h a l f b a k e r yYeah, I wish it made more sense too.
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Space Elevator Crawler/Balloon
A lot of energy has been poured into building a "crawler" for a proposed space elevator--use a tethered balloon instead | |
There's been an annual science contest to determine
what kind of crawler can be built to climb a proposed
space elevator to orbital height. For most of
that distance one might be better served by building a
tethered zeppelin that, despite high winds and buffeting
at higher altitudes, might
be a better choice for lifting
bulk cargo. I strongly suspect you could get to at least
100,000 feet with little difficulty using a tethered
zeppelin moving up and down a set of carbon nanotube
lines. At a predetermined height, the cargo-carrying
LTA car would dock with some facility that would then
transfer the cargo to crawlers for the rest of the cabled
journey to geosynch or to be tossed beyond that, etc.
Benefits would include that the crawler design could be
optimized for the upper length of the cable and that you
could potentially build this facility in stages--
either the lower step first, as a proof-of-concept, or the
upper portion first, as a skyhook. So, this concept is
scalable. Also, it renders irrelevant the whole *building
death lasers at the base of a relatively fragile tower*
thing refered in the story below.
Story on NASA contest for laser power-beaming
http://www.digitalt...e-elevator-contest/
[cloudface, Sep 29 2010]
[link]
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One of the best things about a space elevator is that you can, in theory, recover most of the energy used to go up, on the return journey. |
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A balloon would be a free ride on the way up, but costly on the way down. You'd either have to tug it down again or empty the gas. |
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Also seems like a lot of hassle for a short distance, if the crawler has to carry it the rest of the 22,000 miles. |
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What [mitx] said. You aren't going to save any significent percentage of the trip by getting to the top of the atmo, and balloons aren't a free ride. |
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And you would still need the lasers. What were you planning to use to power the elevators for the rest of the trip? You might, however, want to put in a balloon platform for your lasers, and get them up above most of the air. |
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// crawler design could be optimized for the upper length of the cable // Um, what? A pod that is designed to function in a vacuum is probably strong enough to work in air, and a crawler isn't going to be streamlined enough to be a weight handicap. |
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By the by, I am opposed to the entire space elevator idea. An elevator would be useful only in getting to geosynchronous orbit and departure orbits past its end, and would require clearing all orbits other than those of everything that might possibly strike the tower. It would take a zillion space flights to build it, then all near-Earth flights, satellites and debris would have to be eliminated for the tower's safety. |
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Surely the car would be heavy enough to count on
gravity to bring it down and generate energy on the
way. |
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The space elevator is such a huge undertaking, that I believe the most feasible idea, as incredible as it sounds, is to first build a ~100 mile tall tower (Andes?) to get the crawler above the atmosphere. Kind of like an ephemeral, skeletal, upside-down horn, with tensegrity mooring lines. |
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Actually, I think that there should be an contest to prove that the space elevator won't work on Earth. For instance, only pure carbon nanotubes are thought to have the tensile strength to reach geosynchronous orbit, but all carbon bonds - (C-C, C=C, CÎC) - degrade under UV rays. Even a C-H coating on the surface, like a polished diamond, seems to be too heavy. |
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Assuming that that can be overcome, your idea seems to make a lot of sense, only I think the ribbon should only extend down to the ~20 mile high tethered balloon, which could simply haul things up to it by a pulley. |
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[Wily] You can build a space elevator out of anything, it's just that the Taper Angle gets to you (I forget what it is for steel, but basically the bit in GeoSynch is bigger than the planet). |
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Nanotubes seem to be the best, because the carbon bonds in them allow a taper somewhere in the range of 12-14 to one (If I remember right). This means that with a little additional taper, it can hold extra material such as a shielding layer, or something for the climber to grab on to. |
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Thanks, [MechE], you've given me something to think about/study, and the possibility that a space elevator on Earth is still feasible, really up-lifted my spirits. |
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However, until I study this more, I remain Healthily Skeptical. (Skepticism being the immune system of science, of course.) But thanks for the tapering idea, I'd never heard/thought of it before... |
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I have to ask on this one -- carbon nanotubes, as lightweight and strong as they are, would still weigh a staggering amount in the terms discussed -- a 22,000 mile length? My first thought: how would that affect the rotational velocity of the Earth? Second, I totally agree with [bb] -- everything would have to be cleared out of its orbit, and supposing something *did* happen, say, 5000 miles out, and the cable was severed... 5000 miles of cable crashing back down to Earth could be kind of a big deal, no? Then, when the remaining 17,000 miles of cable realizes that it's not being "pulled taut by centrifugal force" any longer, it'll be pulled back down to earth. It's not just going to fly off at a right angle -- the bit that's 22,000 miles out is moving fast enough at sufficient distance from the Earth's gravity well to stay in orbit, but what about the reverse tapering 17,000 miles of cable it's dragging around? That sounds like a deal breaker to me. |
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Some random thoughts inspired by [CaptainClapper], and [MechE]'s excellent arguments. |
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First: because the "tapered" ribbon of the space elevator will eventually be broken - (it will!) - we need a cut-off at just over atmospheric level. If all conductivities are broken, then the 'cut-off mechanism' severs the ribbon, dropping a ~150(?) mile portion of the ribbon to Earth, doing the damage that it might - (it's almost two orders of magnitude lighter than silk, by mass) - but leaving the upper broken bit of ribbon in orbit. It will eventually fall, but the cut-off will buy us time to do ... whatever. |
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Yeah, I agree with [baconbrain]'s dismal observation, space junk will just render the Space Elevator ribbon Swiss cheese. The constant replacement of the ribbon - long before UV failure, etc. - must be part of the equation... |
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Also, if the ribbon becomes severed, the tiny above geosynchronous height-level capsule (to pull the ribbon taught), would try to run away; we need emergency rockets on it to correct it's altitude (and attitude :) |
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[CaptainClapper] had an interesting observation. Because of the mass of the Space Elevator (what is it?), and sticking up so high, what would be the New center-of-gravity that the Earth rotates around? It might only move millimeters (micrometers?), but because the Space Elevator is so high - (close to three times its diameter) - with the heavy part at the top, it might be more... |
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The elevator's mass should balance at geosynchronous orbit. Treat that as center and do the math, I think. |
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I don't think that a cut-off just above the atmosphere would be the right place, but the thought is interesting. Somewhere further up should be a point at which a chunk of elevator debris would fall into an elliptical orbit that just brushes the atmosphere. That and anything further up could be said to be in orbit, but the stuff 300 miles up is certainly not in orbit. |
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In the event of a total disaster, the elevator could be chopped into segments. The lower section should probably be left intact to keep it as close to the base as possible, but might be sheared into a chunk that would land in an ocean. Above that is material that will drop in a failed ellipse and burn up on re-entry, above that is stuff that will be in an elliptical orbit and is not an immediate problem. |
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If it isn't chopped, the lower sections will be dragging down stuff above, right? Keeping it tethered to the ground after it breaks at geosynchronous would be bad. The elevator probably should be rigged with cutters all over (adding to weight and opportunities for sabotage), and a controlled-detonation plan and system in place for dealing with breaks at any possible point. Shredding lower parts of the thing would reduce ground damage. |
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The idea of safety-destructing the elevator is new to me, but the math should be manageable. |
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The elevator is still bad idea. |
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A space elevator having such an emergency is depicted in the novel Red Mars by Kim Stanley Robinson. |
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I just found that Wikipedia has a good article on Space elevator safety, but that's getting away from the posted idea even further than I already have. |
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A space elevator cable has to be under tension to work properly. Doesn't that mean that if you break the cable, the bulk of it will move AWAY from the planet, rather than falling to the ground? |
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That depends on where it breaks. |
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The bulk of it is in orbit, if you look at it right. The center of mass is in geostationary orbit. But there are bits hanging down toward the ground, counterbalanced by bits centrifuging outward beyond geostationary orbit. |
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So the problem is where it breaks. |
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In short, if it broke off at the base, yes, it would go up and away. But any break above the base would leave a strand with no tension above it to hold it up, still fastened to earth, and a whole lot of released tension snapping it back down like a rubber band. |
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Seriously, go get a bungee cord and a brick. Whirl the brick around on the end of the bungee, then let go. You'll be fine, though the windows might not be. Now do it again, but this time use a sharp machete to cut the bungee anywhere but next to your hand. Smarts, don't it? |
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Any part of the elevator below the break will be coming back down. Even if the bulk of it stays up, as you say, twenty thousand miles of it could be heading down. If, as discussed above, demolition charges cut it in the right places, most of that might stay up in orbit. |
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To say that any break will leave the bulk in orbit isn't correct. Careful disaster management might keep the bulk in orbit, yes. But even a small fraction of it is a lot to have dropping in from above. |
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