Can Quiet, Efficient 'Space Elevators' Really Work?(space.com)
space.com
Can Quiet, Efficient 'Space Elevators' Really Work?
http://www.space.com/24739-space-elevator-tether-technology.html
11 comments
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.
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.
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.
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.
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.
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)
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.
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.
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.
With that, you could potentially send up as many simultaneous climbers as the cable could support.
With some engineering, you could have dual-use climbers that double as habitat segments.
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.
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.
> 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.
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.
Regarding the second point, building two cables in parallel would certainly not double the project price. (It could however pose additional safety challenge).
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.
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.
Building the second one requires you to push the "Copy" button.
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.
Regarding 1: most designs have a large space station as a counterweight at the geostationary position that helps keep the cable steady.
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...
Can't you leave the climbers at the end to add to the counter-weight? Or am I misunderstanding something?
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
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
We already have reason to use space. The big problem is that it's prohibitively expensive to get there. A space elevator solves that problem. Of course the elevator itself is prohibitively expensive too, but if it's possible at all, those costs will eventually be recouped.
Asteroid mining.
Not in the short term see (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.
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.
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.
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.
> 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.
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.
Although it does make getting into orbit very slow.
In "Blue Mars" by Kim Stanley Robinson, we get to a point where travel between Earth and Mars is faster than the elevator ride from the top of Earth's elevator to the bottom.
In "Blue Mars" by Kim Stanley Robinson, we get to a point where travel between Earth and Mars is faster than the elevator ride from the top of Earth's elevator to the bottom.
That's kinda missing that the whole point of researching space elevators is to dramatically change the value proposition.
Space will be economically uninteresting until somebody manages to drop the price-per-pound by a few orders of magnitude, and then an awful lot of things becoming economically interesting. A viable space elevator might just accomplish that.
Space will be economically uninteresting until somebody manages to drop the price-per-pound by a few orders of magnitude, and then an awful lot of things becoming economically interesting. A viable space elevator might just accomplish that.
Can you expand on which things you think might become economically interesting?
I can't see the cost of delivering and supporting humans in space dropping below the cost of, say, delivering and supporting humans underwater in SCUBA gear. Living in a space hab is like living in a deep-sea hab in many logistical ways (including no need for decompression time).
The only major uses of SCUBA I'm aware of are 1) the military; 2) mineral exploration; and 3) tourism. Is there really any reason to think there will be "an awful lot" of other economically interesting things to do in space?
I can't see the cost of delivering and supporting humans in space dropping below the cost of, say, delivering and supporting humans underwater in SCUBA gear. Living in a space hab is like living in a deep-sea hab in many logistical ways (including no need for decompression time).
The only major uses of SCUBA I'm aware of are 1) the military; 2) mineral exploration; and 3) tourism. Is there really any reason to think there will be "an awful lot" of other economically interesting things to do in space?
There are some big differences between space and the deep sea: easier communication, there's abundant precious metals in space, there's plenty of sunshine to generate power, and the view is a lot better. The only real similarity is that they're both really hostile environments where people need serious protection.
But you don't have to send people. There's lots of stuff in space that can be done by robots. Easier than in the deep sea, because of easier communicaton.
And if you're not talking about the deep sea, but fairly shallow seas, then doing things there isn't really all that expensive at all, and driving the costs of operating in space down to that level is going to be incredibly attractive.
But you don't have to send people. There's lots of stuff in space that can be done by robots. Easier than in the deep sea, because of easier communicaton.
And if you're not talking about the deep sea, but fairly shallow seas, then doing things there isn't really all that expensive at all, and driving the costs of operating in space down to that level is going to be incredibly attractive.
Forming monocrystals and other delicate processes, for one.
Asteroid mining.
Mine some asteroids for iron, carbon, silicon, water. Now you have the raw materials for the biggest solar powerplant ever made. Ship the power down the space elevator and you have cheap renewable energy without the downsides of it being earthbound. Ship any excess ores down to earth for more profit. Some of those asteroids could have billions, if not trillions of tons of raw materials ready to use.
It's a bit far out there as ideas go, but so was visiting the moon at one point!
Mine some asteroids for iron, carbon, silicon, water. Now you have the raw materials for the biggest solar powerplant ever made. Ship the power down the space elevator and you have cheap renewable energy without the downsides of it being earthbound. Ship any excess ores down to earth for more profit. Some of those asteroids could have billions, if not trillions of tons of raw materials ready to use.
It's a bit far out there as ideas go, but so was visiting the moon at one point!
We have; here's an example. A solar panel in geostationary orbit only has a few hours of shadow per year. The incoming power is so much greater than that at the surface (about 4x the best desert) that a solar array in space will have a positive energy balance even when launched with current rockets. With a space elevator, orbital solar might be the most lucrative source of electricity. http://physics.ucsd.edu/do-the-math/2012/03/space-based-sola...
How would geostationary orbit have only a few hours of shadow per year? My understanding is geostationary orbit requires you be pretty much over the equator, which would mean the satellite spent as much time on the far side of the earth as the near.
Geostationary orbit is 36,000 km above the earth surface. The earth is 12,000 km in diameter. The sun is 150 MILLION km away. The about of time the something in geo-stat stays in earth's shadow is tiny.
Imagine a base runner in baseball after a home run. Spends half the time on the far side of the pitcher from the homeplate umpire, but he is only occluded from the homeplate umpire by the pitcher for a very short amount of time. That's (very!!) roughly the scales involved here.
Imagine a base runner in baseball after a home run. Spends half the time on the far side of the pitcher from the homeplate umpire, but he is only occluded from the homeplate umpire by the pitcher for a very short amount of time. That's (very!!) roughly the scales involved here.
Not only that, but thanks to the fact that the Earth's rotation (and thus the plane of the equator) is tilted off of the orbital plane, even equatorial satellites far below geosynchronous orbit will only pass through the Earth's shadow at all close to the equinoxes (twice a year). Most of the time, you have eternal sunshine throughout an orbit. It's as though the metaphorical baseball player takes a flying leap so that the umpire can see him over the pitcher's head.
Thank you. That answers it. I had no idea geostationary orbit was so high, but I probably should have.
"The incoming power is so much greater than that at the surface (about 4x the best desert)"
But with 22k miles (3x earth diameter) of transmission losses.
But with 22k miles (3x earth diameter) of transmission losses.
You don't have to transmit the power. You can manufacture energy-intensive things right there and then transmit the end products.
It's really easy to send stuff from orbit to Earth. The trick is keeping it from burning up on the way in.
It's really easy to send stuff from orbit to Earth. The trick is keeping it from burning up on the way in.
And that's where the space elevator is very handy. It's good for getting satelites and factories up there, but it's just as good for getting the end products down again.
The losses when transmitting through vacuum are pretty negligible; it's only the last fifteen miles that really cost you.
That certainly might be the case, but I don't immediately see why.
I'm not a physicist, but my understanding is that there are two ways a wireless photon-based energy transmission system loses energy in transit.
First, each molecule has a specific set of frequencies where photons are "easy" to absorb because they correspond to transitions between quantum states. Any system whose goal is to deliver energy wirelessly through the atmosphere will obviously not use those frequencies.
The other way for a photon to lose energy to the medium it's traveling through is, if the medium contains molecules like water that have different parts with positive and negative charges, the electromagnetic wave will move the differently charged parts in opposite directions. The energy to do that is lost [1].
[1] "Lost" is an application-specific notion. This mechanism is also how a microwave heats food, and in this case the energy imparted to the water molecules in the food can't be considered "lost" because it's supposed to go into the food!
First, each molecule has a specific set of frequencies where photons are "easy" to absorb because they correspond to transitions between quantum states. Any system whose goal is to deliver energy wirelessly through the atmosphere will obviously not use those frequencies.
The other way for a photon to lose energy to the medium it's traveling through is, if the medium contains molecules like water that have different parts with positive and negative charges, the electromagnetic wave will move the differently charged parts in opposite directions. The energy to do that is lost [1].
[1] "Lost" is an application-specific notion. This mechanism is also how a microwave heats food, and in this case the energy imparted to the water molecules in the food can't be considered "lost" because it's supposed to go into the food!
A third way is by diffraction- any beam, including a laser, will spread out as it travels. You get less spread with short wavelengths and large transmitters, but both of those imply expense. If you use long wavelengths and small transmitters, you need huge receivers to get all the beam energy back.
Ah, you intend to keep them photons. I had been imagining converting to electrons and running down cabling attached to the elevator cable. The other approach might very well be better, but doesn't really require a space elevator does it?
I'd imagine they'd go with super-conducting cables personally. Space is extremely cold, so achieving a super-conducting state is simply a matter of shielding the cable from the sun. Shouldn't be too difficult... At least, not in comparison to building a space elevator in the first place!
> Space is extremely cold, so achieving a super-conducting state is simply a matter of shielding the cable from the sun.
Not quite so simple. If we're talking about a cable near the earth, where the sun sometimes shines, there are always two heat paths -- from the sun to the cable, and from the cable to deep space. It's not easy, nor is a way self-evident, to make the cable fall to deep space temperatures at a reasonable cost, given that the sun is providing heat energy that must be diverted.
Not quite so simple. If we're talking about a cable near the earth, where the sun sometimes shines, there are always two heat paths -- from the sun to the cable, and from the cable to deep space. It's not easy, nor is a way self-evident, to make the cable fall to deep space temperatures at a reasonable cost, given that the sun is providing heat energy that must be diverted.
Space is cold, but getting rid of heat is not easy in a vacuum. Not sure what that means with respect to this particular project, though.
> Space is cold, but getting rid of heat is not easy in a vacuum.
On the contrary, getting rid of heat in a vacuum is easier than getting rid of heat in an atmosphere. Here's a diagram of human heat loss at the surface:
http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/coobod.htm...
According to this source, at the surface of earth, under the atmosphere, a person loses:
Perspiration: 17 watts
Conduction: 11 watts
Radiation: 133 watts
Without an atmosphere, the direct radiation of heat energy into space becomes more efficient (no greenhouse effect), and it's always the most efficient way to radiate heat.
It is the efficiency of direct radiation of heat energy that explains why objects at the surface can fall below air temperature overnight under a clear sky, as they surely do.
On the contrary, getting rid of heat in a vacuum is easier than getting rid of heat in an atmosphere. Here's a diagram of human heat loss at the surface:
http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/coobod.htm...
According to this source, at the surface of earth, under the atmosphere, a person loses:
Perspiration: 17 watts
Conduction: 11 watts
Radiation: 133 watts
Without an atmosphere, the direct radiation of heat energy into space becomes more efficient (no greenhouse effect), and it's always the most efficient way to radiate heat.
It is the efficiency of direct radiation of heat energy that explains why objects at the surface can fall below air temperature overnight under a clear sky, as they surely do.
Hm, that's contrary to what I'd heard, but I'll certainly defer to you on the question.
> Hm, that's contrary to what I'd heard
I think some of the confusion arises because of vacuum thermos bottles, which are really efficient at holding onto their heat. But how they work is a bit complicated. They deal with conduction and convection losses by having the vacuum barrier between the contents and the outside. As to radiation, they rely on a reflector that's part of the vacuum bottle, which has the effect of greatly slowing the rate of heat loss by radiation.
So the vacuum thermos avoids radiation heat loss, not because of the vacuum, but with a reflector. That works in space too -- many orbiting telescopes use reflectors to keep the sun's heat energy from heating up the sensors and spoiling their performance.
But a vacuum is a pretty good medium for heat loss by radiation. The moon's surface, heated to several hundred degrees Celsius during the lunar daytime, drops to 26 Kelvins after a few (earth) days of darkness (26 degrees above absolute zero). That's a new figure, lower than had been realized, and seven degrees colder than the surface of Pluto.
I think some of the confusion arises because of vacuum thermos bottles, which are really efficient at holding onto their heat. But how they work is a bit complicated. They deal with conduction and convection losses by having the vacuum barrier between the contents and the outside. As to radiation, they rely on a reflector that's part of the vacuum bottle, which has the effect of greatly slowing the rate of heat loss by radiation.
So the vacuum thermos avoids radiation heat loss, not because of the vacuum, but with a reflector. That works in space too -- many orbiting telescopes use reflectors to keep the sun's heat energy from heating up the sensors and spoiling their performance.
But a vacuum is a pretty good medium for heat loss by radiation. The moon's surface, heated to several hundred degrees Celsius during the lunar daytime, drops to 26 Kelvins after a few (earth) days of darkness (26 degrees above absolute zero). That's a new figure, lower than had been realized, and seven degrees colder than the surface of Pluto.
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?
It would be at risk, yes, and probably more so than most space vehicles. Gladly our space junk hasn't yet reached the point where surprise cascades happen so we can track debris well in advance. But we'll be needing real countermeasures, active spacecraft that can clean up our orbits.
One interesting aspect of the space elevator remains its failure modes though. Cable failure, for any reason, is going to be a problem. We'll need a safety concept both for the ground as well as the payload if humans are going up there. I hope this isn't going to be one of these things where everybody ends up agreeing that it's not feasible to save lives in the event of catastrophic failures.
One interesting aspect of the space elevator remains its failure modes though. Cable failure, for any reason, is going to be a problem. We'll need a safety concept both for the ground as well as the payload if humans are going up there. I hope this isn't going to be one of these things where everybody ends up agreeing that it's not feasible to save lives in the event of catastrophic failures.
"We'll need a safety concept both for the ground"
Think about it... without a working active stabilization system and a carefully engineered for re-entry shape, "stuff" that re-enters usually doesn't make it very far in the atmosphere before being turned into dust.
We have a long way to go until an elevator can be as large as a dino-killer asteroid. Existing designs have spectacular surface are to volume ratios, there won't be much left...
This is, however, a very serious problem on the moon or perhaps mars. And because the moon's gravity is so low compared to the earth, we're almost certain to have elevators all over the moon before the first one on the earth. A moon-vator is so small and light we don't even need new tech to pull it off, just drop a couple billion and we can do it within perhaps 3 years if done privately. Maybe 30 if NASA runs it.
So yeah, its a problem, but not for legacy earthlings.
Think about it... without a working active stabilization system and a carefully engineered for re-entry shape, "stuff" that re-enters usually doesn't make it very far in the atmosphere before being turned into dust.
We have a long way to go until an elevator can be as large as a dino-killer asteroid. Existing designs have spectacular surface are to volume ratios, there won't be much left...
This is, however, a very serious problem on the moon or perhaps mars. And because the moon's gravity is so low compared to the earth, we're almost certain to have elevators all over the moon before the first one on the earth. A moon-vator is so small and light we don't even need new tech to pull it off, just drop a couple billion and we can do it within perhaps 3 years if done privately. Maybe 30 if NASA runs it.
So yeah, its a problem, but not for legacy earthlings.
The space elevator would be flung out to space, in the event of breakage.
Except for the part that comes down to Earth, depending on the place where the ribbon breaks.
I've seen simulations. Parts of it are ejected far away, but other parts whiplash to the ground with great energy. Luckily, a large part of the whip may burn out in atmosphere, but there's still a large remaining portion moving at hypersonic speeds that you need to worry about.
From what I've read of carbon tube based cables, most of the length would burn up in the atmosphere before reaching the ground.
Edit: VLM does have a point in that a falling cable might be a problem on Mars. The moon doesn't have enough rotation for a fixed skyhook (space elevator) anyway. :-) More advanced rotating elevators in a moon orbit, maybe. But it might be hard because of the moon mascons (the moon is lumpy internally with different density, which results in gravitational anomalies).
Edit: VLM does have a point in that a falling cable might be a problem on Mars. The moon doesn't have enough rotation for a fixed skyhook (space elevator) anyway. :-) More advanced rotating elevators in a moon orbit, maybe. But it might be hard because of the moon mascons (the moon is lumpy internally with different density, which results in gravitational anomalies).
The moon doesn't have enough rotation for a fixed skyhook (space elevator) anyway.
You're just fairly restricted in where you can put it- you can only put one near the sub-Earth and anti-Earth points, such that the cable passes through the L1 or L2 Lagrange points, rather than anywhere along the equator. The cable would have to much, much longer than for a space elevator on Earth, but the Moon's lower gravity actually makes the engineering challenges Not As Bad- i.e., we could build a lunar space elevator with current materials.
OK ok, but it is economical insanity.
On the moon, an electric rail launcher with 5-10G only needs to be a few km long.
(Iirc, 1G acceleration of four minutes is the launch velocity of the moon? ~ 2.4 km/s.)
Edit: This assumes local materials for the electric launcher. If everything is skipped from Earth, it might be different. But, consider -- if you need to launch so much stuff from the moon, you must already have the infrastructure to build things there.
On the moon, an electric rail launcher with 5-10G only needs to be a few km long.
(Iirc, 1G acceleration of four minutes is the launch velocity of the moon? ~ 2.4 km/s.)
Edit: This assumes local materials for the electric launcher. If everything is skipped from Earth, it might be different. But, consider -- if you need to launch so much stuff from the moon, you must already have the infrastructure to build things there.
I've always wondered about the aggregate effect of wind pressure across the length of the cable. It seems like it could be significant.
The cable is 62,000 miles long. The atmosphere goes up maybe 100 miles (0.16%) of it, and meaningful wind only goes up about ten miles (0.016%). The pressure doesn't even register.
That said, I've got to imagine that bottom .1% of the cable will be made differently to counteract all the water and oxygen around it.
That said, I've got to imagine that bottom .1% of the cable will be made differently to counteract all the water and oxygen around it.
I wonder about the rotational speed changes of the payload mass more than the wind. The counterweight will tend to speed up and slow down as the mass of the payload is accelerated and decelerated (during climbs and descents respectively), but I keep hearing that the counterweight is just a mass.
If you think the counterweight will move down when you thrust a mass up, you have a misconception. The tether will be consistently under tension, so unless the acceleration mA is sufficient to surpass MV^2/R, that won't happen.
These are all great questions but why do we have to imagine the answers, can we just simulate the damn thing and settle if it would work or not? It seems almost all parameters are known for such simulation except the material used for cable is not invented yet.
Every lower orbit will eventually intersect with the cable, so the cable would actually have to dodge most existing satellites. I've read estimates that it would only need to dodge something on average once or twice a year, but I don't know if that number is correct.
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?
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?
"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.
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.
What tower has been built a mile tall? According to Wikipedia, the tallest radio tower ever was the Warsaw radio mast at 646.38 meters.
You are correct. I had the peculiar idea that Ostankino was 1800 meters tall (well over a mile) its actually 1800 feet. Which is still pretty tall. There are other towers taller than Ostankino now. This comes up a lot at a telecom company, "You think climbing a 100M tower is a PITA, there's one in Russia thats.. etc etc"
If anything it would work like a lightening rod and discharge the currently gradually so it never builds up in the first place.
And, could that current in the tether be used as a source of energy? And, would stopping lightning cause other problems for Earth?
> 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.
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.
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!)
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!)
Devil's advocate:
Rockets are still less safe. Rockets still produce tremendous accelerations. 12% payload fraction is still much less than 50%, or 90%.
Rockets are still less safe. Rockets still produce tremendous accelerations. 12% payload fraction is still much less than 50%, or 90%.
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.
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.
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.
Multiple SF authors have covered this unpleasant contingency. It seems more of an example of why we can't have nice things to me.
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.
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.
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.
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.
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
[1] http://en.wikipedia.org/wiki/Rods_from_god#Project_Thor
Project Thor has got to be one of the most appropriately named projects ever.
Being suddenly slung into outer space sounds pretty terrifying.
It is something the elevator cars will be designed for, and just by the nature of the elevator we'll have a lot more orbital infrastructure ready to receive the cars. IIRC, much like airplanes at takeoff and landing, there are some windows where it is impractical to recover from a detached elevator that occurs at precisely the wrong moment, but they are relatively small. (Depending on the precise numbers, it may be possible to build a passenger car that has no such windows, but cargo cars will probably just run the risks since it greatly increases their payload.)
I won't promise NOTHING WILL EVER GO WRONG!!!1!, but there are a lot of engineering options to mitigate disaster.
I won't promise NOTHING WILL EVER GO WRONG!!!1!, but there are a lot of engineering options to mitigate disaster.
> Being suddenly slung into outer space sounds pretty terrifying.
Definitely a breathtaking experience.
Definitely a breathtaking experience.
Does the possibility of a bomb exploding in a nuclear reactor also seems to you as an example for why we can't have nuclear reactors, while in practice we have hundreds of nuclear reactors and zero bomb explosions?
Seriously, of all the possible problems with constructing and managing a space elevator, this one seems like a non-issue.
Seriously, of all the possible problems with constructing and managing a space elevator, this one seems like a non-issue.
In theory, nuclear power should have been a vital part of a comprehensive strategy to stop climate change in its tracks.
In practice, human greed and stupidity makes them more dangerous and expensive than they should be as well as reflexively feared by the marching morons who prefer the status quo of belching coal into the atmosphere without limit and inadvertently providing them with more radiation exposure than nuclear power.
http://www.scientificamerican.com/article/coal-ash-is-more-r...
So I think nuclear power is a great example actually of what really happens when the monkeys get their hands on otherwise cool technology.
In practice, human greed and stupidity makes them more dangerous and expensive than they should be as well as reflexively feared by the marching morons who prefer the status quo of belching coal into the atmosphere without limit and inadvertently providing them with more radiation exposure than nuclear power.
http://www.scientificamerican.com/article/coal-ash-is-more-r...
So I think nuclear power is a great example actually of what really happens when the monkeys get their hands on otherwise cool technology.
I think you have missed the point of the analogy.
Engineers, like myself, are able to design systems that do not fail spectacularly when faced with predictable threats.
It can be (and arguably should be) designed so that tension at the bottom is zero. Cut it at the base, and it would float, and maybe drift slowly.
If it's cut above that, it depends. In most scenarios, part of it would shoot out into space, the rest would whiplash around the Earth, mostly burning in the atmosphere, some of it (pretty short chunk) would hit the ground at very large velocities.
If it's cut above that, it depends. In most scenarios, part of it would shoot out into space, the rest would whiplash around the Earth, mostly burning in the atmosphere, some of it (pretty short chunk) would hit the ground at very large velocities.
Robert Zubrin's space elevator book analyzes this situation and concludes that most of the mass would either escape or burn up in the atmosphere. Only the bottom-most section would hit the ground.
His design contemplates anchoring to a movable ocean-going platform, both for this kind of safety issue and because it means you can steer it away from big-but-easy-to-track tropical storms.
His design contemplates anchoring to a movable ocean-going platform, both for this kind of safety issue and because it means you can steer it away from big-but-easy-to-track tropical storms.
Can you recommend SF books which contain space elevators?
Arthur C. Clarke's "The Fountains of Paradise" is about building a space elevator. It's probably one of the first science fiction books to include it. There's also Heinlein's "Friday" and Kim Stanley Robinson's "Red Mars". Those are just the ones off the top of my head.
Fountains of Paradise, Red Mars, Blue Remembered Earth
There's a lot but these are 3 off hand. FoP was the one that really popularized the idea.
Red Mars and it's sequels have a small part about what can go wrong.
BRE just uses it as it would really be used. A piece of generic ordinary transportation technology that people used to it don't think twice about.
There's a lot but these are 3 off hand. FoP was the one that really popularized the idea.
Red Mars and it's sequels have a small part about what can go wrong.
BRE just uses it as it would really be used. A piece of generic ordinary transportation technology that people used to it don't think twice about.
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
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.
(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.
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#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.
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#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.
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".
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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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?
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.
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.
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.
Also, it can be easily deployed by unrolling the tether in both directions simultaneously from the geosynchronous orbit.
It doesn't have to be "far far out". It can be closer, but heavier. A big rock.
Isn't it that there's usually a huge counterbalance in a Clarke orbit that would keep the elevator cables straight?
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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It is nice to see people finaly talking about orbital 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.