Design Options and Analysis - of Variable Grav-ity Systems in Space

Paul A. Penzo Rodica lonasescu

Z

Jet Propulsion Laboratory --. - I

California Institute of Technology ---I &.? r -1

Pasadena, California C: I 3 W

27th Aerospace Sciences Meeting \/ January 9-1 2, 19891Ren0, Nevada

For permission to copy or republish, contact the American Institute of Aeronautics and Astronautics 3 370 L'Enfant Promenade, S.W., Washington, D.C. 20024

DESIGN OPTIONS AND ANALYSIS OF 89-0100 VARIABLE GRAVITY SYSTEMS IN SPACE*

Paul A. perno** and Rodica Ionasescu+ Jet Propulsion Laboratory

California Institute of Technology Pasadena, Caljfornia

Abstract

Long term human exposure to microgravity can produce harmful effects if measures are not taken to overcome them. These countermeasures can take the form of medication, exercise, or artificial gravity (e.g., a centrifuge), or be a combination of all three. Currently, only the first two have been studied to some extent. The latter requires rotation to simulate gravity through centripetal force. Such systems cannot be too small because then the head-to-foot change in gravity may be too great, or the Coriolis force may be too large, for comfortable movement in the living environment. The use of tethers permits the rotating system to be as large as desired without greatly penalizing the total mass. This paper reviews earlier concepts of rotating systems, and discusses a number of design options for tethered systems which can produce a variable gravity living environment. Realization of such systems will require a research and technology program, ground based and in space, leading to a greater understanding of the role of variable gravity relative to man's permanent presence near Earth, and his habitation needs for solar system exploration.

Introduction

TWO major challenges must be met if permanent manned presence in space (and human flights to the moon and planets) are to become reality. The first is to reduce the cost of delivering payloads into orbit. The second is to provide humans with living and working environments, so that they can remain healthy and effective. All space- faring nations are aware of the launch-to-orbit cost problem, and some have long range plans for bringing the

* This research performed by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.

** Mission Design Section Staff, Systems Division

+Member of the Technical Staff, Mission Design Section

This paper is declared a work of the is not subject to copyright protection

U.S. Government and in the United States.

cost down. The latter challenge has been researched only to the extent necessary to ensure success for the manned missions flown so far.

This situation will change soon. Commitment by the U.S. and international partners to build the Space Station ensures that the space habitation problem will be addressed and dealt with. Also, interest in a manned Mars mission, possibly early in the 21st century, emphasizes that habitation and health for long duration in space (years) must become important factors.

Considerable animal and human research by the life scientists, ground and space based, will be required to correctly solve the long duration human habitation in space problem. In the near term, adapting to microgravity using a multitude of countermeasures will be the most direct approach. In the far term, and particularly if countermeasures to microgravity are time consuming (e.g., hours of exercise per day) and lead to adverse side effects, artificial gravity produced by rotation will be the only solution.

In general, coping with the microgravity problem can involve three levels (or kinds) of activity: 1) the response level, where little effort is made to understand the reasons for microgravity effects, 2) the understanding level, where deliberate research is conducted to gain the necessary understanding, and 3) the solution level, where research, engineering, and design are used to understand and solve the space habitation problem. Some activities at each of these levels, in the order of increasing concern, are the following:

Pemnse Levek 1. Ignore problem to extent possible 2. Use diet, medication to lessen physiological

symptoms 3. Propose moderate exercise to maintain

conditioning 4. Schedule intensive exercise to relieve symptoms 5. Institute astronaut screening 6. Develop some in-space medical capability 7. Provide a significant gravity level in space

understand in^ Level: 1. Perform animal and human experiments,

Shuttle and ground based 2. Extend experiments to Space Station 3. Provide a life sciences module (and centrifuge)

on Space Station 4. Develop a good in-space medical capability

Solution Level: 1. Develop dedicated life sciences programs and

facilities 2. Provide a controlled environment to maximum

extent 3. Institute maximum life sciences instrumentation

and data gathering 4. Construct a variable gravity research facility 5. Allow experimentation with separation of

important parameters 6. Perform habitat design for maximum livability

Except for limited space experiments to understand the effects of microgravity on biological systems, current efforts are at the response level. This level circuits understanding, and instead concentrates on alleviating symptoms. Astronaut screening, for example, would lead to selection of individuals less affected by (or more adaptable to) mimgravity. This is then supplemented by diet, medication, and exercise. If serious problems arise, some in-space medical procedures may alleviate the effects. It is interesting that at this level, the whole problem of microgravity can be bypassed by providing a suitable gravity level in space.

The understanding level is self-explanatory. It concentrates on acquiring the fundamental medical and biological knowledge of microgravity effects and the means of counteracting them. Presumably, with this knowledge, variable gravity systems, if found to be necessary, may be better designed and constructed.

This understanding has begun to a minimal degree. One recent example is experimentation on five rats flown on the Soviet Cosmos Biosatellite, for U.S. researchers, for a period of two weeks.l Examination indicates not only muscle atrophy, but also damage to muscle fibers and nerve endings, resulting from a pathology similar to that found in human nerve-muscle diseases. Also, normal muscle repair processes, such as exercising, may not work in space as they do on Earth. Complete muscle recovery may require hormones that stimulate protein production, or perhaps anabolic steroids. But then, these compounds may have undesirable side effects which complicate the whole recovery process. Certainly, more studies are needed in this area, and preferably with humans.

The solution level accepts the fact, if indeed this turns out to be me, that variable gravity will be necessary, or at least desirable (and perhaps even cost effective) , for long duration space habitation, and therefore begins immediately with a program to provide it. A natural way to begin would be to perform the activities listed in the understanding level, and to provide dedicated variable gravity life sciences facilities. This added investment

becomes logical because of the conviction that variable gravity systems will be required at some time in the future. It makes sense to proceed to develop the programs and facilities for this long term goal.

Parameters of Rotating Systems

Rotating systems for manned space habitation was considered very early in the space program. To be sure, the concept of providing artificial gravity by rotation goes back to the early pioneers: Hermann Oberth, Wernher von Braun, and Arthur C. Clarke. For the most part, these visionaries did not anticipate any physical drawbacks to mimgravity, and in fact, considered the absence of gravity to be therape~tic.~

Serious consideration by medical experts came with the early animal and manned flights in the 1958-1962 time period. Several key papers were written on the subject of rotating manned space stations, and they will be discussed here. These writers considered possible designs for rotating systems, as well as the parameters which should be considered for maximizing comfort and habitability.

In these early days, emphasis was on learning more about man's dependency on gravity. Hill and Schnitzer in 19623, for example, suggested having a non-rotating section on a rotating station in order to carry out experiments simultaneously on zero gravity as well as variable gravity. In this manner, it would be possible to determine what we need to know about gravity for many space missions, including living on the Moon or Mars. They present a figure showing the major important rotation parameters for a variable gravity station. Their figure is similar to Figure 1 shown here.

The equations associated with this figure are the following, given the rotational radius, r (in meters), and the desired g-level, a, in units of Earth "g's",

where g = 9.8 m/sec2. The angular rate then is,

and the rotational rate is,

The rim speed is,

Two other important parameters are the percent change in gravity from head-to-foot, and the Coriolis effect which is an additional force incurred due to astronaut motion. The Coriolis force is discussed later in detail. The head-to-foot variation is given by,

ANGULAR VELOCITY (RPM)

( 1 Coriolis force with 1 m/s movement. In % of apparent weight.

Figure 1. Rotational Parameters for Artificial Gravity Systems

where h is an assumed astronaut height, taken here as 1.8 meters. The rim speed, shown as a dashed line, is important in determining propellant requirements for starting and stopping the rotation of the station. Figure 1, shown as a logarithmic plot, is able to accommodate short radii, as emphasized in earlier studies, and the longer radii now being considered with the use of tethers.

For non-tether designs (e.g., rigid or inflatable structures), the rotational radius can be expected to be between 10 m and 100 m. For tether designs, the radius can be an order of magnitude greater. From Figure 1, this implies that for a g-level of 0.4, say, the angular velocity is 2-6 rpm, and the head-to-foot gravity variation can be from 2% to 18%. In the upper region of this figure, with radii greater than 100 m as with a tethered system, the angular velocity ranges from 0.6 to 2 rpm, and the head-to-foot variation is less than 2%.

In addition, the motion in a rotating environment is subject to the effects of Coriolis forces and cross-coupled

angular accelerations, which can be disorienting and thus degrade the system habitability and the motor performance of the crew members. The magnitude of these effects is highly dependent on the system radius of rotation, its angular velocity, and the direction of motion of the crew members inside the habitation module as governed by the centripetal or gravity" force and the Coriolis force.

The Coriolis force in units of "g's" is given by the vector cross product,

wheae vman= velocity of crew member with respect to the rotating reference frame, m/s

The direction and magnitude of the Coriolis force depend on the vector relationship between the spin axis and velocity of the crew member relative to the habitation module. This is best visualized by diagrams similar to those illustrated in Reference 4 and shown in Fig. 2. Motion in the radial direction will give rise to a tangential

a) Tangential Coriolis Force with Radial Motion Superimposed on a Varying Gravity Force Field (Fg ('1) * Fg ('2))

0 W 4--5

b ) Radial Coriolis Force with Tangential Motion at Constant Gravity

c) No Coriolis Forces with Axial Motion

Figure 2. Coriolis Forces Resulting from Astro- naut Motion (white arrows)

Coriolis force superimposed upon a varying gravity force dependent on the distance from the spin axis. The result is a force that pushes the astronaut sideways, as he ascends or descends a ladder (Fig. 2.a). A tangential walking motion (Fig. 2.b) tends to increase (with rotation) or decrease (against rotation) the g-level which he feels. Finally, the Coriolis force is zero only for motion parallel to the spin axis.(Fig. 2.c).

The use of tethers represents an innovative approach to the design of an -cial gravity facility in that it can generate the desired gravity level by varying the length of the rotation radius rather than by changing the angular velocity of the vehicle. The benefits from using a larger radius will be significant, yet the impact of a longer tether on the mass of the system will be minimal. As the radius of rotation increases, the head-to-foot gravity gradients decrease and, at the same time, the need to spin the vehicle faster in order to achieve a certain gravity level is eliminated. The result is a smaller angular velocity with reduced Coriolis force effects on the vestibular function which conmls orientation and locomotion.

Early Studies of Rotating Systems

The realization that tethers may be used to provide a large system radius without significant mass penalty has emerged only in the past 10 years. Two previous periods of activity existed. One was the early 60's when NASA Langley Research Center was engaged in studies on rotating manned space stations. Certain ground rules limited the scope of these studies, the most restrictive being that the station must fit into a single booster payload stage. Automatic erection was also essential, since on-orbit assembly was not yet an option.

Some configurations which were considered at that time are listed in Table 1, along with the more obvious advantages and disadvantages. The complete flywheel model (torus plus cross members) seem to provide the best prospects. Designs considered inflatable structures, but quickly moved to total rigidity when micrometeoroid hazards were considered5 A more detailed design, with a countermtating zero gravity laboratory at the hub, and a multiple docking arrangement, resulted in a total mass of 125,000 lb. Addition of a ferry vehicle, command module, service module, and additional interface structure would increase this to 155,000 Ib (about 70,000 kg). This could readily be launched by the advanced Saturn into a 300 nmi orbit, and serviced by the Titan I11 for crew transfer and supplies.

The torus design is clean and esthetically pleasing, and perhaps, as illustrated in science fiction and by early visionaries, will eventually become the design of choice. -

But even in these early studies, alternate concepts were suggested, such as those based on the dumbbell configuration, as noted by ~ o r e t ~ He mentions an early concept suggested by Ehricke in 195g6 (see Table I), who proposed adapting the booster configuration into a dumbbell system. The booster would serve as the rigid

Table 1. ARTIFICIAL GRAVITY CONFIGURATIONS: NON-TETHERED SYSTEMS

CONFIGURATION A D V A N U G E S DISADVANTAGES FIGURE

CROSS (1) The moment of inertia about the spin axis (Iz) is much greater than the moments of inertia about the x and y axes (Ix=Iy).

RIM (TORUS) (1.2)

Very stable configuration, has constant g-level along outer wall.

The space available is pooriy organized. Going from one branch to another would require transfer through zero g.

COMPLETE Presents the same FLYWHEEL advantages as the torus (TORUS+CROSS) and cross configurations. (1) Docking is possible

anywhere along the spokes.

Docking is possible only on the rotation axis. Traffic in the tangential direction with respect to the axis of rotation is subject to a maximum Coriolis force. Person will experience variations in the g-level dependent on the direction of motion.

CYLINDER (1) Easy to place in space. Unstable due to unequal moments of inertia Ix and Iy, with Ix=IZ (spin axis is along z direction).

MODULES Constant g-level inside PARALLEL TO the modules. AXIS OF ROTATION (1)

Size of modules has to be kept small, otherwise the configuration becomes unstable due to increased Ix with respect to IZ. Large os~illations occur when the motion takes place parallel to the axis of rotation (z axis).

Table 1. ARTIFICIAL GRAVITY CONFIGURATIONS: NON-TETHERED SYSTEMS (cont inuat ion)

S DISADVANTAGES FIGURE

MODULES IN THE PLANE OF ROTATION (1)

Increased stability over previous configuration.

Extensive radial traffic.

DUMBBELL (2)

AXIALLY EXPANDED DUMBBELL (2) ( 2 symmetrically- opposed crew compartments plus one along the spin axis, and 2 radial shafts)

MODIFIED AXIALLY- EXPANDED DUMBBELL (Pseudo Geogravitational Vehicle) (2)

Crew compartment and dead weight countermass.

Most traffic occurs parallel to the spin axis at a constant g-level. Orienting the crew bunks and control consoles parallel to the aisle would not conflict with the optimum space utilization. This configuration can be optimized for size, and has minimum visual conflict.

Only one of the cylinders would be used as a crew compartment. The second compartment would be used as countermass and shelter for vehicle components which would eliminate radial traffic. The design parameters lie within the human comfort area as defined by Loret, with an artificial gravity level of 0.9 g.

The lateral dimensions of the crew compartment are limited. It is possible to expand radially, but that would introduce different values of g at each level.

Extensive radial traffic: Could use the second crew compartment as countermass.

connecting structure with a crew compartment at one end and a nuclear power source as countermass at the other. The simplicity would minimize subsystem development costs, and provide an early launch opportunity.

Extending the crew compartment in the axial direction allows major traffic to be parallel to the rotation axis, which minimizes the Coriolis effects. This "arrow" configuration is shown in Table 1 and was proposed by Kramer and ~ ~ e r s ? Building this station, though ideal in many respects, would be a major undertaking. Loret therefore suggests the modified axially expanded dumbbell configuration (also shown in Table 1).

This final configuration modifies the axially expanded dumbbell to a single crew compartment balanced by a nuclear auxiliary power source at the opposite end. The design radius is 180 ft, and a central non-rotational hub provides a stable platform for equipment as well as docking. A modem design of this configuration would look quite different today.

The second period of artificial gravity station design activity occurred in the late 60's and early 70's. The debate at that time was whether to build a space station or the Shuttle. Artificial gravity for the station was a serious contender, as indicated in a 1969 letter by Gilruth, Director of the Manned Spacecraft enter.^

Based on ground experiments with a centrifuge it was felt that 2 rprn represented a minimum angular rate and that, with adaptation, it could be higher. The hope was that this could increase to as high as 10 rprn: (For a 1 g-level, this would imply a radius of 10 m.) Further ground based experiments were recommended, as well as experiments in space. An elaborate program to study physiology, human performance, and habitability, was part of the overall space station recommendations.

The designs themselves were, of course, based on Skylab and the Apollo systems and hardware available at the time. For Skylab, an artificial gravity experiment was proposed where the mission profile included 42 days with a g-level of 0.41. The rotation rate would be 6 rpm, based on a 33 ft radius. For a space station configuration, the required rpm would be lower due to increasing the radius to about 90 f t

The short radii assumed in studies of this phase was also reflected in artificial gravity papers written at the time. ~toneg, for example, reviews the rotation effects on human performance and physiology in some detail, but claims that radii as short as 50 ft would be suitable. Interestingly, this short a radius was not acceptable to most workers in the earlier period of activity.

Recent Studies of Rotating Systems

The long term commitment to, (and the costly development of) the Space Transportation System, of which the Shuttle is the main element, precluded until

recently any other major NASA development, such as a space station. Also, Shuttle philosophy has carried into Space Station philosophy. That is, just as the Shuttle is to provide launch services to customers, so the Space Station is to provide in-orbit services to customers. The customer application may be scientific, commercial, or military. Obviously, such a broad charter is incompatible with any unique use, such as artificial gravity which, by its nature, would place very restrictive requirements on the station's construction.

Space Station Freedom will not be rotating to produce artificial gravity; however, there remains interest in such systems. One of the earliest suggestions of using Space Station technology to construct a rotating system was made in 1985 by one of the authors.1° The conceptual design proposed uses a retractable tether, and is shown in Figure 3. This system could accommodate two manned modules of the type planned for the space station. A liquid propellant system could provide spin as the modules are being deployed to the desired length.

The counterbalance would consist of a platform housing the tether deployer and other spacecraft systems, and de- spun solar arrays. Power and communications would be carried to the modules via the tether line. At the end of the experiment, the tether would be retracted, and the system de-spun. Exchange of personnel, and resupply could be accomplished with servicing by the Space Station or the Shuttle.

The rationale for fulfilling a life sciences long time goal of studying artificial gravity came with the publication of the report by the National Commission on Space in 1986.11 This report recommended setting long-term U.S. goals in space, such as sending men (and women) to Mars. Further, it suggested using rotating tether systems to provide artificial gravity for the long journey there and back. These recommendations were followed up by a formal NASA study in 1987 to design such a vehicle. The results of this preliminary study are summarized in Reference 12.

A manned Mars mission is very complex, involving aerocapture, landing, launching, and orbiting systems. Figure 4 illustrates a concept of a partially deployed rotating system which would provide artificial gravity. An aerobrake is utilized as the counterweight. The dimensions shown are in feet. At full deployment, the tether extends for 2058 feet, and the rotation rate is 2 rpm.

Of course, setting long range goals, and performing preliminary studies for a manned Mars mission does not constitute the whole story. The appropriate technologies and infrastructure must be developed. Aerocapture designs -. and tests in space must be made. Life support systems must be developed. On-orbit assembly will be necessary. Tether systems must be designed and tested, and artificial gravity must be studied in space.

TETHER SOLAR ARRAYS REEL

SYSTEM (DE-SPUN)

A

DEPLOYED LENGTH RPM g-LEVEL -

I 200m 3.00 1 , O O END MASSES ASSUMED EOUAL AND ROTATING

400m 236m 2.13 0,OO

ABOUT COMMON CENTER RETRACTABLE 270m 1.63 0.40 TETHER

340m 1.03 0,20 SOLAR ARRAYS ARE DE-SPUN AND SUN 4OOm ,65 0.12

ORIENTED END TETHER TETHER

TETHER MASS 900 kg MANNED MODULES MASS DIAM MASS

FOR KEVLAR 29 AND PROPELLANT/MOTOR / 3000ks 13 ,4m 90kg SAFETY FACTOR OF 10 ( AV - 30m/s) 10,000kg 24,4111n 290kg (DIAMETER = 42 m)

30,OOOkg 42 0mm 900 kg

Figure 3. Conceptual Design of a Tethered Artificial Gravity System

Figure 4. Manned Mars Artificial-G Vehicle Partially Deployed

A beginning on the latter has been made in a life sciences workshop held at NASA Ames Research Center in March 1988. l3 Questions were addressed concerning human physiology, performance, and habitability relative to the artificial gravity environment, and how a Variable Gravity Research Facility (VGRF) in Earth orbit could answer these questions. A clear consensus emerged that a long radius would be preferable, so .as to provide a rotation rate low enough to avoid stimulating the neurovestibular system. Use of a tether system can provide this capability. In operation, the scenario of a typical VGRF mission would be as shown in Figure 5.

Related to this workshop are two papers which further summarize the results. These are References 14 and 15, which look at human factors in more detail, and which perform some interesting design tradeoffs for VGRF.

References

"Muscles in Space Forfeit More than Fibers," Science Vol. 134, No. 18,29 Oct. 1988.

Clarke, A. C., The Ex~loration of w, Harper & Brothers, 1951.

Hill, Paul R. and Schnitzer, Emanuel, "Rotating Manned Space Stations," Astronautics, Sept. 1962.

Loret, Benjamin J., "Optimization of Space Vehicle Design with Respect to Artificial Gravity," Aerospace Medicine, May 1963.

Berglund, R. A., "AEMT Space-Station Design," Astronautics, Sept. 1962.

Ehricke, K. A., "Manned Outposts in Space," Astronautics, April 1959.

Kramer, S. B., and Byers, R. A., "A Modular Concept for a Multi-Manned Space Station," In Proceedings of the Manned Space Stations Symposium, Institute of the Aeronautical Sciences, New York, 1960.

Armstrong, William O., and Hammersmith, John L., "Artiii~ial Gravity Experiment Definition Study," NASA Advanced Mission Program Office of Manned Space Flight, Dec. 31, 1969-April 29, 1970.

Stone, Ralph W. Jr., "An Overview of Artificial Gravity," Fifth Symposium on the Role of the Vestibular Organ in Space Exploration, Pensacoh, FL, Aug. 19-21, 1970.

10. Penzo, Paul A., "Tethers and Gravity in Space," presented to the Life Sciences Advisory Committee, July 19, 1985.

11. Paine, Thomas 0.. Ed., Pioneerine the So= Frontier, Report on the National Commission on Space, Bantam Books, 1986.

12. Schultz, David N., Rupp, Charles, C., Hajos, Gregory A., and Butler, John M. Jr., "A Manned Mars Artificial Gravity Vehicle," NASAIGeorge C. Marshall Space Flight Center, Huntsville, AL.

13. Lemke, L. G., Welch, R. B., "Workshop on the Role of Life Science in the Variable Gravity Research Facility," NASA Ames Research Center Final Report, Mar. 27-30, 1988.

14. Tillman, Barry, "Human Factors in the Design of A Variable Gravity Research Facility," Intersociety Conference on Environmental Systems, San Francisco, CA, July 12, 1988.

15. Wercinski, P. F., Searby, N. D., Tillman, B. W., "Space Artificial Gravity Facilities: An Approach to Their Construction," For Space 88 Engr Construction & Operations, Aug. 29-31, Albuquerque, NM (ASCE).

B"

Release Launch . by shuttle \ \ - Release into orbit and system checkout Spin-down

Habitation by crew and deployment

Spinup duration: 5 minutes b 1 - -

Operation for at least 90 days

Spin-down to grav-stabilized position

Refurbish, change crew

Redeploy

Figure 5. Variable Gravity Research Facility Mission Sequence