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There is a question How do astronauts turn in space?, and related questions here and on physics.stackexchange, that detail how astronauts might maneuver by spinning their upper and lower body separately. Wouldn't it be easier to spin a fidget spinner, and to preserve angular momentum, spin your body in the opposite direction?

Camille Goudeseune
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StefanH
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2 Answers2

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This is exactly how it works and how the orientation of many satellites is controlled. For example, the Hubble telescope has 4 fidget spinners installed, pointing in different directions - although they are commonly referred to as "reaction wheels".

Unfortunately a typical fidget spinner is a bit too light to be really useful: We have to compare the moment of inertia of the spinner and a human. For the fidget spinner I found values around $3\cdot10^{-5}\,\mathrm{kg\,m^2}$ and a human in a stretched pose around $3\,\mathrm{kg\,m^2}$. I.e. a fidget spinner turns about 100,000 times faster than a human. If we can get the spinner to 10,000 rpm (which is rather high), the astronaut would turn once in ten minutes.

fred_dot_u
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asdfex
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    Spinners can reach about 100,000 rpm before self-destructing, but that requires extra equipment. – Camille Goudeseune Dec 31 '20 at 17:54
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    You had me at "bigger space fidget spinners" – Comic Sans Seraphim Dec 31 '20 at 23:13
  • +1 I'm imagining one with a crank to spin it up, and a patent to go with it. – uhoh Jan 01 '21 at 02:39
  • @CamilleGoudeseune: Is that max rp measured in (and reliant on) gravity conditions though? Just curious whether 0G would improve their max rpm. – Flater Jan 02 '21 at 02:00
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    @Flater A rotating body requires centripetal tension forces to keep from flying apart. Or, in terms of the "fictitious" centrifugal force, spinning causes a force pulling the object apart. These forces exist independently of gravity, and once they exceed the tensile strength of the object, the object will break apart. – Acccumulation Jan 02 '21 at 02:22
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    Accumulation's point. At such intra-fidget forces, terrestrial gravity is mere noise. ISS microgravity is noise within noise. – Camille Goudeseune Jan 02 '21 at 03:10
  • @Acccumulation: My point was more that regardless of specific orientation, in gravity conditions some part of the spinner is under increased stress (i.e. the part that holds up the spinning part), and it was unclear whether the spinners break because of centripetal/fugal forces or the friction because of that increased stress due to gravity. If the former, 0G doesn't matter; but if the latter, it probably does. – Flater Jan 02 '21 at 10:15
  • @Flater To get a Fermi estimate of the gravity contribution, consider a taking bucket, supporting it with a cable that goes through one of the holes of a fidget spinner, and then filling the bucket with spinners. How many spinners do you think you would have to put in the bucket before the spinner holding the bucket breaks? The reciprocal of that is roughly what portion of the stress would be due to gravity, rather than centripetal force. – Acccumulation Jan 02 '21 at 20:27
  • Centripetal acceleration is (angular velocity)^2*radius. $10,000 \text{ rpm}$ is $ 2 \pi \times 10 \text k\frac{\text{ rad}}{\text{min}}$ (a revolution is $2 \pi$ rad) or about $60 \text k\frac{\text{ rad}}{\text{min}}$ or $1 \text k\frac{\text{ rad}}{\text{sec}}$. Square that, and you get $1\text {M} \frac {rad^2}{sec^2}$. If our radius is $0.01\text{ m}$, then a= $10\text{k} \frac{\text{m}}{\text s^2}$ (radians are dimensionless units, so we can drop them to get a), which is about 1000 times the gravitational acceleration at the surface of the Earth (1000g). For 100k rpm, it's 100,000g. – Acccumulation Jan 02 '21 at 20:31
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It's possible to use reaction wheels to arbitrarily alter one's attitude, but a major limitation with reaction wheels is that if an object has rotational momentum and one wants it to maintain a constant attitude, the reaction wheel will have to spin forever unless or until one gives up on holding a constant attitude or one can transfer rotational momentum via some means (e.g. by using rockets). If there's any friction between the reaction wheel and the space craft, then as such friction slows the reaction wheel down, it will transfer the rotational momentum from the reaction wheel back to the rest of the space craft.

supercat
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  • No, it doesn't need to spin forever (unless there is an extra momentum of the whole system it has to keep). – fraxinus Jan 01 '21 at 20:19
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    If the system has acquired unwanted rotational momentum, the combined system (reaction wheel plus remainder of space craft) will need to have that rotational momentum unless it can be transferred to something outside the system (e.g. rocket exhaust). As long as the system has non-zero rotational momentum, at least some parts of the system will need to be rotating. – supercat Jan 01 '21 at 21:32
  • I'm wondering if there are ways around the forever spinning wheel (even if impractical). E.g. spin the wheel, don't slow it down (so you keep spinning), disconnect the wheel from you, then have the wheel stop itself e.g. using monopropellant (matching your rotation), then reconnect. Hugely impractical, but theoretically possible, no? (just not infinitely reusable, given the finite monoprop supply) – Flater Jan 02 '21 at 02:02
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    @There is no way to alter the total angular momentum without some outside object. If you have rockets that can alter the wheel's angular momentum, you can just use them to alter the whole ship's angular momentum directly. – Acccumulation Jan 02 '21 at 02:26
  • @supercat it is a good point. If one starts with some unwanted rotation and a space fidget spinner in one's pocket, think the solution is to time the spin-up just right so that one's rotation is nulled and one is facing one's crew-member, shout "think fast!" and toss it to them :-) – uhoh Jan 02 '21 at 03:18
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    @uhoh if you're in a space station you can just grab one of the handles. Your station has a bigger fidget spinner than you do. – John Dvorak Jan 02 '21 at 17:13
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    @Flater well having the wheel reducing the rotational momentum by itself using thrusters is similar to slowing it with respect to the space craft, and using the attitude control thrusters on the spacecraft to counteract. The difference is really that implementing everything in the reaction wheel, including some mechanism for detaching and especially reattaching is a lot more difficult - that's why it's not done. Normal spacecraft do that once in a while actually in order to desaturate the reaction wheels – Apfelsaft Jan 02 '21 at 21:06
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    @Flater on second thought - you could have "one way reaction wheels" - like a magazin of spare wheels, and when the one in use gets too fast, you just eject the fast rotating disk into space. That said - to my knowledge spacecraft (at least modern satellites) are designed to not lose anything to space, in an effort to minimize debris (hazard) – Apfelsaft Jan 02 '21 at 21:12
  • @CarlBerger that's a fascinating idea, ejectable angular momentum, or L-shedding. – uhoh Jan 02 '21 at 21:58
  • @CarlBerger: Oh I'm well aware that it's not efficient or reusable, I was just wondering about the theoretical possibility. And yes, your one way reaction wheels are essentially going through half of the motions I listed and then not bothering to reconnect - even less reusable ;) – Flater Jan 03 '21 at 00:26