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Space Ladder

  1. Sep 28, 2005 #1
    I read a newspaper article today about trials being approved for a "space ladder." Apparantly they can drop a line from a sattalite that orbits above a set point on earth. Does anyone know much about these. Obviously there are some limitations to simply dropping down a loop of rope attached to a motor or NASA wouldn't spend millions on rockets but I can't imagine what those limits would be.
     
  2. jcsd
  3. Sep 28, 2005 #2
    Yes its being discussed in another thread but has grown off topic. a company called liftport group has set there dead line for 2018. They plan to drop a ribbn of carbon nano tubes from earths orbit down and anchor is somwhere in an ocean. Space crafts with attach to the ribbon and drive themselves to the top then use a thurster of some sort to postition them selves in orbit.
     
  4. Sep 28, 2005 #3

    JesseM

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    The usual name for this is a space elevator--there's a good wikipedia article about it here, and an article from NASA's site here. To build a strong enough cable they'd need a material made of long strings of carbon nanotubes--they're not nearly there yet, but there's been a lot of progress in this area. Still, even if it could work it's probably a long way away.
     
    Last edited: Sep 29, 2005
  5. Sep 28, 2005 #4

    DaveC426913

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    There's an entire newsgroup with scientists who discuss the intimate details of the space elevator. I was on it for a while.
     
  6. Sep 28, 2005 #5
    well what are your opinions of it, is it possible?
     
  7. Sep 29, 2005 #6
    Thanks, links were excellent
     
  8. Sep 29, 2005 #7
    It's not possible yet, it's expected that with advances in material technology it may become theoretically possible.

    However actually building the thing is another matter. As I understand it the plan would be to have another huge mass on another tether "above" the orbiting station. You could then let the lower tether down while letting the higher one up to keep the system in balance.
    However I've no idea how you would catch the lower end of the 200 mile rope when it's swinging in the breeze!

    There's also the very real prospect of something going wrong, what kind of damage could a 200 mile cable do to those beneath it should it fall!
     
  9. Sep 29, 2005 #8

    JesseM

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    This problem was analyzed too, I believe most of it would burn up in the atmosphere.
     
  10. Sep 29, 2005 #9
    It's been a while since I read up about this so I'll follow up the links you suggested above.

    One thing I couldn't find out before was why the cable had to be attached to such an impossibly tall tower. Is this just to minimise the length or to avoid weather conditions?
     
  11. Sep 29, 2005 #10

    JesseM

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    The wikipedia link says the thickness of the cable has to increase exponentially from the base to geostationary orbit, so one way to minimize the maximum thickness is:
     
  12. Sep 29, 2005 #11
    I should have checked first!
    Thanks.
     
  13. Sep 30, 2005 #12

    Chronos

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    No known material has the strength to weight ratio necessary to make such a cable.
     
  14. Sep 30, 2005 #13

    JesseM

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    Not true, the calculations suggest that carbon nanotubes would do the trick, although they'd have to find a way to make much longer ones than they've made so far.
     
  15. Sep 30, 2005 #14

    Chronos

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    Show me the material! I'm an engineer. If I can't abuse it, I can't use it.
     
  16. Sep 30, 2005 #15

    JesseM

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    They are able to make carbon nanotubes in small amounts, if that's what you're asking. It used to be that they could only make them a few nanometers in length, but recently they discovered a new technique which allowed them to make nanotubes up to 4 cm long, and according to the researchers there's no ultimate limit on the length of nanotubes that could potentially be produced using this technique. See the article here:

    http://www.trnmag.com/Stories/2004/110304/Nanotubes_lengthen_to_centimeters_Brief_110304.html
     
  17. Sep 30, 2005 #16

    Chronos

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    Still a bit short to tether it to an earth orbit. Nanotubes are too brittle... at least by current standards... agreed?
     
  18. Sep 30, 2005 #17

    HallsofIvy

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    Arthur Clark wrote a science fiction novel, "The Fountains of Paradise" about this idea- he called it a "space elevator".

    HOWEVER, you can't have one end in "stationary orbit" around the earth! The point of stationary orbit is that for an object in such an orbit, the force necessary to keep the object in orbit at that speed is precisely the gravitational force at that height. If a little below, gravity pulls you down. If a little higher, gravity is not enough to keep you from going higher still.

    But every part of the "ladder" hanging between the satellite and earth would have gravity pulling it down- a satellite in stationary orbit can't offset that- there is no upward force on it to do that. What you would need is a satellite well above stationary orbit so that the total "upward pull" (i.e. gravity is not enough to maintain orbit) of the part above stationary orbit will offset the downward pull of gravity on the part below. Since gravity falls off as 1/r2 the satellite "anchoring" the ladder would have to be many times "stationary orbit" away.
     
  19. Sep 30, 2005 #18

    russ_watters

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    Well, either that or it would just have to be a little further away and enormous.
     
  20. Sep 30, 2005 #19

    turbo

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    It's goin to have to be enormous anyway, isn't it? After all, it has to provide enough tension on the tether to support the full weight of the tether, the weight of the climbing craft (elevator), AND the weight of payload, plus the inertia involved in accelerating the craft and payload.

    Maybe it would be easier to construct a very large rail-gun near the equator, facing east and keep the entire launch mechanism anchored to the ground. It might be possible to put up small payloads at regular intervals and assemble things in near-Earth orbit, instead of launching big chemical-fueled rockets every few months.
     
  21. Sep 30, 2005 #20

    JesseM

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    Brittle? I hadn't heard that brittleness was a problem, do you have a source? I thought the problem was just being able to make sufficiently long nanotubes, and since that new technique increased the maximum length by a factor of 10 million or so, I don't think it's so implausible that with people continuing to pour money into this (carbon nanotubes have many more practical applications besides space elevators) the maximum length will continue to increase rapidly.
     
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