Can Unequal Masses in a Tethered Spacecraft Simulate Gravity Effectively?

[Jaziel]

TL;DR Summary

Conundrum about the use of different masses revolving around a common centre.

In discussions about simulating gravity in a spaceship by the use of tethered masses revolving around a common centre, the assumption appears to be that these masses must be equal. Would instabilities occur were this not the case? Or would the problem go away simply by placing the hub at an optimum point on the tether the so as to balance out the different masses?

 

[Ibix]

Is the system open or closed?

If it's open, what forces are acting on it?

If it's closed, what do you know about the motion of the center of mass?

 

[Vanadium 50]

Two words: USS Corkscrew.

 

[sophiecentaur]

TL;DR Summary: Conundrum about the use of different masses revolving around a common centre.

at an optimum point

The CM? All the massive component of the station could be kept there - where the dock would be. The total angular momentum would be minimum in that position. But it may be better to have most of the mass at the periphery to reduce changes in angular velocity as crew and heavy objects were moved around.
I can't imagine that a flexible tether would be the best thing; a network of tethers would maintain the relative positions of each end.

What would be the prime factor in choosing the mass of a station? kg cost a lot of money for a space structure; perhaps reaction wheels and an active control would be best value. A high 'virtual moment of inertia' would be possible to build in for control of rotation.

 

[Jaziel]

Thanks for that. What prompted the question in the first place was wondering how it would be if the parts of a spacecraft (not a space station) were tethered in such a way so that the life support section would be at one end of the tether while the the rest of the ship was distributed along the rest of it. The puzzle arose when thinking how a balance between the masses would be met. One would imagine that a spacecraft engine (especially were it to be a fusion reactor, say) would be quite a weight compared to the life support module. Then there are the fuel/propellent tanks, which wouldn't be fixed masses anyway.

None of this was properly addressed in the articles on tethered spacecraft I'd chanced upon. Instead they stuck to the basic formula that any given tethered spacecraft comprised of two items, both of equal masses and both positioned at an equal distance from the tether's central hub or node. That didn't seem to make much sense somehow, not based on how things would be in the real world. Unfortunately, having a poor visual imagination made it hard to picture how the alternative would work out and whether it complied with the physics of the situation - which was the rub.

Since then examples from nature have sprung to mind - planetary motions around stars, for instance. True, these aren't perfect circles, but talk about unequal masses orbiting around a common centre! Anyway due apologies for delivering this molehill to Physics Forums. It seemed rather big at the time, but no longer.

 

[sophiecentaur]

both of equal masses and both positioned at an equal distance from the tether's central hub or node.

I don't see how equal masses and equal radial distances are at all relevant. The only need for rotation is to produce artificial g and that would only be needed for living quarters. The whole would of course rotate about its CM and a low mass living quarters would allow a long radius from the CM. All the heavy parts could be at the far end and the engines at the CM.

planetary motions around stars, for instance

Orbits rely on gravity so there is no real parallel with rigid structures.

the articles on tethered spacecraft I'd chanced upon

There is a lot of garbage out there so you can't rely on much of what's posted.

 

[Jaziel]

There is a lot of garbage out there so you can't rely on much of what's posted.

You're telling me.

 

[A.T.]

Instead they stuck to the basic formula that any given tethered spacecraft comprised of two items, both of equal masses and both positioned at an equal distance from the tether's central hub or node. That didn't seem to make much sense somehow, not based on how things would be in the real world.

Real world is always more messy than some idealization. In the real world the CM, and thus the center of rotation, would shift around, when you move stuff around in the station.

 

[sophiecentaur]

the center of rotation, would shift around, when you move stuff around in the station.

It wouldn't be too hard to reduce the effects of shifting small masses in the crew's quarters with an active system for moving the 'heavy' end in and out. If reaction wheels were used as an energy source it wouldn't be too heavy on energy use.