#### Science Forum

sonhouse
Science 24 Dec '14 06:48
1. sonhouse
Fast and Curious
24 Dec '14 06:48
I understand the part where you have an extremely strong cable going up to geostationary orbit and such but I am having difficulty visualizing the part where you extend the cable to 100,000 km or so past geostationary orbit.

I can see if the cable was a super strong tower and the whole thing spins around the Earth and such, the whole thing swinging around in a straight line.

But you have a flexible cable and then extend it out further, way further.

The orbital velocity of something up 100,000 km is a lot lower than at geostationary altitude.

So how can the cable maintain itself to be straight? Is it just centripetal force keeping it straight? The whole cable would have to be in a straight line I would think but how do you keep the far end at the faster velocity of the lower sections?

It looks to me like the far end of the cable would lag behind and so would start some kind of whipping effect.

Am I missing something here? I keep thinking something like a rocket at the top would have to add velocity to keep up with the lower altitudes like at geosynchronous height and if so would a one time thrust be enough or would the whole affair be unstable and require velocity adjustments every now and then like the ISS has to use rockets maybe once a month to keep it at the right altitude.

Just don't know. Anyone with more knowledge of this subject out there?
2. 24 Dec '14 07:511 edit
Originally posted by sonhouse
I understand the part where you have an extremely strong cable going up to geostationary orbit and such but I am having difficulty visualizing the part where you extend the cable to 100,000 km or so past geostationary orbit.

I can see if the cable was a super strong tower and the whole thing spins around the Earth and such, the whole thing swinging aroun ...[text shortened]... t the right altitude.

Just don't know. Anyone with more knowledge of this subject out there?
The orbital velocity of something up 100,000 km is a lot lower than at geostationary altitude.

If there was no cable involved, yes. But you have to imagine the counterweight for the cable (attached to the end of the cable ) at very roughly ~100,000 km altitude (which is beyond geostationary altitude ) being artificially made to have an orbital velocity much greater than that by it being attached to the cable and constantly pulling at it as the counterweight is constantly 'trying' to swing further out. It is this centripetal force from the counterweight that keeps the cable under tension and straight (and no need for rockets to keep it straight )
3. 26 Dec '14 05:37
Put a ball on a string and swing it around. The string remains roughly straight and the ball appears to defy gravity. As humy says, the string changes the physics from orbital gravity physics to string swinging physics.
4. sonhouse
Fast and Curious
26 Dec '14 15:28
Put a ball on a string and swing it around. The string remains roughly straight and the ball appears to defy gravity. As humy says, the string changes the physics from orbital gravity physics to string swinging physics.
So there needs to be an initial acceleration of the end piece to keep tension. A one time affair.
5. 27 Dec '14 06:53
Originally posted by sonhouse
So there needs to be an initial acceleration of the end piece to keep tension. A one time affair.
Yes, but it can be provided by the earth when the device is built. Stick a ball on a string attached to the rim of a bicycle wheel and rotate the wheel constantly and the ball will naturally try to get to its maximum distance from the wheel which equates to a perpendicular space elevator.
I believe a real space elevator has significant mass in the 'string' so the physics will not be so simple and it may not be entirely vertical.
6. 27 Dec '14 12:01
Yes, but it can be provided by the earth when the device is built. Stick a ball on a string attached to the rim of a bicycle wheel and rotate the wheel constantly and the ball will naturally try to get to its maximum distance from the wheel which equates to a perpendicular space elevator.
I believe a real space elevator has significant mass in the 'string' so the physics will not be so simple and it may not be entirely vertical.
If one day we have a space elevator - what will happen with the satellites in orbit, especially with the geostationary satellites? How to avoid the wire?
7. 27 Dec '14 15:37
Originally posted by FabianFnas
If one day we have a space elevator - what will happen with the satellites in orbit, especially with the geostationary satellites? How to avoid the wire?
If they're geostationary, they'll avoid the equally stationary elevator by remaining, well... stationary.

The big problem is the non-stationary satellites. I don't think that problem has been solved yet, and I suspect that's because it's mainly an organisational and (above all) political issue.
8. 27 Dec '14 17:281 edit
Originally posted by FabianFnas
If one day we have a space elevator - what will happen with the satellites in orbit, especially with the geostationary satellites? How to avoid the wire?
the geostationary satellites would have the same orbital speed as the part of the cable that is at the same altitude as them so there should be no risk of a collision there.

As for the satellites orbiting lower down which will have a faster orbital speed than the cable; either the cable will have to be positioned where all the orbits of those satellites will miss the cable or those satellites that would be in collision course with the earmarked position of the cable would have to have their orbits altered.

Perhaps a bigger problem would be how to stop space junk hitting the cable as the position of each bit of space junk tends to generally be a lot harder to monitor than that of satellites and I guess would also tend to be a lot harder to do something about it if it is inconveniently in collision course with the cable. -and that is not even to mention the several other big potential hazards to the cable!
9. 27 Dec '14 22:31
Originally posted by humy
the geostationary satellites would have the same orbital speed as the part of the cable that is at the same altitude as them so there should be no risk of a collision there.

As for the satellites orbiting lower down which will have a faster orbital speed than the cable; either the cable will have to be positioned where all the orbits of those satellites will ...[text shortened]... he cable. -and that is not even to mention the several other big potential hazards to the cable!
You mean that there will be no geostationary satellites in the immediate vicinity of the wire? Well, that sounds likely.

But I wonder how much protection is needed to for the wire from the low orbit satellites and space junk. The different of velocity will be bigger than that between two satellites colliding.

Because if the wire is damaged and split, then the outer part will be flung out and lost, and the lower part will fall down, a mighty fall, an uncontrolled fall.
10. sonhouse
Fast and Curious
28 Dec '14 03:01
Originally posted by FabianFnas
You mean that there will be no geostationary satellites in the immediate vicinity of the wire? Well, that sounds likely.

But I wonder how much protection is needed to for the wire from the low orbit satellites and space junk. The different of velocity will be bigger than that between two satellites colliding.

Because if the wire is damaged and spli ...[text shortened]... l be flung out and lost, and the lower part will fall down, a mighty fall, an uncontrolled fall.
I think if the cable broke above the geosych height it wouldn't just come down, maybe get unstable but I don't think it inevitable it would just fall.
I imagine they would have to have repair facilities for such an eventuality though, fast repair options, double, triple cables and such for redundancy.
11. 28 Dec '14 11:13
Originally posted by sonhouse
I think if the cable broke above the geosych height it wouldn't just come down, maybe get unstable but I don't think it inevitable it would just fall.
I imagine they would have to have repair facilities for such an eventuality though, fast repair options, double, triple cables and such for redundancy.
Not only at the geosynk orbit it will fall down uncontrollably, but way further up. Because of the weight of the cable.

And you don't have time for a repair crew if the cable brakes. It lower part will fall and the upper part will go at the tangent's direction outwards.

Unless you have a shield around the cable that can withstand a collision. But that on the other hand will make the cable much more heavier. And expensive.

I'm not very excited about an space elevator. the thought is fine, but in reality it is not economically feasible.
12. sonhouse
Fast and Curious
28 Dec '14 17:46
Originally posted by FabianFnas
Not only at the geosynk orbit it will fall down uncontrollably, but way further up. Because of the weight of the cable.

And you don't have time for a repair crew if the cable brakes. It lower part will fall and the upper part will go at the tangent's direction outwards.

Unless you have a shield around the cable that can withstand a collision. But th ...[text shortened]... ed about an space elevator. the thought is fine, but in reality it is not economically feasible.
They may build in safeguards like having cutters cut just above the geo orbit height so that doesn't happen. It will be in a stable orbit, I guess you are referring to the tension on the cable pulling down the geo end. That might be. But I think they will build redundancies for such a contingency though, obviously not at first but they would continue to reinforce the original cable I would think.

It is all a moot point however since the cable right now can't be made in more than a few cm in length. They need hundreds of thousands of km of the stuff before they can even dream of building an actual elevator though.

Not only do they need such enormous lengths, they also need a way to safely splice lengths together, something we have no idea how to do ATT.

That said, I wonder if they could run such an elevator cable all the way to the moon, say to the north or south pole of the moon on a tower with a way to pull in slack and let it out as the distance to the moon changes from day today. Wouldn't THAT be a trick! ride a cable to the moon!

One thing about the outer counterbalance that is interesting, if the thing can be made stable and such, a car riding up the cable can be flung off at any distance to give it more than escape velocity and with the proper timing could enable interplanetary trips, at least for cargo to say Mars or Europa or some such, since the cable would be sweeping out a curve all the way round Earth more or less in the ecliptic, there would be a launch window once a day for such flights, free velocity for space crafts.

Don't know if THAT has been discussed but it seems obvious to me such a system could work for interplanetary travel, at least to reduce the amount of fuel such a craft would have to use getting from point A to point B in the solar system.
13. 28 Dec '14 20:00
Originally posted by sonhouse
They may build in safeguards like having cutters cut just above the geo orbit height so that doesn't happen. It will be in a stable orbit, I guess you are referring to the tension on the cable pulling down the geo end. That might be. But I think they will build redundancies for such a contingency though, obviously not at first but they would continue to rei ...[text shortened]... ount of fuel such a craft would have to use getting from point A to point B in the solar system.
(1) The end of the cable must be way beyond the geo-stationary orbit. So the end of the cable has already a velocity way beyond the limit to be flung out from the earth. Just let it go and it will leave by itself. But the question I would like to know - how far out is the minimum distance from the surface will the space-lift reach? Theoretically with infinite strength and infinitely low mass and no margin of safety - at the geostationary orbit - but when we take the mass of the wire, and a necessary safety? Halfway to moon? I haven't seen any calculation on that.

(2) It is not possible to attach a cable from Earth to the moon of obvious reasons. There is no point on earth where the moon is above the horizon all the time the year round. The cable will wind itself around the planet. When in the far future (more than a billion of years) the earth's rotation is syncronized with the orbit of the moon, then it will be possible, but not until then.
14. 28 Dec '14 20:07
Originally posted by FabianFnas
It is not possible to attach a cable from Earth to the moon ... ...
...When in the far future (more than a billion of years) the earth's rotation is syncronized with the orbit of the moon, then it will be possible,.
No it won't! That is because the Moon is orbiting around the Earth in the opposite direction from the direction of the rotation of the Earth around its own axis thus it will always appear to be moving around the earth from the viewpoint of any point on the Earths surface.
15. Soothfast
0,1,1,2,3,5,8,13,21,
28 Dec '14 20:37
Originally posted by humy
No it won't! That is because the Moon is orbiting around the Earth in the opposite direction from the direction of the rotation of the Earth around its own axis thus it will always appear to be moving around the earth from the viewpoint of any point on the Earths surface.
Eh? Viewed from above the north pole, the Earth rotates and the Moon revolves in an anticlockwise direction.