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Thread: What diameter does a rotating space station have to be to generate useful gravity

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    What diameter does a rotating space station have to be to generate useful gravity

    Sorry - I meant to say space station in the title

    First question - I've heard that to generate useful artificial gravity a rotating space station (like in the 2001 movie) one has to have a quite large diameter spin platform so that you can stand erect and conscious without your blood draining from your head to your feet. Are there some available metrics on this (i.e. what's the ideal diameter and spin velocity?)

    Second question - if you ran really fast counter to the direction of spin, could you reverse the artificial gravitational effect?

    Thanks, Steve
    Last edited by Swift; 2020-Oct-22 at 01:29 PM. Reason: fix title

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    See here for a review and some graphs.
    The issue isn't really with blood draining to your feet--you're body's pretty good at preventing that happening in a normal gravity field, and having less "gravity" around your head would make that easier to do, rather than harder.
    The limiting factor would appear to be Coriolis pseudoforce, which affects the balance organs when you move your head. Studies in the 1960s suggested that 6prm was a limit to the rotation rate, above which people experienced nausea on moving their heads. That set a limit of 12-24m radius for the habitat, in order to produce 0.5 to 1g. There's a little calculator here that lets you play with the values.
    As detailed in the article I linked to, later experiments have suggested that people can habituate to 10 or even 24rpm, which would mean a correspondingly smaller habitat would be viable. As I detail here, that would make the 35-feet diameter Discovery centrifuge a viable option as portrayed in the movie, with 1g acceleration at the rim.

    Running counter to the direction of spin would make you lighter, and eventually you wouldn't be able to run any faster because you'd lose traction. If you could somehow "hit the ground running", so that you were actually revolving in the opposite sense to the centrifuge, then you'd start to get heavier the faster you ran. The Skylab astronauts actually managed to run around a set of circumferential lockers in their habitat, generating a very mild centrifugal gravity despite the fact Skylab wasn't rotating. There's probably footage online somewhere.

    Grant Hutchison

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    Oh, and if you want to see some discussion and visual examples of how Coriolis would affect the trajectory of moving objects in a rotating habitat, see my article Coriolis Effect In A Rotating Space Habitat and its supplement.

    Grant Hutchison

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    Quote Originally Posted by Cheap Astronomy View Post

    First question - I've heard that to generate useful artificial gravity a rotating space station (like in the 2001 movie) one has to have a quite large diameter spin platform so that you can stand erect and conscious without your blood draining from your head to your feet.
    Your blood would be fine, as long as the spin didn't produce more that 1G of centrifugal force*. As Grant points out, you can get dizzy in a small diameter spin. Like being on a fast amusement park ride.

    * Call it a force or momentum, whatever.

    And the running-around-a-ring can, if you run fast enough, generate a quarter to a third of Earth's pull. As long as you can keep up the sprint, that is.
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

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    What diameter does a rotating space station have to be to generate useful gravity

    Quote Originally Posted by grant hutchison View Post
    The Skylab astronauts actually managed to run around a set of circumferential lockers in their habitat, generating a very mild centrifugal gravity despite the fact Skylab wasn't rotating. There's probably footage online somewhere.

    Grant Hutchison
    Hereís the video; itís at the 4:12 mark.

    https://youtu.be/JZNKVnDvQY4

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    Quote Originally Posted by Cheap Astronomy View Post
    Second question - if you ran really fast counter to the direction of spin, could you reverse the artificial gravitational effect?
    Another issue here is that, by running counter to the spin, you would increase the centrifugal gravity for everyone else. By conservation of angular momentum, the station would spin a little faster. It would then regain its original spin rate when you stopped running.
    James White got this wrong in Dark Inferno / Lifeboat. He had his characters stop the rotation of their escape pods by crawling around inside them, treating it as some sort of cumulative effect, rather than a temporary one that would only last while they were crawling.
    I did something similar in a short story that never got into a remotely publishable form--had my characters run en masse around a 2001-style space station, in the direction of spin, in order to slow it enough to allow a drifting piece of orbital debris to pass between the spokes without striking the structure.

    Grant Hutchison

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    I also wonder if you need to get to 0.5 or 1.0 g to be useful. Mars' surface gravity is 0.38 g, so for a trip to Mars, it would probably make sense to aim for that in your spacecraft.

    What is the minimum you need to avoid the physiological effects of microgravity? I suspect it is less than 1 g, but I don't know a more specific value.

    I found this article, which discusses the issues and describes some work to determine such things, but doesn't reach a conclusion about a target value.
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    Quote Originally Posted by Swift View Post
    I found this article, which discusses the issues and describes some work to determine such things, but doesn't reach a conclusion about a target value.
    That's actually the same one I linked to in my first post--yours is the version on the NCBI website, mine on the journal's website.

    Grant Hutchison

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    They did it on Gemini 8, but decided to abort.
    A: "Things that are equal to the same are equal to each other"
    B: "The two sides of this triangle are things that are equal to the same"
    C: "If A and B are true, Z must be true"
    D: "If A and B and C are true, Z must be true"
    E: "If A and B and C and D are true, Z must be true"

    Therefore, Z: "The two sides of this triangle are equal to each other"

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    Quote Originally Posted by Swift View Post

    What is the minimum you need to avoid the physiological effects of microgravity? I suspect it is less than 1 g, but I don't know a more specific value.
    Just not enough information. We'll need a variable speed spinner in orbit to find out long term.
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

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    Quote Originally Posted by 21st Century Schizoid Man View Post
    They did it on Gemini 8, but decided to abort.
    They did what? If I remember correctly the spacecraft was tumbling end over end because of a stuck thruster. They stopped it with other thrusters and then came down as soon as possible.

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    Quote Originally Posted by Hornblower View Post
    They did what? If I remember correctly the spacecraft was tumbling end over end because of a stuck thruster. They stopped it with other thrusters and then came down as soon as possible.
    Yes, that's correct.

    I believe they created about 4g of artificial gravity, in quite a small capsule, before successfully turning the gravity off.
    A: "Things that are equal to the same are equal to each other"
    B: "The two sides of this triangle are things that are equal to the same"
    C: "If A and B are true, Z must be true"
    D: "If A and B and C are true, Z must be true"
    E: "If A and B and C and D are true, Z must be true"

    Therefore, Z: "The two sides of this triangle are equal to each other"

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    Quote Originally Posted by Hornblower View Post
    They did what? If I remember correctly the spacecraft was tumbling end over end because of a stuck thruster. They stopped it with other thrusters and then came down as soon as possible.
    You remember correctly. The problem occurred while Neil Armstrong and David Scott were docked with the Agena target vehicle and got worse when they undocked. Neil Armstrong saved the craft by shutting down the orbital maneuving thrusters and using the reentry control system to regain control. Use of the RCS thrusters dictated a mission abort.
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    Ah-ha. I'd assumed that it a reference to the Gemini XI tether experiment with the Agena, when they deliberately induced centrifugal gravity. About 0.00015g, but still.

    Grant Hutchison

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    Quote Originally Posted by grant hutchison View Post
    Ah-ha. I'd assumed that it a reference to the Gemini XI tether experiment with the Agena, when they deliberately induced centrifugal gravity. About 0.00015g, but still.

    Grant Hutchison
    That's the one I was thinking of. NASA description here.
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