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ChaosInc
2005-Jan-11, 03:24 AM
The space elevator topic has come-up a few times, but one (among many)of the reasons it seems to go away is the tethers always seem to require some exotic material. My question is, could a pilot system be tried in orbit about the moon? I think that we might be able to produce the tether with something more ordinary, like graphite composites. A tether orbiting the moon would also seem to be useful for future science missions.

The Supreme Canuck
2005-Jan-11, 03:29 AM
I actually brought something like this up before:

http://www.badastronomy.com/phpBB/viewtopic.php?t=17165

Some interesting information in that thread. Hope it helps.

ChaosInc
2005-Jan-11, 03:43 AM
Yes, I should have dug around a bit before posting. Thanks!

But wouldn't it be possible in direct opposition to the earth? Not an ideal location for maximizing utility.

Another question I've had about tethers is wouldn't they be able to generate their own power, like the space shuttle experiment?

The Supreme Canuck
2005-Jan-11, 03:48 AM
I just remembered that I have a few reference materials:

http://flightprojects.msfc.nasa.gov/pdf_files/elevator.pdf
http://www.space.com/businesstechnology/technology/space_elevator_020327-1.html
http://www.firstscience.com/site/articles/elevator.asp

A lunar space elevator seems to be possible only if you place it at one of the Moon-Earth L points. Is that what you mean by opposition?

And yes, the tether should be able to generate it's own power; it can also generate thrust by running current through it.

the_shaggy_one
2005-Jan-11, 06:46 AM
If you ran a space elevator through one of the earth-moon L points, the gravitational forces on it wouldn't be nearly as strong as they would with one on the earth. I think I remember reading somewhere that we could build one with kevlar. I'll look for a link.

Nicolas
2005-Jan-11, 03:36 PM
Blank spot in my mind here, but I thought the shuttle tether generated power because it was moved through the magnetosphere. How would a tether close to the moon generate power? Is solar radition there the source and is it strong enough?

sidmel
2005-Jan-11, 05:27 PM
Just think, you could combine the idea behind a thermal generator and space elevator to generate Gigawatts of power and beam them to the Earth! Image the thermal differentiation between the Earth's atmosphere to space! :D

***Edit to add the possesive to Earth.

The Supreme Canuck
2005-Jan-11, 11:20 PM
Blank spot in my mind here, but I thought the shuttle tether generated power because it was moved through the magnetosphere. How would a tether close to the moon generate power? Is solar radition there the source and is it strong enough?

Oh, crud. I was thinking of a medium Earth orbit to high Earth orbit tether. #-o

It wouldn't work for one anchored to the ground. Thanks.

Van Rijn
2005-Jan-12, 11:31 AM
Blank spot in my mind here, but I thought the shuttle tether generated power because it was moved through the magnetosphere. How would a tether close to the moon generate power? Is solar radition there the source and is it strong enough?

Google on "electrodynamic tether"

Some text from here: (http://www.islandone.org/LEOBiblio/SPBI1ET.HTM)


The electrodynamic tether is an aluminum wire orbiting the Earth. Electrons are extracted from the adjacent ionosphere, enter the wire at one end, and leave it at the other end. The Earth magnetic field generates voltage in the wire. The electrodynamic tether is similar to an electric motor; it can either convert its orbital energy to electric energy (generator), or it can convert electric energy to orbital energy (propulsion). It is the most efficient when it operates in the area of the highest conductivity of the ionosphere: the altitude of 350 km and the sunny side of the Earth. The conductivity drops to 20% of the maximum value at the altitude of 1500 km.

That's simplified - it isn't just an aluminum wire - but that's the idea.

There are some tricks where you could extract energy by stealing energy from the Earth's rotation. There are also tricks with momentum transfer: Take rock from a conveniently located asteroid and send it in the "down" elevator, then extract the energy just as you do with water through a dam.

Energy for a lunar tether? You could use a nuclear reactor, solar panels at the "zero g" point, or beamed power. I like the rotovator and other relatively short tether concepts over the fixed Lagrange point designs.

Nicolas
2005-Jan-12, 11:43 AM
Energy for a lunar tether? You could use a nuclear reactor, solar panels at the "zero g" point, or beamed power. I like the rotovator and other relatively short tether concepts over the fixed Lagrange point designs.

I think you missed what I was referring too. I thought, and it seems I was correct, that the tether generated power because it moved through a magnetic field (ionosphere), just like a wire moving through a magnetic field generates electricity in a generator. Therefore, I thought it would generate very lttle to no electric energy when located at the moon, because the magnetic field around the moon is much smaller compared to the ionosphere of the earth. Your examples as energy sources for the elevator could work of course, but I was referring to the tether in itself generating power, as with the shuttle experiment.


There are some tricks where you could extract energy by stealing energy from the Earth's rotation. There are also tricks with momentum transfer: Take rock from a conveniently located asteroid and send it in the "down" elevator, then extract the energy just as you do with water through a dam.

Can you explain how you would practically do either of those? (I assume you meant a new way to extract energy from the earth's rotation, and not the electrical effect as it is used (tested) now)

Van Rijn
2005-Jan-14, 10:37 PM
I think you missed what I was referring too. I thought, and it seems I was correct, that the tether generated power because it moved through a magnetic field (ionosphere), just like a wire moving through a magnetic field generates electricity in a generator.


Sure, just to be clear: It is a specific kind of tether, and to generate the power the spacecraft loses momentum.




There are some tricks where you could extract energy by stealing energy from the Earth's rotation. There are also tricks with momentum transfer: Take rock from a conveniently located asteroid and send it in the "down" elevator, then extract the energy just as you do with water through a dam.

Can you explain how you would practically do either of those? (I assume you meant a new way to extract energy from the earth's rotation, and not the electrical effect as it is used (tested) now)


Practical? Well, we haven't built these things yet, and we don't have the materials to build an advanced one yet.

Here (http://en.wikipedia.org/wiki/Space_elevator#Launching_into_outer_space) is one way you can "steal" energy from the Earth's rotation. Essentially, you extend a fixed Earth space elevator far beyond Geostationary orbit. As a pod continues to move past the Geostationary point it gains momentum from the Earth's rotation. If you don't want it to fly off at maximum possible velocity - say, only going to the moon, you can retard the speed with a linear generator and use that energy for whatever you want, including lifting payload up below the Geostationary point. Of course, it would take a lot of space elevator traffic and a very long time to have a noticeable effect on the Earth's rotation.

As for the second, it is simply a matter of moving mass down a gravity well. For a rotovator or other orbiting (not fixed) tether scheme, you can balance mass going up and down to keep the tether at a fixed altitude. For a fixed tether system, you can dock a cargo craft to the tether, then move the mass (whatever it may be) down to generate power. Some asteroids have fairly low delta-v requirements, so you could imagine ion drive cargo tugs bringing material back to earth or lunar orbit. There are a number of other possibilities.

ChaosInc
2005-Jan-15, 12:26 AM
What I had in mind by “opposition” was just that, on the opposite side of the moon. If one can calculate a geosynchronous orbit (Lunosynchronous?) for the moon and place the tether in orbit over a point on the lunar equator opposite of the earth, then wouldn’t that be at least quasi-stable? Since the moon’s orbit and spin have the same period I thought it might stay on the opposite side of the moon from the earth.

As for generating power I had the idea that it would have some electrical inductance or coil effect and would induce a current by sweeping through earth’s magnetic field, even if the field is very weak.

Van Rijn
2005-Jan-15, 01:25 AM
What I had in mind by “opposition” was just that, on the opposite side of the moon. If one can calculate a geosynchronous orbit (Lunosynchronous?) for the moon and place the tether in orbit over a point on the lunar equator opposite of the earth, then wouldn’t that be at least quasi-stable? Since the moon’s orbit and spin have the same period I thought it might stay on the opposite side of the moon from the earth.


Yes, you run the tether through the L2 point from the lunar farside. A fixed lunar elevator can either be on the nearside and go through the L1 point or on the farside and go through the L2 point. This isn't "synchronous" in quite the same way as a Geosynchronous Earth elevator is - the Lagrange points exist because of the interaction of the earth and moon. If the moon were a free body, the Lagrange trick wouldn't work, and would depend on the rotation rate. Venus, for example, with its extremely slow rotation, would require an insanely long synchronous elevator.

Anyway, for more information see

http://www.badastronomy.com/phpBB/viewtopic.php?t=17165

Also google on "lunar elevator L1" or "lunar elevator L2"



As for generating power I had the idea that it would have some electrical inductance or coil effect and would induce a current by sweeping through earth’s magnetic field, even if the field is very weak.

No, it is only practical near the earth, and there are other ways to get energy: solar, nuclear, and momentum transfer.

The Supreme Canuck
2005-Jan-15, 02:39 AM
I've always wondered how damage to a space elevator would be repaired. Say a meteor punches a hole in the cable without severing it. How is that fixed? Is it made of detachable segments that can be replaced?

eburacum45
2005-Jan-16, 12:45 AM
A wacky idea I have had is to build an elevator to the Lunar L2 point, and send material down it to the far end (beyond L2);
the elevator could be used to fling material anywhere in the solar system.

You could use this elevator to send aluminium mirrors to the Earth/Sun L1 point to build a combination solar power swarm/sunshade;

the sunshade could act as a cooling agent for the Earth if or when it heats up due to human impact on the atmosphere and environment.

Who says there isn't a technological fix for everything...