09-10-2001 00:35									#31
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Also if you put the bukey cable on the equator imagine the massive ecodamage it would cause to operate the stuff (I mean, going through a snow storm just to launch a satellite )!

Slowing down the Moon eh? Well, good luck trying ...

	09-10-2001 01:48									#32
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I read once that in the earth's early years...say 4.4 billion years ago, that the Moon was still molten, and it's orbit was significantly closer to earth. As such, it looked the size of a frisbee held at arms length and was brilliant red.

=S=

	09-10-2001 08:25									#33
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Op! And there is always the problem with the moon- it also has severe gravitational influence on our immediate space.

This thing would be a LOT harder to build than we think.

	09-10-2001 10:52									#34
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It would be *complicated*, I shudder to think of the calculations & motions required to keep the thing from 'falling'. But hey, what do we have computers for?

Also the cable would ossiciliate slightly, harmonic motion is often more stable than no motion. Then the computers just account for the moon (& other graviational anomolies), random collisions, atmospheric drag and the rouge spitball and have the relevant thrusters fire to keep the system stable.

In space even really complicated stuff is often simple as long as you can handle the calculations, in the same vien it should be possible to have a space mirror put in an orbit and spinning in such a way as to always reflect light onto a certain part of the planet. The dynamics are as complicated as hell, but computers can handle it.

	09-10-2001 15:56									#35
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quote: Originally posted by Blake But hey, what do we have computers for?

I thought the end all, be all of computer existance was so we could play CIV?

But you are right- its not an impossible task. If they can get man on the moon, surely they could build an elevator half-way to it!

But as is always true: the more complex a thing, the more critical failures become possible. This Space Elevator better have BOOKOO amounts of sub-systems, back-up systems, and quadruple back-ups to the back-ups (and cable TV; or do you think sattelite TV?).

	09-10-2001 16:08									#36
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quote: Originally posted by Anunikoba I thought the end all, be all of computer existance was so we could play CIV?

No, being able to play Civ is the point of MY existance!

quote: But as is always true: the more complex a thing, the more critical failures become possible. This Space Elevator better have BOOKOO amounts of sub-systems, back-up systems, and quadruple back-ups to the back-ups (and cable TV; or do you think sattelite TV?).

Yeah, all the elevators systems would be considered 'critical', as in system failure is not acceptable. (much the same as Nuclear powerplants).
Altough it would be *reasonably* stable, could probably stay up for a week or year even with no power.
And the elevator wouldn't reach halfway to the moon, it would reach to slightly above geo-sync orbit. (my bad)

Tho if they tried to build an elevator on the moon it would reach out past earth, which would present some problems

	09-10-2001 16:44									#37
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Oh, a reply to the electromagnetic cableless idea.
It's called a massdriver. There is a couple of minor problems:
Extreme Acceleration turns humans into mush.
Atmospheric drag. This causes a large energy loss.

However it could still be used to launch mineral packets in airless enviroments.

It would work well on the moon. Also the astroid part of the elevator could have a mass driver mounted - but care would have to be taking to balance launching/recieving to prevent the astroid getting knocked out of orbit. (or packets could be timed so their launch keeps the astroid in stable orbit - free corrections)

Mass drivers have a couple of other uses, mineral packets could be used to bombard, or it could be rigged to work as an ion cannon.

(In the book Moon Rise, and it's sequal/prequel (I forget which) by Ben Bova a mass driver is built on the moon)

The moon cable idea isn't that crazy, because one side of the moon always faces earth just "dangle" the cable off, and have a counterweight close enough to earth to counter the moons gravity. Problem is the moons orbit isn't perfectely circular... so the cable would tend to sway. Altough it should be possible to have stable oscciliations.
Seeing the moon has fairly low gravity and no atmosphere it could be just as easy to use mass drivers and rockets.


I read another book (I forget what it's called, and who wrote it - yeah yeah, I've forgotten almost as much as I know... ) with a 'space fountain' constructed on mars. The theme being hundreds of big iron hoops get lobbed into space, fall back down to the lobber on mars, get lobbed back up in a cyclic perputal motion type idea (mantained by a powerplant), the hoops generate massive magnetic and electromagnetic fields which could be used to propel 'fountain shuttles' at a very low cost. The physics in the book was fairly dubious (IMO) so I have no idea if the idea would be possible or not.

	09-10-2001 16:55									#38
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While I'm spamming this thread I may as well clear up the little confusion over the length of the cable.

I was thinking of a space elevator on Mars, which would be three times the length of the Martian equator (may not even be 3 times... my memory is a bit hazy on this... but it is longer than the equator).
Earths one would be somewhat shorter (just above geo-sync).
Anyone else get those two planets mixed up ?

	09-10-2001 21:54									#39
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Here is a RL lower-tech variation.

Virtual Beanstalk Project

	09-10-2001 22:47									#40
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Hmmm. About Smack's Elecoaster "idea".
How about using the cable as a sling? To propel things around. Earth does rotate at 100,000 kph, IIRC.

And how much material would you need for a cable ethat is, say, 20 metres thick and 100,000 kilometers long. Something like 18 million (Counted in my mind, beware) cubic metres of mass.

But that's small, everyone ever read Orbitsville?

	10-10-2001 08:44									#41
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I don't think that the Earth rotates quite as fast as 100,000kph, more like 1300kph (1000 miles per hourish).


But I do like the 'dangling moon cable' idea. But not really. The reason is: how do you slow your self down once you approach the moon? You will not be in orbit, but heading straight towards the moon, full gravitational effects being applied. Ouch!

	10-10-2001 21:07									#42
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I suppose that some engineers have thought about the effect that the Moon would have on the system, but it seems like it would be a major problem in that it would greatly disturb the gravitational balance. When the Moon was over the cable, it would be adding to the outward pull of the counterweight and when the Moon was on the opposide side, it would be working againist the counterweight. Since we are talking about a cable length of at least 3 times Earth's diameter, the forces would be perhaps be over 49 times (2 cablelengths + 1 EarthDiameter = 7 Diameters; squared = 49) greater than those of the ocean tides, which move a lot of water around at just 1x power.

The counterweight would have to be big enough to balance the system when the Moon was on the opposite side (or else the cable would go slack). That would mean that the entire cable and the EarthStation would have to bear the extra 49 tides worth of force when the Moon was overhead.

Am I missing something, or is this all true, but still managable?

There is also the effect due to the Moon's orbit being in a different plane than the Equator, but I would think that this effect would only be, as I think was mentioned in a previous post, to swing the system in some kind of oval around the true vertical and probably would not be particularly problematic in itself

	10-10-2001 22:59									#43
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Hmm, perhaps the cable itself would be able to adjust for the differences in the monthly cycles, and other things like the temperature change from night/day. Sort of like an antenae.

Of course, a very complex one.

	11-10-2001 03:02									#44
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I wasn't just spamming this thread.
I did mention that I got earth and mars mixed up. Earths elevator .

Anyway, a quick google search suggest earths geo-sync point is
35,786 km above earths surface
earths diameter is 12,756km
Which happens to be 2.8 times earth diameter, not 3 times. So there. (argggggggg )

Mars geo-synch 17,200km
6,800
for 2.5 times Mars diameter...

Yegads. Turns out I was right afterall. I hate it when that happens....

Anyway. I think the solution to the tidal effect is to make the cable ossciliate, like a big pendulum (actually, like a big skipping rope....) with a cycle of one day, this would have the effect of pulling the astroid 'up' and 'down' over the period of one day, in time with the tidal effects of the moon and the sun. I knew it had to ossciliate for some reason, and I bet the tidal effects is it.... this would mean it would be moving through the atmosphere, but considering that most of the motion of the cable would be at the halfway point, which is much much higher than earths atmosphere the friction from the cable swaying should still be quite minimal.

	11-10-2001 07:52									#45
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We've had several threads on space elevators and mass drivers in OT, but I'm not sure if they're accessible, now...interesting stuff.

	11-10-2001 11:02									#46
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Blake,
There may be something to that oscillating idea, but it's too late here for me to be able to think it through. I think the cable would have to be moving in more or less a single plane to make it straighten and contract to move the counterweight, instead of a creating a jump-rope un-bent banana shape which wouldn't necessarily change the distance between the ends (unless you all jump rope differently down there). Wouldn't it take a lot of energy to keep moving the counterweight back and forth, or is there some magic restoring force I'm not thinking of. I can see how the jump-rope motion might not take much energy to maintain, but as I said before, I don't think it does the job. Reeling in the weight and letting it out again could do the trick if the natural forces would do the work in one direction and the energy could be stored and if it didn't have to be moved so much each day that something would wear out and break before long.

	11-10-2001 12:31									#47
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I think that tidal forces will take care of themselves; over the course of eighteen thousand-odd miles, even tidal forces fourty-five times greater than normal will not produce that much additional stress on the cable.

	11-10-2001 15:52									#48
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Okay. You've got an asteroid a bit above geostationary orbit (the lighter it is, the further out it has to be), with the same orbital period as if it was in geostationary orbit. That means that it's moving too fast for a stable orbit at its altitude, so you need the cable to stop it from flying further away. The Earth-asteroid system becomes two masses attached by the cable, spinning to keep it taut.

That puts some tension on the cable, but there's a much greater source of tension - the weight of the cable itself. For instance, consider a short piece of string (10 cm or so). The string might weigh about 0.1 grams, in which case 36 000 km of it (to get to geostationary orbit) would have a mass of 36 tonnes, which is rather more than a simple piece of string can suspend without breaking. In practice it wouldn't actually weigh that much, because the force of gravity decreases as you approach the altitude of geostationary orbit to 7% of its value on the surface (by F = Mm/r^2), and the fact that the cable would be rotating would allow some of the weight to act as a centripetal force, but you still end up with over 10 tonnes (rough estimate, can't be bothered doing the integrals at the moment) of tension in a simple piece of string.

This is why you don't use ordinary string for the cable. Very few materials have the tensile strength to be used in this way - graphite whiskers come close, spider silk could probably manage it but would be too difficult to refine into such a cable, and some forms of carbon nanotubes (buckytubes) should have the strength but can't yet be manufactured in bulk. And this is after you use a few fancy tricks like thinning the cable in the parts that suffer the least tension, so as to reduce the stress on the rest of the cable.

Kim Stanley Robinson (iirc) correctly noted in his Red/Green/Blue Mars trilogy that it would be quite a lot easier to build a space elevator on Mars because of its lower gravity, and the resulting more modest requirements for the cable.

By the way, I spent some time a while ago trying to work out the optimum shape of the cable - how its thickness should vary with altitude so as to minimise the requirements for the material. Some interesting results are that the thickness reaches zero on the Earth's surface, and has a maximum at the altitude of geostationary orbit. If I thought anyone was interested I could probably dredge up or recreate the program I used to calculate it, with the supporting physics.