Gravity & Rotational Forces: Is It Possible?

[vjk2]

Hi,

Doing a quick scan of wikipedia, a radius of 22m going at 10 RPM would produce 1g in gravity on the edge.

However, wouldn't other forces of the spin, torque and angular momentum, create huge problems? Intuitively, if I hold a spinning bicycle wheel, it is highly unstable and the slightest movement will result in a strong gyration in some direction.

I've forgotten most of my physics and am in the process of getting reacquainted, so forgive any mistakes.

 

[Simon Bridge]

A spinning bike-wheel should be pretty stable in that it returns to it's initial state after a small perturbing impulse. If you try to turn reorient the axle, it will resist - if you are on platform free to rotate, it will do so to conserve angular momentum.

There are a lot of quirks associates with rotational "gravity" - it probably should not be thought of as gravity at all.

To spin-up your space station, something else has to go the other way (maybe use rockets along the rim or have two, counter-rotating, stations). You get coriolis effects. Falling from the center, there is no gravity effect until you touch the side. If you throw a ball, it does not follow a parabolic trajectory. You can have a lot of fun with this stuff.

 

[vjk2]

well, what I mean by the bike wheel, okay it is stable compared to many things, but if you hold it while spinning, you can feel several different forces that require some force on your part to stabilize against.

Those forces would be present in a centrifugal force space system.

I also read on wiki that there's something like if you walk against the direction of spin you will...feel different.

 

[Simon Bridge]

well, what I mean by the bike wheel, okay it is stable compared to many things, but if you hold it while spinning, you can feel several different forces that require some force on your part to stabilize against.

That would be a badly balanced wheel then. I've found new bike wheels to require little effort to stop them wobbling.

Those forces would be present in a centrifugal force space system.

When tires are fitted to motor-vehicle rims, some effort is expended to make sure the total wheel is balanced to avoid wobbles. The same would be needed in the design of a spinning space-station.

The mass distribution would be an important design consideration for the 22m station in your description. It has less of an effect the bigger you go ... recall: the Earth is spinning (and wobbling).

I also read on wiki that there's something like if you walk against the direction of spin you will...feel different.

The coriolis effect pushes you radially so you could feel slightly heavier going one way and lighter going the other way.

You can work out how big that effect would be for the 22m/10rpm station if you like.

 

[Filip Larsen]

The site [1] seems to hold a nice list of interesting papers on the subject. I remember reading some of the papers years ago, but haven't followed the subject much since.

[1] http://www.artificial-gravity.com/

 

[vjk2]

Thanks for the link. Will read more.

Actually, I severely doubt that a space station would be be as evenly balanced as a badly skewed bicycle wheel.

edit:this is a pretty good article on the subject

http://www.artificial-gravity.com/IAC-10-D1.1.4.pdf

 

[K^2]

I recall research suggesting an upper limit of 2RPM for humans. While centrifugal effect is a good one to simulate gravity, Coriolis forces are a problem. At higher revolution speeds any movement of person's head messes with vestibular system responsible for balance. Basically, people get motion sick.

2RPM limit would put minimum diameter of the station at 450m. That's over quarter of a mile. This is beyond our ability to construct for the moment.

 

[vjk2]

Is that full 1g Earth gravity? Because I've read that like 1/3 earth-gravity should be enough to stave off any negative health effects

Apparently the ISS was supposed to get a centrifugal test module in 2011 but it was cut for budget reasons.

 

[K^2]

Yes, 1g. But it scales linearly with size, so g/3 would still be a huge station.

As far as experiments, my favorite by far is the track they built on Skylab. So as you can see, for an exercise room you don't need much. Maybe they had something along this scale in mind for ISS.

 

[bahamagreen]

The center and axis of rotation will need to be "maintained".

We are so used to a rock steady gravitational acceleration that being on a boat makes some people sick, especially for long periods.

When people and equipment move from place to place within the rotating station, the redistribution of mass will cause the center/axis of rotation to vary and the people inside will travel an ellipse causing a cyclic variation in their experience of the artificial "gravity", similar to being at sea.

A control system could be used to measure and correct the effect by moving additional mass within the station; a primary system of tanks of water (which they need anyway) and some pumps might be enough to correct the alignment of the center of rotation to the center axis of the station. Maybe a secondary system to correct for the angle between the axis of rotation and the axis of the station for when everyone is attending a meeting along the same wall.

I'm thinking the primary system would need three tanks in a triangular formation to correct an offset to the center of rotation, and the secondary system would need six tanks, three in triangular formation for each +/- z half of the station... although clever design would allow the "secondary" system to also perform the corrections of the primary... so a minimum of six tanks. I do not have a proof of this...

 

[Borek]

well, what I mean by the bike wheel, okay it is stable compared to many things, but if you hold it while spinning, you can feel several different forces that require some force on your part to stabilize against.

That would mean barycenter is not on the axis - but it doesn't matter for a free rotating object.