1. lordofhats
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    lordofhats Contributing Member

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    Orbital Elevators

    Discussion in 'Research' started by lordofhats, Jan 17, 2009.

    I'm planning on adding the concept of Orbital Elevators to my next entry in my 003 universe but am having a problem writing it. I like to at least know when I'm writing things in an unrealistic fashion so I can prepare for that, but for the Orbital Elevator I'm having a scale problem.

    What I've read says that the cable needs to be thin but nothing says how thin. Does anyone know? Are we talking thin as in real thin, or thin in a big way? I also don't understand exactly how the counterweight is organized. In relation to the cable, what kind of supposed weight is needed at the end? Are we talking little weight like boulder sized or huge weight like sky scrapper sized? Mostly its a proportion problem for me and I do want to at least have a semi depiction of realism when writing.
     
  2. Cogito
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    Cogito Former Mod, Retired Supporter Contributor

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    The idea was originated by Arthur C. Clarke, and is featured in his novel The Fountains of Paradise. Larry Niven also explored the idea in The Descent of Anansi. Both of these books will proivide a lot of background in the requirements of the technology (for one thing, you want the anchor points to be on the equator).

    The head of the elevator must be in geosynchronous orbit, which is about 25000 miles out. It needs to be massive enough not to be disturbed from its orbit by the combined weight of the transport cable and the payload. The transport cable should be as low mass as possible, because all the mass elements along its length each have their own prefered orbital velocities, and constraining them not to follow these orbits adds stresses to the system. Still, the cable must be strong enough to withstand these forces and those produced by raising or lowering the payload.

    This might also give some insight into the difficulties in building it in the first place.

    To give a sense of scale, 25000 coincidentally also happens to be the circumference of the earth. So aimagine the weight of even the thinest cable you can imagine circling the equator, and imagine that it is at least strong enough to support that weight.
     
  3. Wreybies
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    Wreybies The Ops Pops Operations Manager Staff Contest Administrator Supporter Contributor

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    The Fountains of Paradise by ACC is one of the first novels I remember reading. I have it translated into Russian as well.

    What I find appealing about the idea of a space elevator is that while it has a rediculous initial cost, cost of use is significantly, significantly lower than current methods to reach Earth orbit. I don't think economics get taken into account sufficiently in the writing of science fiction. Just because we can do a thing doesn't mean we can afford to do that thing.

    ...or that we will afford to do that thing, for that matter. Economics are complicated and can push events in directions that would seem counterintuitive. Over a hundred years after their introduction, cars still run on rotten stegosauri even though there are, and there have been, better methods around...

    I like the idea of a space elevator. Once you get passed the technological hurdles mentioned by Cog, it makes economic sense.

    Space Elevator Economics
     
  4. lordofhats
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    lordofhats Contributing Member

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    The economics were one the reason's I wanted the idea. In the previous bits of the series things got into space the old fashion way and got back onto the planet the old fashion way, but I figured, economically an elevator is more efficient so wouldn't those greedy space corporations want to reduce their long term costs :p?

    Thanks for the book suggestions Cog. I'll see if I can find some copies.
     
  5. Mesuno
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    Mesuno Member

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    I would think that space elevators require significant industrialisation of space to be already in place. The counterweight needs to weigh in the order of millions of tonnes - maneuvering an asteroid into orbit is really the only effective way to do this. How do people on earth feel about a corporation or government putting an asteroid that close to earth if it could conceivably wipe out all life on the planet if it fell?

    Also, the idea of 'weight' is a bit strange when you start talking about orbiting asteroids and cables hanging from space. The asteroid will actually be pulling away from earth (not resting downwards on the cable) so the cable needs to be exceptionally strong under tension.

    You'll also need a lot of power (in terms of thrust) to keep the anchor asteroid in its orbit. The cumulative effect of wind against the whole length of the cable will be significant.
     
  6. Cogito
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    Cogito Former Mod, Retired Supporter Contributor

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    The "counterweight", as you call it, need only be sufficiently massive to not be greatly disturbed by the mass of the payload and the cable.

    There is no danger of te elevator head falling. If you sever the cable, the elevator head will continue to follow a geosynchronous orbit. It would take a tremendous amount of energy to cause it to strike the earth - you would need to cancel nearly all the angular momentum of that orbiting mass, or it would simply stabilize to a lower orbit.

    You do NOT need to continue to provide thrust to keep the head of the elevator in orbit. The cable is NOT facing a significant wind shear. In fact, the base of the elevator is at a fixed point, and most if not all of any wind effects would be observed as forces on the anchor point on the earth.

    The cable DOES need to be strong. If you shorten the cable, somewhat, te tension on the cable will increase, and you can hoist a greater payload with less mass at the elevator head. If the cable does break, the elevator head will stabilize at the higher orbit.
     
  7. Etan Isar
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    Etan Isar Contributing Member Contributor

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    The "Mars" series by KSR(I think) has an elevator, and it mentions a lot on the economics and mechanics.
     
  8. lipton_lover
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    lipton_lover Contributing Member

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    Wasn't there actual serious talk about building one some time ago? If so, any information from then would be useful.
    Anyways, here's the wiki info on the most likely material choice if one was made. It's not a project-ready material, but is being worked on.
    http://en.wikipedia.org/wiki/Carbon_nanotubes
     
  9. Mesuno
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    Mesuno Member

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    I think I rushed off my comment without re reading.

    Agreed, but the weight of the elevator cable hangs down, below the counterweight. To counter this the centre of mass of the counter weight has to be beyond the distance of geostationary orbit, providing an outwards centripetal force.

    Agreed, once it was in place it would be stable but could you envisage the process of getting a massive asteroid into place? Deflecting it from whatever solar orbit it is in currently so that intersects with the earth's orbit sufficiently closely, and with sufficiently low relative velocity, that it can be captured. I would suggest that the technological problems inherent in that are vast, especially as relatively insignificant deviations in path can have major and catastrophic consequences further down the line.

    Also, what would the political implications be of one nation/company etc having control of such a large mass in orbit? Would other nations simply accept their superiority?

    Peter F. Hamilton, in his book Fallen Dragon describes a situation where an orbital asteroid is destroyed deliberately using a large nuclear explosion. The resulting debris spreads out causing a chain reaction of collisions shattering the rock into a layer of dust/gravel shrapnel which would take thousands of years to burn up. In that time access to space would be totally blocked as any ship trying to pass through would be ripped apart by the debris.

    mesuno blushes
    should have thought that one through better

    Wind sheer will cause turbulent effects on the lower end of the cable which, as Cogito says, will affect the anchor point, not the counterweight. Serves me right for extrapolating from a Mars based Elevator (lower counterweight, atmosphere is thin but extends further into space) direct to earth.


    So, in principle the idea is very plausible provided you have sufficiently strong materials (carbon nanotubes have been suggested as being strong enough I believe) but I stand by my point that there are significant social and political hurdles to actually putting one in place.
     
  10. Cogito
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    Cogito Former Mod, Retired Supporter Contributor

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    I find this premise highly improbable The quantity of debris spread out in a spherical shell around a planet would require a great deal of initial mass, even assuming you could shape the explosion such that all the debris would form stable orbits of roughly te same kinetic energy (and thus the same orbital altitude). Even intuitively, the math seems way off, even for a fairly low orbit.
     
  11. lipton_lover
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    lipton_lover Contributing Member

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    I thought of something else that should play a big role in any story with an orbital elevator. Being such a massive project, every precaution would be taken to keep it safe. This includes maintainance and some way of keeping it from destruction from asteroids. As far as I know no location on earth is safe from an asteroid hit, and it wouldn't take a direct hit to seriously damage the structure. Also if tethered by an asteroid pulled into orbit, that's an even bigger target. Let your imagination run wild with that in mind :)
    Nate
     

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