This document proposes a solution for transferring personnel and supplies between a large, rotating habitat wheel that provides artificial gravity and a central, non-rotating utility core in an orbital space station. The solution involves a dedicated subway system with cars that travel on tracks between the rotating wheel, moving at 31.3 m/s, and the stationary core. The subway cars would slow or speed up using linear induction motors to match speeds, providing a safe way to cross the speed differential while keeping the rotation of the main habitat and stationary core unchanged.

1. A NOVEL APPROACH TO INTRA-STATION TRANSFERS BETWEEN
LARGE CENTRIPETAL HABITATION WHEELS AND STATIONARY
UTILITY CORES
Stephen A. Sywak, PE1
Abstract:
In previous studies of centripetally induced artificial gravity for use in orbital habitats
and other long-duration space facilities, large diameter rotating habitats are shown to
provide suitable living and working quarters for long-term missions in space.
One of the important practical issues typically missing from these discussions is the
need for a central, non-rotating hub for docking resupply and crew-exchange
vehicles. The issue is how to move equipment, supplies, and personnel between the
crew quarters and facilities on the habitat’s revolving rim and the docking and
loading platforms in the stationary core.
This presentation proposes a solution to this issue.
Introduction:
The idea of a large, revolving ring or wheel for long-term habitation in zero-gravity
space has been around probably as long as people have been thinking and writing
about space exploration (Noordung, 1929). A space asset this large presumes a
relatively large crew complement, with the attendant needs to support and supply that
crew. Recent discussions regarding the commercialization of space through space
tourism (Collins, 2000) also beg the question of how to move personnel and supplies
from a fixed docking location to the rotating portion of the habitat.
Presentations of tethered stations (Lansdorp, et. al., 2003; Gemini/Agena experiment,
1969), which rely on a counterweight on the opposite end of a large tether from the
habitat of interest (similar to the Argentinean “bola”), have failed to address any need
to access the crew quarters on a real-time basis. The rotation of the entire assembly
must be stopped before any crew or material exchange can be made. This may be an
acceptable risk (as regards tether stability) for short-duration, Low Earth Orbit
testing, but could pose substantial problems regarding both safety and energy
management if this approach were to be used for long duration or permanently
manned habitats.
1
ASME Member; BSME, Washington University, St. Louis, MO; Senior Mechanical
Engineer, M. G. McLaren Engineering Group, 100 Snake Hill Rd; West Nyack, NY
10994; SSYWAK@MGMCLAREN.COM; Phone: (845) 353-6400 x350:
FAX: (845) 353-6509
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2. Even a recent student paper on an “Orbital Hilton” (Applebaum, et. al., 2002)
mentions a zero gravity hub as part of a ring-style station, but does not mention a
means of accessing that section of the station.
The film 2001: A Space Odyssey (Kubrik, Clark, Trumbull, 1968) provides the image
of a large, revolving, wheeled space station with a synchronously revolving hub, and
a manned vessel docking with this rotating target. As visually intriguing as this
imagery may be, the impracticality of docking to a constantly rotating target (Sywak,
1989) will need to be addressed if centripetally driven habitats are to be constructed.
This paper proposes a possible design solution, which consists of two steps: the first
step would be to counter-rotate the central utility core of the centripetally rotating
habitat, such that the center remains stationary with respect to an external, inertial
reference frame (such as an approaching vehicle in an Earth-orbit trajectory). The
second step would be to provide a means of crossing the gap between the rotating
wheel and the counter-rotating central core. The means of moving from the rotating
reference frame to the stationary one would be placed at a distance far enough from
the central axis to be relatively free of the unique rotational problems associated with
small-radius centripetal effects, and to allow sufficient flow of people and supplies
such that the transfer point does not become a bottle-neck, regarding either simple
logistics or personnel safety. In addition, future space station designs may require the
central core area to be reserved for storage, docking facilities, equipment rooms,
laboratories, or the transfer of power, data, air, or other utilities.
Selection of principal operating values:
Studies on the effects of artificial gravity have been performed since 1960. These
studies attempted to predict a “comfort zone” in terms of minimum and maximum
percentages of Earth-normal gravity, habitat angular and tangential velocities, and
maximum gravity gradients. This information is collected in Table 1 (Hall, 1997).
The goal of this paper is not to reexamine the collected information; but to establish
reasonable dimensions (radius and speed) as nominal “design criteria” for a
centripetal habitat, and then to explore a means of moving from that centripetal
habitat to a stationary central structure. This section will determine a nominal radius
and angular velocity for further discussions within this paper.
Year Minimum Maximum Maximum Minimum Maximum Maximum
Published Gravity Gravity Gravity Radius Angular Tangential
Author Gradient Velocity Velocity
Clark & Hardy 1960 - - 0.1 rpm -
Hill & Schnitzer 1962 3.5% G 100% G 14.3 m 4.0 rpm 6.0 m/s
Gilruth 1969 30% G 100% G 12.5 m 6.0 rpm -
Gilruth (Optimum) 1969 30% G 90% G 8% 67.1 m 4.0 rpm -
Gordon & Gervais 1969 20% G 100% G 8% 12.5 m 6.0 rpm 7.0 m/s
Stone 1973 10% G 100% G 25% 15.2 m 6.4 rpm 10.2 m/s
Cramer 1985 10% G 100% G 3% G 22.3 m 3.0 rpm 7.0 m/s
Table 1: Comfort Zones in an Artificial Gravity Environment
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3. Gravity Gradient is the difference in centripetal acceleration that a person would
experience between their head and their feet, due to their location at different radii.
The basic equation: gradient = [w2Rf – w2(Rf-1m)]/ (w2Rf)
Solves as: gradient = 1m / Rf
Or: Rf_min = 1m / gradient
(where “w” is angular velocity in radians/sec; Rf is the radial distance from the
habitat’s rotational axis to the floor surface, in meters)
With this in mind, an 8% gradient yields an Rf_min = 12.5m; a 25% gradient yields an
Rf_min = 4.0m. Cramer’s 3% G delta yields a 33.3m minimum floor radius for 1G
nominal habitats, For a 30% G nominal habitat, Cramer’s 3% G delta represents a
10% gradient, and therefore a 10.0m minimum floor radius.
The Maximum Angular Velocity of the habitat bears directly upon the likelihood of
inducing motion sickness in the occupants. This motion sickness is related to
gyroscopic coupling of normal head rotations of the occupants within the rotating
frame of reference of the centripetal habitat, as it affects the vestibular fluid within
their inner ears (Graybiel, 1975). Graybiel states that, “at 1.0 rpm even highly
susceptible subjects were [relatively] symptom-free. At 3.0 rpm subjects experienced
symptoms, but were not significantly handicapped. At 5.4 rpm, only subjects with
low susceptibility performed well and by the second day were almost free from
symptoms.” A two-day acclimation period for subjects with “low susceptibility”
would appear to place 5.4 rpm as an upper limit, with 3.0 rpm as a reasonable value.
Since Earth-based centrifuge testing naturally imposes a 1G reference, it cannot be
known if this reference adds an offset (physical or perceptual) to the test subject’s
sensitivity to the rotational cross-coupling effects. In addition, these tests dealt
primarily with acclimation, and not long-term health effects.
A maximum angular velocity of 3 rpm is therefore selected.
In general, the “comfortable” gravity range falls within 30% to 100% of Earth-normal
gravity. Hill & Schnitzer’s value of 3.5% G has been classified as more of a
mathematical limit than a true physical requirement (Hall, 1997). What none of the
previous estimates were able to determine, however, was the minimum value
acceptable for long-term health of the inhabitants (Czarnik, 1999). In the absence of
these additional data, and in light of the known value of a 1G environment, a value of
100% Earth-normal gravity is proposed.
The expression for centripetal acceleration is straightforward:
Acent = Rf* w2
Solving for Rf: Rf = Acent/w2
Using the previously selected values, a floor radius of 99.41m is achieved.
3

4. However, note that for a value of:
w= Acent / R f = (9.81m / s 2 ) /(100m) = 0.3132 rad/sec = 2.991 rpm
a direct relationship between radius and %G is established, such that a floor radius of
100m yields 100% G, a radius of 90m yields 90% G, and so on.
The values used will therefore be:
w = 3.0 rpm
R = 100m
A = 9.81 m/sec^2 (100% G)
V = 31.3 m/sec tangential velocity at the habitation level
These values meet the current, known criteria for artificial gravity “Comfort
Boundaries.” Future studies may refine minimum allowable gravity levels for long-
term health—and so reduce the radius and/or the angular velocity requirements.
On moving from a large centripetally rotating habitat to a stationary utility core:
As early as 1929, Hermann Noordung wrote of a large rotating habitat wheel
designed to provide sufficient centrifugal force to create an artificial gravity
environment in a space station in orbit around the Earth. He also described a 2-3m
diameter “air-lock,” which could be rotated in a direction counter to the main wheel
(“spun down”) to provide a fixed target for docking maneuvers. The air-lock would
rotate at an angular velocity at exactly the same magnitude as—but in the opposite
direction from—the main wheel, and thus provide a stationary loading point for
personnel and supplies.
A 1963 NASA paper described a large, wheel-shaped habitat with a central air-lock
and docking structure (Loebel, 1963). As with Noordung’s concept, this central
structure is spun down on an as-needed basis. However, the NASA paper is far more
specific regarding docking procedures. Up to seven vehicles would be allowed to
dock to their structure—but only in radially symmetric patterns. While the ships are
docked, this central structure would be required to spin back up to full station speed
(nominally 1/3 radian/sec, the same speed considered in this paper) in order to allow
for the transfer of personnel and materials to the large wheel. Even though the NASA
paper considered the “de-spin/spin-up subsystem” as critical to both logistical support
and crew safety, no further mention was made of it other than the initial description
of its operation. Issues of momentum and energy balance, stresses on docking and
support mechanisms developed from centripetal and tangential accelerations, and
methods for accommodating an asymmetric docking condition (such as a single ship)
were not presented.
Noordung’s “air-lock,” then, remains a possibility as a means to transfer from a
rotating to a non-rotating element of a space station. In his book, “2001,” (written
concurrently with the film’s screenplay), Arthur C. Clark describes the docking
platform of the earth-orbiting space station as permanently counter-rotating with
respect to the habitation wheel, and uses a Noordung-style air-lock to bring the
crew—a few at a time—across the rotating interface.
4

5. But suppose we need to transfer personnel and supplies at a radius roughly the same
as the living quarters? For the purposes of this paper, that would be at a radius of
100m from the central axis. As previously stated, the tangential velocity of a point
100m out and running at nominally 3.0 rpm would be 31.3m/sec (about 70 miles an
hour). The goal is to provide a design that will safely and repeatedly bring a payload
from 31.3m/sec to zero speed, and back again, on an as-needed basis. This design
should affect neither the speed of the centripetal wheel, nor the speed of the stationary
core. Additionally, the final design needs to consider energy/momentum balance, as
well as eccentric loads and their dynamic-balance issues.
The concept presented incorporates a dedicated “Subway System” to move payloads
from rotating to stationary elements and back again (as shown in Figure 2). A special
subway car would run on a dedicated track at a given radius from the center of the
station. The track is built onto a concentric platform (“shelf”) extending from the
side of the stationary central core. Multiple cars can be provided by incorporating
more concentric tracks at lesser and greater radii, with one car per track. The wheels
of the cars are held to the track in the same manner as the track systems used for
roller coasters, since the cars are normally parked on the zero-G side of the space
station. Motive force for the cars would likely be from a linear-induction motor, with
the car providing its “rotor” field either from a series of permanent magnets, or a
wound coil. Friction drives are to be avoided, for reasons to be mentioned shortly.
Some combination of contact-conductor rails or inductive power transfer rails would
provide data and electrical power to the car.
Energy/momentum balance issues would be handled through the use of momentum
wheels in the stationary core of the station, or electromagnetic torquers, common to
many of today’s satellites.
Eccentric imbalance issues would be resolved through the use of shuttling
counterweights on adjacent tracks, servo-controlled to remain 180º opposed to the
active subway cars, or through the use of mechanical linkages tying a number of cars
together in a radially symmetric (and therefore balanced) pattern on a common track.
Access platforms to the car would exist on both the rotating and non-rotating sides of
the track. Sliding partition doors, similar to those found on the Japanese subway
system, would prevent people and materials from falling on to the track. The cars
themselves would possess their own sets of sliding doors. The cars would be fitted
with seats and seat belts, as well as cargo tie-down rails and related equipment.
Interior surfaces of the cars would be padded to accommodate the zero-G
environment when the cars are parked at the stationary core.
Alignment pins would extend from the edge of the platforms on command, and
engage receiving holes on the car when it was stationary against that particular
platform. The holes would be elongated vertically, to allow slight vertical dynamic
misalignment from any eccentricity in the local track or overall assembly of the space
station main elements.
The subway system might also be used to solve a problem not yet mentioned:
maintaining pressurization of the rotating and stationary sections of the space station.
Given a space station with rotating and non-rotating components, there is a choice of
5

6. how to maintain pressurization within the habitable areas. Specifically: does the
interface between the rotating and stationary components maintain the atmospheric
pressure differential with respect to the vacuum of space (such as through the use of a
large sealing ring), or do the two components maintain their own atmospheric
pressures independent of each other? Assuming the latter, the subway system would
also perform the duty of an air-lock between the rotating and stationary components
of the space station. The cars would be independently pressurized, and the tunnels
they run in would be at vacuum. The platform doors and the subway car doors would
be built to withstand the 1 atmosphere differential, and a sealing/docking mechanism
would be built into the interface between the platforms and the subway cars.
Axis of Habitat Rotation
"Subway Car" Rotating Habitat
Sliding Doors
Possible Air-Lock (typ.)
(Not To Scale)
100m
Docking Occurs This Side
Effective Gravity Vector
To Outside of Wheel
Hallway
Alignment Pin
Hallway Carriage Wheels/Track
Linear Induction Motor
Stationary
Core Inductive Power/Data Transfer
Figure 1: Illustration of the "Subway System"
Method of Operation:
For the purpose of this discussion, a station occupant would start out in the 1G
Habitation Ring; the rotating part of the space station. Upon arriving at the “Subway
Station” area, he or she would press the call button for the car. Through the windows
in the platform door, and through windows to the left and right of the door along a
common wall, the far side of the space station can be seen moving past at about
31m/s (70mph). Every 20 seconds, the subway car—stationed at the far wall—
swings by. In 20-second vignettes, the subway car is seen to slow down from its
31m/s speed, until it has come to a full stop aligned with the nearby platform door.
The car hasn’t really slowed down, though: it has sped up from 0m/s to 31m/s in
about 30 seconds (roughly 10% G uniform tangential acceleration), until it has
synchronized with the rotating section of the station.
The alignment mechanism extends the locking pins into the receiver slots in the car.
These pins keep the car aligned to the entry door, with the tracks below moving by at
31m/s. With the locking pins engaged, a failure in the induction drive system will not
6

7. cause a danger to the passengers or equipment transferring into the car. A friction
drive failure would not be so forgiving, since it could lock the car to the rack.
An airtight seal is made between the car and the platform. The entry doors open—
first the platform doors, then the matching doors on the car itself. A slight vertical
movement is noticeable in the car. The tracks are part of the stationary core, and the
slightest eccentricity in the huge bearings joining the fixed core to the revolving ring,
or in the track itself, shows up right here—as a subtle vertical oscillation.
The passenger enters the car, the doors close, the seal is broken, the alignment pins
retract, and the car starts to pull away from the Habitat Ring Station. The passenger
feels acceleration as they start to move, but in reality they are decelerating. The
Habitat Ring Station is moving at 31m/s with respect to the stationary core; the car is
slowing down to synchronize with the core. As it slows down, the car also starts to
lose the centripetally imposed artificial gravity. In the 30 seconds it takes to go from
the Habitat Ring reference to the Utility Core reference, the passenger has also gone
from 1G to 0G. There’s been a 10% G tangential deceleration acting during that
time, but it is also gone once the movement has stopped.
The car stops, another set of locking pins engage, another airtight seal, and the
opposite doors open. The passenger floats or pushes out into the zero-gravity
environment of the Utility Core. Though the windows on the Utility Core Platform, it
is possible to see the far wall of the subway tunnel—the wall of the Habitat Ring
Platform—passing by at 31m/s.
Another design option is to place the track closer to the center of rotation. For
example, at a distance of 50m from the axis of rotation, the peak G-level is 50%, the
tangential velocity is 15.5m/s (35 mph), and the tangential acceleration/deceleration
will be 5% G for the same 30 second change in velocity. The length of track, and its
associated equipment and costs, is also reduced by 50%
Conclusion
The proposed “subway car” system, and its attendant subsystems, solve a number of
design issues presented in this paper. It allows for the exchange of a moderate
number of passengers and payload between a rotating habitat wheel and the stationary
docking core of a large-scale space station. It addresses redundancy issues by
allowing for the placement of parallel systems through the use of concentric tracks. It
addresses gravitational and logistics issues by locating the means of transfer away
from the central axis of rotation. It addresses energy balance and dynamic imbalance
issues through the use of counterweights, flywheels, and symmetric design. And it
also presents a possible solution for the maintenance of module pressurization, a
solution that does not require the use of large, dynamic pressure seals.
However, the biggest obstacle to realizing this system may not be purely technical.
The cost for such a large habitat ring, a stationary utility core, and the support
mechanisms to make it function is currently prohibitive. It can only be hoped that at
some point in the near future, economic forces would make such a large space habitat
a profitable venture, and an idea such as this might in some way be beneficial to that
habitat’s overall design.
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8. REFERENCES:
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