Rotating Space Station Proposal

By Roy Harvie
July 2018

This is a proposal for a rotating wheel style space satation or space ship that provides a microgravity working environment in the hub and normal 1G living environment in the rim. The design is using existing or near-term technology with the expectation that it could be built in the next 10 years if desired. Although I say 'could be', I don't really expect such a station to be built so soon. The main premise that I have assumed here, that living in a 1g environment 50% of the time will solve adverse effects of full time weightlessness and also transiting back and forth each day is not disorienting. This is a safe assumption, but it has to be proven, so a small version needs to be built for testing this premise, and this small version could also be spun at different speeds to test health effects for long term living at Moon or Mars gravity.

Since the 1970s we have been expecting such a space station to be built but the only proposals I have seen have been grandiose sci-fi types like in the movies 2001 and Elesium. All big monolithic structures. In order to be practical it has to be modular so piecess can be built on earth and launched with rockets and assembled in space with the fewest operations possible. This shows how a modular design looks like. The consensus from several studies is that the rotation speed must be 2 rpm or slower to prevent dizziness by the occupants in the rotating section. Actually my information may be oudated as newer studies now suggest that 4 to 15 rpm is "tolerable". This design is for 2 rpm. This requires the rim to be about 225 meters from the hub. I believe it would be easier and cheaper to support the rim by kevlar Hoytether cables than solid tubes or girders. A network of cross-connected cables similar to spokes on a bicycle wheel would provide stable positioning keeping the hub and rim ridgedly alligned. This design is for about 240 people and larger future designs would probably be 1 rpm or less with at least 890m radius.

Some people have pointed out that this style of station will not be ideal for microgravity research or manufacturing because of vibrations caused by people, equipment movement and visiting spacecraft docking. They are absolutely right. I feel sure that there are many situations where abslolute vibration free environment is required and also many where merely low gravitational forces are sufficient. No station with humans in it will be vibration free and for those situations that require an abolutely vibration free environment, I believe the best solution is fully automated free flyers, such as the proposed Dragon Lab missions. There could be some advantages to operating these out of a large station, in that they will not have to endure the rigours of Earth landing before being examined and evaluated. Also significant cost savings on re-using these free flyers if they just have to berth and deploy from a space station instead of return to Earth to be unloaded, reloaded and re-launched.

I have shown a completely assembled station with 56 modules in the rim, and for cost reasons it could be built with incomplete ring. However incremental build will be much more expensive in the long run. If a manufacturer has an order for 56 modules then they order parts for 56 units and set up a minimal assembly line. If they get an order for 4 units, then two years later another 4 etc. parts procurement, paper work, shipments could be 14 times higher, and hand assembly would be used instead of assembly line approach and savings. It would also be difficult, but not impossible to add units to the rim while it is spinning. Some say "just stop the spinning, do the work, and re-spin". Not so easy after people have moved in, and all movable items have to be locked down, like beds, blankets, pillows, dishes. Conventional cooking, showers, toilets etc. would be inoperatable until work was done etc.

Unfortulately much of this design still has to be fabricated in space, namely the large wheels and attached structure for supporting the elevators. This station would require about 100 rocket flights, most of which would require large heavy lift rockets like Starship rocket from SpaceX. This new rocket is designed to be re-useable and promise significant reduction in cost to launch space hardware. It took about 40 flights to assemble the ISS and I feel confident that with new comericial rockets and comericial manufacturers such as Bigelow Aerospace and Tethers Unlimited the cost of this station should be close to and hopefully even less than the cost of the ISS despite it being much bigger and supporting many times the population.

Illustration 1: Non-Rotating Hub with Rotating Tether Attachment and Elevator Cradles

The central hub does not rotate with the outer ring. This hub would be mainly made up of Bigelow 2100 modules and their design can be seen at the WikipediA web site. One would be modified with docking ports on opposite sides for the elevator modules. A pair of robotic arms, one for each port, are used to move the elevator module from the port to the cable system to go "down" to the outer ring modules. Two large rings at the ends of this group of 3 modules have magnetic bearings to allow them to freely rotate and also be equiped with an inductive transformer system to transfer power. The picture shows 2 disks in gray with the inner one attached to the hub and the outer one free to rotate. These large rings have mounting points for the cables to suspend the outer rotating ring of modules and framework for top end of elevator lift system. Attached to some of these cables would be power cables to deliver electricity from the central modules to the rim modules. The central hub would provide docking ports for visiting spacecraft, support solar panels for power generation and provide work space for all micro-gravity activities such as research in a microgravity environment, manufacturing, weightless entertainment area, and possibly other activities such as Hollywood style film studio for spacey sci-fi movies. This space station design allows people to work in a microgravity environment, but go "home" to a normal 1G enviroment for eating, sleeping etc. The expectation is that everyone would spend at least half of their day in a 1G environment and should not have to suffer adverse effects of continuous weightless environment.

Basic premis of this design is to provide a continuous path around the entire ring so push carts and hand dolleys can be used to easily move items from one module to another. Service elevators (not shown) at the center of each module would be used to move large or heavy items to upper or lower floors. Lower service accessible by removing floor plates on main level and a section of the upper floor could be designed to lower down to main floor. These service elevators would not be conventional in nature (having no walls) but would be rarely used, and all but invisible in stowed condition. For safety reasons this service elevator would require two people to operate with up/down buttons on both levels and a person at each level holding the appropiate button in to move the elevator. Only with both buttons pressed does the elevator move and the two operators can warn others of possible danger. These service elevators would be rarely used and people would use the staircase to go to upper or lower levels.

The rim modules are based on the Bigelow designs, but structually different as they are intended to support a 1g environment. I call them BA1400 to give an indication of size. These are 18m long and 10.6m id and 12m outside diameter. These modules would be built with floors, walls and as much support equipment for ECLSS, water, oil balast, pumps, plumbing, electrical etc., all pre-installed on Earth before being launched. This means that the modules must be pre-expanded, not launched in compressed format like the Bigelow BA330 and BA2100 designs. These will require new larger fairings for Starship rockets, bigger than the current upper payload area of Starship, but I believe this is just an engineering exercise and these modules should not exceed the mass capability of the rockets.

Illustration 2: "Office Module, Offices on left, common area right. Staterooms above.

Each module has an air lock at both ends which makes construction of incomplete ring versions easy and allows a damaged module to be sealed off from the rest.

Lower utility level is for electrical, plumbing (sewage & fresh water), ECLSS, HVAC etc. and large tanks for oil balast. All modules would be linked with pipes and pumps for oil transfer including diametrically across the main ring at the elevator positions. This would be an automated balancing system pumping oil from side to side as it detected mass change by people and items arriving via elevator or moving from module to module. As mass is moved down one elevator from the central hub, a compensating mass of oil would be pumped to the other side of the ring. To keep the imbalance to a minimum, the elevators to opposite sides of the ring would always operate synchronously being both at central hub or at the rotating ring at the same time. If there are passengers/load only in one, the other operates anyway.

Illustration 3: Upper Level Stateroom Layout

There are two versions of the rotating ring modules, one with an upper floor for state rooms, and the other without. The version without the upper floor are agricultural modules and the intent is that there would be many more agricultural modules than living/working modules. I believe a major goal of any large rotating space station/ship is to become completely self-sustaining. Starting with growing own food, chickens (for eating and eggs), fish, even orange and apple trees. Too much of a stretch for cattle, so beef/milk/cheese/ham would be an imported luxury. The goal of self sustainability is in addition to the goal of providing work space for research and manufacturing. Growing its own food (and re-cycling CO2 to O2) will be a required function of any Moon or Mars based colony, perfecting this capability close to Earth where upgrades and compensation for short fall are relatively easy is IMHO the best way to proceed before attempts at colonization. An additional benefit is reduced shipment of food to the station. Based on a full ring of 54 modules, 16 to 24 living modules would give 38 or 30 agricultural modules. The staterooms on the upper level are small as I worked to cram in as many as possible, but they have queen sized beds, full bathroom, desk for makeup or computer, dresser, and hanging closet. The end unit has two bedrooms with the intent that the larger is a master bedroom and the other suitable for a child. Based on 10 staterooms per living module and most being single occupancy, so assume 15 people per module * 16 = 240 people. 160 staterooms total. Larger, more comfortable living quarters could of course be designed, this design just shows maximum density possible. I think it is important to note that this spacestation design is intended for people to stay long term and with their family. For childless couples, the small stateroom would be doable. I don't see a system working where a husband or wife lives on a space station and the other on earth.

Illustration 4: Cafeteria Module with stairs to staterooms.

As drawn there are two living groups of modules opposite each other, that is across the diameter of the ring from each other. The remaining modules are agricultural modules. There is one cafeteria module for each side. This module has the main floor devoted to the kitchen and eating area, roughly 50% each. The kitchen is divided into two parts with the center aisle between.

Illustration 5: Agricultural Module with stairs to lower Utility Level.

To travel from the hub to rim and back, I propose an elevator system that is like a small space ship or capsule in that it is designed to work outside in the space environment. This design not only provides the transportation but also solves the problem of how to transfer from a non-rotating hub to the rotating rim. The elevator modules have complete autonomy in the sense that they have ECLSS, small solar panels and a back room normally locked, but containing emergency equipment including a toilet, small external air lock, tools, etc. The modules are driven up and down by two cables on each side of the module. These Hoytether cables are flat and designed to withstand dammage from small debris and still be functional. The cables are edge on to the module and are aligned with the center of mass (front to back). These cables form a long belt that goes around drive wheels located at the central hub end, spring loaded to control tension and idle wheels at the rim end. The motor and drive and belt drive wheels are mounted within a cage that is part of the rotating structure attached to the central hub. This structure also has the attachment point for the support cables to the rim. When the elevator gets to the "top", i.e. within the rotating cage, it is moved from the support rack attached to the cables and placed in a cradle for easy access by the robotic transfer arm mounted on the hub. This second robotic arm moves the module from the cradle to the port in the non-rotating hub. The robotic arm will have only about 10 seconds to grab the module, then move the module sufficently inside the cage to avoid collision with any part of the rotating cage. Not much time but with proper placement of the cradle, very little distance required. The robotic arm will have to be strong and powerful, not the light-weight style of the ISS CanadaArm.

Illustration 6: Elevator docked at Rim.

There are 4 elevators spaced equally around the rim. Two are centered in the living modules and the other 2 at the agricultural modules. I propose a simple flat-surface mating of the ports (with gaskets) and the airlock assemblies will be pressed tightly together with hydraulicaly actuated cam locks. Advantages are that loss of hydraulic power will not matter after locked in position due to the cam design. This allows minimal air space between the two air lock doors. This space will be filled with air and pressure verified to be stable before the doors are opened. I think this process should take less than a minute. The small space also means that minimal air will be lost when dis-engaged.

Some people have expressed the idea of a “tunnel”, a tube going from the hub to the rim pressurized with the elevator inside. Their argument is that somehow this would be safer and avoid the need for air locks and docking. When I asked how the joint to the non-rotating hub works, they said simple, just like the shoulder joint on the space suits. Creating an airtight joint about 8” dia. that has to operate reliably for hours with intermittant speeds up to 10cm/sec is a major accomplishment. To scale this up to about 6m dia, 60cm/sec constantly for years in a configuration that is almost impossible to service, I think is too much to expect. I should explain here that I belive that any large space station should be designed for a minum of 100 year lifespan. If we are aiming for permanent habitation of space, future designs should be capable of 1000 years operation with proper maintenance. Also the tube would provide a larger target for space debris than the elevator, so in this case I argue that the elevator is safer.

The space station could be placed in an orbit just above the current ISS orbit and well below the Van Allen radiation belt. This orbit will have slightly lower drag than the ISS but unfortulately higher orbits get farther into the range of most debris orbiting the Earth. The ISS orbit is 400km, and the main mass of debris is between 600km and 1100km altitude. Much higher orbits, beyond the debris and Van Alan Belt may also be desirable in terms of being a better jumping off point for deep space operations. In order to not have the Space station wobble due to coriolis forces, the orbit must be on the Earth - Moon - Sun plane and the axis perpendicular to this plane i.e. parallel to the surface of Earth and perpendicular to direction of orbit. As the Moon orbiting the Earth is not exactly the same plane as the Earth orbiting the Sun there will have to be some compromise here, but the end result will be very little wobble. Another solution to preventing coriolis forces causing wobble has been proposed by some, and this is to have a counter-rotating flywheel of equal rotational momentum to the rim or in some versions, two living rings counter-rotating. I want to head this off right away. In terms of solid objects, this works, but any loose objects inside the rim, such as people, would still be subjected to the coriolis effect and be thrown against the walls. This is not a solution. I should point out that wobbling would not be felt by people living in the space station, but would make docking by visiting spacecraft more difficult. So the reason for choosing this orbit is to make docking easy.

Illustration 7: Debris Distribution

Visiting space ships would dock at the non-rotating hub in much the same manner as currently done at the ISS. The round ports at the end opposite to the solar panels represent docking ports. I suggest small ion-drive rockets mounted on the rim for spin-up and to maintain spin lost to air drag. Also ion-drive rockets on the hub to compensate for air drag and also to maneuver to different orbits, or possibly even for inter-planetary travel. Momentum compensating flywheels would be used to cancel angular momentum from changing solar panels as they are oriented to point at the sun, and for any other situation where the average momentum change over a year or less is expected to be about zero.

A space station this large or larger can support a large enough comunity to include such things as a nursing station staffed with a doctor, cooks to prepare food, farmers, maintenance workers etc. I also think that this design would be suitable for interplanetary transport, with continuous ion drive, even small rockets can efficiently create velocities for Earth to Mars in a few months. Of course there would be acceleration for the first half of the trip and decelleration for the second half. The low acceleration would not disturb the people in the rim. This could then be a mother ship with small rockets capable of ferrying people from this station in Mars orbit to/from the surface. Mars exploration could then take place with facilities such as medical care available in orbit, not months away on Earth. Self-sustainability and long term reliability would already have been proven out while in Earth orbit.

People such as Jeff Bezos are keen to see humanity expand to space and are working to promote an economy such as asteroid mining and microgravity manufacturing. To provide economic viability to this concept, I believe a rotating space station such as described is a necessary part of this dream.

Contact: rotatingspacestation@gmail.com