A new, rotating, space station
Space.com’s publication of the Russian company Energia’s patent for a space station capable of creating artificial gravity through rotation, prompted a post on LinkedIn by Philippe Coué and a comment from Paul Titze. This induced me to work further on this concept. I present hereunder what I think about it (calculations performed with the help of claude.ai).
A rotating space station in Earth orbit with volumes offering different levels of gravity would, contrary to what Paul Titze writes, offer several advantages: The people living on board would not suffer from the harmful effects of weightlessness. They could still work at the center of gravity in a volume where they could study these effects. They could also study the comparative effects of different levels of reduced gravity by moving further or closer to the center of gravity. The station could also be used as a stopover for people coming from deep space (Mars, for instance) to allow them to recover their ability to live in Earth’s gravity and also to be isolated for treatment in case they would be carrying infections that nobody would want to transmit to people on Earth.
But what should the characteristics of this station be? In other words, is the Energia project optimal, or can we design something more interesting? Let’s first look at the constraints:
1) The gravity generated by rotation must be minimal. It’s difficult to say what is medically necessary over time, because unfortunately, we have serious experience only about weightlessness (besides very short stays on the Moon, of course!). However, we observed through short missions, that lunar gravity (0.16g) makes astronauts clumsy. A stay in a habitat subjected to this gravity could therefore be used to prepare for longer missions (settlement?) on the Moon. Similarly, subjecting astronauts to Martian gravity (0.38g) could be useful before going to live on Mars and at the very beginning of the exploration of this planet, to observe any potentially negative effects of such specific low gravity. Finally, testing 0.50g could be useful in preparing Jeff Bezos’s plan to build giant stations located at Earth’s L4 or L5 Lagrange points, where thousands of people could live, as envisioned by Gerard O’Neill in the 1970s.
2) The head-to-feet gravity differential must be minimum. This differential results from the distance from the center of gravity, but it is all the more felt as the rotation speed and the artificial gravity are high in absolute value. From a medical standpoint, it is imperative that for a distance to the center of about 100 meters, the differential be certainly less than 10% and preferably less than 5%, with a 2% gradient being the target.
3) Likewise, the Coriolis force is more pronounced as the rotation speed increases. This force is just disorienting, but anyway very disruptive to our instinctive behavior.
4) An optimal size for the station. A station several kilometers in diameter, like Gerard O’Neill’s « Island 3, » is not realistic for our time. Conversely, the Russian station for which Energia obtained a patent, which rotates at 5 rpm and whose distance from the habitat to the center is only 40 meters, would not be habitable (head-to-feet gradient would be too high and Coriolis force too strong).
5) We must plan for some « engines » to maintain the station in the correct attitude, rotation speed, and altitude, because we will be going to and from the station, and the Earth, with its gravitational pull and atmosphere, will remain close. These engines will be small tangential thrusters for attitude and rotation, and a more powerful thruster for altitude correction.
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Suggestions for the structure of the ideal station:
1) On the periphery of the center, in concentric circles:
Set a living volume (a torus) at a distance of 112 meters from the center of gravity. At this distance and at a rotation speed of 2 revolutions per minutes (rpm), gravity will be 0.50g (as in the Energia station) and the feet-to-head gradient for a 180cm tall person shall be 1.6%. This will be dimensioning for the other horizontal elements.
Set a second torus at 85 meters so that its internal gravity will be 0.38g (the head-to-feet gradient will be 1.7% gravity) and a third torus at 36 meters so that its internal gravity will be 0.16g (the head-to-feet gradient will be 4%, but it’s better not to move the first torus further than 112 meters from the center).
The tori would be held from the center of gravity by metal tubes. There could be four of them, separated by 90° angles.
2) At the center, on a linear axis orthogonal to the plane defined by the tori, would be the following vertical modules:
A ‘crossroads’ globe serving as a passageway in all directions, both via the aforementioned tubes and through the line of modules forming this central axis. This globe could have a diameter of 4 meters.
Above it, in order, a cylindrical module for studying the behavior of masses and various mechanical and chemical reactions in a weightless environment (four meters in diameter, 8 meters long); a spherical ‘cupola’-type module with peripheral portholes (four meters in diameter instead of two); a 4m x 6m cylindrical module for the station’s computer system and data center; a 4m x 6m cylindrical module for telecommunications; and two telecommunications antennas.
Below it, in order, a 4m x 6m cylindrical module for various storage needs. A docking sphere 6 meters in diameter with three equally spaced access points (below the plane parallel to that of the tori to limit the risk of spacecraft snagging on them); a cylindrical waste disposal module, 4m x 4m; a propellant (fuel and oxidizer) tank module, 4 meters in diameter and 12 meters long; and a propulsion module (engines and nozzles), 4m x 4m. Note: the hemisphere where the docking ports would be located should be mobile to facilitate spacecraft approach and contact.
The whole station will be pressurized at 0.5 bar (and not 1 bar as in the ISS currently) in order to reduce stress on the module walls. This pressure should be tolerable for the crew. In return, the oxygen level should be doubled (42% instead of 21%). Of course, the docking module could be depressurized, if necessary, as it would include exits that would necessarily be open during a rocket launch or landing (or during an EVA for inspection or work outside the station). It would therefore be closable via airlock doors. The high oxygen level will necessitate specific adaptations to the risk of fire (fire-resistant coatings, monitoring of metal tool friction, etc.). Throughout the station, there will be fire doors that can be closed in case of depressurization, a fire in any component, or the need for sanitary isolation.
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Habitats:
The ‘lunar’ and ‘Martian’ tori will consist of a series of 3-meter diameter steel modules. These will serve as experimental areas and, secondarily, as passageways. The study will focus on observing human behavior, as well as the fluidity of gases and liquids in order to regulate them, and the practice of life support. Indeed, in reduced gravity, hot air doesn’t rise naturally, and CO₂ can accumulate in dangerous « pockets. » Therefore, appropriate ventilation is necessary. It’s not essential at 0.5g, but it must become increasingly powerful as gravity decreases. In the 0.38g torus, food cultivation experiments can also be conducted, as it will be necessary to rely on Martian planetary resources for sustenance when living there. The effect of Martian gravity on the recycling of organic waste will also need to be studied (and adapted to work effectively!). In the 0.16g torus, these capabilities will be less critical because we know that growing crops on the Moon would be very difficult, and we can always obtain supplies from Earth (due to its proximity and the possibility of rapid access year-round).
The third torus will also serve as a workspace, but primarily as a passageway, connecting a series of modules and forming another torus outwards from the third one. The station will consist of Bigelow modules, alternating between habitat modules with sanitary facilities and functional modules. Access to all these modules will be via airlocks built into the wall of the third torus (and, of course, the only one within the module in question). The functional modules will support life (regeneration of the atmosphere, water, organic matter, various recyclable chemicals, and food production). The food produced will be plants (hydroponics), fish and shrimp (tanks), and lab-grown meat (cell-based). Portholes overlooking the heart of the station could be installed at regular intervals (approximately every 40 meters). This would allow for visual monitoring of the rest of the station and mitigate the feeling of confinement.
The tori, like the connecting tubes, will all have a diameter of 3 meters. This diameter is necessary because they will be lined internally with thermal insulation (fireproof) and will have a floor and a ceiling. Pipes will run beneath this floor to supply clean water and breathable gases throughout the station and to remove wastewater and non-breathable gases (CO2, CH4). Fluid circulation will be accelerated by pumps (especially in the low-gravity tori and tubes). Heating and electricity will be circulated in the ceilings. Radiation protection will be contained within a coating on the exterior of all modules (except the docking module), including the connecting tubes. The aim is to limit the effect of impacts on the metal of the modules (and potentially from micrometeorites or small pieces of space debris) and to avoid reducing the habitable space inside. Between the lunar and Martian tori and the tubes, arrays of solar panels will be installed with dual exposure, facing upwards and downwards (in orbit around the Earth, sunlight from below will not be zero at certain times during the orbit, especially since the Station will be inclined relative to the Earth’s surface). These panels will be orientable so that the angle of solar radiation is always as close as possible to perpendicular to the panels. There will be plenty of space available, i.e. 18.600 sq m2. With 400W/m2 available on the upper face of the panels and 50 to 100 W/m2 on their lower face, we could get 8,4 MW, whereas we would need only 0.75 to 1.00 MW. This means that we could use only part of the surface or accept a lower performance of the panels.
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The entire station will orbit between 480 km and 520 km above the Earth’s surface and around the equator. This altitude, higher than that of the ISS (414 to 419 km) and on a constant orbit, unlike that of the ISS, is chosen in order to avoid being too close to the Van Allen Belts, and in order to be as high as possible above the Earth’s atmosphere to minimize the effects of Earth’s gravity and atmospheric drag.
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The data considered above (dimensions, speeds) are the minimum required to achieve a gravity suitable for human life, and to reduce the head-to-toe gradient and the effects of the Coriolis force to an equally acceptable minimum. However, this will still result in a very large station. With a radius of 112 meters, the circumference of the third torus will be 703 meters long (the ISS at its longest does not exceed 110 meters). At the end of construction, the station will be able to house and support around thirty people (compared to only seven on the ISS). Regarding mass and transportation for the construction, I got the following figures from claude.ai: total mass 2575 tons, construction in 2.5 to 3.5 years with 20 to 25 Starship payloads. Note: These figures are only given to provide an idea of what the reality could be.
That said, this station would be feasible with today’s technologies, since I assume that Starships could be used to transport equipment into orbit and that construction would be carried out by robots, including humanoids assisting human crews. I do not underestimate the difficulties of assembly in space, but that should be the subject of another article.
Of course, the investment would be more expensive than that required to build the Energia station or even that already invested in the ISS. To put things in perspective, it’s worth remembering that the ISS cost $150 billion. It would be rash to give a figure today for the new station. We can only hope it won’t be more than double. This isn’t impossible if we use starships and robots. Note: the starship will have the advantage of a huge transport capacity and, hopefully, reduced transport cost due to economies of scale (compared to the rockets used for the ISS). In any case, it will be ‘expensive,’ but in the field of research, what’s the point of spending money on something with expected mediocre performance when we can do better technically?
Illustration by claude.ai. Note that the length of the vertical axis is not to scale with the distance of the tori from the core.
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Adjustment (june 10th):
I’ve been thinking about the diameter of the cylindrical modules (toroids and tubes) and I believe it would be better to increase them from 3 m to 3.6 m. The difference would allow for greater comfort and more possibilities for activities. Given the importance of the piping, thermal insulation, and fireproofing coating, this wouldn’t be a bad thing.
I discussed this with claude.ai and learned that the diameter in the ISS varies between 3.50 m and 4 m. I think we shouldn’t go as high as 4 m because the size and weight would be too great. Indeed, with a 3.6 m diameter, we can have a total mass for the station of less than 3000 tons (2800 to 2900 instead of 3100 to 3300). Furthermore, the number of Starship flights carrying payloads could be limited to 22-28 instead of 26-33. This is because it’s conceivable to fit three 3.6 meter tubes in parallel within the hull diameter of a Starship, something impossible with 4 meter tubes. Construction time would then be reduced from 3 to 4.5 years down to 2.8 to 4 years.
Therefore, a diameter of 3.6 meters is optimal.
copyright Pierre Brisson
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