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The Guardian reports on a conference on space elevators being held in Santa Fe. There's at least one wonderful howler in the report:
While I'm not up to doing the maths to work out how strong the cable would need to be, I have seen figures from people whose mathematical skills I trust, and my engineering training tells me that this stuff is still pure Unobtainium. Thanks to everyone's favourite buzzword, "carbon nanotubes", we can just about predict the possible development of a material that might just be strong enough. Right now, the main ingredient for this material costs about a zillion dollars per gramme, and is only available in minute quantities. Which also have poor quality control. In short, we still can't make pieces of this material big enough to do sensible mechanical testing to work out if it's as strong as we think it will be.
Of course, you're going to need at least 50,000km (probably more) of cable, which is probably going to be several metres thick, minimum. It's going to mass hundreds of thousands of tonnes. You also need to get this cable into geostationary orbit. At this point I will just say "launch capacity" and leave it at that.
Finally, if the material is only just strong enough, there's alwasy the risk it will break. You do not want a space elevator cable to break. It won't land gently in a nice neat little pile around the ground end. Remember, this thing will be long enough to wrap around the planet with cable left over. In any case, the higher parts of the cable will hit pretty hard when they land. How many cities are there within, say, 250km of the equator?
So, while the basic science is sound, it's still in la-la land from an engineering perspective. I'd love to see one, but I'm sceptical that we'll have the engineering capacity in my lifetime. From the Guardian again:
At [...] 36,000km from Earth [...] objects take a year to complete a full orbitAhem. Yes. Try "24 hours". Apart from that, it's mostly on-the-ball.
While I'm not up to doing the maths to work out how strong the cable would need to be, I have seen figures from people whose mathematical skills I trust, and my engineering training tells me that this stuff is still pure Unobtainium. Thanks to everyone's favourite buzzword, "carbon nanotubes", we can just about predict the possible development of a material that might just be strong enough. Right now, the main ingredient for this material costs about a zillion dollars per gramme, and is only available in minute quantities. Which also have poor quality control. In short, we still can't make pieces of this material big enough to do sensible mechanical testing to work out if it's as strong as we think it will be.
Of course, you're going to need at least 50,000km (probably more) of cable, which is probably going to be several metres thick, minimum. It's going to mass hundreds of thousands of tonnes. You also need to get this cable into geostationary orbit. At this point I will just say "launch capacity" and leave it at that.
Finally, if the material is only just strong enough, there's alwasy the risk it will break. You do not want a space elevator cable to break. It won't land gently in a nice neat little pile around the ground end. Remember, this thing will be long enough to wrap around the planet with cable left over. In any case, the higher parts of the cable will hit pretty hard when they land. How many cities are there within, say, 250km of the equator?
So, while the basic science is sound, it's still in la-la land from an engineering perspective. I'd love to see one, but I'm sceptical that we'll have the engineering capacity in my lifetime. From the Guardian again:
[Arthur C] Clarke [...] once said a space elevator would only be built "about 50 years after everyone stops laughing"Well, on Sky News' "tomorrow's front pages" thing last night, they had a grand old giggle.
A couple of thoughts
Launch capacity: We only need to get *one* thin line to orbit, then we use that to pull up the second line and build a loop, then we just hook things on one line and pull on the other and everything goes up in a hoist (or just fix the one line and run little climbing robots up the line ... either take them apart at the top for raw material, send them down the line, or put them in a box and throw them back down (yike!)
The "several metres thick" bit totally depends on how strong the cable is ... the mythical "sinclair molecule chain" was strong enough for a single molecule wide wire to self support, we'd want more thickness than that for extra cables (emergency spares) plus maybe an up and down side, plus something for the elevators to grab onto ...
... falling down on Milton Keynes ... well, whereever ... put small charges every 10/100 metres along the cable ... if the cable breaks, hit the *bang* button and deal with lots of smaller pieces instead (maybe!)
plus if the cable breaks nearer the ground than up in orbit (which I think is much more likely ... hit by aircraft, torn by storms etc.) the length that will fall is the bottom section, the top section is still attached to a geosync station (and if they aren't quick and balance the other side, then they are going to start going UP with the cable!)
Re: A couple of thoughts
There's nothing about the material there, although they've been bullish about it in the past. IIRC it would have to be several tens of times stronger that anything available, which is actually less than I'd expected.
Their proposed timescale though - fifteen to twenty years - isn't anything I'd put money on.
And as for the fifty years quote - I think NASA stopped laughing a few years ago when they started doing engineering studies. Fifty years or so might not be an unreasonable guess.
Re: A couple of thoughts
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The numbers KSR uses for the elevator collapse in Red Mars seem to show that one collapsing on earth, no matter how thin, would dump a lot of energy into the atmosphere.
Engineeringwise, we're actually not that far off. We could actually build one today, using current materials. It would be big, and bulky, but it can be done. It's the smaller, dropped from orbit, versions that require more exotic materials. Oh, and an asteroid sized counterweight...
There is an interesting alternative, which is a lot cheaper, requires much lower tensile strengths, and that's the rotating skyhook. Imagine a rotating 2500 km cable in a Molniya orbit that dips the end of the cable in for a few minutes every few hours... and then hook stuff onto it as it passes through the atmosphere. It'll be moving quite slowly, so wouldn't even require complex supersonic pickups...
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There's another kind?
Elsewhere:
Yes, the less ambitious skyhooks look more feasible from an engineering perspective, though hooking something onto the end of even a subsonic cable-end in mid-air would still be lively.