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ohh - is there anyone here who can answer a question for me. I have been writing a science fiction story which features large rotating space stations like the

by sgt101 7y ago

ohh - is there anyone here who can answer a question for me. I have been writing a science fiction story which features large rotating space stations like the one in 2001 and I wondered... What is the force on a rotating structure in space. Is it possible to build a space station that rotates with 1G force on people at the rim, and if so what scale of structure could be built with current, or plausible technology? Where does a structure go from "imagined future" to "magic"?

parliament32 7y ago
There's nothing particularly difficult about building a spinning structure that has 1G of force at the outside (you see this all the time in carnival rides and the like -- if anything, it would be easier in a 0G environment, because you just have to start spinning and it'll keep spinning by itself forever). It's just not particularly useful -- astronaut comfort isn't really a priority, considering how expensive (and prone to problems) it would be.
- dredmorbius 7y ago
  This is far less about astronaut comfort and far more about health. Zero G turns out to be harmful in numerous ways, in addition to the other problems of space environments (radiation, iosolation, contamination). My sense is that the most valuable information to come from ISS is of astronaut adaptation and long-term space effects on health.
dmd 7y ago
http://curious.astro.cornell.edu/about-us/150-people-in-astronomy/space-exploration-and-astronauts/general-questions/927-can-artificial-gravity-be-created-in-space-intermediate http://curious.astro.cornell.edu/about-us/150-people-in-astr... tl;dr: it's trivial from a materials point of view, but hard from a "but you have to build it in space" point of view. We're still not great at building large structures in space.
tshannon 7y ago
If you haven't yet you should check out Isaac Arthur on youtube (https://www.youtube.com/channel/UCZFipeZtQM5CKUjx6grh54g https://www.youtube.com/channel/UCZFipeZtQM5CKUjx6grh54g) Specifically this video: https://youtu.be/86JAU3w9mB8 https://youtu.be/86JAU3w9mB8
johngalt 7y ago
http://www.artificial-gravity.com/sw/SpinCalc/ http://www.artificial-gravity.com/sw/SpinCalc/ The math pencils out, but the station has to have a fairly large radius away from the axis of rotation. For example a station rotating once a minute with a radius of 500m produces 0.6g. This means that you are looking at a 1km diameter if the station is symmetrical, or possibly less if some sort of counterweight is setup. Not at all implausible with current technology. Two habitats connected with a tether would do it.
theothermkn 7y ago
This is a straightforward calculation, with modest bookkeeping requirements, that is possible with undergraduate physics and calculus. As a sci-fi author, your craft will be improved by forcing yourself to do it. You'll be concerned with centripetal acceleration, yield stress, density, pressure, free body diagrams of pressure vessels, summation of forces on a differential element, and fairly basic integration (after taking symmetry into account).
LorenPechtel 7y ago
It's trivially easy. You almost certainly have the stuff around your home. Take a piece of rope, spin it above your head. If the angle of the rope to the ground is less than 30 degrees you have at least 1g at the end of it. There is a size limit, though--the required material strength scales linearly with the radius. Note that if you'll accept some fancy engineering this can be avoided: Instead of making it self-supporting you surround your station with a non-rotating ring. You can make it as thick as you need to provide the required strength. Wheels can't take the speeds involved, the connection will have to be magnetic levitation. Yes, this means maintenance but so long as you have an adequate safety margin you can power down and take apart any given piece for upkeep or even replacement. Just don't take apart too many at once.
- aidenn0 7y ago
  If you use two balanced rotating masses with a single tether, the required material strength scales only with the total mass of the system; regardless of the radius, the acceleration of the two rotating masses is 1g. At some point the tether mass will become non-negligible and it will scale linearly, but that should be quite large for any reasonable tether material, particularly since only the very tips of the tether are accelerating at 1g.
zaroth 7y ago
There’s discussion on /r/spacex that two Starships could be teathered nose to nose and set in a rotation during their ballistic flight to Mars, in order to provide at least Martian (~.3g) level artificial gravity along the way. This is important because even things like convection in the air stop working well at 0g, so for example, you can suffocate on your own CO2 if you fall asleep and there isn’t a fan running. Funny enough the hardest part about this doesn’t even seem to be the tethering or rotation, but two things; 1) you have to make slight course corrections along the way because the initial burn is never precisely accurate enough to target something as far away as a planet, and 2) occasionally you have to put as much mass as possible between you and the sun due to solar storms spewing radiation, so you have to stop the rotation occasionally to point the engines sunward to use your fuel as a radiation shield, or to course correct.
- dredmorbius 7y ago
  Water supply as rad shield might be better, presuming you have a large water store. Designing a chamber within the water supply would put mass between you and radiation.
jerf 7y ago
The tricky bit isn't spinning to get to 1G, the tricky bit is building something where the axis is large enough that the Coriolis force isn't making your inhabitants sick. I've heard mixed reports about this; this, for instance https://www.nasa.gov/vision/space/livinginspace/23jul_spin.html https://www.nasa.gov/vision/space/livinginspace/23jul_spin.h... suggests that even a small room is something we can adapt to fairly quickly, but I've seen suggestions I can't seem to source right this second that we may need to go out to hundreds of meters to get it low enough for long-term habitation. But it was never clear to me whether those were numbers for what the human body can tolerate, or whether they were trying to get the force down to "imperceptible". Personally I wouldn't be all that worried about that; if we can develop "sea legs" I'm sure we can deal with Coriolus force, it's only if we inevitably get motion sick that it's a problem. But merely getting up to 1G is trivial. I think the reason we don't have it today is that while getting "something" up to 1G is trivial, all the other engineering things that it interacts with get to be non-trivial, and right now, we're pretty tapped out with what we've got in space. If we continue to get more comfortable with space engineering and we continue to get better at putting stuff up there, I wouldn't be surprised we start seeing some spin-for-gravity relatively soon now.
yummypaint 7y ago
The formula you want is a=r*w^2 where r is the radius and w is the rotational rate. 1g is ~10m/s^2 The challenge in realizing it almost entirely an issue of getting materials into orbit, which is the problem space tethers are targeting. The cost of getting to geosynchronous orbit is around $30k per kilo, so at the moment economics is limiting us rather than tech when it comes to building crazy space stations. If we could bring the cost per kilo down 100x or so then so many more possibilities open up.

by sgt101 7y ago

parliament32 7y ago

There's nothing particularly difficult about building a spinning structure that has 1G of force at the outside (you see this all the time in carnival rides and the like -- if anything, it would be easier in a 0G environment, because you just have to start spinning and it'll keep spinning by itself forever). It's just not particularly useful -- astronaut comfort isn't really a priority, considering how expensive (and prone to problems) it would be.

dredmorbius 7y ago
This is far less about astronaut comfort and far more about health. Zero G turns out to be harmful in numerous ways, in addition to the other problems of space environments (radiation, iosolation, contamination). My sense is that the most valuable information to come from ISS is of astronaut adaptation and long-term space effects on health.

dmd 7y ago

http://curious.astro.cornell.edu/about-us/150-people-in-astronomy/space-exploration-and-astronauts/general-questions/927-can-artificial-gravity-be-created-in-space-intermediate http://curious.astro.cornell.edu/about-us/150-people-in-astr... tl;dr: it's trivial from a materials point of view, but hard from a "but you have to build it in space" point of view. We're still not great at building large structures in space.

tshannon 7y ago

If you haven't yet you should check out Isaac Arthur on youtube (https://www.youtube.com/channel/UCZFipeZtQM5CKUjx6grh54g https://www.youtube.com/channel/UCZFipeZtQM5CKUjx6grh54g) Specifically this video: https://youtu.be/86JAU3w9mB8 https://youtu.be/86JAU3w9mB8

johngalt 7y ago

http://www.artificial-gravity.com/sw/SpinCalc/ http://www.artificial-gravity.com/sw/SpinCalc/ The math pencils out, but the station has to have a fairly large radius away from the axis of rotation. For example a station rotating once a minute with a radius of 500m produces 0.6g. This means that you are looking at a 1km diameter if the station is symmetrical, or possibly less if some sort of counterweight is setup. Not at all implausible with current technology. Two habitats connected with a tether would do it.

theothermkn 7y ago

This is a straightforward calculation, with modest bookkeeping requirements, that is possible with undergraduate physics and calculus. As a sci-fi author, your craft will be improved by forcing yourself to do it. You'll be concerned with centripetal acceleration, yield stress, density, pressure, free body diagrams of pressure vessels, summation of forces on a differential element, and fairly basic integration (after taking symmetry into account).

LorenPechtel 7y ago

It's trivially easy. You almost certainly have the stuff around your home. Take a piece of rope, spin it above your head. If the angle of the rope to the ground is less than 30 degrees you have at least 1g at the end of it. There is a size limit, though--the required material strength scales linearly with the radius. Note that if you'll accept some fancy engineering this can be avoided: Instead of making it self-supporting you surround your station with a non-rotating ring. You can make it as thick as you need to provide the required strength. Wheels can't take the speeds involved, the connection will have to be magnetic levitation. Yes, this means maintenance but so long as you have an adequate safety margin you can power down and take apart any given piece for upkeep or even replacement. Just don't take apart too many at once.

aidenn0 7y ago
If you use two balanced rotating masses with a single tether, the required material strength scales only with the total mass of the system; regardless of the radius, the acceleration of the two rotating masses is 1g. At some point the tether mass will become non-negligible and it will scale linearly, but that should be quite large for any reasonable tether material, particularly since only the very tips of the tether are accelerating at 1g.

zaroth 7y ago

There’s discussion on /r/spacex that two Starships could be teathered nose to nose and set in a rotation during their ballistic flight to Mars, in order to provide at least Martian (~.3g) level artificial gravity along the way. This is important because even things like convection in the air stop working well at 0g, so for example, you can suffocate on your own CO2 if you fall asleep and there isn’t a fan running. Funny enough the hardest part about this doesn’t even seem to be the tethering or rotation, but two things; 1) you have to make slight course corrections along the way because the initial burn is never precisely accurate enough to target something as far away as a planet, and 2) occasionally you have to put as much mass as possible between you and the sun due to solar storms spewing radiation, so you have to stop the rotation occasionally to point the engines sunward to use your fuel as a radiation shield, or to course correct.

dredmorbius 7y ago
Water supply as rad shield might be better, presuming you have a large water store. Designing a chamber within the water supply would put mass between you and radiation.

jerf 7y ago

The tricky bit isn't spinning to get to 1G, the tricky bit is building something where the axis is large enough that the Coriolis force isn't making your inhabitants sick. I've heard mixed reports about this; this, for instance https://www.nasa.gov/vision/space/livinginspace/23jul_spin.html https://www.nasa.gov/vision/space/livinginspace/23jul_spin.h... suggests that even a small room is something we can adapt to fairly quickly, but I've seen suggestions I can't seem to source right this second that we may need to go out to hundreds of meters to get it low enough for long-term habitation. But it was never clear to me whether those were numbers for what the human body can tolerate, or whether they were trying to get the force down to "imperceptible". Personally I wouldn't be all that worried about that; if we can develop "sea legs" I'm sure we can deal with Coriolus force, it's only if we inevitably get motion sick that it's a problem. But merely getting up to 1G is trivial. I think the reason we don't have it today is that while getting "something" up to 1G is trivial, all the other engineering things that it interacts with get to be non-trivial, and right now, we're pretty tapped out with what we've got in space. If we continue to get more comfortable with space engineering and we continue to get better at putting stuff up there, I wouldn't be surprised we start seeing some spin-for-gravity relatively soon now.

yummypaint 7y ago

The formula you want is a=r*w^2 where r is the radius and w is the rotational rate. 1g is ~10m/s^2 The challenge in realizing it almost entirely an issue of getting materials into orbit, which is the problem space tethers are targeting. The cost of getting to geosynchronous orbit is around $30k per kilo, so at the moment economics is limiting us rather than tech when it comes to building crazy space stations. If we could bring the cost per kilo down 100x or so then so many more possibilities open up.