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Author Topic: How large would a rotating spaceship need to be to create artificial gravity equal to that on Earth?  (Read 6087 times)

Offline Michael Peoples

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Michael Peoples asked the Naked Scientists:
   
I've read many science fictions that involve large structures in space where people live on the inner wall of the rotating "spaceship". 

Now, I'm one of those who cannot tolerate being "spun" in any way (a child's merry-go-round can make me ill). So my question is this: 

How large would the inside of the enclosure have to be so that the centrifugal force would create artificial gravity equal to that experienced on Earth, but produce no perception of motion on the part of the observer?

In other words,  it would be just like standing on our planet, except the sky overhead would be a view of your neighbours.

What do you think?


 

Offline DoctorBeaver

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If there were no way to view outside then there would be no indication of motion no matter what speed the craft revolved at. Remember, any person on the craft would be in the same frame of reference as the craft itself & therefore would be travelling with the same rotational velocity.
 

Online syhprum

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According to Mach the rotation is relative to the whole mass of the universe but this is not proven it may be relative to the local galaxy or the dark matter.
Please Mr. Einstein can we have our ether back?
 

Offline graham.d

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To create the artificial gravity you could have a small diameter ship rotating quickly or a large one rotating more slowly as DD says. The small one would give a significantly different force over the height of your body, which may be disturbing. The Coriolis force may be unbalancing if you were jogging round, as the astranauts did in the film 2001. Really the larger the diameter the better.

Another (unrelated) factor in rotating craft in orbit is so that they get evenly cooked from the sun. The heating effect can be significant so rotation helps to even out the hull temperature.
 

Online syhprum

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Experiment would have to be carried out to see how much difference in G is tolerable between ones head and ones feet.
A less ambitious design than a torus could be two cylindrical accommodation units connected by a tunnel which could contain a well shielded section for radiation emergencies and would be easier to construct from rocket sections.
 

Offline Vern

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It might help to have large view screens that keep the outside scene motionless instead of actual view ports to the outside.
 

Offline that mad man

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Would the Coriolis force be strong enough to have much effect?
 

Offline Bored chemist

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Re
"Now, I'm one of those who cannot tolerate being "spun" in any way"
I have bad news for you concerning the Earth.
 

Offline Madidus_Scientia

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lol

Maybe he needs to see a spin doctor :p
« Last Edit: 06/10/2009 10:41:42 by Madidus_Scientia »
 

Online yor_on

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Awh and here I was, wondering what was tripping me all the time. Thanks BC :)

Ahem.
 

Offline Michael Peoples

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It's me, the person who asked the question.  Thanks for the responses.  I think a few things about the "parameters" of my question.  So, I'll elaborate.

The Coriolis effect (wherein the friction of a rotating body against some "fluid" mass (or least something that can 'bend') causes the portion of that mass closest to the rotating body to move at a higher rate than that farther away from the rotating body), is really THE critical factor in terms the user's experience.  If you've ever been on a children's merry-go-round, and had it move suddenly under you, your legs move immediately with the merry-go-round, but your upper torso's mass tends to stay put, potentially taking you off your feet.*

Being in a small ship rotating at high speed would be very difficult to stand in.  The Earth, in the context of this questions, does not count.  While centrifugal force has a finite, measurable, effect on people, the Coriolis effect can't really be perceived directly by people. 

This question is, at least at our present level of technology, purely theoretical.  This isn't about an actual design, but rather, what diameter of cylinder would be needed to simulate Earth gravity (1G), without an "average" observer perceiving the rotational motion.  Yes, I realize that would depend upon the observer's sensitivity to motion, but work with me here.  So, some bright mathematician, physicist, engineer out there needs to decide on some "acceptable" difference in inertia between the observer's feet and head.  Then, based upon that acceptable difference, how big would the cylinder's inside diameter be?  I realize it's not a easy question, but that's why I asked all smart folks who listen to the Naked Scientist.


* Yes, I know, the people who write into this forum could probably quote Gaspard Coriolis' principles from memory, but I thought I'd "hold forth" with an oversimplification.

 

Offline graham.d

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Well that's what I said MP, but as for making a value judgement for what is acceptable...

I think it would have to be a compromise between the costs of the larger diameter and what people would accept. Mostly they would not be fare-paying passengers so they would have to put up with whatever was on offer - I guess some gravity (despite Coriolis problems) is better than none. It's surprising what people can get used to.

I'd have a guess that 40 metres diameter (giving 10% difference between the force on the feet compared with the head for a tall person) would be acceptable. I wonder what the long term effects on the physiology would be too.
 

Offline Michael Peoples

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Graham,

Cost isn't a consideration, nor is hull temperature (although it would be in a discussion of a "practical" solution).  I was thinking more like "Rendezvous with Rama" (Arthur C. Clark).  Kilometers in diameter.

I get the feeling this question may not get an answer, because of the variable nature of "perceived" motion.  I suppose we could decide on an acceptable amount of lateral force on the upper part of your body.  I was watching "Top Gear" om BBC America, and I saw a device mounted in a car that showed the G forces exerted during a turn.  It consisted of a hollow, plastic ring filled with a red liquid.  As the car turned, the liquid tilted in relationship to the "plane" of the camera.  So if we decide on an arbitrary tilt of 5 degrees, it would would represent some measurement of lateral force.

How much force, I can't say.  Perhaps our mathematician could calculate what that force would be.  Once that is known, we can then assume a person of average height (180 cm) and weight (77 kg), with a normal distribution of that weight along the body.  The calculations for this is probably complex.  The key result of that calculation would be the speed of the ground (acceleration) on the person's feet.

With that value as a "limit", the other calculation would be the down force (gravity) and the centrifugal force needed to produce it.  From that, the diameter might be calculated.  But like I said, I have nothing close to the math needed to figure this out. 
 

Offline graham.d

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If cost is not a consideration, nor thermal effects, then (as I also said before) the bigger it is the better. The force felt due to the rotation is simply prportional to the radius, so for a 1.8m person, the top of his body feels a different force compared to is feet which (as a percentage) is (998.2/1000)*100% (= 99.82%) for a 1 km radius. My guess would be that this would probably be not noticable and certainly not damaging. Rotation would have to be once per 63.44 seconds to emulate earth gravity with the outer rim moving at 99.05 m/s. I think I got the sums right :-)
 

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