Space Elevator

[mkillio]

I had to read 3001: The Final Odyssey for my Astronomy. And in the book there were space elevators that were conneceted to earth at 4 different points on the equator. As far as I could tell they were then connected in space to eachother and millions of people lived in them. I now have to right a paper on the space elevator and how feasable it is and how close we are to making one. Through my research I found out that it should cost anywhere from $5-$15 billion dollars and could be up with in 2 decades. It's really an interesting concept and seems to be very cost efficient, compared to shuttles, and will pay its self off in 5-10 years.

Here are some links, for more specific details.

http://www.space.com/businesstechnology/technology/space_elevator_020327-1.html

http://flightprojects.msfc.nasa.gov/fd02_elev.html

http://www.wired.com/news/technology/0,1282,57536,00.html

 

[3dilettante]

Twenty years sounds pretty optimistic, though to me anything short of a century or two is pretty optimistic.

In 3001, the elevators spanned all the way to geostationary orbit, which is 35,786 km above the equator. In theory, the elevator cores would be made of carbon nanotubes or diamond (at least with Clark's vision).

Problem is, the longest carbon nanotube fiber ever made stretched about 30 centimeters, which is a bit further than any diamond ever stretched.

So right now current technology is about 1/89465000 of the way there.

There's no guarantee that nanotubes will work. They're only 60 times as strong as steel at a quarter of the weight. I don't know of any steel cables that span anything close to 1/60 of 34,786 km.

Even Clark said such a technology will be available "50 years after everybody stops laughing". Unfortunately, there are a more than a few giggles here and there even now.

 

[Guden Oden]

Um, as a geostationary elevator would be gravitationally balanced, that means one end would pull pretty much exactly as much as the other (a little more in the outer end to prevent the thing to come crashing down I would think ), so wouldn't the thing therefore (according to your comparison) only need steel cables 1/30th of 35k km. Still a pretty extreme distance of course.

Anyway, 60 times steel is tensile strength, right? So what about compression strength then? If we build a f'n tall tower - which we could build several km tall even today without particulary fancy materials if we wanted - to cover some of the distance rather than make a cable that hangs down from space, that would help too.

 

[sytaylor]

There are also politics to consider. It is possible to do almost anything and push any boundary with enough research... but research costs money and either requires humans to unite behind a common purpose Star Trek style or a government to pile everything it has into a project Man on the moon style.

Neither of which are likley in the current political climate, it is worth remembering that while the cold war sparked the space race it also sucked all the funds out of it.

 

[Citrous]

Even if a space elevator could be built, there would be more forces plotting on how to destroy it then what it's best uses would be. More work needs to be done here on earth unfortunately.

 

[Nathan]

Anyway, 60 times steel is tensile strength, right? So what about compression strength then? If we build a f'n tall tower - which we could build several km tall even today without particulary fancy materials if we wanted - to cover some of the distance rather than make a cable that hangs down from space, that would help too.

Buckling would become an insurmountable issue. While you can load the vast majority of materials higher in compression than tension, it's impossible to produce anything on a space elevator scale that would have the required symmetry of construction and loading to give optimal resistance to buckling. And even if you could, the first hint of a breeze/solar wind would be enough to bring the whole thing crashing down.

Space elevators are a nice idea, but they'll never happen. Using carbon nanotubes for a project of this sort of scale is not an option - and probably never will be. All materials contain defects that severely reduce the amount of stress that they can withstand. The more material you have, the more likely it is that there will be a very bad defect that will limit the strength of the material. I really doubt that anyone can make 30,000Km x whatever cross-sectional area of carbon nanotubes without any defects whatsoever.

Regarding the comparison between nanotubes and steel - bollocks. I'd bet money that they're comparing microscopic nanotube material properties to macroscopic steel properties. It's been proven many times over that the strength on any material will rise significantly when only a small amount of material is tested. For instance, an aluminium wire of 0.01mm diameter will be able to take considerably higher stress on average than a wire 100mm in diameter. This is, again, due to the inclusion of defects in the material. Spider webs are not stronger than steel either.

 

[MPI]

Um, as a geostationary elevator would be gravitationally balanced, that means one end would pull pretty much exactly as much as the other (a little more in the outer end to prevent the thing to come crashing down I would think ), so wouldn't the thing therefore (according to your comparison) only need steel cables 1/30th of 35k km. Still a pretty extreme distance of course.

Ponder this: how much rope could you haul down an infinite abyss?

 

[Bouncing Zabaglione Bros.]

Surely the point is that you don't build a massive tower into space with all that weight standing on the ground like a skyscraper - you put the upper end in geostationary orbit, and you string the elevator between the top end (effectively a space station) in orbit and the ground.

Heck, the elevator doesn't even need to be a rigid tower. You just need to be able to connect the elevator car to the main guide cable/tower, and then haul it up and down with more cable - kind of like a ski lift or mountain cable car. If the elevator tower curves or bows, it doesn't matter as the elevator cars just follow the curves across however many miles it takes to get to the top of the elevator.

 

[Fred]

Nathan is right, the problem is stress. Imagine a small rotation at some point along the line (a tiny perturbation if you want). Now, add another one at some other point and so on and so forth.

Effectively stress propagates somewhat like a wave, all you need is for constructive interference at a few points, and you can calculate the chance for this, and it doesn't look good unless material science goes through a quantum leap in technology..

Nevermind that in principle carbon nanotubes may be enough, (though i've seen people argue against this convincingly), the manafacturing of such a thing in large quantities is way out of reach, and might always remain out of reach.

 

[epicstruggle]

ive been a big fan/supporter of space elevators for years. Ive commented on this board for a while on it.

What will most likely happen is that we create the first space elevator on the moon. Why?
-it will not take an exotic material to create the elevator
-doesnt need to be as tall
-fits the revitalized moon aspiration

epic

 

[digitalwanderer]

Ponder this: how much rope could you haul down an infinite abyss?

A lot!

 

[Xenus]

As long as someone doesn't get the brillant idea of trying to conncet the earth to the moon with one.

 

[Nathan]

ive been a big fan/supporter of space elevators for years. Ive commented on this board for a while on it.

What will most likely happen is that we create the first space elevator on the moon. Why?
-it will not take an exotic material to create the elevator
-doesnt need to be as tall
-fits the revitalized moon aspiration

epic

My back of the envelope calculations (which could be wrong) put the geosynchronous orbit radius at about 93,000km. Subtracting the radius of the Moon gives approx 90,000km. I'm not even going to try calculating the stresses required for a cable that long, but I'd guess that they will be a bit lower than for a space elevator on Earth. Probably not enough to allow good 'ol fashioned engineering materials to do the job though.

Actually, on second thought, the radius would have to be greater than 90,000km to stop the weight of the cable from pulling the space station back to the Moon. Though this applies to an Earth space elevator as well, so I think we can ignore it for the purposes of comparison.

Anyway, it looks like a Moon space elevator needs to be 3 times as tall as an Earth one, and places a fairly similar level of stress on the cable. But, hey, it it helps the Moon's economy, then I'm all for it.

 

[epicstruggle]

Nathan, your numbers dont make sense to me. (im not good in this area, so i might be wrong) The moon has "less gravity" than the earth, woulnd it mean that the elevator would not need to be as tall to escape the moons gravity?

epic
edit: quick googling:
http://www.universetoday.com/am/publish/lunar_space_elevator.html

The advantage of connecting an elevator to the Moon instead of the Earth is the simple fact that the forces involved are much smaller - the Moon's gravity is 1/6th that of Earth's. Instead of exotic nanotubes with extreme tensile strengths, the cable could be built using high-strength commercially available materials, like Kevlar or Spectra. In fact, Pearson has zeroed in on a commercial fibre called M5, which he calculates would only weigh 6,800 kg for a full cable that would support a lifting capacity of 200 kg at the base. This is well within the capabilities of the most powerful rockets supplied by Boeing, Lockheed Martin and Arianespace. One launch is it takes to put an elevator on the Moon. And once the elevator was installed, you could start reinforcing it with additional materials, like glass and boron, which could be manufactured on the Moon

edit2: the author replies to the slashdot story on his article:

I wrote the article, and now I'm reading through the Slashdot comments, and they're killing me. Didn't anyone actually RTFA?!?

Let me clear these up...

1. The cable would be 58,000 km long. This is the distance from the Moon to the L1 point, which is the balance point of gravity between the Earth and Moon. The Earth pulls the elevator straight using its gravity. If you looked at the Moon from the Earth, the space elevator would always be at exactly the same place on the Moon, always pointed directly at us, like we're tugging at it with the Earth's gravity. This has nothing to do with centrifigal force, like an Earth-based elevator where the counterweight keeps the cable taut.

2. Because of low gravity on the Moon, you could build the elevator with commercially available materials on the market today, like Kevlar or M5. The cable would be light enough that it could be launched on a single heavy lift rocket available from Arianespace, Boeing or Lockheed Martin.

One launch = one lunar space elevator

3. You could connect a second cable to the Moon's south pole, so the two cables form a V, and then bring up water ice from the south pole. This would put water, air and rocket fuel into high Earth orbit at a fraction of the price of bringing it up from Earth.

4. As you make the cable longer, it allows you to kick objects into high-Earth orbit. You could transfer materials from the Moon into orbit for relatively little fuel.

 

[Nathan]

As I said, I could be wrong.

The main thing is that the Moon's day is an interminably slow 29.5 Earth days, so the elevator must rotate at that speed. Disregarding the Earth's influence does give a radius of 90,000km. However, what is proposed in your linked article is to use the Earth's gravity to help provide some of the centrifugal acceleration required to hold the cable up, which allows the elevator to be considerably shorter, yet still be geostationary. Quite clever really.

 

[3dilettante]

With regards to using Lagrange point 1, I hope they've already factored in the effect of thermal expansion and contraction on a cable that is 58,000 km long.

The elevator's head might find itself a couple dozen kilometers off of position quite regularly, or worse it might get warped to a side, accellerating it relative to the moon's surface.

I'll admit I'm not an expert, but I get leery when big numbers are brought up and the expectation is that all the minor inconveniences small examples suffer from won't scale similarly.

 

[Nathan]

That's an interesting point. Given the listed a material properties for M5:

Elongation due the load
If we assume that the load only increases the stress in the cable by 1% (16MPa) of its compressive strength (1.6GPa) then the elongation would be 2.65 km for a 58,000 km cable. Seems completely reasonable to me.

Elongation due to thermal expansion
Estimating the coefficient of thermal expansion at 5 microns/m/°C and a 200°C temperature delta gives a thermal expansion of 58 km. I really have no idea whether this would cause a problem, or if the numbers I guessed are even close to the ballpark. In the end, it's still only 0.1% of the total distance, which would probably get ignored by every engineer I know.

[edit]I do wonder what will be at the top of the elevator. The article mentioned capturing an asteroid. Whatever it is, it'd have to be fairly massive to help reduce the amount of cable required and to give some stability under varying loads. However, it must also be able to move with thermal expansion of the cable. Since it will undoubtedly need some propulsion to keep it in a stable orbit, I suppose that could be used to adjust the tension in the cable.[/edit]

 

[3dilettante]

I think expansion and elongation might prevent a totally passive stabilization scheme. The changes in position shouldn't be too extreme, but they would probably accumulate over time, requiring periodic adjustments.

Since the first Lagrange point is not stable, in the sense that being just a little bit off will mean that the head of the elevator will either begin accellerating towards Earth or towards the Moon, there could be problems with forces being exerted over 58,000 km of cable.

I'm not an expert on kinetics so I can't pretend to understand what the exact effects would be.

 

[Nathan]

I don't see the instability of the Lagrange point as an issue. The asteroid would not actually be located at the Lagrange point anyway. Rather, it would be closer to the Earth, so that the Earth's gravity is trying to pull the asteroid towards it, which would counteract the effect of the cable trying to pull the asteroid towards the Moon.

I imagine the elevator would be designed such that the bottom of the cable always had a bit of tension in it. This would hold the asteroid in place and prevent the cable from being able to flop around. It would probably have quite a nice stablizing effect too. Sort of like a pendulum.

The size of the asteroid would have a large effect on the dynamics of the elevator though. A larger asteroid would allow the cable to be shorter because the asteroid would have a greater attraction to the Earth, but could also cause issues with thermal strain. If the cable goes into shadow and starts cooling, then there will be an associated stress that must be relieved by allowing the asteroid to drift closer to the Moon. This is not a big problem as long as the rate of cooling is not large. The asteroid will lazily drift towards the Moon under the slightly increased tug of the cable. If the rate of cooling is large, then the asteroid will need to react quickly to prevent the cable from breaking under the stress caused by it shrinking. A comparatively massive asteroid simply won't be able to do this.

 

[3dilettante]

If the asteroid weren't at L1, then would it remain stationary with respect to the moon's surface?

L1 apparently is stationary from the surface, but if the asteroid were a hundred or so kilometers distant from that point, would this position impart a slight relative motion?