Communists on the moon ! - Page 7

Suicide Junkie · #61 December 19th, 2003, 02:22 AM

Quote:

Originally posted by geoschmo:

quote:
Originally posted by Suicide Junkie:
Assuming everything stays inside the station, the center of mass will remain in a consistent position.

No, I don't think this is right either. Loser is right when he says there will be some reaction to people walking around the inside of the station. As they exert force against the deck plates to go in one direction the station will have an equal reaction in the opposite direction. I just think he's way over estimating the amount the reaction will affect the station. The difference in mass between the person and the station means that almost all the energy moves the person and almost none of it moves the station. So while there will be a force applied to the station it would be imperceptable I believe.

Geoschmo

Ah, but they can only keep going in that direction for so long before they hit the far wall.

Then you cancel out the velocity you and the station had while you were on your trip.

The biggest effect you could get is if everybody ran from one side to the other all at once. (Much faster than a rotation of the station, for simplicity.)
If the station's mass is 100 times what the people total, then the people moving 100m would cause the station to move 1m in the other direction, and the center of mass (people + station) would still be in the same spot (right smack dab on the L-point where it started)

narf poit chez BOOM · #62 December 19th, 2003, 06:44 AM

what it would do is viberate, maybe more, maybe less as forces bounced around inside. but, baring momentum transmitted to space dust, the viberation would center on an orbiting point.

JurijD · #63 December 19th, 2003, 10:10 AM

Quote:

Originally posted by Loser:
I will stand by my contention that the constant, semi-random displacement forces involved in real people living their lives inside a spinning-wheel space station are sufficient to make the concept wholly impractical without significantly different means of station-keeping. Every bit of thruster fuel has to be lifted to orbit as well, and I contend, as well, that it will be expended at a fairly rapid pace.

We had this discussion before

A while back when we were talking about sending a manned mission to Mars. And I explained then that If you have a rotating enclosed space station with people inside the people inside CANNOT (I repeat CANNOT) affect the total rotation or linear momentum of the whole system. (Now I don't know what happened to space lab, maybe it was just built badly and that was the reason for its orbit deteriorating, the people inside jumping about surely weren't !) Just look at Mir... those russians had some wild parties up there

EXPLANATION:
Say we have a roating wheel in space and its hollow on the inside. Now lets say that we have an astronaut that starts to run on the inside of that wheel in the same direction the space station is spinning. Running like that would produce a force by his legs that would work against the space station's rotation. BUT! the overall rotational momentum of the space station and the person inside wouldn't change as this is an inclosed system with no outside forces acting upon it and the sum of the rotation energy of the astronaut and the space station would be conserved.
1. space station and astronaut spinnign together: D1 = m*r^2*W1 + a*r^2*W1
D1.. total rotational momentum at the beginning
m.. mass of the space station
a.. mass of the astronaut
W1.. beginning rotational speed
r.. space station radious

2. astronaut start running and achieves a speed of Wa=W1+dW.
D2=m*r^2*Wss + a*r^2*Wa
D2=D1 (the total rotational momentum is conserved)
and with some rearangements we can see:
Wss= W1 - (a/m)*dW
D2.. total rotation momentum at the end
Wa.. ending speed of the astronaut
Wd.. the increase of the astronauts speed over his beginning speed
Wss.. ending speed of the space station

So you see we end up with an astronaut runnin dW FASTER than the speed of the original space stations rotation AND a space station rotating (a/m)*dW SLOWER then before.

But the funny thing is that the moment the astronaut STOPS running the space station will return to ist original rotating speed W1!

Friction and all other forces produced by the astoronauts legs touching the ground of the wheel will make no difference here as this is an inclosed system and it respects the law of the conservation of momentum (linear and rotational).

If this doesnt make sense to you try to think of it this way. Lets say we enclosed you in a hollow sphere in space and gave u the task of movig it around. You woulnd't be able to do it. You could bounce off one of the walls and the spere would go the other way but the moment u hit the opposing wall the spere would stop again. And the center of gravity of the spere and yourself put together would't change in this manuever... so even if you would be in orbit around the earth you couln't deteriorate the speres orbit IN ANY WAY by jumping on the inside of the spere AS THE CENTER OF GRAVITY OF THE SPHERE AND YOURSELF REMAINS CONSTANT AT ALL TIMES

[ December 19, 2003, 08:21: Message edited by: JurijD ]

JurijD · #64 December 19th, 2003, 10:37 AM

Quote:

Originally posted by geoschmo:

quote:
Originally posted by Suicide Junkie:
Assuming everything stays inside the station, the center of mass will remain in a consistent position.

No, I don't think this is right either. Loser is right when he says there will be some reaction to people walking around the inside of the station. As they exert force against the deck plates to go in one direction the station will have an equal reaction in the opposite direction. I just think he's way over estimating the amount the reaction will affect the station. The difference in mass between the person and the station means that almost all the energy moves the person and almost none of it moves the station. So while there will be a force applied to the station it would be imperceptable I believe.

Geoschmo

Oh no you're totally wrong here geoschmo, and Suicide Junkie is completely right. The center of mass (gravity) of the whole system (station+people inside) cannot be changed by movements inside the station.

Think of it this way: when we exert force on the deck plates in one direction we also have to exert the equal but opposing amount on some other deck plate to stop ourselves from flying out of the space station...

so... we bounce off one wall: we start moving in one direction FAST and the space station in the other just SLOWER as it has greater mass. But the POINT of gravity doesn't change.

Say you're holding a rock in space and you throw it away... that action pushes you back when the rock flies away... BUT. The center of gravity of the rock flying away and your body flying away in the opposite direction (just more slowly) DOESN'T CHANGE .. EVER!.

Now say you and the rock are tied together with a piece of string (similar situation as if you were inclosed in a hollow sphere). When the rock is so far away that the string is taught the rock will stop as the string will pull it in the opposite direction of its movement ... BUT so will u! the rock will stop your movement throught the string too! So the result is just a change in the geometry of the system NOT in its center of gravity. And if the rock and you and the string were in orbit the orbit would't change either.

[ December 19, 2003, 08:39: Message edited by: JurijD ]

geoschmo · #65 December 19th, 2003, 01:05 PM

Ok, I see what you and SJ are saying now Juri. Actually I wasn't talking about moving the base to a different orbit, but causing it to tumble, wobble etc, which would make docking difficult. I thought that's what Loser was saying too, but I reread it and see he was talking about actually changing position in space, so I was incorrect when I supported his point there. Movement of people around the inside won't cause the base to move to a different location, but it can have a minor effect on the orientation of the base, as demonstrated by the skylab astronauts. But as I was saying to Loser originally , even that effect would be infinetesimal when dealing with a base as large as what would likely be placed at an L4 L5 point.

narf poit chez BOOM · #66 December 19th, 2003, 01:15 PM

with flexible docking tubes and rotational slats to keep out the sunlight, it doesn't really matter if your spacestation wobbles on it's axis.

Suicide Junkie · #67 December 19th, 2003, 05:10 PM

Quote:

(Now I don't know what happened to space lab, maybe it was just built badly and that was the reason for its orbit deteriorating, the people inside jumping about surely weren't !) Just look at Mir... those russians had some wild parties up there

I think it was just in a low orbit, so the very slight atmospheric drag caused it to drop deeper and deeper over the months/years without thruster boosts.

Narrew · #68 December 20th, 2003, 12:27 AM

Quote:

Originally posted by geoschmo:
The mass distribution could be compensated for by pumping fluid around from point to point in the structure. There will be plenty of fluid available, water, fuel, waste products. It would be a fairly simple matter of having resevoirs at various points around the station connected by tubes and pump it back and forth to keep things in balance. We do it now on airplanes and ships. It can even be done automatically by the computers.

NO, no, no, your almost correct, I read this real late Last night and was too tired to let you all know my master plan to solve the vibration issue...JELLO yep, thats right jello, that will take care of any vibration issue and a little treat for maintenance workers, heck there could be some radiation absorbing material added to the jello for shielding.

Loser · #69 December 20th, 2003, 02:41 AM

Y'all are correct. I have no idea what I was thinking, it's been a while since college and I'm afraid I haven't used physics much since then.

I still think there would be problems with orientation, which would not be corrected by placing the station in a Trojan orbit, but these would be more easily corrected. They could, in fact, be corrected without the expenditure of thruster fuel, but rather through the use of a powered gyroscope driven by whatever power source the rest of the station uses.

One more thing about the spinning wheel. Small changes in the distribution of mass around the wheel will change the point around which the wheel spins. This will change the strength of the illusionary 'gravity' throughout the wheel and could cause problems for any machinery sensitive to changes in it own 'weight'. Amusingly, the more mass you put on one side of the wheel the less 'weighty' things on that side will become.

geoschmo · #70 December 20th, 2003, 02:48 AM

The mass distribution could be compensated for by pumping fluid around from point to point in the structure. There will be plenty of fluid available, water, fuel, waste products. It would be a fairly simple matter of having resevoirs at various points around the station connected by tubes and pump it back and forth to keep things in balance. We do it now on airplanes and ships. It can even be done automatically by the computers.