Symposium on the Role of the Vestibular Organs in Space Exploration (1970)

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Certain Aspects of Onboard Centrifuges and Artificial Gravity RALPH W. STONE, JR., W. M. PILAND, AND WILLIAM LETKO NASA Langley Research Center SUMMARY Artificial gravity is an exceedingly complex environment within which man must work. Clearly, habituation to the environment will be required. There appears to be the potential of a physically induced ataxia tendency, a tendency to leg heaviness while walking, and numerous unnaturally induced acceleration phenomena. Design criteria for artificial gravity are presented which show a vehicle of 55 feet in diameter may meet potential criteria. Some limited data indicate, however, that smaller vehicles may be satisfactory. Onboard centrifuges, which have a relatively short radius, require much larger centrifugal forces than are anticipated for basic artificial gravity and exceed some of the tolerable limits for artificial gravity. Thus a restriction of movement on centrifuges is required. INTRODUCTION The influence of weightlessness on man and on his performance during extended space mis- sions remains an enigma. The possible in- fluences involve physiological habituation which includes effects on the cardiovascular, muscular, and skeletal systems and involve the possible inability to perform tasks effectively during ex- tended exposure to weightlessness. It is not the purpose of this paper to examine these effects of weightlessness, but to examine certain aspects of imposing artificial gravity by rotation either of the entire vehicle or a portion of it to alleviate or eliminate these potential influences. These aspects encompass the anomalies or side effects of the environments imposed by rotating vehicles such as that of figure 1, or by onboard centrifuges such as shown in figure 2, as well as the effects of these environments on mission goals. SYMBOLS a acceleration, ft/sec2 C\ to C% criteria d distance, feet 6' gain of human balancing system g 32.2 ft/sec2 (Earth's gravitational accel- eration) h height above floor of e.g., feet / man's moment of inertia, slug-ft2 m man's mass, slugs M moment, ft-lb p pressure, lb/ft2 r radius, feet rpm revolutions per minute t time W weight, Ib p density, slugs/ft3 0 angle of tilt, radians W angular velocity, radians/sec Subscripts: 0 initial value 7* threshold U unbalance r radial 1 tangential V vehicle R relative /• floor // head ob object A dot over a symbol indicates its first derivative with time. 331

332 THE ROLE OF THE VESTIBULAR ORGANS IN SPACE EXPLORATION FIGURE 1. - MORL in spinning mode. FIGURE 2. —Short-radius centrifuge concept. THE ROTATING ENVIRONMENT Those features of a rotating environment that cause artificial gravity to be different from real gravity are listed below. The factors listed are those that create the anomalies of concern relative to the use of artificial gravity and on- board centrifuges. We are all generally familiar with these factors and are conversant with their qualitative effects. The quantitative as- pects of these factors, necessary to be known for spacecraft design, will now be considered. Factors of Rotating Environments (1) Artificial gravity level (2) Coriolis forces (3) Artificial gravity gradients (4) Hydrostatic pressure gradients (5) Cross-coupled angular acceleration Artificial Gravity Level The fundamental purpose of artificial gravity is twofold: to eliminate or reduce the physio- logical adaptation to the weightless state and to allow the astronaut to function as nearly as possi- ble as he functions in Earth gravity. The level of artificial gravity for steady exposure required for the reduction or elimination of the physio- logical habituation due to weightlessness is not known and remains a prime area of research. Periodic exposure to artificial gravity by use of a simulated onboard centrifuge (ref. 1) has indi- cated that 2 to 4 g-hours have an influence on the habituation to bed rest. The g-hours are the product of the g at the feet times the exposure time on the centrifuge. Thus one has a starting point for the therapeutic use of a centrifuge but no data regarding continuous artificial gravity. One can assume that with any value of artificial gravity less than 1 g, some habituation will occur. As noted previously, research is required to establish the degree of physiological change with g-level. From the standpoint of allowing an astronaut to function more or less normally in artificial gravity, several factors shown on table 1 are related to the convenience with which astronauts perform within the spacecraft. No real data exist to establish desirable g-levels of any of these fac- tors; the remarks in table 1 indicate certain as- pects of relevance for each problem. The most critical problem may be that of mobility. There has been a great deal of study of walking in simu- lated 1/6 g (lunar gravity) which demonstrates that walking at 1/6 g seems readily possible. It should be noted that such simulations have limita- tions as to the freedom of the subjects, a factor that must be always considered. There is a possibility that a functional ataxia may exist, which may derive from the sensory-system lim- itations, particularly the threshold limits of those systems involved with balance. An elementary

ONBOARD CENTRIFUGES AND ARTIFICIAL GRAVITY 333 TABLE I.—Artificial Gravity for Convenience Item g-level Remarks Walking >l/6 Simulator studies on a firm flat surface have indicated the adequacy of 1/6 g. Less than this may < 1 be adequate, but insufficient data exist. Other means of mobility, as jumping, also may be desirable. Studies on Langley rotating space-vehicle simulator should examine these areas. Placement of >0 Assuming linearity of frictional forces with weight, objects would have identical position objects. stability at any g-level, except objects on flat surfaces will tend to move to the largest radius. An object's stability when placed improperly would be identical at any g-level. Upsetting stability would vary linearly, however, with g-level decreased. Wider bases on objects may be required. Waste collection >0 Natural expulsion processes with any amount of g-level and proper systems design should transport the material and hold it in place. Proper collector design would be required to minimize such problems as splashing which would be effected by radius and rotational rate. Gas-liquid >0 Any amount of g will maintain separation once it is accomplished. Sloshing from perturba- separation tions, however, will vary with g-level. During a separation process, the rate of separation systems. will be a function of the g-level, and thus system size will be influenced by g-level. If all systems are designed for zero g, as they most likely will be, then with proper orientation any g-level will augment the system process. A study of the influence of g-level on systems seems desirable. General convec- >0 Forced convection and filtering are common in \-g systems, as in aircraft, and will certainly tion of the be used in spacecraft, especially as they will undoubtedly be designed for zero g. Thus, settling of dust. any amount of g will supplement the system if it is properly oriented. etc. Manual applica- >0 The ability to push parallel to the floor would vary linearly with g-level, assuming linearity tion of forces of frictional forces, and become nil at zero g. Lifting ability increases with reduced gravity. and moments. The application of torques by using one's weight will decrease with g-level. Proper use of the floor and other accouterments should allow manned application offorces and moments relatively unaltered by g-level, provided sufficient g exists for man to conveniently arrange himself to apply the forces. mathematical model which examines the effects of balancing, on one foot has been developed. Three equations are used to model the situation shown in figure 3; these are mgh / sinh (1) which applies where the angle of tilt ij> is between zero and that value 4>T for which the sensory system exceeds its threshold. l C-l C-l ipT COS ; — l)mgh ',-l)mgh •• sin y (G-l)mgh — — — - . (2) which applies where the angle of tilt $ is between <£r and that value <£i for which the person be- comes unbalanced and can recover only by stepping, and cosh + T mgh / . , sinh mgk , / ,„, (3) which applies after he becomes unbalanced and until he takes a step. The factor G in equation (2) is the gain of the human balancing system in shifting the point of application of the floor reaction on the sole of the foot. Figure 4 shows some preliminary results calcu- lated for 1 g and 1/6 g, using a threshold sensitiv- ity of 0.01 g. These results indicate that an indi- vidual who can retain balance, eyes closed, on one foot in Earth g, may not be able to do so at

334 THE ROLE OF THE VESTIBULAR ORGANS IN SPACE EXPLORATION FIGURE 3.— Elements for mathematically modeling in ataxia test. 5r , DE6 lg l/6g 3.44 DEG SENSORY THRESHOLD AT l/6g / I 89 DEG POINT OF UNBALANCE 573 DEG SENSORY THRESHOLD AT Ig .2 3 4 TIME, SEC FIGURE 4. —Calculations of variations of tilt angle with time for an ataxia test in Earth and lunar gravity. 1/6 g. These results are based strictly on the physical aspects of the problem and do not consider any psychophysiological effects that may occur at reduced gravity levels. The funda- mental problem from this elementary example is that the individual becomes unbalanced before the sensory stimulus reaches the threshold value. Figure 5 shows angle of tilt required for sensory stimulus at various artificial-gravity levels and at assumed threshold values of A g of 0.005.0.010, and 0.015. Also shown is the angle of tilt for unbalance. These results show the potential increase in balance difficulty that may be en- countered in reduced gravity. These results may not be significant, however, as one normally will not stand in artificial gravity with his eyes closed. The stimulus to the eyes is not considered g dependent and balance with this stimulus as ordinarily used for balance on Earth by labyrinthine-defective persons may undoubtedly suffice. It is evident, however, that the situation tends not to be normal as the artificial-gravity level decreases and some adaptation and learning must occur. Some limited studies of walking in artificial gravity on the Langley rotating space station simulator (fig. 6) have been made. In this simu- lation, the freedom of the subject is just in the sagittal plane, and certain aspects of reduced gravity may not be evident. Further discussion of these results is included in the next section. THRESHOLD STIMULATION LEVELS Ag DEG 3 UNBALANCE ANGLE V005 .2 4 .6 .6 10 ARTIFICIAL GRAVITY LINE, g UNITS FIGURE 5.— Effect of gravity level on the angle of tilt required for threshold stimulation for an ataxia test.

ONBOARD CENTRIFUGES AND ARTIFICIAL GRAVITY 335 FIGURE 6. —NASA Langley rotating space station simulator. Coriolis Forces This section deals with those factors imposed when a linear motion within a rotating environ- ment is attempted. These are distinguished from the effects of angular motions in a rotating environment, which are also commonly called Coriolis effects. The effects of angular motions will be discussed under the section titled "Cross- Coupled Angular Accelerations." The influ- ences of the Coriolis forces are well known, as when someone or something moves tangentially or radially in a rotating environment, a force perpendicular to the motion relative to the rotating environment exists. Relative to self-locomotion, the movement will primarily be on the floor either axially or tangen- tially. In the latter, the astronaut, because of Coriolis forces, will feel heavier or lighter depend- ing on the direction of motion. The radial acceleration involved is ar = — (4) where reo? is the basic artificial gravitational force due to the vehicle rotation and ro>« is the relative velocity due to movement in the rotating environ- ment. The value for o>« would be positive if motion is with the rotation and negative if against it. The tangential acceleration involved It would seem that with some normal velocity of locomotion, 3 or 4 ft/sec, the total effective radial force clearly should not be less than 1/6 g, which seems to be adequate for locomotion (ref. 2) and not be more than 1 g to which man is normally accustomed. The tests on the Langley rotating space station simulator (ref. 3), previously mentioned, have been performed at 0.1, 0.2, 0.3, and 0.5 artificial- gravity levels with two subjects. The subjective impressions of these conditions are listed in table 2. These results were obtained with a 20-foot radius which influences certain aspects of them. Generally, the subjects felt the floor of the simulator was closing in on them from the front for, in fact, the floor curves up in front of them. Walking at 0.2 g seemed to give good subjective results, although walking against the rotation was not so good as walking with the rotation. It is interesting to note that when walking with the rotation at 0.1 g at 3 ft/sec, the actual total effective gravity, because of the Coriolis force, was about 0.18 g, whereas walking against the rotation at 0.2 g at the same walking speed gives an effective gravity of 0.12 g. These results tend to corroborate the finding of good walking conditions at lunar g (0.167 g). The leg heaviness found at 0.3 and 0.5 g may be caused by the Coriolis forces that result from the relative leg velocity which is larger than the walking speed. As the leg is swung forward, it tends to become heavier. The maximum foot accelera- tion is expressed in an elementary equation as arfoot- (6) is (5) where it is assumed the maximum foot velocity is twice the walking speed. This is a radial force but, in addition, as the leg is raised and lowered, a tangential force also exists to compli- cate the picture, causing the leg to be forced forward as it is raised and backward as it is lowered. A comparison of the accelerations on the feet as compared to the nominal artificial gravity and that of the body is shown on figure 7. At a radius of 20 feet, at which the walking tests previously discussed were performed, the foot probably weighed from H to 2 times its expected weight. At 0.6 g the maximum foot acceleration is nearly

336 THE ROLE OF THE VESTIBULAR ORGANS IN SPACE EXPLORATION TABLE 2.—Subjective Results of Walking in Artificial Gravity Artificial gravity level, g Comments Walking with rotation Walking against rotation 0.1. 0.2. 0.3. 0.5. Slow initiation of walking Light on feet No leg heaviness Good walking when stepping rate is 3 ft/sec Good walking condition Start and stop well Legs have heavy sensation Heavy legs Stable sensation Tend to walk on heels Not as comfortable as 0.2 g Laborious Heavy swinging leg sensation Firm heel down steps Tend to soar or float with no control Develop large tilt to initiate Good walking condition Light on feet Less tilt required than for 0.1 Easier than at 0.2 g 9, UNITS .6- -ACCELERATION AT FOOT -ACCELERATION OF BODY - NOMINAL ARTIFICIAL GRAVITY 60 BO IOO RADIUS, FT FIGURE 7.— Acceleration of the foot compared to that of the body and the nominal artificial gravity level. Walking speed, 3 feet per second. that in Earth gravity. Clearly, this is greatly dependent on the radius of the vehicle; the effect demonstrated decreases markedly with in- creasing radius. Further study of leg heaviness is required, not only that the foot is heavier than normal but also that it varies in weight during a step which may have a distracting effect. A mathematical model more elegant than that of equation (6) should be developed, and leg heaviness could be a significant criterion for vehicle design. In addition to the problems of locomotion just discussed, the astronauts will experience un- usual effects while moving objects within a rotating spacecraft. Such objects are affected in accordance with equations (4) and (5), and research to determine the degree of such condi- tions must also be considered. Another potentially annoying related factor is that objects when dropped will not fall where one would normally expect them to. Human tol- erance or adaptation to this situation may require study. The distance that an object strikes the floor from the expected spot is expressed as (7) where r>- is the radius of the floor (or table top) and r« is the radius from which the object is dropped. It should be pointed out that within the confines of an onboard centrifuge, most of the factors discussed in this section do not have application as general mobility is not possible. However, as some work tasks, such as small repair and assembly jobs and waste management, may be performed, certain aspects that relate to the handling and dropping of equipment, parts, etc., are applicable to centrifuges. Artificial-Gravity Gradients This is a subject that has received considerable discussion but for which there appears to be no accepted effect or applicable criteria to establish acceptable gradients. The artificial gravity

ONBOARD CENTRIFUGES AND ARTIFICIAL GRAVITY 337 varies, of course, directly with the radius for any given vehicle. From a general standpoint, for vehicles of different radii but having the same artificial gravity at the floor, the gradient is inversely proportional to the floor radius, ex- pressed as follows: da ~dr _ f desired (8) From the physical standpoint, objects would weigh more on the floor than on a shelf and from the physiological standpoint, a person would be heavier when squatting than when standing erect. The significance of these factors relative to adequate astronaut performance seems rather vague, particularly in that the maximum artificial- gravity level probably would be much less than 1 g. Figure 8 shows variations of the gravity gradients per foot of height for consideration of handling objects and per height of man when considering psychological implications. These results show that with a radius as small as 20 feet, the g-level at the head would be 70 percent of that at the feet. The significance of this difference is not clear, and the simulations previously discussed and presented on table 2 were performed at a 20-foot radius without a clear influence of this factor. Hydrostatic-Pressure Gradients The pressure distributions along the body, RATIO OF GRAVITY AT HEAD TO THAT AT FEET .50 .70 .80 85 90 933 .OJ g" LEVEL ATFL DOR .16 1 GRAVITY GRADIENTS I PER FOOT OF RADIUS .12 l c iRAVITY GRADIENTS DERMAN HEIGHT (6 FT) r .a \ \ GRAVITY \ 1 GRADIENT \ \ \ .08 - 1 1 \ \ 1 \ '\ \ \ \ x .04 \ \ N N *" "V "*--. t V ^ --._- V _\ J. & --- o "*• -» — — ^S aBSS^ 40 60 80 RADIUS, rF, FT FIGURE 8. — Gravity gradients in rotating spacecraft. I00 particularly in the cardiovascular system, may have more physiological significance than the gravity gradients just discussed. The circulation of blood to the head and from the lower extrem- ities is a significant element in man's well-being and is influenced by the hydrostatic pressures present. In artificial gravity these pressures vary differently from those in the Earth's gravi- tational fields. In Earth's gravity there is a linear variation in hydrostatic pressure in a standing man, whereas in artificial gravity the pressures vary with the squares of the radii and are expressed as P=f 0-*-r (9) where p is the fluid density and TH radius at the head. The variation of the head-to-heart and the head- to-foot pressure increments with radius for vari- ous artificial-gravity levels are presented in figure 9. These results show relatively large varia- tion of pressure with radius up to about 40 feet of radius after which the pressure (at 1 g) tends to asymptote that in Earth g. The head-to-foot pressure becomes 90 percent of that on Earth at about 30 feet of radius, whereas the head-to- heart pressure becomes 90 percent of that on Earth at about 60 feet of radius. It is not evident that these variations would have any critical I40r 120- I00 PRESSURE mm Hq 80 60 40- 20 I.0 EARTH V I0g HYDROSTATIC PRESSURE HEAD TO HEART HEAD TO FEET 3g - ' INEARTH "9" ..Ig -3g 20 30 40 RADIUS, FT 50 60 70 FIGURE 9. — Hydrostatic pressures in a standing man in Earth and artificial gravity.

338 THE BOLE OF THE VESTIBULAR ORGANS IN SPACE EXPLORATION influence on the man or his systems. It is inter- esting to note, however, that at a given artificial- gravity level, the shorter radii would have less influence in challenging the cardiovascular system than the larger ones. With the onboard centrifuge, if used as a thera- peutic device to challenge the cardiovascular system, as simulated in reference 1, gravity levels up to about 4 g may be required. Because of the confinements of space, onboard centrifuges will have radii of the order of 10 feet or less. For these conditions of high g's and short radii, concern for blackout and leg pain and petechia exists. The results of references 1 and 4, and with consideration of the data in reference 5, indicate that blackout occurs when the head-to- heart hydrostatic pressure gradient is about 134 mm Hg, and the lower leg pain and petechia of the feet occur about when the head-to-foot hydrostatic pressure is 485 mm Hg. Figure 10 presents these conditions in terms of g-bound- aries for a seated man in a short-radius system. The therapeutic boundary is based on four short exposures per day with 4g at the feet as outlined in reference 4. Longer exposures at smaller artificial g-levels may also be adequate. Be- cause of the characteristics of hydrostatic pres- sure variations shown on figure 9, and provided the hydrostatic pressure in the blood system is 140,- I2O 100- 80- M 60- 40- 20- BLACKOUT BOUNDARY LOWER LEG PAIN AND PETECHIA BOUNDARY INSUFFICIENT THERAPEUTIC VALUE 6 8 RADIUS, FT 10 FIGURE 10. — Critical boundaries in short-radius rotating systems related to hydrostatic pressure variations. The results are for a seated man. the important therapeutic factor in preventing adaptation to weightlessness, lesser g at the feet probably may be adequate as the radius of rota- tion increases. The 4 g's noted in references 1 and 4 may be the largest g-values required for therapeutic purposes as the centrifuge used was of minimum size. Cross-Coupled Angular Accelerations This area is that which has received the most concern relative to vehicles with artificial gravity. The psychophysiological aspects of cross-coupled angular acceleration, often referred to as Coriolis acceleration, were a part of the previous three Symposiums on the Role of the Vestibular Organs in Space Exploration, as it is a part of the current symposium. The emphasis on this aspect of the problem of artificial gravity is well founded as it may be the most critical aspect physiologically in that the endpoint can be serious motion sickness. The basic equations for calcu- lating the angular stimulus involved are included in reference 6. These equations are independent of radius, however, because of a possible hydraulic,, mechanical, or neural interaction between the otoliths and the semicircular canals (refs. 7 and 8, for example); the entire stimulus situation may be more complex than is expressed in reference 6. Because of this complexity and the broad treatment of the subject by others, no discussion of this basic physiological problem will be made in this paper. It appears only that subjective tolerance and maintenance of perform- ance in cross-coupled angular acceleration probably range between 1 to 4 rad/sec2 (refs. 6 and 9, for example). A related phenomenon also exists for astro- nauts in artificial gravity. When objects are rotated by the astronaut out of the plane of vehicle rotation, the astronaut must apply a moment to the object to prevent it from rotating in an unnatural and undoubtedly undesired direction. The magnitude of the angular accel- eration he must impose on the object to prevent the undesired motion is directly proportional to the angular velocity he has purposefully applied to the object and the vehicle rate of rotation. The degree of difficulty is a function of the moment man can apply and the moments of

ONBOARD CENTRIFUGES AND ARTIFICIAL GRAVITY 339 inertia of the object rotated. The maximum moment required may be expressed as A/ = /obo>ro>ob (10) which certainly must not exceed and, desirably, should be appreciably less than the moment he could apply M — T *"» ft i \ — *ot>it>ob (11) clearly then OH' (12) where this ratio is approximately equal to the cyclic frequency of the motion applied to the object, assuming one starts and stops the object in the desired motions. Differences in moments of inertia about the axis of the initiated motion and the axis of the cross-coupled acceleration complicate this simple analysis. THE INFLUENCE OF ARTIFICIAL GRAVITY ON MISSION GOALS In examination of the influence that artificial gravity may have on the mission goals of space vehicles, let us first examine two configurations: one representative of a simple vehicle and the other representative of a complex vehicle. The first configuration that will be discussed is the MORL in its spinning mode (fig. 1). Here, the basic laboratory is separated from the SIV—B launch stage by a system of cables, and with the SIV-B stage acting as a counterweight, the entire configuration is rotated to achieve the desired gravity field within the laboratory. As an example, at a radius of 70 feet from the common center of mass of the spinning con- figuration to the outer floor of the laboratory, a gravity level of 0.333 g can be achieved by rotating the deployed system at 4 rpm (ref. 10). The inclusion of this spin capability in the basic zero-gravity MORL has considerable effect on the laboratory design. The major impact is the increase in weight of the structure, reaction control, and flight electronic systems to accom- modate this additional operating mode. The total changes in dry launch weight of the labo- ratory/SIV—B combination amount to 3400 pounds and require about 600 pounds of addi- tional propellant to circularize the orbit from an initial elliptical orbit. Therefore, the impact of the spin capability on the initial launch of the laboratory involves a decrease in discretionary payload capability of approximately 4000 pounds. Fewer consumables, experiments, etc., can thus be carried on the initial launch, and more severe demands are placed on the subsequent logistics schedules. In addition to the initial launch penalties, the spin capability includes a major increase in re- action control propellant consumption rate. Increased drag and moments of inertia, deploy- ment, and spinup requirements all increase the orbital propellant requirements. For the MORL in the spinning mode, the orbit-keeping require- ments are increased by about 200 pounds of propellant per month, and the attitude control expenditures are raised by almost 400 pounds per month. These increases in overall propellant consumption are approximately 80 percent over the basic zero-gravity configuration. Other than the impacts on system design, there are also other factors which must be con- sidered. One such factor was investigated in the MORL studies for which a typical prelimi- nary experiment program for the mission was selected (ref. 11). The experiment program for- mulated covered all the major scientific/technical disciplines. As illustrated in figure 11, 40 per- cent of the total 157 experiments would demand almost absolute zero gravity, 43 percent would require a significant increase in design com- plexity for artificial-gravity performance, 16 percent would be gravity independent, and only 1 percent would actually require some level of Experiments on which artificial gravity imposes significant design complexity (43%) Experiments which require zero gravity (40 *) Experiments which are insensitive to gravity (16 %) Experiments which require gravity (1 *l - FIGURE II. —Typical Earth-orbital experiment program.

340 THE ROLE OF THE VESTIBULAR ORGANS IN SPACE EXPLORATION gravity. Since the primary purpose of most future space missions will be the performance of a meaningful experiment program, perhaps the experiment program itself will be the domi- nating factor in the decision of "to g or not to g." A possible method of providing a continuous gravity force for the crew as well as satisfying the experimental requirements is illustrated in the second rotating spacecraft concept. A pro- posed, large, rotating, manned orbital space- station configuration is shown in figure 12. There are other concepts, but generally each is basically a 24-man station, rotating to provide artificial gravity at the operational floor levels. Zero-gravity-dependent experiments could be provided for in the counterrotating hub where the gravity level goes to zero. For these larger rotating vehicles, there are similar, but more complex, problems than those associated with the MORL spinning mode. Besides a tremendous launch weight, the aero- dynamic and gravity gradient torques and the orbit-keeping requirements will involve very high propellant consumption rates, although a lesser number of spin/despin operations will be involved since docking would be accomplished at the zero-gravity hub. Although these are highly complicated vehicles requiring subsystems of increased complexity to support the mission, there may be a requirement for such vehicles in the future. It is, however, difficult to justify the initiation of any space mission using such an elaborate vehicle without first thoroughly estab- lishing and understanding the true requirements for artificial gravity. FIGURE 12.— Large rotating space station. Three-radial module configuration. An onboard centrifuge also imposes on the mission capability of a spacecraft. The basic added weight could range from about 300 pounds to 3000 pounds, depending on the intent of the device. The mere application of acceleration for therapeutic purposes would require the lesser weight, whereas developing a centrifuge with an experimental capability of measuring vestibular responses, sensitivities and thresholds, cardio- vascular reactions, performance, reentry pro- ficiency, etc., as was proposed in reference 12, would require the greater weight. Another significant factor is that the astronaut time directly associated with artificial gravity or the onboard centrifuge results in a direct reduc- tion in time available to perform experiments or operate the vehicle. Basically, artificial gravity requires no specific astronaut time except that, for experiments requiring weightlessness, the system must be despun or the astronaut must transfer to the weightless portion of the vehicle. A centrifuge being used for therapeutic purposes requires from 4 to 6 man-hours per day for riding and monitoring the centrifuge, which time is lost from other activity. DESIGN CRITERIA FOR ARTIFICIAL GRAVITY Based on the previous discussion of the rotating environment, eight preliminary criteria for the design of space vehicles with artificial gravity have been developed. These criteria include upper and lower limits of gravity levels, maxi- mum tolerable rates of rotation, rotational radius, percentage change in gravity gradient, and the degree of Coriolis acceleration tolerable and cross-coupled angular acceleration. It must be first noted that the minimum amount of artificial gravity to maintain good physiological tone and to prevent or allay the reconditioning that may occur in weightlessness is not known. These criteria are designated as Ci through C«. Figure 13 is a plot of radius of rotation versus rate of rotation for artificial gravity and indicates the influence of the sundry criteria on potential design values. These criteria incorporated in figure 13 form a design envelope illustrating limits that might be imposed on an artificial

ONBOARD CENTRIFUGES AND ARTIFICIAL GRAVITY 341 g LEVELS I ANOI/6g BOUNDARIES CORIOLIS ACCELERATION BOUNDARIES RATE Of ROTATION LIMITS 0 10 20 30 40 50 60 70 80 90 I00 I10 120 I30 I40 I50 RADIUS (r), FT FIGURE 13. — Design envelope for manned rotating space vehicles. gravity vehicle design. The criteria presented are not as completely supported by experimental evidence as desired. However, if conservative selections are made, an arbitrary rotating vehicle can be defined and an estimation of its practi- cality can be made. The mathematical expres- sions used to establish curves on figure 13 are listed on table 3. From the figure, the maximum tolerable rate of rotation seems to be the most critical and restrictive criterion. The next most significant criterion is the amount of Coriolis force that is tolerable. These two criteria gen- erally will set both the rotational rate and radius of the vehicle, since the amount of Coriolis ac- celeration to be experienced varies with spin rate and radius. From experimental observa- tion, the other criteria would allow vehicles of smaller radii and higher rates of rotation than those just discussed. The maximum tolerable rate of rotation, as noted previously, has been the subject of ex- tensive research. Generally, a value of about 4 rpm has been suggested as safe, although values of 10 rpm have been tolerated during many tests. It is believed that a relatively conservative value would be about 6 or 7 rpm. A value of 6 rpm more than halves the radius required at 4 rpm. Additional research and flight experiments will be required to establish the actual value to be used. Relative to the Coriolis force that may be tol- erated, the results previously discussed indicate that rather large values of this ratio (C\) of the TABLE 3.— Expressions for Criteria for the Selec- tion of Vehicle Radius and Rate of Rotation for Artificial Gravity Criteria Mathematical bases ( 1 + C, ) i » (l-C,) r pirH (*?)"* (n,«. + nA \ /i+r„ i i"ob ior — = Or =£ Cg order of 0.5 seemed to be readily tolerated pro- vided the basic g-level is about 0.2 g or more. In figure 13, the value of C\ represents the toler- able percentage increase or decrease of man's artificial weight as he moves at 4 ft/sec within the vehicle. The correct value of C\ must be established by experiment, but in figure 13 a graphical representation of values of C\ from 15 percent to 50 percent are shown. C-i represents the percentage weight change in an object when moved. C-i was selected as 25 percent when the object's velocity is 4 ft/sec. The value of €3 is the distance from the local vertical which an object falls to when dropped. €3 was selected as 1 foot for objects dropped from 3 feet above the floor. This indicates that at a radius greater than 44 feet, such an object will strike the floor not more than 1 foot from where it was expected to fall. The value of d is the ratio of the weight of an object on the shelf to its weight on the floor. The value of €4 was chosen as 0.5 for a height above the floor of 6 feet, indicating that an object

342 THE ROLE OF THE VESTIBULAR ORGANS IN SPACE EXPLORATION so positioned will weigh not less than one-half its artificial weight on the floor. Figure 13 shows that this condition will be satisfied at radii of rotation greater than 12 feet. C5 is the criterion for the gravity gradient limit which would be comfortable for man in the rotating environment. For the purpose of figure 13, C5 was selected as 0.15, indicating that the centrifugal acceleration at the head will not be less than 85 percent of the value at the feet. This condition is satisfied at radii greater than 40 feet for a 6-foot individual. It should be noted that this value is not substantiated by data and that larger values may be readily tolerated. C6 is the ratio of fluid pressure at the heart level to that at the foot level. It was selected to be at least 90 percent of the same ratio when in the Earth environment. It is further assumed that the heart is 4.5 feet above the floor level. The ratio equals 90 percent at a vehicle radius of 26 feet and becomes more like the Earth's envi- ronment at radii greater than 26 feet. As before, this value of 90 percent is not substantiated. For the cross-coupled angular acceleration experienced when a man rotates an object, the value of C^ was selected as 0.50. This indi- cates that the cross-coupled acceleration will not exceed 50 percent of the angular accelera- tion that the man can impose on the object. It was assumed that a man can impose a maximum angular acceleration to angular velocity ratio of 16 per second. Eight rpm is the upper limit at which this condition is satisfied. The maximum rate of steady rotation tolerable to man was taken to be 6 rpm (Cg). However, in figure 13, several values are shown for complete- ness. The crosshatched area in figure 13 is bound by an assumed value of Coriolis-force ratio of 0.25 (Ci = 0.25), a maximum rate of rotation of 6 rpm (Cs = 6), and an acceleration not exceeding 1 g when moving about the vehicle. This design envelope allows a minimum radius of about 55 feet. If the artificial-gravity level selected falls with- in the design envelope of figure 13, the environ- ment produced will satisfy the requirements of walking, performance of everyday tasks (table 1) and, in addition, will simplify some of the mechanical systems designs needed aboard the space vehicle. The crosshatched area as noted is based on a value of Coriolis-force ratio (Ci) of 0.25. In the walking experiment previously discussed herein, values of Coriolis-force ratio of the order of 0.5 appeared tolerable. With this value, radii of less than 30 feet would be possible. The foot- heaviness problem discussed previously is not considered on figure 13, although it will be experienced generally in the crosshatched area. Further experiments to establish a tolerable level of foot heaviness seem necessary. REFERENCES 1. NYBERG, J. W.: ET AL.: Modification of the Effects of Recumbency Upon Physiological Functions by Periodic Centrifugation. Presented at the Aerospace Medical Association Meeting, Las Vegas, Nev., Apr. 1966. 2. HEWES, D. E.: AND SPADY, A. A.. JR.: Evaluation of a Gravity Simulation Technique for Studies of Man's Self-Locomotion in the Lunar Environment. NASA TN D-2176. 1964. 3. HEWES. D. E.: AND SPADY, A. A.. JR.: Status Report on Recent Langley Studies of Lunar and Space Station Self-Locomotion. Presented at the AGARD 24th Aero- space Medical Panel Meeting. Brussels. Belgium, Oct. 24-27, 1967. 4. WHITE, W. J.; ET AL.: Biomedical Potential of a Centri- fuge in an Orbiting Laboratory. Prepared under Air Force contract no. AF04(695|-679 by Douplas Aircraft Co., Inc., July 1965. 5. Bioastronautics Data Book. (Paul Webb, ed.) NASA SP-3006. 1964. (t. STONE, RALPH W.. JR.: AND LETKO, WILLIAM: Some Observations on the Stimulation of the Vestibular System of Man in a Rotating Environment. The Role of the Vestibular Organs in the Exploration of Space, NASA SP-77, 1965. pp. 263-278. 7. BENSON. A. J.; AND BODIN. M. A.: Interaction of Linear and Angular Accelerations on Vestibular Receptor in Man. Aerospace Med., vol. 37, 1966, pp. 144-154. 8. GUEDRY. FRED E.. JR.: Orientation of the Rotation-Axis Relative to Gravity. Its Influence on Nystagmus and the Sensation of Rotation. Acta Oto-Laryngol.. vol. 60, 1965, pp. 30-49. 9. NEWSOM. B. D.; AND BRADY, J. F.: A Comparison of Performance Involving Head Rotation About Y and Z Cranial Axes in a Revolving Space Station Simulator.

ONBOARD CENTRIFUGES AND ARTIFICIAL GRAVITY 343 Aerospace Med.. vol. 37, 1966. pp. 1152-1157. 10. DOUGLAS AIRCRAFT Co., INC.: Report on the Optimization of the Manned Orbital Research Laboratory (MORL) System Concept. Vol. XIII, Laboratory Mechanical Systems—Artificial Gravity Systems, prepared under NASA contract no. NAS1-3612, Sept. 1964. 11. DOUGLAS AIRCRAFT Co., INC.: Report on the Development of the Manned Orbital Research Laboratory (MORL) DISCUSSION Dietlein: One of the final common pathways of basic neurophysiological research in the vestibular area relates intimately to applied investigative efforts which ultimately influence greatly and sometimes dictate optimal spacecraft design. Such efforts are destined to assume an increasingly important and even critical role in this country's future long-term space flights, inasmuch as the present consensus, at the Manned Spacecraft Center at least, holds that some form of artificial g must be provided to space stations or space-station-like vehicles. The reason for this position is simply to facilitate the simple day-to-day chores of work and habitation, to obviate the need for painstaking relearning of even the most pedestrian of activities and maneuvers, and to add immeasurably to crew effectiveness in increasing their work capacity in space. Mr. Stone's presentation dealt with those advantages and problems of providing artificial gravity and/or centrifugal devices in space stations. Graybiel: Dr. Jones left for Washington at noontime, and just before he left he asked me if I would open up the dis- cussion on Ralph Stone's paper and point out some of the things that seem to me to be worth doing using an onboard centrifuge. By way of introduction, let me say that when I was talking with Ralph early on about his presentation, I was particularly desirous that he include the figure shown in his next to the last slide. This nomogram contains a great deal of information, and I am going to ask permission to use it as soon as it is published. There are a few things which I might call to your attention, and they are meant to supplement the material Ralph has just presented. The important question from the bio- medical point of view is, What are the operational problems which must be solved in planning for manned space flights measured in years? More specifically, what are the effects of exposure in a weightless spacecraft for this period? With regard to health hazards, the President's Science Advisory Committee has called attention to the need to look for subtle alterations in addition to the manifestations with which we are already familiar as a consequence of short exposure in weightlessness. These subtle effects which might be revealed at the cellular, subcellular, and molecular levels could not be investigated extensively on man for obvious reasons and would require the use of animal subjects. Cur- rently. Dr. Jones is supporting preparatory studies with the object of sending unattended rhesus monkeys into orbital flight for long periods of time. The chief object of interest would be extensive biopsy and postmortem studies which might reveal the effects of chronic exposure to weightlessness System Utilization Potential. Final report prepared under NASA contract no. NAS1-3612, Jan. 1966. 12. STONE, RALPH W., JR.; LETKO, WILLIAM: AND HOOK, W. RAY: Examination of a Possible Flight Experiment To Evaluate an Onboard Centrifuge as a Therapeutic Device. Second Symposium on the Role of the Ves- tibular Organs in Space Exploration, NASA SP-115, 1966. pp. 245-256. not manifested in clinical studies and laboratory determina- tions. This initial probe, which could be conducted sometime soon, might be extended later on by the use of rotating capsules to generate different subgravity levels and, indeed, by conducting studies aloft when animal facilities are estab- lished in manned orbiting laboratories. The onboard centrifuge used as a countermeasure would solve the problem of health hazards due to weightlessness, and its contribution to habitability of the spacecraft would depend a great deal on its size and appointments. An on- board centrifuge would be extremely helpful in defining the benefits of different subgravity levels, not only in preventing loss of fitness but also in restoring fitness once it is lost. It would have value in determining postural stability while standing, as a function of the-level of artificial gravity (in a rotating spacecraft), but would have limited value in determin- ing stability under the same conditions while walking, due to inability to simulate the force environment in a rotating spacecraft. The worrisome problem of fitness for reentry after pro- longed exposure aloft could be handled not only by using the centrifuge as a therapeutic device but also by using it to simulate the g-profile on reentry. From the scientific side, the opportunity to investigate physiological and behavioral effects as a function of g-levels ranging from weightlessness, at the one extreme, to super- gravity levels, at the other, would contribute enormously to our knowledge. In my opinion, one of the biggest oppor- tunities is to determine the role of the tonic otolithic activity resulting from the stimulus due to gravitational force and the role of the resting activity of the sensory receptors in both the otolith organs and the semicircular canals. This raises the question as to the exact role of the resting discharge of the semicircular canals under natural terrestrial conditions. Is this role simply to preserve the functional integrity of the canalicular system? With motions of the head they function as angular accelerometers, but with head fixed in what way do they contribute to our well-being? The situation is very different in the case of the otolith organs. Like the canals, they also act as accelerometers responding to linear accelerations. In addition, it is generally agreed that change in position of the head in the gravitational field constitutes adequate and purposeful stimulus. I would like to add that we have demonstrated persistence of be- havioral responses when the head is fixed either in the gravi- tational or in a gravitoinertial field. Dr. Miller has had some persons tilted for several hours during which time he made

344 THE ROLE OF THE VESTIBULAR ORGANS IN SPACE EXPLORATION repeated measurements of the amount of counterfoll of the eyes. There was only a little falloff in the magnitude of this effect, indicating little in the way of adaptation. The magni- tude of the roll is a function of subgravity and supragravity load under otherwise static conditions. In an experiment conducted with Dr. Clark, subjects demonstrated little change in the settings of the oculogravic illusion over periods of 2 to 3 hours. In this experiment subjects were exposed to a change in the gravitoinertial vertical with respect to themselves while observing a dim line of light in the dark. When the change of the vector is in the frontal plane, the line appears to rotate, and its new position accords with the change in this vector. In short, it appears that with head fixed in the gravitational or in a gravitoinertial field, receptors are continually stimu- lated by a gravitational or gravitoinertial force. Only when these are lifted, as in weightlessness, will the true resting discharge of the otolith organs become manifest. I believe a major scientific opportunity awaits investigators in an orbiting laboratory studying vestibular problems. Such studies and many others as well will require the advantage of varying the g-loading such as might be accomplished with an onboard centrifuge. Newsom: I should like to make a comment on Dr. Gray- biel's presentation. It appears as if the thing to do is to put up an artificial-gravity station and then have the centrifuge spin in the opposite direction so the level can be decreased from one instead of increased from zero up. I think this was one of Dr. Graybiel's suggestions when I last visited Pen- sacola. This is a very interesting concept and one I have given a great deal of attention to. It is interesting that the only way this can be done, however, is for the centrifuge to have a common axis as that of the spinning space station; otherwise cross-coupled accelerations are produced and not pure angular accelerations. But the concept itself is certainly a very good one, and one I know that is going to be pursued. I am wondering if it is correct to base our criteria for angular velocity and g on a physiological requirement at this time. We know practically nothing about that, but there are other things we do know about. What I am thinking of is the ad- vantages of an artificial-gravity system for the mechanical system, such as life support and general habitability. Cooling of components in zero-g is a problem. With an artificial- gravity station one then has convective cooling. Fine particle control and water separation can be studied. We should be able to get some of these values from a theoretical basis. These might help us to decide what the level of artificial g should be. Another very important factor for some life-support systems is the behavior of fine particles. Some of these types of things we might learn a little more about from some of the work that is planned in the Apollo Applications Program. So these physical factors might be used to de- fine a g-level with a higher level of confidence than what is now being proposed for the physiological systems. Perhaps a group such as ours should also be thinking about these other problems associated with rotating systems and then see how they might affect the vestibular organs. Lowenstein: Dr. Graybiel, we must not forget that the resting discharge both in the semicircular canals and in the otolith organs is gravity independent: it is metabolic. There- fore, I would not envisage any large-scale disappearance of the resting discharge under weightlessness. So the otolith weight is immaterial. In fact, experiments have been carried out, I cannot quote the reference out of hand, where otolith membranes have been centrifuged off without concomitant tonus loss. You ask what is the significance of the resting discharge in the semicircular canals. They are highly im- portant tonus pumps as well. You can eliminate a single semicircular canal and get lasting tonus asymmetries in a state of rest. Graybiel: You still have not answered the questions I raised regarding the changes in behavioral responses as a function of g-loading primarily on the otolith organs. Their nature and accuracy imply statotonic reflex activity which has no counterpart in the case of the canals. Lowenstein: Yes. In such a situation, of course, the otolith organs are more efficient. Graybiel: Then there must be some increase or decrease in firing as a function of g-loading. Lowenstein: In the stationary individual. Graybiel: That is really my point. We all agree the otolith organs act as accelerometers. The point is, do they also have a tonic output related to g-loading? Lowenstein: Yes. That depends on the spatial position of the individual. Take the utriculus; when it lies at 90° to the gravitational pull, I would not anticipate very great changes. But if the individual in his resting position is in fact slanted, and when then weightlessness or additional gravitation exerts its influence on the shearing element in the otolith organ, then you get your effect. Graybiel: Dr. Miller and I have varied g-loading without varying the direction of g, and these effects are still there. Lowenstein: In a position with the utriculus at 90°? Graybiel: We were able to manipulate independently the magnitude and direction of the gravitoinertial force vector on a human centrifuge by controlling the position of an other- wise free-swinging gondola on its trunnions. By this means we demonstrated behavioral changes which could be varied as functions of magnitude or direction of the force vector. Lowenstein: I think, as Alice in Wonderland, I would like to see this all on paper. One should then discuss it at length. Waite: I have seen several reports, indeed numerous pro- posals, in the last several years, primarily from the engineer- ing side of NASA, which have proposed changing the g-level and angular velocity of a rotating vehicle many times in a 2- or 3-day-long mission. In my opinion this is putting us in a position of changing the stimulus before any kind of adapta- tive response has had time to fully complete itself. I would like to ask Dr. Graybiel if he would care to suggest a minimum duration during which g and angular velocity should be maintained constant before changing them, assuming our goal is to discover the full effect of rotation in space on one's physiology for purposes of extrapolation to longer duration missions and not merely the mission at hand. Newsom: The work that I presented at the first symposium and that which Dr. Graybiel presented here at this meeting offer a nice solution to that. A stepwise increase in angular velocity allows your engineers to study incremental increases

ONBOARD CENTRIFUGES AND ARTIFICIAL GRAVITY 345 in g. and permits adaptation. This appears to me the way to satisfy both the engineer and physiologist. Waite: For how long do you maintain ag-level or an angular velocity before you can be assured that you are not con- taminating that level's physiological responses with the new level that you are imposing? How long does it take for adaptation to take place or to reach asymptote, if 1 can speak of a general physiological adaptative asymptote in this environment? Graybiel: This is a difficult question to answer without knowing more about the initial symptoms experienced as a result of moving the head or walking about at a particular angular velocity. If the Coriolis illusion and nystagmus are present, the former will usually be the first to go during the process of adaptation. Mild symptoms of motion sickness usually disappear soon unless head movements are restricted. Postural equilibrium is regained as a consequence of walking and there is abolition of vestibular symptoms on moving the head. After all overt symptoms have largely disappeared, a further period of hours at least or even a day may be neces- sary before all homeostatic adjustments have been made. Money: I would like to suggest that although there is good reason to believe that astronauts could adjust to rotation especially if the spacecraft were increased in its rotational speed stepwise as Dr. Graybiel has just shown, successful adjustment is not a certainty. Before such a rotating space- craft is sent away for a period of 2 years or something like that, it would be really important to have first a rotating space- craft orbiting the Earth in a position to come back right away to be sure that these people can adapt before sending them away. Graybiel: That is in the cards. They thought of that.