Clarke Station:

An Artificial Gravity Space Station at the Earth-Moon L1 Point

University of Maryland, College Park

Department of Aerospace Engineering Undergraduate Program

Matthew Ashmore, Daniel Barkmeyer, Laurie Daddino, Sarah Delorme, Dominic DePasquale, Joshua Ellithorpe, Jessica Garzon, Jacob Haddon, Emmie Helms, Raquel Jarabek, Jeffrey Jensen, Steve Keyton, Aurora Labador, Joshua Lyons, Bruce Macomber Jr., Aaron Nguyen, Larry O'Dell, Brian Ross, Cristin Sawin, Matthew Scully, Eric Simon, Kevin Stefanik, Daniel Sutermeister, Bruce Wang

Advisors: Dr. David Akin, Dr. Mary Bowden

Abstract

In order to perform deep space life sciences and artificial gravity research, a 315 metric ton space station has been designed for the L1 libration point between the Earth and the Moon. The station provides research facilities for a total of eight crew in two habitats connected to their center of rotation by 68 m trusses. A third mass is offset for stability. Solar arrays and docking facilities are contained on the axis perpendicular to rotation. A total of 320 m² of floor space at gravity levels from microgravity to 1.2g’s are available for research and experimentation. Specific research capabilities include radiation measurement and testing, human physiological adaptation measurement, and deep space manned mission simulation.

Introduction

Space is a harsh and unforgiving environment. In addition to basic life support requirements, radiation exposure, cardiovascular deconditioning, muscle atrophy, and skeletal demineralization represent major hazards associated with human travel and habitation in deep space. All of these hazards require special attention and prevention for a successful mission to Mars or a long duration return to the Moon. Greater knowledge of human physical response to the deep space environment and reduced gravity is required to develop safe prevention methods.

An artificial gravity space station would provide a facility for exploring these issues. The primary purpose of the station will be to explore the ability for humans to live and work in artificial gravity in deep space across a wide range of gravity levels up to 1.2g. In preparation for a future mission to Mars, the station will also simulate a full-length Mars mission. The simulation will acquire valuable data about the body’s adaptation to Mars gravity, and will allow astronauts to test technologies at Mars gravity. Artificial gravity also provides opportunities for life sciences and advanced technology research with application to Earth based needs.

Positioning this station at the Earth-Moon L1 point provides an ideal location for study of the deep space environment. A human presence at the L1 point, over 300,000 km from Earth, will require the closed loop life support systems and increased radiation protection common to any deep space mission. An artificial gravity station at the L1 point could also serve as a transportation node for Mars missions, providing storage, supply, and crew recuperation in artificial gravity.

In 1961, Arthur C. Clarke predicted the establishment of a space station at L1 in his book "A Fall of Moondust.” Clarke’s “2001: A Space Odyssey” portrayed yet another incredible station with artificial gravity. In tribute to Arthur Clarke’s vision and inspiration, the University of Maryland L1 habitat is named Clarke Station.

Challenges

Artificial Gravity

In 1966, astronauts Conrad and Gordon achieved a low level of artificial gravity when they tethered together the Gemini capsule to the Agena target vehicle and rotated slowly for 2 ½ orbits around the Earth. While artificial gravity production through rotation has been demonstrated on a small scale, knowledge of the ability for humans to live and work in a large scale rotating artificial gravity environment is limited. Research conducted in centrifuges on Earth has concluded that humans can adapt and live for extended periods to rotation rates as high as 8.5 RPM. To ensure that astronauts can live and work comfortably, Clarke Station will have a maximum rotation rate of 4.0 RPM. Changes in gravity level are accomplished through control of the rotation rate. Since the station generates a maximum gravity level of 1.2g, or 12m/s², and has a maximum rotation rate of 4 RPM, the radius of the station is 68.4 m. The station must also be capable of accepting a docking vehicle while it is spun up in order to dock to the station without disturbing the science missions and to reduce propellant expenses.

Floor Space

Clarke Station will support 8 crewmembers during normal operations and has the capability of supporting 16 crewmembers for short durations during crew transfers. The crew must have enough space to live and work effectively for long durations. Because Clark Station has gravity, floor space area requirements, not volume, must be considered. For long duration space flight, the minimum floor space per crewmember is 40 m². The open floor space requirement is 8 m² per crewmember. This requirement results in a station with a total floor space of 320 m².

Radiation Exposure

Space radiation consists mainly of high energy-charge particles such as protons and heavy ions. At the L1 point, shown in Figure 1, beyond the protection of the Van Allen Belts, radiation from Galactic Cosmic Radiation and Solar Particle Events threaten the health of Clarke Station inhabitants. Galactic Cosmic Radiation (GCR), originates from outside the solar system, and consists mainly of hydrogen. GCR is indirectly related to the 11-year cycle of the Sun, where its maximum is at the solar minimum. During a solar minimum, an unshielded dosage is about 60 rem/year, and a factor of 2.5 lower at solar maximum. Solar flares are explosions on the Sun that generate Solar Particle Events (SPEs), and shoot them into outer space. SPE’s occur once or twice a solar cycle. One of the largest solar flares occurred in 1972, producing a dose of 350 rem for several hours.

According to NASA requirements, maximum radiation dosage for Blood Forming Organs is 50 rem/year. Since solar flares can occur throughout the solar cycle, the worst-case scenario is a large solar flare occurring during solar minimum, when GCR is largest. This scenario requires shielding against GCR along with sufficient shielding for a solar flare.

Mission

Bone and Muscle Research

In microgravity, a significant number of bone forming cells die, and healthy bone cells produce fewer minerals. Muscle size decreases dramatically and there is a reduced capacity for muscles to burn fat for energy. Clarke Station science will determine the rate, location, and magnitude of bone and muscle loss as affected by gravity level. Changes in muscular performance as related to gravity level will be documented. Equilibrium bone/muscle levels, and the extent of bone loss reversal due to increases in gravity level will be determined. Exploring the relation between bone loss and decreasing muscle strength at other than Earth's gravity will aid in developing protocols for long duration space missions. Physical measurements and performance measurements, Dual Energy X-ray Absiorptiometry (DEXA) and ultrasound scanning will provide accurate measurements of bone structure and density.

Human Physiology Research

In addition to causing changes in bone and muscle strength, microgravity is known to cause drastic changes in the lungs and heart. Central venous blood pressure decreases, baroreflexes are impaired, and heart rate increases. There is a shift in body fluid toward the head, blood volume decreases, and red blood cell count decreases. Experiments in the cardiovascular field will help understand cardiac and circulatory hemodynamics, biochemical changes, baroreflexes, and dysrhythmias at different gravity levels. Reduced gravity environment adaptation and circadian rhythms will be analyzed and related to performance. Immunology research will focus on the ability for astronauts to respond to and recall antigens at different gravity levels. Neurotransmitter and overall neurosensory changes in response to a change in gravity remains incomplete. Experiments designed in the field of neuroscience will aim to understand space motion sickness, how sensory motor skills are affected, and rotating environment effects on the neurovestibular system.

Radiation Science

Radiation science experiments will provide accurate radiation monitoring and measurements to assess and reduce health risks of the crew as well as chart the radiation environment of deep space. Dosimetric Mapping will provide a quantitative description of the radiation field inside and outside Clarke Station. Active dosimeters will measure localization of charged particles and the energy spectrum of radiation, and the crew will wear passive dosimeters to measure absorbed dose. Outside the station, the Phantom Torso, a torso and head constructed from a muscle-tissue plastic equivalent with over 350 passive dosimeters embedded in it, will be used to measure organ level radiation doses. The Bonner Ball Neutron Detector (BBND) uses six detector spheres filled with He₃ to determine neutron radiation effects. Results from these experiments will provide more accurate and reliable radiation prediction models for future missions.

Mars Simulation Science

Mars simulation missions will allow for valuable experimentation and learning in preparation for a future mission to Mars. Physiological changes resulting from long-term exposure to Mars gravity will be documented. Communication time delays that would occur on a Mars mission, of 21 minutes maximum length, will be simulated. Astronauts will utilize the Range, an open area of approximately 10 m², for Mars suit mobility testing, structure building, and interaction with autonomous robots. To prove the ability to grow plants for consumption at Mars gravity, as necessitated in the Mars Reference Mission, three plant growth modules totaling 3 m² of growth area will be on Clarke Station. These plants will also be analyzed on the cellular level in the biology lab. Completion of the full-length Mars simulation in 2012 will allow time to integrate the lessons learned from the simulation into a Mars mission design for the opportune window of 2016-2018 when travel durations will be as short as 130 days.

Advanced Technology – Future Research

After the full-length Mars simulation, Clarke Station will transition to a life sciences and advanced technologies station. Biotechnology, Microbiology, Materials Engineering, Reproduction and Development, Lunar research, Electrical Engineering, and Exobiology research will further help scientistics understand the human response to the space environment, the composition of the solar system, and lead to important medical and technological discoveries that have benefits on Earth.

Gravity Level Timeline

Table 1. Gravity Level Timeline
Year	Gravity Level	Duration (months)
Year 1 (2007)	Lunar (.17g)	2
	Mars (.38g)	2
	½ Earth	2
	¾ Earth	2
	Earth	2
	Maximum	2
	Crew Change
Year 2 (2008)	½ Earth	3
	Mars (.38g)	6
	½ Earth	3
	Crew Change
Year 3 (2009)	Microgravity	3
	Mars (.38g)	6
	Microgravity	3
	Crew Change
January 2010 – July 2012	TBD	5
	Mars (.38g)	21
	TBD	4
	Crew Change
July 2012	TBD	TBD

Table 1 shows the station gravity levels for the first six years beginning with initial station operation in January 2007. Crew rotations occur once a year for the first three years. Gravity level step increases are conducted the first year to study adaptation and living abilities of astronauts at various gravity levels. The second year is a short-term Mars mission simulation. This short-term simulation assumes the astronauts will have a ½g artificial gravity transfer vehicle. The third year is devoted to another short-term Mars mission simulation. Mars transfer in this simulation is in microgravity. A comparative study of the second and third years will give scientists valuable insight into the transportation needs for a Mars mission. Following the short-term Mars mission simulations is a full-length Mars mission simulation. The gravity level for the first 5 and last 4 months of the full-length Mars simulation will be decided based on information gathered over the initial three years and Mars mission plans in 2010. The durations for the full-length Mars simulation match the durations of the long stay fast transit mission outlined in the Mars Reference Mission. Completion of the full-length Mars simulation in 2012 will allow time to integrate the lessons learned from the simulation into a Mars mission design for the opportune window of 2016-2018 when travel durations will be as short as 130 days. The station gravity levels following the full-length Mars will be selected based on experience gained from the critical six-year period and to accommodate research needs.

Systems Design

General Configuration

The crew and equipment for conducting these experiments is distributed into two manned habitats at equal distances from their center of rotation. To allow docking while spinning, a non-spinning truss was placed on the axis through the center of rotation, perpendicular to the plane of rotation (Fig. 2). The station fixed coordinate system used to describe the location of station components uses the spin axis as the z axis. The z truss serves two purposes: to eliminate the relative rotational motion of the rotating section from the docking procedure and to serve as a sun-tracking axis to accommodate solar array pointing with minimal support structure mass. Thus, the z truss will rotate at a rate of approximately 1°/day with respect to an inertial frame. Stiff trusses were chosen in order to adequately transmit torques required during station keeping and docking.

In order to maintain a stable spin situation, the rotating section of the station must have its center of gravity at the center of rotation and the station must be spinning about its minor or major principal axis. Modeling the station as a gyrostat, a dual-spin system with an axis-symmetric z truss, showed that having two collinear masses (habitats, labs, or other mass) spinning about the z truss is unstable because the spin axis would then be the intermediate principal axis. Therefore, three spinning masses were required to maintain spin stability.

Using expended transfer vehicle boosters for the third mass minimizes the expense of delivering additional mass to L1 while providing for station stability. Based on the assembly and delivery schedule, 5 expended boosters with 3 tons of inert mass each will arrive at Clarke Station. Because this mass totals only 15 tons compared to the 42-ton habitats, the habitat trusses must be at an angle of 160° from one another, and the boosters at an equal distance of 68.4 m from the center of rotation. By making use of this excess mass, only about 1½ tons of extra truss will be needed to connect the boosters to the rotational center.

The Z-truss

The z truss is actually two separate free-spinning trusses, the +z truss and the –z truss, which are attached to each side of the rotating section perpendicular to the plane of rotation. The –z truss rotates with the habitats the entire way to the docking system and is sun-tracking from the docking collar to the –z end of the station. The attachment points will have rotational interfaces as described below. Although only the +z truss contains the solar arrays, both sections will track the sun to maintain alignment of the reaction control thrusters, which are housed at the ends of the z trusses.

The angular momentum of the rotating section will be on the order of 10⁸ kg-m²/s. The reaction control thrusters are placed 30 m from the rotating plane on the + and - z trusses in order to produce a sufficient torque to adjust this angular momentum. The transfer vehicle docking system was placed 25 m away from the rotational section to reduce plume impingement on the structures from the transfer vehicle thrusters.

Depending on whether one, two, or no transfer vehicles are docked to the station, the center of mass and moment of inertia will change. The moment of inertia for the station was calculated using the coordinate system shown in Table 2, which depicts the center of mass and moment of inertia as a function of the number of docked transfer vehicles.

Table 2. Station Center of Mass and Moment of Inertia

Center of Mass (m) as a function of Transfer Vehicles Docked	Moment if Inertia
No x-fer vehicle docked: (0, 0, -4.6)	Ix = 3.9 X 10^8
1 x-fer vehicle docked: (0, <<1, -9.2)	Iy = 0.84 X 10^8
2 x-fer vehicles docked: (0, 0, -13.8)	Iz = 4.6 X 10^8

Habitat Modules

An inflatable structure was chosen for the habitats because of its low weight, small packaging volume, strength in terms of pressure, ability to withstand impact of micrometeoroid debris and better radiation shielding as compared to conventional modules. The inflatable habitat is mounted longitudinally to the truss and has the inflated dimensions of 5.4 m radius, 7.6 m length, and 0.3 m wall thickness. The habitat interior consists of two floors with 2.5 m ceilings and 1m storage space located above the upper ceiling and below the lower floor. The floors are connected by a 3.5 m diameter core. The habitats were designed to accommodate crewmembers from the 5^th percentile Japanese female to the 95^th percentile American male.

The habitat’s internal pressure creates both longitudinal and transverse pressurization loads on the habitat wall. In addition to the longitudinal pressure loads, the habitat also sees longitudinal loads due to centripetal acceleration. The habitat shell consists of multiple layers of woven Kevlar that are responsible for the module shape, loads, and protection from micrometeoroid debris. The micrometeoroid protection is made up of alternating layers of woven Kevlar and polyethylene foam. Inside those layers are bladders made up of viton to hold water for radiation shielding. The innermost layer is Nomex cloth protecting the viton bladders from scuffs and scratches. This design has a safety factor of 3 and a margin of safety of 1% for transverse stress and 2.7% for longitudinal stress. The total mass of the module is 42,000 kg, which consists of 15,000 kg empty mass, 23,000 kg radiation shielding mass, and 4,000 kg of equipment.

Truss Structure

The truss is the main structural backbone of Clarke Station. It is separated into three Rotating Truss (RT) spokes and two Z-Truss (ZT) elements (positive and negative). The truss provides a pass-through for the transfer tunnel and hard mounts for attached payloads. The RT passes around the hub module by means of a spoke interconnect structure, thereby decoupling the hub from reacting station bending and axial loads.

Both the RT and the ZT are 6m box trusses having four tubular main spars of outer diameter 250 mm and cross-members of 130 mm diameter. The main spars are two concentric tubes of a 1.5mm thick composite laminate. The laminate is Toray M55J/Fiberite 934-3 carbon/cyanate ester in a [90/±30/±15/0]_s symmetric fiber orientation. The tightly woven plies offer superior micrometeoroid impact resistance and superior corrosion resistance. Furthermore, the laminate possesses ultra-high dimensional stability under thermal cycling.

The truss is weakest in its resistance to buckling. Bending in the RT due to angular rate adjustment thrusting loads truss members in compression and causes lowest M.S. on buckling. The truss design was driven both by resistance to buckling and resistance to natural frequency excitation in bending of the RT spokes. Longitudinal and Bending Natural Frequencies were calculated for both the RT and ZT spokes and are tabulated in Table 3. For the operational load environment, margins of safety are presented in Table 4 below.

Table 3. RT and ZT Bending Natural Frequencies

Natural Frequencies	RT Spoke, Habitat at End	RT Spoke, Offset Mass at End	+Z-Truss	-Z-Truss, 2 Transfer Vehicles Docked
Longitudinal, Hz	4.5	7.5	46.8	5.9
Bending, Hz	0.37	0.63	10.5	1.3

Table 4. Truss Margins of Safety

Transfer Tunnel

The transfer tunnel provides crew passage between the habitats and docking areas. It consists of four major parts: an inflatable tunnel, consisting of eight layers of material that are similar to the layers of the habitat module but without water filled bladders for radiation shielding; aluminum stiffening rings; Kevlar stringers attaching the tunnels to the trusses; and aluminum lockout doors located at every 10 m of the tunnel to maintain pressurization of the tunnel in the event of a breach in one section of the tunnel wall (Fig. 5).

Transfer through the tunnel will be by use of ladders or a winch mechanism. Two 10 m ladders will be placed along either side of the tunnel wall in each 10m section of the tunnel. The winch is a 12 VDC planetary gear winch for carrying loads and crewmembers up and down the tunnel.

The major loads on the transfer tunnel, given in Table 5, are the force of the lockout doors on the walls from centripetal acceleration, the pressure loading on the tunnel walls, the stress on a closed lockout door due to pressurization, the stress in the Kevlar stringers due to torsion in the truss, and the maximum stress on the aluminum stiffening rings.

Table 5. Transfer Tunnel Margins of Safety

Load Considered	Applied Stress (MPa)	M.S.
Force of Lockout doors on walls from centripetal acceleration	54.0	21.23
Transfer Tube Pressure Load, Hoop stress walls @ 101 kPa	108	10.13
Stress on Lockout door from pressurization @ 101 kPa	31.9	Figure 5. Transfer Tunnel 6.91
Maximum Forces on Kevlar Stringers	63.4	0.39
Maximum Forces on Stiffening Rings	121	1.09

Rotational Interface

Figure 3

Rotational interfaces are located on the positive and negative despun trusses to allow these sections to rotate independently of the rotating section. The –z interface has three modes of transmission: electrical, consisting of both power and data; biological, or life support and human passage; and structural resistance to moments created by spinning up and down the rotational section, thrusting for attitude and station keeping, and docking the transfer vehicle. The +z interface will need to handle only power, data, and structural loading. The –z interface is also the junction between the –z tunnel and Airlock-Docking System (ADS), which will be separated by an airlock that can be opened during transfer times. Therefore, the ADS will have a separate atmosphere control that will also be used for EVA pre-breathe.

Main loading on the rotational interfaces stems from either impact with the transfer vehicle or from thruster firing. Forces from docking are about 2250 N on a 1 m moment arm on the –z truss, and the thrusters fire at about 500 N on a 30 m moment arm on the +z truss. Thus, the greatest loading on the rotational interfaces comes from the thrusters on the +z section. Using this information, the thickness of each bearing collar and its flanges must be at least 0.045 m of aluminum. Stainless steel shims are used inside on contact surfaces to minimize the coefficient of friction.

Each rotational joint will also have a bearing assembly to overcome friction losses on the rotating interface and a vacuum seal to separate the internal atmosphere and the outside vacuum of space. To maintain constant relative angular velocity, the interfaces will also contain two redundant constant-spin motors.

Hub

The station hub serves as a storage center as well as a pass-through from the two spokes and the –z truss (Fig. 6). The hub shell is designed to handle only pressurization loads. To handle a maximum internal pressure of 101 kPa, the total thickness of inflatable material is 0.024 m. Since the hub and the transfer tunnel share similar functions and loading environments, their inflatable weaves are identical. Accounting for attachment points, the hub final dimensions are 7.0 m in diameter and 5.5 m high with an interior volume of 210 m³.

Airlocks

The station is designed to handle 2 person EVA’s on a daily basis. Most EVA’s would be for upkeep and repair of the station. In order to facilitate ease of mobility and safety about the trusses, an airlock is placed next to each habitat and one by the docking collar. The airlocks on the rotating section needed to accommodate two astronauts and their EMU’s, so the dimensions of the chambers are 4 m diameter and 2.5 m high. An access tunnel allows the astronaut