For Phyl: Space Elevators

>> Thursday, March 18, 2010


Phyl asked (some time ago - sorry I took so long): What do you think of that "elevator" idea of getting off the earth?

I'm glad you asked. In short, not much. Now, after waiting so long, you deserve a better answer than that, and you'll get it, but I thought I'd cut to the chase.

The idea, for those of you who aren't familiar with it, is that one could construct a cabling system that reach all the way out to geosynchronous orbit, a point that would rotate directly overhead at the equator. You'd need a substantial mass on the other side to balance the total (cable and slide(s)) going down to the surface so that the center of gravity would stay at geosynchronous. z

If you could construct this, the idea goes, you could just ride an elevator up to orbit without requiring rockets and such-like, bringing yourself to orbital velocity and making it far easier to launch stuff out into outer space.

If it worked.

The general thought, for those excited by this notion, is that, if we could just get a material strong to take bear 36,000 km of its own weight but light enough not to make the amount of mass required above geosynchronous less than onerous, this would be workable. Nanotubes look promising, but they're only microscopic so far. Putting them in a resin matrix or whatever drastically reduces their strength. (I could tell you the ridiculously high strength they need but it won't mean much to most of you: 65–130 GPa tensile strength. Kevlar gets up to ~4, most steel is <2 and quartz gets up to 20).

You'll have to pardon me if I seem skeptical. Say you solved all your material problems and ended up with a cable that only weighed, say 500 metric tons. A 13mm kevlar rope 38,000 km long weighs more than 3240 metric tons. That's a bit less than 10X the mass of the ISS sent far far higher, geostationary.

The first question that comes to my mind is, if you need to send three million metric tons into high orbit to get this up and running, what exactly are you saving on rockets later? Once I've fired 10 station's worth of hardware up there, I might as well have just launched from the ground. And, even having all of this in space doesn't change how much energy you need to put into the system. To get into geostationary orbit, even via elevator, requires the same amount of energy to raise the potential and increase the speed.

You can't get something for nothing. You have to put energy into whatever you put into orbit to get it there. A lot of energy. If you use the energy of the elevator to pull it up, you will slow down the elevator and/or lower the orbit.

Truth is, I just don't like it. I don't think we're close to this and, if we can do it, I'm not sure it will buy us as much as we think it will.

Truthfully, if I was going to look for something to help us get into orbit without pure rockets, I'd be looking for an electrodynamic rail system to launch something to several times the speed of sound and then just use a rocket to kick it up the rest of the way. In this way, without rockets, one could take great masses up fairly high (perhaps put the rail up a mountain) and accelerate it at a controlled rate over a long distance. Still uses energy, of course; can't get away from that. You also have a large infrastructure to build, but it's built here on earth where that's relatively easy. And you can use methods we've already used. The US Navy has built a rail gun capable of accelerating a 3.2 kg projectile to Mach 7. It's only a matter of scale to make it do more.

Hope that helps.

7 comments:

  • Project Savior
     

    As far as the counter weight I've put some thought into it for a different problem. An asteroid could be captured and put into geo.
    With an Earth intersecting asteroid at most you would need to change its velocity by 8 mph. Doable with todays, almost, technology to get it into an orbital capture path using the moon's gravity to slow it down.
    With enough time you could maneuver it to get it to the right orbit.
    You still have the problem of material strength, buckytubes are only 5,000 times as strong as steel (not enough).
    You also have the problem of building anything useful (even a straight rail) 26,000 miles long. Even if you managed to build it at a mile a day, no simple task it wouldn't be completed for 71 years.
    Did I just say a large engineering task was impractical, I must be slipping?

  • Jeff King
     

    It does help.

  • Stephanie Barr
     

    Project Savior, I like ideas that are cool, but I like smarter engineering, not bigger for coolness sake. And this is just my opinion.

  • The Mother
     

    Yeah, but we can dream, can't we?

  • friendly_nelson
     

    This comment has been removed by the author.

  • friendly_nelson
     

    Hi, I thought that perhaps providing a hyperlink to the description of a different type of space elevator may answer some of the questionable proposals that seem to plague a centrifugally spinning CNT tether.

    The proposed system is still very much in the theoretical stages and the inventors are not English speakers, but it is worth looking at.

    http://spaceshaft.org

  • Phyl
     

    Stephanie! Now it's my turn to be very late getting back here! Thank you so much for that answer.

    It's been a while since my physics courses, but I did have the nagging feeling that the elevator idea was still trying to get something for nothing, and that you couldn't really shortcut the energy needed to get out to space -- because that's simply how much energy you need, whatever system you're using.

    I've been sceptical about the idea all along myself. So I'm really glad I know someone who can talk about it with the sort of background to be able to analyze it realistically.

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