Space station artifical gravity

[Shooting Star]

Exactly why I would like to consider other options. (My favorite is the Rama series by Aurthur C. Clarke)

One of my favourites too, as I've mentioned somewhere earlier in the thread. The stairs and the elevator has been discussed in great detail.

Now, does adding torque to rotation make the rotation rate non-constant? I guess what I'm trying to say is, I'd really like to keep the rotation constant and add torque.

You just cannot have the same centrifugal force at every point on the surface of a sphere. It doesn't matter in whatever way you rotate it, and then add some torque to it, and then paint it red or whatever. This has has been explained to you by several members, even using mathematics.

Can't you simply visualize it? Also, there will be instantaneous poles, where the Fc is zero, even if for an instant.

Stick to cylinders, as the Ramans did.

 

[wysard]

First off, I have to apologize to all. My browser cache was doing some very wonky things and as a result I was seeing posts from page 2 as the current page and responding. I realize that might mean that some posters noticed that my posts bore only a fleeting connection to the current part of the thread. (In other words I sounded a bit off my nut...lol) All is fixed now and I can see clearly (for now).

Second. DH, I understand what you were saying about constant angular rotation, what part I missed was if a hollow sphere rotates equally in all three dimensions simultaneously then the center of rotation "r" remains constant. As a result because "r" never changes wrt the inside surface there is no reason not to have a uniform force at the surface. Did I miss something? If you tumble an object in three dimensions the objects radius or location changes? (Not arguing, just scratching my head) That said, the corolis forces could be large indeed depending on the omega involved, so I wouldn't recommend say a running broadjump.

 

[Shooting Star]

Second. DH, I understand what you were saying about constant angular rotation, what part I missed was if a hollow sphere rotates equally in all three dimensions simultaneously then the center of rotation "r" remains constant. As a result because "r" never changes wrt the inside surface there is no reason not to have a uniform force at the surface. Did I miss something? If you tumble an object in three dimensions the objects radius or location changes? (Not arguing, just scratching my head) That said, the corolis forces could be large indeed depending on the omega involved, so I wouldn't recommend say a running broadjump.

There is always an instantaneous axis of rotation for a rigid body. The centrigual acceleration at a point at an instant will be given by \omega^2 R, where R is the distance of the point from the instantaneous axis. \omega is not the same for different points, and hence centrigual acceleration will be different for different points.

Why do I have to invoke angular velocity etc to show that a sphere cannot rotate around infinite number of axes simultaneously?

 

[wysard]

There is always an instantaneous axis of rotation for a rigid body. The centrigual acceleration at a point at an instant will be given by \omega^2 R, where R is the distance of the point from the instantaneous axis. \omega is not the same for different points, and hence centrigual acceleration will be different for different points.

So Omega is different for different points with the same "r" (because all of them share the same non-moving intersection of their axies) I am following this bit. My problem is with the

and hence centrigual acceleration will be different for different points

Which is only true if their are different instantaneous "r"s. Which there is not. Ergo centrifugal acceleration is the same at all points. Does "r" change magically somewhere on the sphere?

Why do I have to invoke angular velocity etc to show that a sphere cannot rotate around infinite number of axes simultaneously?

I don't know. I don't care about an infinite number. Three is sufficient given it is the number of dimensions required to describe a sphere. It's not like I want to tumble a tesseract you know.

Again, not arguing, just having trouble with observed external phenomena and seeing what you are driving at in this thought experiment about the inside of the sphere.

 

[KontaktMan]

You just cannot have the same centrifugal force at every point on the surface of a sphere. It doesn't matter in whatever way you rotate it, and then add some torque to it, and then paint it red or whatever. This has has been explained to you by several members, even using mathematics.

Yes, I appreciate the mathematics although I have a difficult time making sense of some of the calculations. Every message seems to introduce new terms for me to decipher. (Not your guys fault of course) I did a little searching and came across a couple of physics on-line glossaries I'm referencing to try and help me out.

You have to realize that yes, I'm out of my element here and is the reason I asked to have you 'bear with me' in my initial post. I really do appreciate all your replies. Thanks for letting me visit your forum.

Anyway, I believe I stated previously that I accept the fact that not every point on a sphere can have the same centrfugal force. What I'm trying to understand now are the effects created when you give torque to a rotating sphere.

As DaveC426913 said in a previous post "Sure. You'd have a cosmic-scale tilt-a-whirl. It would be little more use than entertainment."

OK, that's something I can picture. I kinda take that to mean, yeah, there would be various degrees of G's on your journey. That leads me to picture this: a man standing on the inside equator of a 2 mile sphere in a constant rotation to give him a feeling of 1G. A slight amount of torque is added, say only enough that it would take 24 hours for him to return to that exact coordinate in space.

What happens to him? How does he feel? Will he instantly feel something is wrong? Enough to induce nasea? Will the universe cease to exist? ;-)

Thanks

 

[wysard]

OK, that's something I can picture. I kinda take that to mean, yeah, there would be various degrees of G's on your journey. That leads me to picture this: a man standing on the inside equator of a 2 mile sphere in a constant rotation to give him a feeling of 1G. A slight amount of torque is added, say only enough that it would take 24 hours for him to return to that exact coordinate in space.

What happens to him? How does he feel? Will he instantly feel something is wrong? Enough to induce nasea? Will the universe cease to exist? ;-)

You need to be clearer here. Do you mean a momentary instananeous torque? Like a bump, or "twist"? All that would do is cause a momentary push, or stumble, but would change the effective "axis" of rotation. Or do you mean add a new small, but constant torque, and in what frame of reference is it applied?

 

[KontaktMan]

You need to be clearer here. Do you mean a momentary instananeous torque? Like a bump, or "twist"? All that would do is cause a momentary push, or stumble, but would change the effective "axis" of rotation. Or do you mean add a new small, but constant torque, and in what frame of reference is it applied?

No, not just a momentary bump but a small constant torque. Not sure what you asking in terms of frame of reference.

The best ways for me to describe what I envision is back to this scenerio:
A man standing on the inside equator of a 2 mile sphere in a constant rotation to give him a feeling of 1G. A slight amount of torque is added, say only enough that it would take 24 hours for him to return to that exact coordinate in space.

What I'm searching for here is a compromise. If you can't have 1G throughout the surface of the sphere simultaniously all the time then can you have a gradual change so that a human might go about his day with say 1G at noon, (time to get in a nice cardio workout), while the G forces gradually lose force until midnight when he'll be weightless. From there the G forces increase again until noon when the pattern repeats itself ad infinitum.

Or an even longer period (and smaller torque) such as a week or a month? Gravity for such a space colony would be a shared resource.

 

[wysard]

It would be easier for him to go west, young man, go west...

Really.

If the man stands at the bottom of the sphere when it is stationary and we then spin the sphere right side clockwise, like the man had to walk forward to stay at the bottom. But he doesn't move because we spin it fast enough that he experiences 1G all the time (so he doesn't notice from an external frame that half the time he is upside down) to lower the G force on the person all they have to do is move at right angles to the axis of rotation. No special torque needed.

EDIT:
Adding a new constant thrust to the sphere (preferably at right angles to the initial spin) doesn't momentarily alter the frame, but adds a constant lateral acceleration factor to the body. Effectively altering where "down" is by the margin of thrust. Not only that because it is not a "bump" that can alter the spin to give your 24 hour turn around time, the effect of a small constant thrust accelerates so that what might have taken 24 hours for the first cycle, will take less and less time to rotate on the axis of the new thrust and would require harmonic calculation to figure out when succeding revolutions will put the person back in the intial position.

 

[KontaktMan]

It would be easier for him to go west, young man, go west...

Really.

If the man stands at the bottom of the sphere when it is stationary and we then spin the sphere right side clockwise, like the man had to walk forward to stay at the bottom. But he doesn't move because we spin it fast enough that he experiences 1G all the time (so he doesn't notice from an external frame that half the time he is upside down) to lower the G force on the person all they have to do is move at right angles to the axis of rotation. No special torque needed.

Yes but if you had an apartment in such a colony there would be the haves and the have-nots. Those that have 1G, those that have less, and those on the poles that have none. That's why I was exploring an alternate method where no matter where you live on the inner surface of a sphere everyone would get their daily allowance of gravity. (Or weekly, or monthly, however long the determined cycle would be)

EDIT:
Adding a new constant thrust to the sphere (preferably at right angles to the initial spin) doesn't momentarily alter the frame, but adds a constant lateral acceleration factor to the body. Effectively altering where "down" is by the margin of thrust. Not only that because it is not a "bump" that can alter the spin to give your 24 hour turn around time, the effect of a small constant thrust accelerates so that what might have taken 24 hours for the first cycle, will take less and less time to rotate on the axis of the new thrust and would require harmonic calculation to figure out when succeding revolutions will put the person back in the intial position.

So, by "contant lateral acceleration you mean the torque, right? Well no then, I'm not suggesting that the torque accelerate. Once the sphere is given torque it should remain at that speed so that the entire cycle always remains the same. (24 hours, 1 week, etc.)

So, is what I'm suggesting possible?

 

[wysard]

I guess. If you imagine two spheres one inside the other separated roller bearings one with a motor attached. Now spin the sphere(s) up to speed. You have 1 G at the equator and 0 G at the poles. If you then turn on the motor so it will make the inner sphere turn 1 revolution in 24 hours it would work. The trick here is that from the motor's frame of reference, neither sphere is spinning because both of them and the motor are all spinning at the same rate. As a result when you turn the motor on, it creates a uniform torque (theoretically) causing the spheres to rotate in opposite directions. If the motor works at right angles to the primary axis of spin then in a given day an object on the inner sphere would experience a constantly changing gravity cycling in a sine wave motion from 0 to 1 G twice per day.

Question is, how would you get in, or out of the colony?


EDIT:
As an afterthought, to ensure there are no "dead" spots or places that do not get their fair share of G forces you need to ensure that the speeds of the spheres are at least relatively prime. For example if you start both spheres spinning at some RPM that is prime, and then engage the motor to turn at some other prime RPM times 2 ( assuming both spheres have the same mass and the stator and rotor are the same mass...that pesky equal and opposite reaction thing...) then you should be good to go. Less of course the "how do I get in or out?" bit.

 

[KontaktMan]

I guess. If you imagine two spheres one inside the other separated roller bearings one with a motor attached. Now spin the sphere(s) up to speed. You have 1 G at the equator and 0 G at the poles. If you then turn on the motor so it will make the inner sphere turn 1 revolution in 24 hours it would work. The trick here is that from the motor's frame of reference, neither sphere is spinning because both of them and the motor are all spinning at the same rate. As a result when you turn the motor on, it creates a uniform torque (theoretically) causing the spheres to rotate in opposite directions. If the motor works at right angles to the primary axis of spin then in a given day an object on the inner sphere would experience a constantly changing gravity cycling in a sine wave motion from 0 to 1 G twice per day.

Question is, how would you get in, or out of the colony?


EDIT:
As an afterthought, to ensure there are no "dead" spots or places that do not get their fair share of G forces you need to ensure that the speeds of the spheres are at least relatively prime. For example if you start both spheres spinning at some RPM that is prime, and then engage the motor to turn at some other prime RPM times 2 ( assuming both spheres have the same mass and the stator and rotor are the same mass...that pesky equal and opposite reaction thing...) then you should be good to go. Less of course the "how do I get in or out?" bit.

I admire your creative solution. This is all quite an elaborate design, one which I have a difficult time comprehending, but would it really take that much complexity?

Maybe it takes that harmonic calculation you mentioned in a previous post. Even in 1 rotating rigid sphere given torque, I would expect at some point a cycle to be established where the man (With 1G becuase he's initially standing at the equator) would return to the coordinates in space he started at. In that case does he return to feel 1G or does the introduction of torque forever take that force away?

 

[wysard]

It's not really all that elaborate, just two spheres one inside the other, like a box inside a box. And the advantages it offers in terms of ease of modelling forces involved and a real world ability to isolate, monitor and control the torque force you specified makes it hard to beat, although I'd love to hear any other ideas.

As to the reason for the concern for the harmonics is that while in your visualization we can make the man start at 1G, and by some torque, float down to near zero and back in a day, try this experiment. Close your eyes and imagine the sphere you want, and then put a torque on it. But imagine you don't know where the man is standing! Suddenly instead of a truly simple solution that fits just where you originally imagined the man, you must now create a solution that always works, no matter where the "mystery man" is standing on the inside of the sphere.

By the same token, if you don't know where the man is standing, imagine he started instead of at the equator, he started somewhere around the 45th parallel. If you make the torque an even divisor of the original rate of spin it is VERY easy to wind up with a scenario where the man NEVER gets to neither Zero, nor to One. Those are the "dead zones".