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Space Elevator May Become Reality
mojotek writes: "The NASA Institute for Advanced Concepts has a study(15Mb pdf) about the feasibility of a "Space Elevator" comprised of a 22,000 mile long cable built out of carbon nanotubes. In theory, it would be able to carry loads of 20 tons to space without using a single rocket engine. Sounded way too sci-fi for my taste at first, but this article at TechTV actually helped fill in the holes."
I did the math and worked out that if you gibbed the cable (say 1m chunks), you'ld wind up with something like 25-30 thousand km (I don't remember the exact figure) of the cable crashing down on earth and the rest flying off into space. However, I didn't figure out if the cable would fall east or west (west would be better, but I think it's less likely). Either way, that's a little over 1/2 way around the world and while the only land mass likely to be hit is Africa, I don't imagine the impact with the water would be particularly fun (possible tsunami).
Bill - aka taniwha
--
Leave others their otherness. -- Aratak
as shown in full gory detail here. note the counterweight too.
To transport you (70 kg) up to an altitude of 200 km would take roughly 140,000 kilojoules of energy (you do the math ... first year physics stuff). However, they can't just lift you, they also have to lift a vehicle containing you. Say the vehicle weighs 500 kg for every person it can carry -- this would take rougly 1,000,000 kilojoules. If they do this electrically (which is one of the more expensive forms of energy), at 100% efficiency it would eat up roughly 300 kWh of energy. At 0.30/kWh (say), that's roughly $100.
Of course, a clever engineer would realize that every vehicle going up eventually goes down ... so the vehicle on the way down could be used as a generator, feeding power to the load of a vehicle going up. Equally obviously, we're not considering the amortization of the construction cost, which would be monumental.
Toronto-area transit rider? Rate your ride.
they want to have a 22,000Km cable to space, but I can't get DSL because I'm 2.3 miles away...
Grrrr
Chaos, Mayhem, and Destruction: Not
Trouble is, if someone farts in the elevator, it's a damn long wait before you can open the door... ;)
Not to dismiss the elevator out of hand, but wouldn't research into efficient space vehicle propulsion yield better long term results?
Not really, because the "efficient" propulsion systems probably won't be able to lift a rocket off the ground. E.g. the DS-1 ion engine, high efficiency but only about 0.1N of thrust - or nuclear engines that would be too dirty to run in the biosphere, but would work fine in interplanetary space.
If a space elevator could be built, the cost of lifting payloads into space could drop dramatically, and that would create a lot more incentive for companies to develop these efficient space-only engines.
The short answer is: Yes.
Physics works everywhere all the time. When you climb a flight of stairs or walk up a hill it slows the Earth's rotation - and it speeds back up as you walk back down.
No - seriously - just as an ice skater's rotation slows or speeds as they extend or contract their arms the same principles apply to all rotating bodies. Everytime we slingshot a space vehicle around the Earth we are effectively transfering some of the planet's energy to the vehicle and that energy has to come from somewhere.
But the amounts here are so small that the effect is not measurable or "effective" in the scale of anything we could notice. It's like the fact that anything with mass has a gravatational field - but you don't notice the effect of the gravity created by your pen.
=tkk
Bill Gates - Creationist?!?
Robert Heinlein (iirc) once commented that low earth orbit (LEO) is halfway to anywhere, and that's even more true of geosynchronous orbit (GEO). It takes a *lot* of fuel to get out of the earth's gravity well, and getting to GEO for the cost of electricity (provided by in-space solar cells!) would profoundly change everything.
If you want to leave earth orbit, you take a second elevator that runs from geostationary station out to the anchor and let go. Depending on the length of this section, you'll have a ballistic launch to anywhere else in the solar system. Well, you'll need a modest amount of fuel unless the plane of earth's orbit is exactly aligned with your destination, but you'll need orders of magnitude less fuel than you need today, and you can get that fuel up to the launch point for the cost of electricity alone.
If you want to leave the solar system, you let go of the upper elevator and hop to the center of a freespinning tether, then inch outward. When you reach the end of this tether, you could be traveling at a few percent of c. You'll be at Alpha Centari within 100 years... and a second tether there could capture you and slow you down. That's too long for passenger traffic, but brief enough that interstellar colonization is a realistic possibility by the end of the millennium.
So all things considered, I think research into carbon nanotube space elevators has better long term potential than anything rocket propulsion technology. Even antimatter propulsion, excluding some unknown mechanism to mass-produce anti-atoms.
For every complex problem there is an answer that is clear, simple, and wrong. -- H L Mencken
I guess this obsoletes my "space escalator" idea then, eh?
This disaster was used (although on Mars) in the plot of in Kim Stanley Robinson's Red Mars (or maybe Green Mars... can't remember). In that case, though, the "beanstalk" was sabotaged as a weapon during a revolution. It wiped out a slice of a city, puncturing the atmosphere of a bunch of buildings, but had no casualties outside the settled areas. Can't have a tsunami in that thin an atmosphere.
"Prepare for the worst - hope for the best."
There are some variations on the idea though,like this one, that are close to being possible with today's technology, and can even be provisionally costed. Basically the idea is to construct an elevated runway about 100km up, and use mass drivers to hurl stuff into orbit. At that altitude the saving from air resistance is huge and mass drivers become very efficient
At this stage, NASA speanding serious time thinking about space elevators is probably no more useful than daydreaming. Thinking about this kind of thing is probably more productiove though, becuase something might come of it in the medium term, and its almost as efficient as an evelator anyway - with the decided advantage of not being able to collapse and strangle the planet.
(Since I heard about this from a NASA researcher, maybe Im being a little harsh to accuse them of daydreaming)
But the weight to payload ratio sucks pretty hard. I imagine the up-front construction costs would be a lot higher than the cost of building rockets. So even if it's cheaper after, say, 10,000 uses, we might not see anyone wanting to build it.
You see? You see? Your stupid minds! Stupid! Stupid!
One big issue they missed is the fact that a carbon nanotube cable still isn't strong enough to support it's own weight without tapering the cable correctly, at the middle it has to be about 10 times thicker because the stresser are highest at geostationary orbit.
The deployment method they're using doesn't take account of the fact that you need the thickest part to always be at the middle - if you simply unroll it the way they suggest then the incorrect thickness profile will result in the cable exceeding it's breaking point and snapping.
What they need to do is unfurl a cable like this from geostationary orbit simultaneously up and down at the same time. The Mechanism to do this would have to be very delicate at unfurling the last kink or the cable will again snap.
The cool thing about this is if you figure out what kind of weight you want the cable to support then you can come up with an idea of the amount of energy stored in the tension. If the cable snapped at any point then the amount of energy released would be pretty phenomenal. From each end of the snap you'd generate a compression wave which would get stronger as it travelled along the cable, after a while of picking up energy it may turn into a shockwave and snap the cable again (essentially shattering the cable). If it doesn't then the wave will have energy equivalent to nuclear weapons when it reaches the endpoints and the waves transmit themselves into the supporting structure....
No, you don't need a counterweight. If the cable is long enough so that the center of mass is in geostationary orbit it will just hang there by itself.
Besides, this "space elevator" would be a giant, provactive, easy target for terrorists.
If we let that stop us, then the terrorists have already won!
___
The way to see by faith is to shut the eye of reason. --Ben Franklin
And it is exactly that, sci-fi. Sure, carbon nanotubes are incredibly strong. And they're also on the order of a few microns long. Now, this cable needs to be a few hundreds of thousands of meters long. You do the math.
The semiconductor industry figured out how to make large single crystals of ultra-pure silicon, then pattern the surface down to a ridiculously fine resolution. The fiberoptic folks figured out how to make glass so clear that a light pulse can go through many many miles of it and still be recognizable at the other end. Molecular biologists can "amplify" single molecules of DNA into macroscopic quantities.
I wouldn't be so quick to say that we will never be able to make carbon nanotubes that are long enough to be useful as structural materials.
I was just out of college (iirc) when the first popular discussion of beanstalks came out (Charles Sheffield, in some long-dead Baen book-zine).
The numbers were so ludicrious that he repeatedly apologized for wasting our time. Of course this was a flight of fancy, the numbers were orders of magnitude larger than the strongest known materials. Yet, if "ultronium" could be developed from some exotic material....
Then buckyballs were discovered. Then buckytubes.
The fact that this is even "just" possible with known materials less than 20 years later is mindblowing. I can only compare it to the confident RSA predictions in Scientific American (which I also remember when it first appeared) that RSA-128 would take millions of years to crack. We all know how well that prediction held up.
Given this perspective, I don't think it's unreasonable for NASA to spend some serious money considering its options if/when stronger materials become available. It's easier to believe that even stronger materials will be discovered (e.g., perhaps by putting foreign elements within the tubes to manipulate quantum properties) than that we've suddenly hit the ultimate barrier.
For every complex problem there is an answer that is clear, simple, and wrong. -- H L Mencken
A sci-fi/sci-fact magazine in paperback form called Destinies had a story about this in their Aug-Sept 1979 edition. The story was called "How to Build a Beanstalk" by Charles Sheffield. He did some research into the material strength required, and to get the stalk to reach down to earth, or somewhere near it required a material with a tensile strength of 2 000 000 kg/cm^2, which was 10 times the current known tensile strength of known materials at the time.
"Beanstalks, originally called skyhooks, are an idea of the 1960's whose time may at last have come. They are used as important elements of at least two novels published in 1979, Authur Clarke's 'The Fountains of Paradise' and my own 'The Web Between Two Worlds' "
http://pcblues.com - Digits and Wood
I've read this paper in full, a couple of months back. According to the paper the actual, demonstrated strength of the carbon tethers is only as strong as Kevlar- it's about 1/10 of the needed strength. The overall weight of the fiber is exponentially related to the strength, so the tether works out maybe 20,000 times heavier than his design- which makes it completely uneconomic.
OTOH, single fibers are almost strong enough, but only if you allow absolutely no 'safety factor'. Most normal engineering uses atleast 2 safety factor, and usually many times that. But as nobody knows how to splice them together into a rope, and doing so would lose atleast 25% strength, it's not enough.
He's got the best architecture I've seen for this by a long way, nice paper study. Not practical right now. Hope somebody sorts out the fibers very soon.
-WolfWithoutAClause
"Gravity is only a theory, not a fact!"I studied this concept as part of a commercial space development group back when I was in college. It's quite compelling.
;)
There're two significant challenges in implementation, though.
The fundamental flaw in the concept lies in conservation of rotational inertia. Think about a spinning ice skater - as she draws her arms in, she spins much faster. The opposite is also true - as a rotating mass extends from its center, its rate of rotation decreases.
The space elevator rotates at a constant geosynchronous rate, but as its payload is raised along that axis, the difference between its linear inertia at the surface of the earth and its linear inertia around the circumference at geosynch altitude (or any significant altitude along that axis) is absorbed by the elevator's structure.
Unless the payload applies some sort of thrust perpendicular to the axis of the elevator, that difference in inertia only works to pull the whole system back down to earth. Effectively, the amount of energy you'd have to put into the system to keep it up would equal the thrust expended to send the payload into orbit by conventional means.
Then there's the whole issue of vibrational harmonics. Accumulated shocks from winds, payloads, and even space dust would propagate up and down the string (any human structure of that incredible length would effectively be a string in tension) and create severe vibration problems. That'd take some *seriously* epic engineering to dampen.
NASA has done some experiments with tethered satellites which address the vibration issues (as well as accumulated electric charge from atmospheric drag), but they were intended more for spinning-wheel satellite applications than for space elevators.
It's a really cool idea that unfortunately is a something-for-nothing scheme. If there were some kind of cool electric thruster system which didn't rely on reaction mass, it'd be feasable, but then we're straying into Area-51 technology.
I didn't do any math for the damage caused by pieces below that mark, but my guess is that anything below a few km wouldn't be any worse than dropping a WWII bomb and the resulting damage would be very localized. between that and several thousand km, the chunks would fall into the water (assuming the builders were smart enough to build close to a coastline on the correct side:). There would be a region above those thousands of km where the chunks would be a bit more of a worry, but above that, they're likely to burn up when they hit the atmosphere.
Beyond all that, buggered if I know :)
Bill - aka taniwha
--
Leave others their otherness. -- Aratak
Another Robert Heinlein observation, this time from _Friday_. The issue is never energy, it's how the energy is stored.
The energy required to lift a ton of cargo to GEO is the same regardless of the mechanism used (and disregarding any power you can extract from descending cargo). But there's a tremendous practical difference in that energy coming down superconducting power lines from a solar array out by the ballast or if it comes from liquified oxygen and hydrogen stored in disposable tanks. It makes a tremendous difference whether you the energy is coming via an existing infrastructure (e.g., power cables) or if if you have to waste some fuel to lift the fuel you need now.
I don't know what the current factors are, but I wouldn't be surprised if putting something into GEO requires 99 kgs of fuel for every kg of payload. A beanstalk would get you there with no "waste" other than the reusable elevator car.
As for harmonics caused by weather... I think this has been dismissed. This cable is under millions of tons of tension, and has a cross section of well under a meter when it's in the atmosphere. The load bearing core will be surrounded by a much larger infrastructure for the elevator, power cables, etc., but since it's not load bearing it can be dampened -- and is still on the order of a few meters. With such a small profile and high tension you aren't going to see much energy transferred from weather systems into the cable. (Earthquakes are another matter.)
And the conservation of momentum issues are real, but I (and others) are skipping many of the fine details for overall clarity.
For every complex problem there is an answer that is clear, simple, and wrong. -- H L Mencken
Then there's the whole issue of vibrational harmonics. Accumulated shocks from winds, payloads, and even space dust would propagate up and down the string (any human structure of that incredible length would effectively be a string in tension) and create severe vibration problems. That'd take some *seriously* epic engineering to dampen.
To some extent those two are each others' solutions.
The low-frequency vibration solves the pull-back problem. Thinking discretely: The weight of the payload on the thether and the taut teather form a loaded "stringed-instrument" string:
Go up a bit, you pull the string back.
Stop and wait a bit, the string accellerates you forward.
Now go up some more while the string is still going forward, providing a "pull" backward that damps the vibration, stopping the string at the vertical position.
Repeat.
In fact you do this continuously, modulating your ascent slightly so the net result is the string stays nearly vertical. When a vibration starts to build up you adjust your speed in sync to damp it.
Similarly the tether and the weight at the end (large compared to the payload) form a pendulum. It's a much more complicated pendulum than one near the surface, due to the varying gravity and the rotating coordinate system, but that's the basic idea. Again thinking discretely:
Go up a bit. The couterweight pulls back.
Stop and hang around. The counterweight starts going forward.
Go up some more. You decelerate the counterweight and bring it to a stop near the top again.
Repeat.
Again you do it continuously, this time keeping the weight at a constant displacement behind the point over the tether's base. The slant of the tether corresponds to a forward accellerating force from the rotation of the earth, providing your angular-momentum transfer by accellerating your payload and decellerating the earth. (Coming down you push the counterweight forward to accellerate the earth and decellerate the payload.)
Now there may be one or more locations along the tether where what you have to do to damp the two modes is exactly opposite. But if you've kept it damped on your way to those spots you should be through before an oscilation builds up. Or run two or more payloads simultaneously and coordinate them so you can always damp both modes. (Multiple coordinated payloads can also provide better damping and trade off each others' effects on the tether to achieve faster travel.)
Of course you have to put your counterweight a bit further above geosync, so lift losses when it is displaced downward slightly don't turn into a positive-feedback collapse.
If you don't have enough payloads in transit you can damp higher-frequency modes against the atmosphere with a few active airfoils spotted along the tether. (REALLY high frequency stuff - like seconds-to-audio - you can damp with a couple small structures attached near the geosync level.)
Effectively, the amount of energy you'd have to put into the system to keep it up would equal the thrust expended to send the payload into orbit by conventional means.
No.
The amount you have to put in is only a small delta above the amount that you would have had to put in to run an electric elevator up an idealized stiff structure of the same height - and the delta approaches zero as your damping approaches perfection.
But once it's up you don't need to power it AT ALL, which I'll get to in another posting.
Bantam Dominique roosters crow a four-note song. Once you've heard it as "Happy BIRTHday" you can't NOT hear it that way
Something I haven't seen mentioned here (is the idea forgotten, or has it been proven to be flawed?) is the "construction ring" method.
Basically you launch your cable fabrication facility and create a *huge* loop of cable. Something long enough to encircle the earth at geostationary orbit. This loop is initially unstable and will require temporary station keeping engines. You don't care about north-south twists, but don't want in-out twists to grow to large. (Read any analysis of _Ringworld_ for details...)
You then turn the cable machines on their side and start laying cable towards/away from earth. The cables will follow local geopotential fields down and up, and eventually you'll have a starter cable touch down. This can be a temporary cable, designed to be discarded, that does nothing but throw mass up the cable to build the ballast and feed additional cable machines that are producing the production cables.
Eventually you have ring in geostationary orbit, plus numerous anchors along the equator. You supplement the ring at geostationary orbit with another ring a bit inside (or outside) of it so that it's always under tension.
Besides solving some construction issues, it eliminates many of the collapse modes. If the cable snaps, the upper portion is kept in place by the ring. Even if all cables are snapped, the ballast weights will keep the ring under tension and survivors can manage station keeping by dumping ballast. (Unfortunately, if all cables snap the rest of the system will have a different net orbital velocity and there could be a big jolt.) Since there are multiple anchors, there's little value to terrorists in destroying any single anchor.
I know that _3001_ mentioned a ring as an endstage after building the first beanstalk, but I thought I've seen papers suggesting they be used as a construction platform.
And the secondary benefits are huge. Let's say the ring is 250,000 km long, and there's a 500m wide band of solar cells attached to that ring. The solar constant is around 1370W/m^2, that's potentially 171 GW of pollution-free power than can be fed down superconducting cables - 540 trillion kWh/year. According to the USGS the US consumed about 9 billion kWh/year of power from all sources in 1998, so even if the ring has only 1% efficiency it would still provide every person in the world 300x more power than the average American consumed in 1998!
For every complex problem there is an answer that is clear, simple, and wrong. -- H L Mencken
"If you detach the cable from the base (Earth-Side) all that happens is you have to reattach it (Assuming the Space-Side can hold the cable in orbit.)"
Essentially, orbit means "centripetal force juuuust matches gravity." If the top is in geostationary orbit, then only the top is in microgravity. Every single inch below the top has a net force pulling downward. The lower your altitude, the faster you have to go to be in orbit (one revolution per day at geostationary, one revolution per hour at LEO). A break at any point in the beanstalk would bring it down.
You could make it tall enough so that the sum of the centripetal force of the end counterbalances the weight of the structure, and this would put the structure under tension instead of compression.
However, if you cut the structure anywhere between the surface of the earth and geostationary, everything below the cut will come crashing down. Fly a plane into it at seven miles, and you have a seven mile structure (about 35 times the height of the WTC) falling towards you. If the US can hit ballistic targets at a few hundred miles up with a kinetic-kill vehicle, Joe Shmoe with his suitcase nuke on a V-2 can hit a stationary target at that altitude. If there's a time-bomb on the elevator that goes off when the elevator floor is at or near geostationary, then we have 22,000 miles of material coming down.
"And the cable itself can withstand the force of multiple nuclear explosions (has to b/c of forces acting upon it)meaning it ain't coming down easy."
Tension, compression, and shear are three different things. Just because a material can withstand one or two of the three doesn't mean it can withstand all three.
And then there's a fourth factor: Heat. This was the WTC's weakness. While the steel structure withstood the airplane impacts, it couldn't survive the heat of the fire. Sure, the beanstalk might be able to survive the blast from a nuke, maybe even a shockwave if it was within the atmosphere, but nothing can survive the heat.
"The cable will wrap around the earth in a straight line from where it was cut."
No. Your main problem here is that you're assuming that all the mass will be at the top of the structure, forcing the structure below it to follow the top along as it comes down. Gravity being what it is, the center of gravity (assuming a structure of uniform density) will be somewhere between the bottom and the half-way point. And because gravity increases exponentially as you go down, taller structures will have their centers of gravity further from the midpoint than shorter ones.
So while you're correct in thinking that each unit length of cable will have to deal with tension in the cable (due to the motion of the rest of the cable) as well as gravity, you're incorrect in guessing what direction that tension will pull. For points in the structure higher than the center of gravity, the tension in the structure will be the stronger of the two forces, pulling the structure down along it's length instead of letting it spiral down in free-fall.
If anything, the top of the structure may fall along a straight line because it got snapped like the end of a whip, giving it more kinetic energy than it would have had if it were just in free-fall (and causing more damage than a free-fall would have done).
"By the second time around the earth the cable will began deterioting and exploding in the upper atmosphere."
First off, you have no idea how large these pieces may be when they break off. Second, all the kinetic energy of hundreds or thousands of miles worth of stuff has to go somewhere. If the actual mass doesn't make it past the upper atmosphere, then the momentum and kinetic energy just gets transferred to the atmosphere, which means a shockwave.
"Also since this has top be placed in a geo-synch orbit it needs to be located close to the equator. I.E. if it falls it hits a whole lotta ocean and not much else."
Tsunamis. Big tsunamis. And most of the world's population lives within 200 miles of the ocean.
Remember, something with the mass of a small island killed off the dinosaurs. What we're talking about is a structure with at least that much mass. While it may not be one big chunk, mass is mass and it's still coming down in a very short period of time.
"Futhermore having breakaway points on the cable itself would allow for only say 1/10 of the cable to impact the earth the rest would break and fly off into space."
Just for the sake of repeating myself, if the cut is anywhere between 0 and 22,000 miles up, anything below it is coming down. Period.