Can Quiet, Efficient 'Space Elevators' Really Work?
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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.
Building the second one requires you to push the "Copy" button.
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.
With that, you could potentially send up as many simultaneous climbers as the cable could support.
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.
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.
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 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.
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
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 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.
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.
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.
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.
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?
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!
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 with 22k miles (3x earth diameter) of transmission losses.
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!
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.
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 do recall someone explicitly stating that radiation is a poor means of losing heat compared to convection and conduction - which seems to just be wrong.
Yes -- it's a common belief, and it's wrong. Under clear skies after dark, objects on the surface that are convectively coupled to the atmosphere will quickly fall below air temperature because of radiation heat loss, which can produce what is called "radiation fog", so named because it's caused by the air being cooled by the ground, which in turn has been cooled by direct radiation into space.
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.
It's really easy to send stuff from orbit to Earth. The trick is keeping it from burning up on the way in.
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.
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?
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.
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.
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.
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.
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.
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.
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.
Lightning might be interesting but wind is no big deal.
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!)
Rockets are still less safe. Rockets still produce tremendous accelerations. 12% payload fraction is still much less than 50%, or 90%.
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.
Multiple SF authors have covered this unpleasant contingency. It seems more of an example of why we can't have nice things to me.
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.
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.
Definitely a breathtaking experience.
I won't promise NOTHING WILL EVER GO WRONG!!!1!, but there are a lot of engineering options to mitigate disaster.
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.
Seriously, of all the possible problems with constructing and managing a space elevator, this one seems like a non-issue.
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.
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.
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.
(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.
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.
Also, it can be easily deployed by unrolling the tether in both directions simultaneously from the geosynchronous orbit.
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.