Journal of Mechanical and Electrical Intelligent System (JMEIS)International Space Station (ISS) Intelligent Human Centrifuge
Kazuhito Shimada1,a,*, Yuji Kasai2,b, and Tomohisa Yuasa2,c1TSUKUBA KOKEN, 2-16-37 Takezono, Tsukuba 305-0032, Japan
2AIST Artificial Intelligence Research Center, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8560, Japan*Corresponding authora<kazuhito.shimada@gmail.com>, b<y.kasai@aist.go.jp>, c<t.yuasa@aist.go.jp>

Keywords: artificial gravity, International Space Station, astronaut, centrifuge, HTV-X

Abstract. The International Space Station (ISS) astronauts need physiological countermeasuresagainst microgravity (weightlessness on ISS). Recent abnormal findings in the eye (Space flight–
Associated Neuro-ocular Syndrome, SANS) after 6-month-stay on the orbit tell us that current exercise
measures are not complete. Ultimate solution should be the Artificial Gravity (AG) for astronauts.
Before closure of the ISS program, which is slated for 2024, we would like to have a test bed on the
JAXA HTV-X cargo ship berthed to ISS, which verifies feasibility and usefulness of AG. ISS
Multilateral Medical Operations Panel is interested in ISS AG for medical operations purpose.
Roscosmos, the Russian space agency, expresses intention to fly a human centrifuge on ISS. The
overall design of AG by short-arm centrifuge with exercise function is not easy to come by, in termsof rotor cost savings. The authors propose to develop a human-powered centrifuge for ISS medical
operations. To make AG effective and safe on manned spacecraft, incorporating advanced information
processing mechanism into the AG machine should be the natural choice. Also, especially deep space
exploration spacecraft which are exposed to high Linear Energy Transfer (LET) ion particle radiation,need to have onboard systems which are equipped with intelligent self-diagnosis and recovery
function. Even low-earth-orbit (LEO) subsystems on ISS are known to be prone to space radiation
damage.
1. Introduction
The environment in manned spacecraft orbiting in Low Earth Orbit (LEO) is different from that on
the Earth. Atmospheric pressure and gas composition are different from spacecraft to spacecraft. The
International Space Station (ISS) uses 1 atm absolute atmospheric pressure with 21% oxygen and 79%
nitrogen, at the same pressure as at sea level, to accommodate animal and other life science
experiments. Space Shuttle cabin was usually at 1 atm abs., but it was occasionally at 0.69 atm abs.,
to facilitate denitrogenating for spacewalk (Extra Vehicular Activity, EVA) astronaut body. Nitrogengas bubbles are the cause of DeCompression Sickness (DCS).
Also, natural space radiation dose is higher in ISS orbit than that on the Earth. The 3-mm thickaluminum alloy wall helps somewhat against radiation. The same wall is for holding atmosphericpressure of ISS. ISS has separate bumper layer to protect crew from hits by orbital debris, which are
immediate hazard to ISS.
The end of mission of ISS has been extended from 2020 to 2024. Current physiological measuresagainst microgravity (weightlessness) work well on ISS, except for ocular problems. ISS medical
problems in space includes space radiation effects, social isolation, confined space environmental
control, and physiological changes due to microgravity. Possible countermeasures in the future include
exercise, medicine, drink water, radiation shield, and centrifuge.
The most potent physiological factor to the astronauts is microgravity (weightlessness) in LEO.
Besides enhanced mobility (3D up and down) in the cabin, no positive physiological change in the
body is reported in microgravity. Fig. 1 summarizes negative clinical changes in the body, for longduration (6 months) human stay in orbital microgravity.
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Journal of Mechanical and Electrical Intelligent System (JMEIS)In Fig. 1, the most recent and best-funded study target is Spaceflight Associated Neuro-ocular
Syndrome (SANS) [1, 2, 3]. In addition, NASA started to investigate, from 2019, that if upper body
venous thrombosis and retrograde blood flow in jugular vein are specific to long-duration ISS
spaceflight [4].

Fig. 1. Physiological changes in microgravity. This figure summarizes symptoms andphysiological problems identified on the International Space Station (ISS). The latest in thischart is the ocular issue (SANS, Spaceflight Associated Neuro-ocular Syndrome), including
ocular fundus abnormality (papilledema) and related changes. Dash one ‘-1’ indicates nosymptom group, ‘-2’ is for affected population [5]. In 2019, NASA started investigation if
upper-body thrombosis is also the result of long-duration microgravity exposure [4].
2. Spacecraft Environment
The era of space station started with Soviet Union Salyut 7 in 1971. Then NASA Skylab followed
in 1973. USSR dominated in the space station domain with Almaz-Salyut in 1974 and Mir space stationin 1986. NASA initially intended to boost the altitude of Skylab module to prolong its use, but thedelay of Apollo program prevented the orbital reboost of the space station. In addition, its orbital decayrate seems to have been quicker than predicted.
NASA considered flying a space station early in space race history. During 1970’s NASA started
planning for an orbital module. Initial detailed design in 1980 is known as Space Station Freedom.
Then budget reduction work was started to write up a design that achieves a more realistic transport
scheme. With addition of Russia as an International Partner, revised Space Station Alpha became theInternational Space Station (ISS) for US, Japan, Europe, and Canada.

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radiation dose

Fig. 2. ISS space radiation dose compared to ground exposures. Long-duration mission on
ISS for 6 months gives a crewmember 60-100 mSv equivalent dose of space radiation, whichcomes from the Sun, distant galaxies, and protons trapped close to the earth. Lifetime dose
limit of radiation is regulated by each ISS International Partner (U.S., Russia, Japan, Europe,and Canada).
2.1 ISS Communication
Holding humans aboard spacecraft for a longer period poses new challenges to space operators.
For a low-earth orbit station, communication with a control center needs to be secured to cope with avehicle emergency that necessitates an early return to base. A set of geosynchronous communication
satellites or a number of ground stations are necessary. For Space Station Mir, Soviet Union placed ahalf a dozen ground station to cover communication period over its territory. This is a similar approach
NASA took to support Apollo flights. For ISS, NASA uses Tracking and Data Relay Satellite System(TDRSS). Currently, three third-generation TDRS’s are active; older ones are kept for backup. Still,time coverage is shy of 24 hours. Its 24-hour coverage is preplanned when there is an important taskschedule, e.g., an ExtraVehicular Activity (EVA, spacewalk). Filling a gap (Zone of Exclusion, ZOE)
with a backup TDRS is always doable, but it is not budget-friendly. Frequency bands are S (2-4 GHz),
Ku (12-18 GHz), and Ka (27-40 GHz). Practically S band audio communication between ISS andControl Center-Houston is 24 hours coverage throughout the year. As TDRSS transponder time isshared by US Air Force and deep space science satellites, Ku band use, which allows ISS video
downlink, is pre-coordinated. Later in ISS program, NASA offered the Russians TDRS time to makeElectroCardioGraphy (ECG or EKG) schedule flexible. With TDRS, Russians do not have tosynchronize EKG recording schedule with the period when ISS is flying over Russian ground stations.
JAXA operated its single DRTS geosynchronous satellite for S and Ka band communication
including ISS Japanese Experimental Module data relay from 2002 to 2017.
ISS communication is between ISS and ground Mission Control Centers at Houston, Tsukuba,
Moscow/TsUP, and Munich. The communication environment on ISS allows immediate voice commwith ground control, but video downlink has to be pre-coordinated.
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Journal of Mechanical and Electrical Intelligent System (JMEIS)As there is no Ku band uplink to ISS, use of S band with low-resolution digital two-way video/voiceis the current practice. Digital video system is required to privatize medical communication between
an ISS crewmember and a Flight Surgeon on the ground. These mean that if a quick medical
consultation is wanted, it starts with private voice, then use of digital two-way video, or digital stillcamera data downlink. Data file downlink is not immediate, it happens a couple of times a day in abulk, usually during crew sleep. Thus, for an immediate care, onboard physician is the key. Physician
crewmember manifest is not mandatory by rules. If there is no physician, one of two dedicated Crew
Medical Officer will be at work. Official emergency return to ground time is 72 hours for ISS, butthere is much risk in spacecraft operations if early return is forced to happen. The consensus is that for
many medical conditions, ‘stay and fight’ would be less risky than to conduct an emergency return
whose landing place could be far to a definitive treatment facility.
Beyond lunar orbit, including Martian surface, communication delay mandates autonomy of thecrew in medical emergency resolution. Delayed communication has been simulated as ISS operational
experiments. During the trial, ground controllers did not respond immediately; they gave ISS crew
more autonomy than usual.
All systems onboard deep space exploration vehicle, including our intelligent human centrifuge,needs to be autonomic.
2.2 ISS Life Support
Next factor for a manned space station is a set of life support systems. It starts with atmospheresupply and air conditioning. As spacecraft air seal is good (ISS has an excellent, i.e. very low, leakrate), supply of nitrogen is enough with a series of pressurized gas tanks. Russian spacecraft always
use 21% oxygen and 79% nitrogen atmosphere with 1 atm abs. pressure, which are the same for thaton the ground. United States spacecraft applied different atmospheric composition. To make spacecraft
lighter, a lower atmospheric pressure is preferred. Space station Skylab used 74% oxygen and 26%
nitrogen at 5 psi (0.34 atm abs). At 0.34 atm abs, physiologically sea-level-equivalent oxygen
percentage is 62%. Thus, Skylab atmosphere had more oxygen than on the ground, which means wemust start to be careful about oxygen toxicity to the lung.
Apollo capsule initially used pure oxygen, then after Apollo 1 fire which happened ground oxygenpressure of 16 psi (1.1 atm abs), NASA converted it to two-gas oxygen/nitrogen system that made thehardware and protocol complex, with some adverse effects [6]. As humans use oxygen and carbon
dioxide is exhaled, the essential function of atmospheric system is to remove carbon dioxide and supply
the amount of oxygen used by crew. When humans are kept in a confined space without life support
system, they die from high concentration carbon dioxide poisoning before hypoxia starts a serious
condition. Principle carbon dioxide removal on ISS is by two zeolite beds. When one bed is absorbing
carbon dioxide from ISS cabin, the other zeolite bed is exposed to vacuum, then absorbed carbon
dioxide will diffuse out to outer space.
Apparently, air temperature control is mandatory. It is attained by heaters in the wall, chiefly forprevention of water condensation, and an air conditioner with foul odor remover. Usually considerable
amount of heat is dissipated by avionics, life support mechanism, and experimental apparatus, heatremoving function is activated on spacecraft. ISS removes cabin heat via water loop, heat exchanger,
ammonium loop, to external radiator.
There are two ways to supply oxygen and water in manned spacecraft. One is the fuel cell systemused by Gemini, Apollo, and Space Shuttle. They were not stable, intimate care was given. When fuelcell system is selected, capsule carries pressurized oxygen and hydrogen tank. Solar panels are notneeded. Electricity is generated by fuel cell. Water for system and crew is a biproduct of chemical
combination reaction. Amount of water exceeded daily requirement, thus jettisoning of excess water
was conducted.
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Water regeneration by ISS Water Reclamation System (WRS), from urine and wall condensate is
not simple. It starts with accommodating high calcium urine, which made early Space Shuttle toilets
with a lot of calcium compound accumulation. Purifying urine to yield potable water is working on
ISS with many troubles. Water regeneration is currently by high pressure and high temperature. In
2010, Sabatier chemical reaction system was activated, to catch cabin air carbon dioxide and generatewater. It consists of a furnace, a multistage compressor, and a condenser/phase-separation system [7].
2.3 Space Radiation
At the orbital altitude of 400 km, ISS is exposed to space radiation. But ISS is also partially shieldedby earth magnetosphere and atmosphere from the radiation. There are 3 sources of space radiation:
galactic, solar, and earth-trapped particles (proton and electron) [6]. Nominally total dose from thesedoes not affect crew significantly [Fig.2]. However, there is always a chance of a large solar flare givesa large dose to ISS crew. It has not happened yet, in the history of ISS, largely due to the smaller
volume angle steradian of the earth area. Not all direction s around the sun are affected equally by asolar flare. ISS crew had to stay in the center of ISS, where the shield from module walls, for a day
due to solar flares. This shield procedure has been exercised twice so far. Wall thickness of ISS
modules is equivalent to about 3 mm-aluminum.
There has been no case of acute radiation sickness in space operations. Each ISS International
Partner regulates their astronaut’s acute and lifetime dose limits.
In the case of Mars mission, not only proton but heavy ion from galaxies are to be considered. For
example, iron ion radiation effect to Central Nervous System (CNS) is in discussion [8].
ISS electronical equipment has shown many incidents due to space radiation. Not only crew, butalso equipment onboard deep space exploration spacecraft needs to be either high-LET radiation
tolerant, or more realistically, to be with self-diagnosis and recovery intelligence. This applies to ourintelligent human centrifuge, too.
2.4 Psychological Considerations
Often media reports on confinement and social isolation effects on ISS astronauts. On ISS,
atmospheric confinement is a big issue at the time of fire or air leak. But there has been no prominent
case of psychological problem case on ISS. Since around 2010, NASA augmented Internet Protocol
Phone (IP Phone) and internet browsing on ISS. At leisure time crew can call out anybody on thephone (receiving call is regulated by mission control center), and browsing speed is at a practical level.
Generally, crewmembers are happier on orbit than in training on the ground. Psychological cases
are occasionally seen on the ground.
Volume of ISS is larger than a regular house. Its volume exceeds the internal volume of Boeing 747.
As they can move in 3D, and each crewmember can occupy almost one module per person, ‘restricted’personal space is not an issue. Sometimes they do not see each other after a morning briefing until
dinner.
For smaller spacecraft, psychological issue is of higher importance. NASA Orion space capsule islarger than Apollo capsule, but not much. For this regard NASA and Roscosmos have a good amountof experience in the past. The key is to pick crewmembers who knows each other well. To do so,
training as a group on the ground for sufficient length is the practical solution.
2.5 Physiological Countermeasures against Microgravity
Apollo lunar missions were only one week long, but exercise machine (Exer-Genie Exerciser) was
already made available, which was first tried in Gemini program (bungee on Gemini IV and isometric
exercise on Gemini VII) [9, 10]. Skylab crew used cyclergometer in medical experiment M171:
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Metabolic activity [11]. They also had Dr. William E. Thornton’s makeshift treadmill that the thirdcrew brought with them to the station. The treadmill device consisted of a Teflon-coated aluminum
plate bolted to the floor of the workshop [12].
During Space Shuttle program, the Skylab cyclergometer was kept in use. It was launched folded,
and was extended on mid-deck floor while they were flying. For ExtraVehicular Activity (EVA)
preparation, grip exerciser was used by EVA crewmembers. For research, Lower Body Negative
Pressure (LBNP) device was extensively tested. The consensus on LBNP then was that it is good for
cardiovascular assessment, but it never became operational in NASA program, while it was required
operationally in Russian program [13].
On ISS, crew physical exercise is the primary physiological countermeasure against microgravity.
Other countermeasures implemented for ISS crew are pharmacological (Vitamin D), and LBNP (forRussian cosmonauts). Food nutrition follows WHO recommendation.
2.6 New Medical Issues
ISS medical operations have not seen new physiological issues for almost 10 years after permanenthuman habitation was started. However, new health issues were identified recently [Fig.1] [14].
A surprise came around 2009, when they found an index case of papilledema, which was not
resolved after landing [1]. Most prominent feature of these cases was the flattering of posterior side of
the ocular globe.
Then in 2017, MRI study revealed brain shift after ISS flights [15]. This may explain themechanism of ocular globe flattering and papilledema [2, 3].
In 2019, NASA started looking into upper torso venous thrombosis. It is a candidate of longduration spaceflight adverse effect [4].
As we do not know what kind of new health issue would be found, we need to seek for a holistic
approach to ill effects from long duration spaceflight.
3. Artificial Gravity as a holistic countermeasure
The differences in environment, between space and that on the ground, to crew onboard spacecraft,are;1) Microgravity2) Space Radiation
3) Atmospheric Confinement
4) Social Confinement.
In this paper, above we discussed microgravity and space radiation. Below we continue to discuss
microgravity effect on crew.
As microgravity onboard spacecraft is the reason for most of health problems for crew, applying
gravity artificially should rectify adverse physiological effects. However, we do not know how muchor how long acceleration (gravity) should be loaded to human body to prevent health problems onboard
spacecraft. Some hints, e.g., for ocular issues, are acquired by animal study [16]. A considerable
amount of research has been done on the ground with centrifuge [17]. An international life science
society research proposal ‘AGREE’ [18, 19] was submitted for an ISS short arm humancentrifuge/cycloergometer, but the proposal was rejected in 2012 by ISS Program [14].
For animal experiment, a large animal centrifuge for ISS was proposed before the first element
launch of ISS in 1998, but it was cancelled [14]. Japanese space agency, JAXA, is running a mouse
centrifuge onboard ISS [14, 20]. Its ‘partial gravity’ run was initiated in 2019, starting with 1/6 G.
As animal response to microgravity has been studied since Sputnik 2 with dog ‘Laika’, more is
known by now (for lunar orbit Apollo 17 flew 5 mice for radiation monitoring [21]), but no ‘partial
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gravity’, i.e., 0 – 1.0 G, spacecraft experiment has been conducted [22]. For human spaceflight, lunar
surface (1/6) and Martian surface gravity (1/3) are of most interest at present. If there is a threshold
either lower than 1/6 G or 1/3 G, sustainable human presence on the moon or Mars could be without
extensive physiological countermeasures.
Today we have no idea if a long surface stay on Mars or the moon would harm the eyes or brain.
That kind of research should be started before ISS comes to the end of mission, with animal and humancentrifuge.

(a) Human body acceleration axis system.

(b) Aircraft acceleration axis system.

.
Fig. 3. Axis nomenclature for Artificial Gravity. There are different axis systems used inaerospace field, (a) In this paper, the most popular human body axis system used inaerospace medicine is employed, Gz axis is from the head to the feet, (b) One of popularaircraft axis systems.
3.1 Artificial Gravity implementation
NASA’s Artemis-Gateway plan would not be able to accommodate large modules. ISS can.
Spacecraft to Mars would need to be equipped with artificial gravity device. In this paper we use theaxis system used in aerospace medicine [Fig. 3].

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3.1.1 Running on Skylab
It is widely known that Skylab crewmembers ran on the circular wall of the spacecraft. They had
difficulty in starting the run under microgravity and continue running (from NASA Skylab video, about
2 m/s, thus at 0.14 Gz at feet) [Fig. 4].
This type of running on the wall in spacecraft, without any vehicle rotation assist, could be the final
configuration for attaining G for crew body as a microgravity countermeasure. However, we do notknow what limits their body movement per vestibular symptoms, or if a crewmember can run fastenough, with simple running shoes, in a smaller diameter volume (to benefit from higher rotationalrate). We need to accumulate experience with a human centrifuge onboard spacecraft.
There is an anecdote that ground controllers did not like crewmember moving this way, as it used
Skylab system fuel for attitude control. Rough estimation of attitude control system load can be madebefore launch, but how crew use their body movement during actual G load should be verified onboard.

Fig. 4. Running on Skylab. By analysis of Skylab NASA Public Affairs Office motion film, thecrewmember was running on the wall (3 m radius) at 9 seconds a revolution (0.67 rpm). At
this rotation speed, the centrifugal acceleration at feet was ca. 0.14 Gz. Starting the run was
difficult, since before they gained velocity, there was no mechanism to stabilize their body inmicrogravity. As the +Gz value at the heart level was very small, no cardiovascular effect was
expected.
3.1.2 Artificial Gravity for Space Shuttle
Spacelab science module in Space Shuttle cargo area once accommodated a short-arm human offaxis centrifuge for ‘Neurolab’ life science payload mission, but subject orientation was only forvestibular study, thus no Gz (acceleration in the axis from head to toe) was loaded [23, 24]. However,their G load reached 1 Gy (sideway acceleration).
3.1.3 Artificial Gravity for ISS
In October 2016 at Tokyo meeting, ISS Multilateral Medical Operations Panel was presented with
HTV-X option of ISS human short-arm centrifuge, and agreed that this possibility should becontinuously monitored. Also, in 2016 Roscosmos was reported to plan a human centrifuge on ISS[25]. ISS human centrifuge is expected to fill in knowledge gaps, and HTV-X AG option fits very wellin AG roadmap [17].
At present, centrifugation on ISS is only for micro-organism and mice [14, 18, 26, 27]. As statedabove, a large rat centrifuge was cancelled before the launch of ISS, and international short-arm humancentrifuge research plan ‘AGREE’ was turned down in 2012: there is no launch plan of a human68
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centrifuge. Currently the ‘Permanent Multi-purpose Module (PMM)’ which AGREE planned to use
for centrifuge implementation and storage is occupied with hardware. But a newer expandable module,
Bigelow Expandable Activity Module (BEAM), was added later [28]. BEAM’s diameter is rather
small at 3.23 m, though.
The scheme for AGREE centrifuge launch/installation was that 1) European Space Agency (ESA)
builds the centrifuge rotor; 2) JAXA launches the rotor; 3) NASA gives out installation volume in
PMM; 4) NASA gives out crew time to assemble the rotor on ISS; 5) research time would be sharedby ISS International Partners (IPs). The weakness in this scheme is that rotor integration becomes
multilateral and complex, and also a long and error-prone onboard assembly of a large machine wasnecessary.
3.1.3.1 HTV-X human short-arm centrifuge option for ISS
We are now at a later stage of ISS Program which financially ends in 2024. A new opportunity
became available after the 2012 AGREE proposal rejection. Since 2009 JAXA has been flying an ISS
cargo ship ‘HTV’, which has the largest internal payload diameter of 4.1656 m, among ISS cargotransfer ships. JAXA is developing its new spacecraft, ‘HTV-X’ that will start launching in 2022.
The prominent difference between HTV and HTV-X is that the latter will have longer orbital life,
which is nominally 6 months (HTV orbital life is 3 months), but could be much longer if required.
The advantages of HTV-X human short-arm centrifuge, compare to the AGREE scheme are:1) HTV-X can accommodate the AG rotor like a new, empty module.
2) JAXA does not have to prepare an extra rocket launch. If it negotiates with NASA for NASA’s
cargo rights (maybe for NASA crewmember participating in AG trial) on HTV-X (about half ofa cargo ship), a launch already on manifest can be allocated.
3) If the cargo ship stays berthed to ISS for 6 months or longer, many of the objectives of AGREE
could be fulfilled.
4) The AG rotor can be assembled and firmly installed in a cargo ship. Welding, which is unable
to conduct on ISS, becomes an option for assembly.
5) Rotor location can be closer to ISS berthing mechanism, than the one planned for PMM.
6) As HTV-X will be thrown away after unberthing, it is easy to launch an improved rotor on
another HTV-X.
3.1.3.2 Human-powered rotor option for ISS AG
The real major reason, estimated, for the reason of cancelling of AGREE was that rotor cost was
too expensive. As AGREE rotor feasibility study showed that the rotor would be very complex, thus
additional effort to reduce hardware vibration was not materialized. The rejection letter from NASAand JAXA management to AGREE Principal Investigator (PI) said that whole ISS vibration was toexceed ‘desirable’ allowance. The AGREE rotor mass estimation grew from the original humanpowered 60 kg machine with two subjects to 320 kg motorized model with one subject. The mass ofcancelled CAM rat centrifuge rotor was 2 ton and was said to have cost 200 million US dollars. The
CAM rotor was at an engineering module stage when it was cancelled, but safety review was notcomplete then. Its design was said to have never met the design safety requirement to tolerate sudden
halt torque of the rotor.
One option to reduce cost and be safer in operation is to choose the subject as power source andcontrol personnel. The energy to start up and brake a human-powered AG rotor on ISS is about the69
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same as riding a bicycle on the ground. With some design, highly reliable racing bicycle component
could be used.
When one of the authors held a Technical Interchange Meeting (TIM) with AG researchers at
NASA Ames Research Center in 2016, attendants from Massachusetts Institute of Technology, ESA,
and Johnson Space Center all agreed that the first human AG rotor abord ISS does not have to be ascomplex as the research centrifuge for AGREE. One comment was “Rather than keep watching AG
proposal ‘rounds, rounds, rounds around realization’ for more than 4 decades, anything that
accommodates a subject to see exactly how much vestibular effects the person feels is welcome.”
There could be unexpected issues; e.g., we do not know if airflow is comfortable for a subject astronautor not. We need to learn about human short-arm centrifuge on ISS before we apply AG for Mars
mission physiological countermeasure.

(a) Center shaft configuration(b) Wall track configuration

Fig. 5. Design options for an ISS human short-arm centrifuge [27], (a) front view of a swing
rotating around a center shaft design. Heavy members sustain the center shaft. If motorized,
this may be easier to build, (b) side view of a monocycle running on a rail on the wall.
Authors’ preference. Weight distribution is for the rail and the truss (triangular for this design
sample). Very similar to riding a bicycle, braking is easy, inherently safe.
3.1.3.3 Requirements for the rotor
Basic requirements for the first-ever ISS human short-arm operational centrifuge are:1) Able to be safely halted by a subject.
2) Safe when a foreign body suddenly stops the rotation.
3) Inner ear position can be adjected to be as close to the center of rotation.
4) Highly reliable, durable mechanics.
5) Able to monitor and adjust rotation rate (that equals to Gz value).
6) Able to adjust distance, as large as possible, between inner ear and center of rotation.
7) Cyclergometer variable loading. Max oxygen uptake load is desirable; at least 200 W range.
8) Can be operated by a subject, no need for a safety monitor person.
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3.1.3.4 Design sample – rail or center shaft
Most of ground short-arm centrifuges are laid horizontally, and have a rotation shaft in the center[Fig.5 (a)]. However, in microgravity, other configurations could be optimal, like use of module wallor rails to run on with a wheel or two [Fig.5 (b)]. In the latter case, holding a wheel on the wall needssome restraint mechanism. At least, to start the rotation, something to hold a wheel in place isnecessary. The same restraint member could be used for correcting riding track and for safety. Whena subject is vigilant, rails may not be necessary after rotation has started. But for a safe sideway
tracking, some restraint member is desirable.
Authors prefer [Fig.5 (b)] wall track/rail approach for our human-powered rotor. One of the tightest
and robust designs for wheel-rail, popular on the ground, is found in roller coasters [Fig.6] [29]. Thismechanism allows cars to sustain G’s from all directions. Many of roller coasters are with a pair ofrails, but monorail system and vibration damping systems are materialized [30, 31]. For this paper wetake a model from roller coaster design. Vibration isolation system for exercise machines on ISS isimportant, but it should be discussed later when more details are defined for rotor design.

(a) Coaster Rails(c) Fake Wheels (black)(b) Wheels (upper, side, & lower)(d) Dual Upper Wheels

Fig. 6. An example of roller coaster rail/wheel system. Those large, wheel-like pieces are fakewheels. Each bench is held with 4 groups of upper two wheels, two outer side wheels, andone under wheel that contact to one of two rails. Each wheel group consists of 5 wheels on
one side.

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3.1.3.5 Design sample – ergometer installation
ISS human short-arm rotor should be both for AG load and leg exercise. As the rotor arm length isshort, steep Gz gradient is inevitable. The Gz value to the heart is hard to reach 1 G. Vice versa, Gz on
feet becomes larger than 1 G. Without lower extremity exercise, venous blood pooling would result;leg exercise is anyway necessary.
There are efforts to shorten the time necessary for ISS countermeasure exercise, which is 2 hours
in time block and exercise time about 1.5 hours besides preparation and clean up, by menu selection[32], but High-intensity Interval Aerobic Training (HIAT) is not yet well accepted for ISS
implementation.
Exposure duration to AG per day was researched on the ground, from 0.5 to 4 hours per day [33].
The cardiovascular and musculoskeletal effect of AG depends not only the loading duration but also
on its combination with load G value. As short as 0.5 hour per day with multiple sessions (e.g. 5 mineach) is reported to have had a significant result. Countermeasure time slot length shortening is well
expected.
A mechanical cyclergometer was used in Skylab and Space Shuttle program. Workload heat wasdissipated to air via metal enclosure. ISS cyclergometer, ‘Cyclergometer with Vibration Isolation
System (CEVIS)’ is electronically controlled [34].
There are other loading devices used for bicycle. One good example is a compact device for indoor
bicycling with permanent magnet [Fig.7].

Fig. 7. An example of permanent magnet bicycle rim loader [V]. Level of load is adjustablewith the dial in hand. These items are sold commercially for serious athlete riders. Its
durability is excellent. The location of bicycle aluminum wheel rim is shown by a book.
Rubber rollers are replaced regularly.
3.1.3.6 Design sample – ISS human-powered intelligent rotor
With design options described above, Fig.8 shows the initial draft of our ISS short-arm centrifuge
for astronaut physiological countermeasure. It combines AG load with cyclergometer exercise. The
design has two rails with firm wheel grips, modeled from roller coaster standard design. Comparedwith a simpler delta truss design in Fig.5, this rail/wheel could be excessively tough and heavy, butshould be a good, safe starting point. We may leave only one fixed rail and one rotating frame, to72
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Journal of Mechanical and Electrical Intelligent System (JMEIS)
reduce mass. After we gain experience with the first rotor, we may depend on subject self-control forcorrect path tracking.
Braking power necessary for this rotor is about the same for sport bicycle. Same components could
be applied. To free subject hands and make it simpler, foot coast brake may be chosen.
Height of subject above the monocycle wheel, or distance between subject inner ears and the centerof rotation, should be adjustable. To accommodate subject of different size, either variable size
pedals/cranks, or alteration of subject posture is needed.
The shape of truss member, or rotating frame, prefers circular shape for ease of operation, andspacing to avoid inadvertent crew contact with the frame when anomaly happens.
This version follows the rotor dimension of AGREE rotor (3.2 m diameter), and assumes up to 30
rpm rotation rate. The larger the diameter, adjustment for subject height becomes somewhat easier, but
attaining higher Gz may become more difficult, as inertia of the cycle becomes larger.

(a) Centrifuge Concept(b) Frontal section view of monocycle

Fig. 8. An initial draft design of ISS short-arm centrifuge [26]. This is a robust, safe design tostart with. The diameter of 3.6 m follows the AGREE rotor dimension, assuming rotation up
to 30 rpm. To reduce mass, one rail and one rotating frame could be removed. To adjust the
distance between inner ears and center of rotation, and receive different height crewmembers,
subject position may have to be altered. Average height male subject needs a large
monocycle wheel. Tire can be air-filled or flat-free type. The former would have advantage in
mitigating high frequency vibration. Counter balance weight could be fixed or dynamicallylocated with intelligent control. Foot coaster brake may be employed, which free hands. The
brake may supplement friction loading components that can be located at each rotating frame
wheel. Distributed workload device has advantage in heat dissipation. Overall intelligent
anomaly detection system should be in place for safety.
4. Conclusion

This paper first reviewed requirements for an ISS human short-arm centrifuge, then examined eachcomponent of centrifuge rotor. Our initial design of the AG rotor chose a human-powered monocycle
configuration with fixed rails and rotating circular frame. Detailed design work should reduce mass of
the rails and wheels.
To reduce mass of safety components and detect anomaly earlier, realization of the function forreal-time intelligent analysis and recovery is desired.
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