12 ms·

Can Quiet, Efficient 'Space Elevators' Really Work?

68 points by acheron 13y ago

pyalot2 13y ago
I think there's two problems nobody seems to be addressing about space elevators. 1) A vehicle starting at the earths surface has the moment of earths rotation. As it climbs the cable it would need to be accelerated to a suitable orbital momentum. If you don't do this, then it'll "tug" on the cable counter its rotation. Edit: To clarify why that is a problem. The climber would be accelerated by the tension of the cable, but as that happens, the cable would be bent at the point of the climber, which might be problematic. Also the counterweight that provides the tension would be decelerated, so the whole cable would "tilt". That is all, as long as the climber climbs. When it stops climbing (or is jettisoned), both counterweight and climber would now start a pendulum motion (at different frequencies). That's probably also not good. 2) To reach geostationary orbit at a speed of say 100km/h would take 2 weeks. Traveling 500km/h it would still take 3 days. Unless you jettison the climber at the top, only one climber could be on the cable at any one time, and that would put a lot of strain on recovering the initial cost of building the elevator by putting an upper bound on how often you can send a climber up to anything between every 6 days to a month. I don't think anybody would engage in a project that cost trillions of dollars to get built, and then can only send 12 missions per year into orbit. Edit: People are pointing out that a second cable would solve this issue. I think it might be challenging to keep the two cables from tangling up across a tens of thousands of kilometers.
- Zikes 13y ago
  I don't think it's entirely unreasonable that you could jettison the climber at the top. It could even be a reusable component of the craft itself. With that, you could potentially send up as many simultaneous climbers as the cable could support.
  - pkinsky 13y ago
    With some engineering, you could have dual-use climbers that double as habitat segments.
- seren 13y ago
  Regarding the second point, building two cables in parallel would certainly not double the project price. (It could however pose additional safety challenge).
  - pilom 13y ago
    Most of the economic analysis of elevators I've seen use one elevator to lift a second cable as their first flight, this is why building every one after the first is cheaper.
    - wikwocket 13y ago
      Just like Star Trek's replicators: Building the first one requires you to batter the rules of quantum physics into submission and bend them to your will. Building the second one requires you to push the "Copy" button.
  - Xylakant 13y ago
    Having two cables in parallel would also allow to regain the energy that's held in the climber that's descending by using it to power the upwards motion of the other. This would further reduce the "launch" cost.
- skj 13y ago
  I don't think that (1) is really such an issue. Two things to consider. First, the energy required to keep the ... err... toppiece? stable is significantly less than the energy required to launch something into the same orbit. Second, I think that the metaphorical centrifugal force will bring the toppiece back to where it began. That is, there is a stable equilibrium with the toppiece at its highest point, and bringing it out of that equilibrium will be resisted. Edit: Certainly the total energy required, from the train and toppiece stabilization, is the same as the energy required to get something into orbit, but to stabilize would require thrusters, while climbing can be done electronically against the chain.
  - ithkuil 13y ago
    total amount if energy won't be the same as to get something into orbit with rockets, first because rockets have to carry their own fuel, second because it's far more efficient to move by displacing a very heavy reaction mass (the earth) rather than displacing a light reaction mass (which the rocket is carrying)
- kisstheblade 13y ago
  Doesn't this also have to do with point 1? http://what-if.xkcd.com/58/ http://what-if.xkcd.com/58/ So it doesn't help to just climb space, you need to gather orbital speed as well?
- chromaton 13y ago
  Regarding 1: most designs have a large space station as a counterweight at the geostationary position that helps keep the cable steady.
- aethertap 13y ago
  I'm probably a bit out of my depth here, but maybe somebody else can jump in and set me straight. Regarding point (1), I think that may be mostly solved by conservation of energy if the vehicle is large compared to its payload. The reason is that, as it climbs, it will have to be accelerated to orbital velocity (I'm assuming this will be done by the cable itself, or more accurately the mass at the end that keeps it in tension). It will then drop its payload, and climb back down. During the climb down, the cable will have to decelerate the climber back to surface velocity, so the energy that went into speeding it up on the climb will be mostly recovered on the way back down. So it seems like the cable would sway backward as it lifts the climber, and be pushed forward almost as far when the climber descends again. If the mass at the end of the cable included some kind of constant low-grade thrust, maybe you could bank up angular momentum between runs to account for the mass of the payloads. I'd love it if someone with more physics background could weigh in on it though.
- Udo 13y ago
  > If you don't do this, then it'll "tug" on the cable counter its rotation. There is tension on the cable, the counterweight at the top will pull the cable straight. > I don't think anybody would engage in a project that cost trillions of dollars to get built, and then can only send 12 missions per year into orbit. That's a good point, single space elevators don't scale very well if there's bidirectional traffic on a single cable. Jettisoning the climber at the top is certainly feasible in the beginning, but once we're really using space (which the elevator could finally enable us to do) we're going to need the elevator as a safe return vehicle. In the long run, we're going to need drastically more space elevators, and it might make sense to have a dual ribbon for each elevator, allowing full duplex. Right now it's about getting someone to build the first one, though. Baby steps.
- VLM 13y ago
  The first one is an engineering problem. If the mass of the cable is immensely higher than the mass of "whatever it is" then it simply doesn't matter. High efficiency (ISP) engines unfortunately stereotypically output very little thrust, while high thrust engines stereotypically waste fuel. So rather than hoisting something heavy in only 9 minutes using incredibly inefficient engines, hoist it in a couple days and then using super efficient engines very slowly get everything back into place. Basically you shatter the old ISP vs thrust tradeoff by immense capital expense. The second problem also frankly doesn't matter much. Aerobraking is cheap and efficient. I don't remember the specs but it scales pretty well, even when small, such that one pound of re-entry vehicle can land something like 20 pounds of "stuff" if you do it right, so rather than building a second elevator you simply "give up" on 5% of the first elevators capacity. Weight of heat shield is immensely lower than weight of fuel to get up there so don't even bother with reusable shields. Also once you get something up there, you should never return it with the exception (possibly?) of people. So keep a stockpile of shipping crates of re-entry capsules up there and never take anything down. Ever. Except maybe emergency medi-vac. If it costs almost nothing to get it up there, send up a solar powered foundry and a solar powered machine tool plant and start squirting out a stockpile of rough spare parts and panels and the like. As for the edit, the hidden assumption is the cables "have to be" side by side. However the fuel cost to travel along an orbit is basically zilch compared to getting up there. And the materials science concerns of basically building a cablecar elevator are limited. So if you really, really insist on installing a "down" elevator, simply up the "up" in Africa and the "down" over south america and if you still manage to tangle them, you must have totally screwed up beyond all recovery anyway.
  - pyalot2 13y ago
    There's something which tells me it won't work, and I can't prove it yet. But I think it's provable with a relatively straightforward simulation.
- mrfusion 13y ago
  Can't you leave the climbers at the end to add to the counter-weight? Or am I misunderstanding something?
- bentcorner 13y ago
  I'm having difficulty wrapping my mind around the magnitude of the forces involved. Wouldn't sending a climber up to the top ultimate result in that much force being "pulled" on the tether at the bottom? The entire tether would be under extreme stress, although maybe it's insignificant compared to the weight of the tether itself...
- Florin_Andrei 13y ago
  These are not addressed in pop-sci articles, but are actually considered quite seriously. There is lateral force from the climber to the cable, yes. But if the net mass flow to/from the top is zero, you don't have to compensate. You only compensate for the net flow. You do that by applying orbital corrections to the station at the top. If the climber is not rising at hypersonic speeds, the lateral force is pretty tiny - literally lost in the error margin, compared to the tension in most of the cable length. The climbers would have to rise fast, and the journey would be long anyway, yes. Basically, we would have to marry high-speed train technology (for speed) with Orient Express or ocean liner amenities (against boredom). The view would be spectacular, and would be a major part of the entertainment. It's probably best to build cables in pairs, one going up, the other down.
Kenji 13y ago
What about space debris? If we build such a large stucture up to the geostationary orbit, won't it be exposed and how can it be protected? The top station can't dodge - or can it?
- sentientmachine 13y ago
  Self healing cables. You could use three parallel cables separated from each other by struts, with sufficient margin of safety that severing just one or two strands still allows the surviving strand to hold the elevator's entire weight while repairs are performed. When a meteorite hits a strand, or two, robots going up and down the cable will immediately repair the break. What about lightning though? When a cloud contains an imbalance of electrons, won't the lightning strike much rather travel through the cable and weaken the material so it breaks?
  - mrfusion 13y ago
    If anything it would work like a lightening rod and discharge the currently gradually so it never builds up in the first place.
    - mmphosis 13y ago
      And, could that current in the tether be used as a source of energy? And, would stopping lightning cause other problems for Earth?
      - Tuna-Fish 13y ago
        > And, could that current in the tether be used as a source of energy? Yes, but the actual continuous power delivered by it would be surprisingly low, making it pointless. > And, would stopping lightning cause other problems for Earth? No, as the effect would be very localized.
  - VLM 13y ago
    "What about lightning though?" We have commercial experience (well, at least in Russia) making communications towers a mile or so tall. Making one five miles tall would not be a huge stretch of the imagination, and those are fairly tolerant of all kinds of ridiculous abuse. Simply attach the elevator at the top of the tower instead of ground level and you've eliminate pretty much any man-portable non nuke attack potential and most mechanized vehicle attacks (or accidents). Ditto the elevator material. We can't afford for weight reasons to make the entire thing "airliner proof" but we can afford to make the bottom 10 miles or so "small cessna proof". So make the bottom couple miles quadruple redundant steel battleship anchor chain or whatever. 99% of it will still have to be light as a feather, but nothing can hit it, so thats OK.
snake_plissken 13y ago
The thing I have never been able to grasp with the space elevator concept is how the cable(s) would be strung so that it isn't torqued/twisted by the Earth's rotation. Or is elevator's centripetal acceleration supposed to straighten it out?
- dkuntz2 13y ago
  Isn't it that there's usually a huge counterbalance in a Clarke orbit that would keep the elevator cables straight?
- rwj 13y ago
  The tether actually extends out past geosynchronous orbit. The section below geosynchronous orbit want to fall to earth, and the section above wants to fly away. Because of this, the tether is under tension.
- deleted 13y ago
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- Derbasti 13y ago
  The tether would extend far far out over a geosynchronous orbit, such that the centripetal acceleration is exactly equal to the force of gravity. Thus, it would be weightless on the ground, maximum tension at geosynchronous orbit, and an easy launching platform at its end. Also, it can be easily deployed by unrolling the tether in both directions simultaneously from the geosynchronous orbit.
  - Florin_Andrei 13y ago
    It doesn't have to be "far far out". It can be closer, but heavier. A big rock.
- VLM 13y ago
  Imagine in your brain a standard, boring geosync comsat. There it is, hovering over some spot of land in Africa or WTF, 24x7. (well, its not quite that simple, but close enough). Now take two jumpropes and throw one down and one up. No problemo its still "motionless" WRT the ground. Now make those jumpropes ridiculously long until suddenly the lower one hits the ground. There's never a point as it stretches where it gets all spun around and stuff. You are tangentially correct in that its a huge dynamic problem to dampen waves. You wanna piss off / terrify legacy earthlings today? Get in an elevator with 500 foot cables and start hopping up and down while its moving. You'll scare them half to death as the whole cabin starts bouncing. Now try a couple thousand mile long cable with multiple cabins all wiggling. Its going to give the control system engineers headaches. Solvable, just a PITA.
varelse 13y ago
So what if a bomb goes off on the thing and manages to sever the cable? Look out below! Up to 62,000 miles of high-speed fun coming your way... Multiple SF authors have covered this unpleasant contingency. It seems more of an example of why we can't have nice things to me.
- 3pt14159 13y ago
  Engineers, like myself, are able to design systems that do not fail spectacularly when faced with predictable threats.
- Zikes 13y ago
  Depending on where the cable is severed, the majority of it is likely to just fly off into space. Remember, it's being actively pulled by centripetal force. However, if it's severed at some point in the middle, say just below the geostationary orbit point around halfway up the cable, then the majority of what falls toward Earth would actually just burn up in the atmosphere. Only about 100 miles of cable would likely reach the ground, and it would probably weigh about as much as your average power line of similar length.
  - jerf 13y ago
    Indeed. The real question about a Space Elevator is not whether it would be safe in a disaster, the question is whether it would work, at all. If Space Elevator terrorism is to become a thing someday, it probably won't take the form of threats to cut the elevator. Especially once you have more than one, that's not a very interesting attack (and even if you only have one, the damage would be primarily economic, not physical). The way you'd do terrorism is to sneak something terrifying into orbit, since it's so much cheaper to do that than it is now.
    - Zikes 13y ago
      Yeah, tungsten telephone poles in orbit could do some serious damage, as much as a small nuke. [1] [1] http://en.wikipedia.org/wiki/Rods_from_god#Project_Thor http://en.wikipedia.org/wiki/Rods_from_god#Project_Thor
Shivetya 13y ago
Well as soon as we find a reason to use space, solving something we can only solve there that is dire enough to warrant the cost, or prove manufacturing there is well worth the expense of the cable, then perhaps the question will matter. Its not like not having the cable prevents from exploiting space based industries, we just haven't come up with the value of the proposition
- Zikes 13y ago
  Asteroid mining.
  - polymatter 13y ago
    Not in the short term see (http://en.wikipedia.org/wiki/Asteroid_mining#Financial_feasibility http://en.wikipedia.org/wiki/Asteroid_mining#Financial_feasi...). It is a lot cheaper, easier and less risky to mine even very rare minerals on Earth rather than do so in space. And will be too for the foreseeable future. Plus any investor doesn't want to dump platinum on the market as then the price would plummet and seriously erase the profit. And you'll want a lot of guaranteed returns to undertake such a financially risky investment. I could see Space Tourism taking off though. Taking space jet skis out for a spin round the moon and back that kind of thing. Perhaps some exclusive space hotel.
- delluminatus 13y ago
  And the space elevator is not something that would be developed by interplanetary travelers but by people who just want to get into Earth orbit. I think it would mainly be useful for people either launching/maintaining a lot of satellites, or living in Earth orbit. Or if someone is producing energy or material in space (orbital solar power stations?) and they want to easily transmit it back to Earth, an elevator would probably be more efficient than weekly collection runs.
  - ef4 13y ago
    > And the space elevator is not something that would be developed by interplanetary travelers but by people who just want to get into Earth orbit. That's not true at all. The vast majority of the energy cost of going to another planet is incurred just getting into earth orbit. A space elevator radically improves the economics of interplanetary travel. In fact, you can get certain transfer orbits "for free" by just extending your tether a bit beyond geosynchronous orbit and using it to fling bulk payloads to Mars, etc.
bhhaskin 13y ago
It is nice to see people finaly talking about orbital elevators
ackydoodles 13y ago
Many problems must be solved before they can exist. (1) Only suitable material for the cable is, at the moment, unobtanium. (2) Cable must be moved continuously to dodge debris and satellites whose orbits cross the equator (all of them except those in geosynchronous orbit). (3) The cable, elevator module and any cargo or people must pass through the Van Allen belts, which will degrade them. People don't respond well to degradation by radiation. Other than that, we're good to go.
- dllthomas 13y ago
  Wouldn't another approach to (2) be eliminating debris and having satellites steer around it? We don't move buildings to dodge cars. This isn't to say that's easy, just that there's more than one currently intractable way we might eventually be able to skin that particular cat. Edited to add: With respect to (3), there's apparently this: http://en.wikipedia.org/wiki/Van_Allen_radiation_belt#Proposed_removal http://en.wikipedia.org/wiki/Van_Allen_radiation_belt#Propos... I don't know near enough about it to say whether it's any sort of a good idea, but it doesn't seem hard relative to building a space elevator.
- Florin_Andrei 13y ago
  The problems created by radiation in the Van Allen belts are vastly overstated. The funny part is, this is due in part to fake Moon landing conspiracy theorists, who use the radiation as a "supporting argument".
  - dllthomas 13y ago
    My understanding is that satellites that plan on spending significant time in the Van Allen belts need to take countermeasures. Is that not the case, or is it only relevant to things like sensitive electronics?
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theothermkn 13y ago
While space elevators make for a fun toy gedanken for space enthusiasts who don't have an engineering background, they will never happen for one primary reason: As soon as you have a material out of which you can build a space elevator, you have a material to make extremely lightweight filament-wound pressure vessels. You essentially get SSTOs with amazing payload fractions and greatly reduced mechanical complexity for free. LH2 gives you tanks that are too heavy for an aluminum craft? No problem. Elevator-grade carbon nanotube composites are 1/1000th the mass. Go nuts. When vehicle mass fractions go from 10% down to .1%, payload mass fractions go from 2% to 12%, minus trace amounts of structure. Was it Asimov who said that space elevators will happen 50 years after people stop laughing? I hope so, because people should never stop laughing at this idea. (Really? We're going to better utilize space--i.e., cramming NEO with nanosats--at the same time we erect a huge hazard in NEO? Come on!)
- Florin_Andrei 13y ago
  Devil's advocate: Rockets are still less safe. Rockets still produce tremendous accelerations. 12% payload fraction is still much less than 50%, or 90%.
  - theothermkn 13y ago
    QED. Your first sentence shows no consideration at all of the effect of material advances on launch vehicle safety. Your use of the phrase "tremendous accelerations" shows no understanding of the accelerations actually produced even by current launch vehicles. Also, you don't mention why this is, or even ever has been, a limiting factor in designing a payload, nor the degree to which it impacts payload design. Noting that 50 or 90 is greater than 12 doesn't change the fact that we're in an entirely different design space from the 2%-mass-fraction vehicle. Like I said, space elevators are a fun little toy gedanken for people who just want to think about The Amazing Future. If this is the best the Devil's Advocate can do, the Saints are going to be just fine.
dublinben 13y ago
They're not perfect, but launch loops solve many of the problems of straight up and down space elevators. https://en.wikipedia.org/wiki/Launch_loop https://en.wikipedia.org/wiki/Launch_loop

68 points by acheron 13y ago

I think there's two problems nobody seems to be addressing about space elevators. 1) A vehicle starting at the earths surface has the moment of earths rotation. As it climbs the cable it would need to be accelerated to a suitable orbital momentum. If you don't do this, then it'll "tug" on the cable counter its rotation. Edit: To clarify why that is a problem. The climber would be accelerated by the tension of the cable, but as that happens, the cable would be bent at the point of the climber, which might be problematic. Also the counterweight that provides the tension would be decelerated, so the whole cable would "tilt". That is all, as long as the climber climbs. When it stops climbing (or is jettisoned), both counterweight and climber would now start a pendulum motion (at different frequencies). That's probably also not good. 2) To reach geostationary orbit at a speed of say 100km/h would take 2 weeks. Traveling 500km/h it would still take 3 days. Unless you jettison the climber at the top, only one climber could be on the cable at any one time, and that would put a lot of strain on recovering the initial cost of building the elevator by putting an upper bound on how often you can send a climber up to anything between every 6 days to a month. I don't think anybody would engage in a project that cost trillions of dollars to get built, and then can only send 12 missions per year into orbit. Edit: People are pointing out that a second cable would solve this issue. I think it might be challenging to keep the two cables from tangling up across a tens of thousands of kilometers.

Zikes 13y ago
I don't think it's entirely unreasonable that you could jettison the climber at the top. It could even be a reusable component of the craft itself. With that, you could potentially send up as many simultaneous climbers as the cable could support.
- pkinsky 13y ago
  With some engineering, you could have dual-use climbers that double as habitat segments.
seren 13y ago
Regarding the second point, building two cables in parallel would certainly not double the project price. (It could however pose additional safety challenge).
- pilom 13y ago
  Most of the economic analysis of elevators I've seen use one elevator to lift a second cable as their first flight, this is why building every one after the first is cheaper.
  - wikwocket 13y ago
    Just like Star Trek's replicators: Building the first one requires you to batter the rules of quantum physics into submission and bend them to your will. Building the second one requires you to push the "Copy" button.
- Xylakant 13y ago
  Having two cables in parallel would also allow to regain the energy that's held in the climber that's descending by using it to power the upwards motion of the other. This would further reduce the "launch" cost.
skj 13y ago
I don't think that (1) is really such an issue. Two things to consider. First, the energy required to keep the ... err... toppiece? stable is significantly less than the energy required to launch something into the same orbit. Second, I think that the metaphorical centrifugal force will bring the toppiece back to where it began. That is, there is a stable equilibrium with the toppiece at its highest point, and bringing it out of that equilibrium will be resisted. Edit: Certainly the total energy required, from the train and toppiece stabilization, is the same as the energy required to get something into orbit, but to stabilize would require thrusters, while climbing can be done electronically against the chain.
- ithkuil 13y ago
  total amount if energy won't be the same as to get something into orbit with rockets, first because rockets have to carry their own fuel, second because it's far more efficient to move by displacing a very heavy reaction mass (the earth) rather than displacing a light reaction mass (which the rocket is carrying)
kisstheblade 13y ago
Doesn't this also have to do with point 1? http://what-if.xkcd.com/58/ http://what-if.xkcd.com/58/ So it doesn't help to just climb space, you need to gather orbital speed as well?
chromaton 13y ago
Regarding 1: most designs have a large space station as a counterweight at the geostationary position that helps keep the cable steady.
aethertap 13y ago
I'm probably a bit out of my depth here, but maybe somebody else can jump in and set me straight. Regarding point (1), I think that may be mostly solved by conservation of energy if the vehicle is large compared to its payload. The reason is that, as it climbs, it will have to be accelerated to orbital velocity (I'm assuming this will be done by the cable itself, or more accurately the mass at the end that keeps it in tension). It will then drop its payload, and climb back down. During the climb down, the cable will have to decelerate the climber back to surface velocity, so the energy that went into speeding it up on the climb will be mostly recovered on the way back down. So it seems like the cable would sway backward as it lifts the climber, and be pushed forward almost as far when the climber descends again. If the mass at the end of the cable included some kind of constant low-grade thrust, maybe you could bank up angular momentum between runs to account for the mass of the payloads. I'd love it if someone with more physics background could weigh in on it though.
Udo 13y ago
> If you don't do this, then it'll "tug" on the cable counter its rotation. There is tension on the cable, the counterweight at the top will pull the cable straight. > I don't think anybody would engage in a project that cost trillions of dollars to get built, and then can only send 12 missions per year into orbit. That's a good point, single space elevators don't scale very well if there's bidirectional traffic on a single cable. Jettisoning the climber at the top is certainly feasible in the beginning, but once we're really using space (which the elevator could finally enable us to do) we're going to need the elevator as a safe return vehicle. In the long run, we're going to need drastically more space elevators, and it might make sense to have a dual ribbon for each elevator, allowing full duplex. Right now it's about getting someone to build the first one, though. Baby steps.
VLM 13y ago
The first one is an engineering problem. If the mass of the cable is immensely higher than the mass of "whatever it is" then it simply doesn't matter. High efficiency (ISP) engines unfortunately stereotypically output very little thrust, while high thrust engines stereotypically waste fuel. So rather than hoisting something heavy in only 9 minutes using incredibly inefficient engines, hoist it in a couple days and then using super efficient engines very slowly get everything back into place. Basically you shatter the old ISP vs thrust tradeoff by immense capital expense. The second problem also frankly doesn't matter much. Aerobraking is cheap and efficient. I don't remember the specs but it scales pretty well, even when small, such that one pound of re-entry vehicle can land something like 20 pounds of "stuff" if you do it right, so rather than building a second elevator you simply "give up" on 5% of the first elevators capacity. Weight of heat shield is immensely lower than weight of fuel to get up there so don't even bother with reusable shields. Also once you get something up there, you should never return it with the exception (possibly?) of people. So keep a stockpile of shipping crates of re-entry capsules up there and never take anything down. Ever. Except maybe emergency medi-vac. If it costs almost nothing to get it up there, send up a solar powered foundry and a solar powered machine tool plant and start squirting out a stockpile of rough spare parts and panels and the like. As for the edit, the hidden assumption is the cables "have to be" side by side. However the fuel cost to travel along an orbit is basically zilch compared to getting up there. And the materials science concerns of basically building a cablecar elevator are limited. So if you really, really insist on installing a "down" elevator, simply up the "up" in Africa and the "down" over south america and if you still manage to tangle them, you must have totally screwed up beyond all recovery anyway.
- pyalot2 13y ago
  There's something which tells me it won't work, and I can't prove it yet. But I think it's provable with a relatively straightforward simulation.
mrfusion 13y ago
Can't you leave the climbers at the end to add to the counter-weight? Or am I misunderstanding something?
bentcorner 13y ago
I'm having difficulty wrapping my mind around the magnitude of the forces involved. Wouldn't sending a climber up to the top ultimate result in that much force being "pulled" on the tether at the bottom? The entire tether would be under extreme stress, although maybe it's insignificant compared to the weight of the tether itself...
Florin_Andrei 13y ago
These are not addressed in pop-sci articles, but are actually considered quite seriously. There is lateral force from the climber to the cable, yes. But if the net mass flow to/from the top is zero, you don't have to compensate. You only compensate for the net flow. You do that by applying orbital corrections to the station at the top. If the climber is not rising at hypersonic speeds, the lateral force is pretty tiny - literally lost in the error margin, compared to the tension in most of the cable length. The climbers would have to rise fast, and the journey would be long anyway, yes. Basically, we would have to marry high-speed train technology (for speed) with Orient Express or ocean liner amenities (against boredom). The view would be spectacular, and would be a major part of the entertainment. It's probably best to build cables in pairs, one going up, the other down.

Kenji 13y ago

What about space debris? If we build such a large stucture up to the geostationary orbit, won't it be exposed and how can it be protected? The top station can't dodge - or can it?

sentientmachine 13y ago
Self healing cables. You could use three parallel cables separated from each other by struts, with sufficient margin of safety that severing just one or two strands still allows the surviving strand to hold the elevator's entire weight while repairs are performed. When a meteorite hits a strand, or two, robots going up and down the cable will immediately repair the break. What about lightning though? When a cloud contains an imbalance of electrons, won't the lightning strike much rather travel through the cable and weaken the material so it breaks?
- mrfusion 13y ago
  If anything it would work like a lightening rod and discharge the currently gradually so it never builds up in the first place.
  - mmphosis 13y ago
    And, could that current in the tether be used as a source of energy? And, would stopping lightning cause other problems for Earth?
    - Tuna-Fish 13y ago
      > And, could that current in the tether be used as a source of energy? Yes, but the actual continuous power delivered by it would be surprisingly low, making it pointless. > And, would stopping lightning cause other problems for Earth? No, as the effect would be very localized.
- VLM 13y ago
  "What about lightning though?" We have commercial experience (well, at least in Russia) making communications towers a mile or so tall. Making one five miles tall would not be a huge stretch of the imagination, and those are fairly tolerant of all kinds of ridiculous abuse. Simply attach the elevator at the top of the tower instead of ground level and you've eliminate pretty much any man-portable non nuke attack potential and most mechanized vehicle attacks (or accidents). Ditto the elevator material. We can't afford for weight reasons to make the entire thing "airliner proof" but we can afford to make the bottom 10 miles or so "small cessna proof". So make the bottom couple miles quadruple redundant steel battleship anchor chain or whatever. 99% of it will still have to be light as a feather, but nothing can hit it, so thats OK.

snake_plissken 13y ago

The thing I have never been able to grasp with the space elevator concept is how the cable(s) would be strung so that it isn't torqued/twisted by the Earth's rotation. Or is elevator's centripetal acceleration supposed to straighten it out?

dkuntz2 13y ago
Isn't it that there's usually a huge counterbalance in a Clarke orbit that would keep the elevator cables straight?
rwj 13y ago
The tether actually extends out past geosynchronous orbit. The section below geosynchronous orbit want to fall to earth, and the section above wants to fly away. Because of this, the tether is under tension.
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Derbasti 13y ago
The tether would extend far far out over a geosynchronous orbit, such that the centripetal acceleration is exactly equal to the force of gravity. Thus, it would be weightless on the ground, maximum tension at geosynchronous orbit, and an easy launching platform at its end. Also, it can be easily deployed by unrolling the tether in both directions simultaneously from the geosynchronous orbit.
- Florin_Andrei 13y ago
  It doesn't have to be "far far out". It can be closer, but heavier. A big rock.
VLM 13y ago
Imagine in your brain a standard, boring geosync comsat. There it is, hovering over some spot of land in Africa or WTF, 24x7. (well, its not quite that simple, but close enough). Now take two jumpropes and throw one down and one up. No problemo its still "motionless" WRT the ground. Now make those jumpropes ridiculously long until suddenly the lower one hits the ground. There's never a point as it stretches where it gets all spun around and stuff. You are tangentially correct in that its a huge dynamic problem to dampen waves. You wanna piss off / terrify legacy earthlings today? Get in an elevator with 500 foot cables and start hopping up and down while its moving. You'll scare them half to death as the whole cabin starts bouncing. Now try a couple thousand mile long cable with multiple cabins all wiggling. Its going to give the control system engineers headaches. Solvable, just a PITA.

varelse 13y ago

So what if a bomb goes off on the thing and manages to sever the cable? Look out below! Up to 62,000 miles of high-speed fun coming your way... Multiple SF authors have covered this unpleasant contingency. It seems more of an example of why we can't have nice things to me.

3pt14159 13y ago
Engineers, like myself, are able to design systems that do not fail spectacularly when faced with predictable threats.
Zikes 13y ago
Depending on where the cable is severed, the majority of it is likely to just fly off into space. Remember, it's being actively pulled by centripetal force. However, if it's severed at some point in the middle, say just below the geostationary orbit point around halfway up the cable, then the majority of what falls toward Earth would actually just burn up in the atmosphere. Only about 100 miles of cable would likely reach the ground, and it would probably weigh about as much as your average power line of similar length.
- jerf 13y ago
  Indeed. The real question about a Space Elevator is not whether it would be safe in a disaster, the question is whether it would work, at all. If Space Elevator terrorism is to become a thing someday, it probably won't take the form of threats to cut the elevator. Especially once you have more than one, that's not a very interesting attack (and even if you only have one, the damage would be primarily economic, not physical). The way you'd do terrorism is to sneak something terrifying into orbit, since it's so much cheaper to do that than it is now.
  - Zikes 13y ago
    Yeah, tungsten telephone poles in orbit could do some serious damage, as much as a small nuke. [1] [1] http://en.wikipedia.org/wiki/Rods_from_god#Project_Thor http://en.wikipedia.org/wiki/Rods_from_god#Project_Thor

Shivetya 13y ago

Well as soon as we find a reason to use space, solving something we can only solve there that is dire enough to warrant the cost, or prove manufacturing there is well worth the expense of the cable, then perhaps the question will matter. Its not like not having the cable prevents from exploiting space based industries, we just haven't come up with the value of the proposition

Zikes 13y ago
Asteroid mining.
- polymatter 13y ago
  Not in the short term see (http://en.wikipedia.org/wiki/Asteroid_mining#Financial_feasibility http://en.wikipedia.org/wiki/Asteroid_mining#Financial_feasi...). It is a lot cheaper, easier and less risky to mine even very rare minerals on Earth rather than do so in space. And will be too for the foreseeable future. Plus any investor doesn't want to dump platinum on the market as then the price would plummet and seriously erase the profit. And you'll want a lot of guaranteed returns to undertake such a financially risky investment. I could see Space Tourism taking off though. Taking space jet skis out for a spin round the moon and back that kind of thing. Perhaps some exclusive space hotel.
delluminatus 13y ago
And the space elevator is not something that would be developed by interplanetary travelers but by people who just want to get into Earth orbit. I think it would mainly be useful for people either launching/maintaining a lot of satellites, or living in Earth orbit. Or if someone is producing energy or material in space (orbital solar power stations?) and they want to easily transmit it back to Earth, an elevator would probably be more efficient than weekly collection runs.
- ef4 13y ago
  > And the space elevator is not something that would be developed by interplanetary travelers but by people who just want to get into Earth orbit. That's not true at all. The vast majority of the energy cost of going to another planet is incurred just getting into earth orbit. A space elevator radically improves the economics of interplanetary travel. In fact, you can get certain transfer orbits "for free" by just extending your tether a bit beyond geosynchronous orbit and using it to fling bulk payloads to Mars, etc.

bhhaskin 13y ago

It is nice to see people finaly talking about orbital elevators

ackydoodles 13y ago

Many problems must be solved before they can exist. (1) Only suitable material for the cable is, at the moment, unobtanium. (2) Cable must be moved continuously to dodge debris and satellites whose orbits cross the equator (all of them except those in geosynchronous orbit). (3) The cable, elevator module and any cargo or people must pass through the Van Allen belts, which will degrade them. People don't respond well to degradation by radiation. Other than that, we're good to go.

dllthomas 13y ago
Wouldn't another approach to (2) be eliminating debris and having satellites steer around it? We don't move buildings to dodge cars. This isn't to say that's easy, just that there's more than one currently intractable way we might eventually be able to skin that particular cat. Edited to add: With respect to (3), there's apparently this: http://en.wikipedia.org/wiki/Van_Allen_radiation_belt#Proposed_removal http://en.wikipedia.org/wiki/Van_Allen_radiation_belt#Propos... I don't know near enough about it to say whether it's any sort of a good idea, but it doesn't seem hard relative to building a space elevator.
Florin_Andrei 13y ago
The problems created by radiation in the Van Allen belts are vastly overstated. The funny part is, this is due in part to fake Moon landing conspiracy theorists, who use the radiation as a "supporting argument".
- dllthomas 13y ago
  My understanding is that satellites that plan on spending significant time in the Van Allen belts need to take countermeasures. Is that not the case, or is it only relevant to things like sensitive electronics?

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theothermkn 13y ago

While space elevators make for a fun toy gedanken for space enthusiasts who don't have an engineering background, they will never happen for one primary reason: As soon as you have a material out of which you can build a space elevator, you have a material to make extremely lightweight filament-wound pressure vessels. You essentially get SSTOs with amazing payload fractions and greatly reduced mechanical complexity for free. LH2 gives you tanks that are too heavy for an aluminum craft? No problem. Elevator-grade carbon nanotube composites are 1/1000th the mass. Go nuts. When vehicle mass fractions go from 10% down to .1%, payload mass fractions go from 2% to 12%, minus trace amounts of structure. Was it Asimov who said that space elevators will happen 50 years after people stop laughing? I hope so, because people should never stop laughing at this idea. (Really? We're going to better utilize space--i.e., cramming NEO with nanosats--at the same time we erect a huge hazard in NEO? Come on!)

Florin_Andrei 13y ago
Devil's advocate: Rockets are still less safe. Rockets still produce tremendous accelerations. 12% payload fraction is still much less than 50%, or 90%.
- theothermkn 13y ago
  QED. Your first sentence shows no consideration at all of the effect of material advances on launch vehicle safety. Your use of the phrase "tremendous accelerations" shows no understanding of the accelerations actually produced even by current launch vehicles. Also, you don't mention why this is, or even ever has been, a limiting factor in designing a payload, nor the degree to which it impacts payload design. Noting that 50 or 90 is greater than 12 doesn't change the fact that we're in an entirely different design space from the 2%-mass-fraction vehicle. Like I said, space elevators are a fun little toy gedanken for people who just want to think about The Amazing Future. If this is the best the Devil's Advocate can do, the Saints are going to be just fine.

dublinben 13y ago

They're not perfect, but launch loops solve many of the problems of straight up and down space elevators. https://en.wikipedia.org/wiki/Launch_loop https://en.wikipedia.org/wiki/Launch_loop