less marked effect on the food and oxygen requirements, as indicated in figure 7. These curves are intended to be roughly suggestive of the dependence of water, food, and oxygen consumption upon ambient air temperature. They are greatly affected by the amount of work activity, and also by such factors as the amount of clothing worn, humidity, acclimatization, and body weight. However, as indicated, the quantity of water required becomes dominant at temperatures in excess of about 70° F. Thus, in a space ship it would be desirable to maintain the living quarters at a comfortable temperature, not only for the comfort of the crew but also to avoid excessive loading of the water-removal or water-recycling equipment.
With respect to the problems of recovery of man's metabolic products for reuse in a sealed cabin, many possible schemes of recovery of man's metabolic waste products have been proposed but none has yet been perfected. The actual type of scheme that would be used would depend upon the length of the projected excursion into space.
First it should be pointed out that the forms of radiation from the Sun other than the sporadic radiation from solar flares can be adequately handled within today's technology. The effects of thermal radiation can be controlled by adjusting the absorptivity and emissivity of the outer skin of the vehicle, and almost any desired skin temperatures can be obtained. As for solar radiation in the visible, ultraviolet, and soft X-ray regions, present data indicate that these do not constitute a direct hazard to crews of space vehicles, as they can easily be stopped or attenuated by thin layers of almost any structural material.
The newly discovered "radiation belts" of the Earth present a problem that can be met either by avoidance or by shielding to reduce dosages to human beings to acceptable levels. This radiation is apparently X-rays produced when high-velocity charged particles impinge upon the material of which a space vehicle is constructed. Manned satellites, to avoid the radiation belts, could orbit the Earth at altitudes lower than three or four hundred miles. The occupants of space vehicles escaping from the Earth could be shielded with fairly thin sheets of dense materials such as lead; or escape routes over the Earth's polar regions might be used to avoid the radiation belts almost entirely. More information about peak dose rates in these belts is needed to establish the best procedures for dealing with the problem, but solutions are available.
However, the hazard of cosmic radiation remains an open question, since there is no satisfactory way of shielding against it. What is still unknown is the relative biological effectiveness (RBE) of cosmic radiation as compared with other forms of radiation we know more about.
Figure 8 displays the generally accepted human tolerances with respect to genetic damage and nongenetic damage by gamma radiation. For example, for persons in the reproductive ages, recommended dosage limits for genetic reasons are: not more than 300 milliroentgens per week, no more than 15 roentgens in any one year, and no more than 5-roentgen-per-year average dose. A man can tolerate much larger dosages without any noticeable (nongenetic) effects, however, ( up to 50 roentgens in an acute dose).
Where would be the effects of the heavy nuclei of cosmic radiation with respect to these limits? It all depends upon the RBE, and at present the data are inadequate to calculate a meaningful RBE. Physical measurement indicates that the dosage that would be received from unshielded cosmic radiation is low.
The commonly accepted view is that there will be no measurable adverse effects for short exposure times but possibly some genetic effects or minor local tissue effects when the exposure is prolonged.
So far, in short-duration experiments with rats, there has been no evidence of cancer produced by cosmic radiation, although black mice and guinea pigs exposed to cosmic radiation at high altitudes for 24 to 36 hours subsequently developed gray spots in their hair, indicating irreversible damages to parts of hair follicles by cosmic heavy nuclei.
The whole question of the effects of cosmic radiation is now being studied intensively and undoubtedly much will be learned in the next few years.
The mechanism of X-ray formation inside a vehicle due to possible impingements on the outside is illustrated in figure 9.
A 5-man crew will require approximately 100 pounds of oxygen for a 10-day trip.35-36 Three possibilities immediately suggest themselves:
35 Ross, H. E., Orbit Bases, Journal of the British Interplanetary Society, vol. 8, No. 1, January 1949, pp. 1-19.
36 Paris, N. S., and S. S. Naistat, Hydrogen Peroxide as a Source of Oxygen, Water, Heat, and Power for Space Travel, Proceedings of the American Astronautical Society, Fourth Annual Meeting, January 29-31, 1958, New York, pp. 31-1 to 31-13.
Liquid oxygen (LOX),37 hydrogen peroxide, and some form of plant life to regenerate the air.
Liquid oxygen normally requires a double-walled insulated storage container, approaching in weight the LOX itself. Also LOX must be humidified by the addition of water prior to breathing.
Hydrogen peroxide (H2O2) does not present so difficult a storage problem, and is somewhat less hazardous. Using 90 percent H2O2, 236 pounds (plus a 40-pound container) would be required to yield 100 pounds of oxygen. The rest of the weight (136 pounds) would appear as water. This would itself be useful, and consequently cannot be counted against a hydrogen peroxide system. Furthermore, energy released during the decomposition of H2O2 to oxygen and water could be used as an auxiliary power source.
For very long flights one would attempt to establish a close regenerative cycle similar to that which exists in nature-man consuming oxygen and producing carbon dioxide, while plants reverse the cycle. Algae appear attractive because they have a high photosynthetic efficiency, have no waste stalks, etc.38 However, algae tanks are usually cumbersome and may require considerable power (about 1,400 watts per man ) .
The inert component of a space vehicle atmosphere is most likely to be nitrogen.39 Suitable means must be provided for filtering and purifying the atmosphere and controlling the moisture content.
Aboard space craft on extended trips it will be desirable for the morale and health of the crew to provide food that is varied in form and of high quality. Water may be recycled, and possibly food also, through the use of algae or various synthetic processes. Nevertheless homegrown food would undoubtedly be desirable. Satisfactory methods of preserving food should (1) not alter consumer acceptability; (2) insure a long storage life, preferably without refrigeration; (3) not require bulky or heavy packaging. Concerning the last, it may also be desirable to lighten the weight of the product further by dehydration.
In addition to conventional canning, freezing, and pickling40 there are three new approaches to food preservation: (1) Gamma irradiation, (2) beta irradiation, and (3) freeze-drying. Referring to the first two, gamma rays or electrons are utilized to extend the storage life of foods by the inhibition of sprouting and the destruction of microorganisms, parasites, or insects. Food preserved by irradiation is subjected to a minimum of temperature rise-normally enzymes are not deactivated unless a very strong dose is administered.
37 Compressed air itself would not be particularly desirable. Man uses up oxygen from the air, replacing it with carbon dioxide. Therefore it is sensible to take along only that constituent which is being consumed.
38 Gaume, J. G., Design of an Algae Culture Chamber Adaptable to a Space Ship Cabin. Proceedings of the American Astronautical Society. Fourth Annual Meeting, January 29-31, 1958, New York, pp. 22-1 to 22-4.
39 Haviland, R. P., Air for the Space Ship, General Electric Co., Document No. 56SD235; reprinted from Journal of Astronautics vol. 3 No. 2, summer 1956.
40 In canning, bacteria are killed and enzymes deactivated by heat; in pickling, the high pH concentration prevents bacteria growth; and in freezing, the low temperature inhibits bacteria growth.
Radiation treatment has been under intensive investigation at the University of Michigan Fission Products Laboratory, and at the Quartermaster Food and Container Institute in Chicago. The latter also subcontracts work. Sixty-nine cooperative Army research-contract holders are listed in the footnote reference below.41 Gamma irradiation can be provided in a number of ways: spent reactor fuel elements, a reactor core surrounded by a blanket containing a liquid with a specific gamma producer, separated fission products, gaseous fission products, and artificial isotopes. As long as there are no neutrons mixed with the radiation, and the energy of the latter is below the neutron binding energy in the target, there should be no induced radioactivity.42
Typical doses required for specific applications are as follows:43 Onion, potato, and carrot sprout-inhibition, 5,000 to 15,000 roentgen-equivalent-physical 44 (r. e. p.); mold inhibition on citrus, 150,000 to 250,000 r. e. p.; trichina irradiation, 50,000 r. e. p.; insect deinfestation of grain, 50,000 to 100,000 r. e. p.; pasteurization, 500,000 to 106 r. e. p.; sterilization, 2 to 4 x 106 r. e. p. Potatoes given 7,000 r. e. p. have resisted sprouting for up to 5 1/2 months at room temperature,45 and well over a year when refrigerated.46 47 Raw ground pork treated with rather high dosages has a refrigerated storage life of 10 to 11 days.48 Irradiated apples may be kept in a refrigerator for several months ( although the radiation tends to lower the total pectin content) .
Beta radiation may be supplied by Van de Graff machines, linear accelerators, etc. Electrons do not penetrate very well, but this may be an advantage in certain cases-i. e., surface mold treatment of peaches or citrus fruit. However, the Army is constructing at Stockton, Calif., a 20-m. e. v. (million electron volts) linear accelerator capable of treating slabs of food up to 6 inches thick (both sides are irradiated simultaneously). Induced radioactivity may be a problem at these energies.
In the case of freeze drying, food is first frozen, then placed in a vacuum and subjected to a pulsed electromagnetic beam (of radar frequencies) to sublime the ice crystals. (If a continuous beam were used, the center might be cooked.) The resulting product will have lost approximately 90 percent of its weight, and both bacterial and enzyme actions are inhibited by the absence of moisture. Refrigeration is not necessary if air- and moisture-proof packaging is available. The Raytheon Co. reports having successfully applied this technique to mushrooms, carrots, beef ribs, steak, veal cutlets, pork chops, lobster, shrimp, fish, strawberries, and peas. In the case of shrimp, the product has the consistency of popcorn. Preparation of the dehydrated
41 The Interdepartmental Radiation Preservation of Food Program, February 15, 1957, the Interdepartmental Committee on Radiation Preservation of Food.
42 Selection of a Food Irradiation Reactor Type-Phase 1, Internuclear Co., Inc.; Report AECU-3319, July 1, 1956.
43 Basic Concepts in the Application of Ionizing Radiations to Foods for Preservation B. H. Morgan. G. E. Donald, G. E. Tripp, and D. F. Farkas, Paper No. 57-NESC-117, Second Nuclear Engineering and Scientific Conference, March 11-14, 1057, ASME.
44 Defined as 93 ergs absorbed per gram of tissue.
45 Potatoes May Be First Food Preserved by Atomic Energy, I. Bialos, Western Grower and Shipper, June 1955.
46 There are some anomalies In sprout inhibition. A dose of radiation sufficient to inhibit sprouting by Spanish onions may accelerate it In the case of white pearl onions.
47 Combining Gamma Radiation, Refrigeration, L. E. Brownell, S. N. Purohit, Refrigeration Engineering, June 1956.
48 See footnote 43.
shrimp for eating requires one-half hour soaking in tepid water, and 2 minutes in boiling water. Although not entirely overlapping, it is clear that this process is in a sense competitive with the other two.
Now let us compare some of the relative advantages and disadvantages of canning, freezing, freeze drying, ß and y irradiation.
Advantages: Generally no refrigeration required, and a long shelf life.
Disadvantages: Low acceptability, increased weight, and high container cost.
Advantages: High acceptability, medium packaging costs, extended storage life as long as freezing temperatures are maintained.
Disadvantages: Freezers are bulky, expensive, and to some extent unreliable.
Advantages: No refrigeration required (if moisture-vapor and oxygen can be sufficiently excluded by the package), long storage life, light product weight, possibly high acceptance. Vitamins and the protein structure remain intact.
Disadvantages: Critical packaging. Even as much as 2 percent moisture by weight will cause "browning"-a nonenzymic chemical reaction. A slightly greater moisture content will activate enzymes and then bacteria.
Advantages: Versatile, can increase the refrigerated storage life of meat and produce, as well as break the trichinosis cycle. If sterilization were possible, refrigeration might be avoided altogether.
Disadvantages: Only a relatively few items can be treated by irradiation without producing undesirable tastes, colors or odors, and generally the stronger the dosage the worse the effects. Sterilizing doses almost always produce undesirable side effects-broccoli, for example, turns gray and limp.
A unique distinction may exist between liquid and solid propellant rockets for launching manned vehicles. Experience to date with liquid rockets shows that virtually all destructive failures are accompanied by rather slow buildup of fires with wide dispersal of the propellants. A manned vehicle built to survive reentry conditions could probably be expected to sustain the environment created by such launching rocket failures. On the other hand, bursting with explosive suddenness is not an uncommon type of malfunction with sold rockets and survival chances may not be good in the event of such a launching rocket failure. Future tests of all sorts of rockets should
be observed for indications of the validity or faultiness of these tentative impressions.49
As a general rule, machinery can be fairly readily designed for operation in a weightless environment. The effects of this environment on humans, however, is not known, although brief tests attainable in aircraft are encouraging.
It is often suggested that, should weightlessness prove to be a problem, it can be obviated by "artificial gravity" through rotation of the space vehicle. This can be done, since rotation produces centrifugal acceleration that can be made equal, at any desired point, to the acceleration of normal gravity. There are, however, certain limitations that must influence vehicle design and may also lead to a requirement for training and adaptation.50 First of all, centrifugal acceleration, for a fixed rate of rotation, depends on distance from the axis of rotation and always acts outward from that axis. If a man were to lie across the center of a rotating space vehicle with his head on one side of the axis of rotation and his feet on the other, he would be simply stretched by the centrifugal force-weightlessness might be better. For rotation to provide a reasonable substitute for gravity the design must be such that the man is at a distance from the axis of rotation that is large compared with his length; i. e., he must be several multiples of 6 feet away from the axis, say, 100 feet. Even at a point 100 feet from the rotation axis, the effective g force varies about 6 percent over the length of a 6-foot man.
Another point of consideration arises from the fact that a man in a space vehicle will want to move, and a moving body will experience some odd effects in a rotating vehicle due to the Coriolis force. This force exists because the vehicle is a rotating reference frame for the man's movements and, therefore, differs from customary nonrotating environment of everyday experience. As a result of Coriolis effects, a man will experience a level of g force that depends upon his direction of movement. If he is 100 feet from the rotation axis and the average g force is the normal sea-level value, he will experience a g force variation of almost 20 percent between a slow walk in one direction and a slow walk in the opposite direction. If he walks toward or away from the axis of rotation, he will feel a g force of about one-tenth normal in a sidewise direction. These effects suggest that a man in the artificial gravity of a rotating vehicle may stagger a bit until he gets his "space legs."
49 Goldsmith M., Suitability of Solid and Liquid Rocket Engines for Placing Manned Satellites In Orbit The RAND Corp. Paper P-1542. November 10 1958.
50 Lawden D. E., The Simulation of Gravity, Journal of the British Interplanetary Society. vol. 16. No. 3, July-September 1957, p. 134.