Is a space elevator a real possibility?

[JamesG]

There is also the important issue of demand - demand for a space elevator is much higher on Earth and even if it was easier to build one for Mars or the Moon, would it be so much easier to be offset by the lower demand?

Not so much the demand but the external factors, mostly legal, that will make a space elevator on Earth first much more unlikely within the next century or two.
So it Earth isn't practicable, where esle is the technique more likely to be used? Places without large populations or environment/ecosystems that can be damaged. No environmentalists or other wachos who will be worried about the "new tower of Babel inciting the wrath of God" etc.

If that criticism was directed at me,

Not really, just at the negative vibe.

 

[NCC-1701]

While reading this topic, I encountered several small errors that I wish to correct. I believe that most of them can be corrected fairly easily. I would like to use this quote as an example, with no offense meant towards Anemazozo.

1. What would the cable be made of?
So far bucky carbon or "Carbon Nanotubes" looks like the right stuff. Although there need to be much more study of the material I have not read about any issues that disqualifies this material. The typical design envisioned using Carbon Nanotubes is a ribbon about a meter wide.

2. Where would the elevator be anchored?
As cjp stated, an oil platform seams to me the best solution. It can help in manuvering the ribbon out of the way of any orbital objects and will not be subject to any potential territorial issues.

3. What about objects in earth orbit striking the elevator?
I think this is the most problematic issue but it is not without solutions although they might be costly. If we (humans) really wanted to clean up the lower orbits we could launch a hand full of automated spacecraft that would collect and de-orbit objects or send them to a higher orbit where they would be less of a risk. Meanwhile establish stricked manuverability requirments for future spacecraft. If we can co-ordinte 2000 commercial aircrat in the skies at any given time why not spacecraft to avoid collision?

4. What if the ribbon is severed and falls back to earth?
Well, it the ribbon is cut within the atmosphere the the whole elevator will fly away from the earth whcih would suck for people on the ribbon but life boat type spacecraft would solve that problem. If the counter weight was severed and the ribbon fell back to earth the ribbon is not substatial enough to do any damage and most of it would burn up on re-entry.

5. How would the space elevator be built?
My favorite solution is to capture a NEO, bring it into earth orbit, land nanotube manufacturing equipment and start spinning a ribbon using the asteroid as feed-stock, slowly droping the ribbon down to earth.
This is also a tricky part because most people i think will have serious issues with deliberatly bringing an asteroid near the earth.

The first point that I would like to touch on is the subject of anchor placement. While I have no doubt that it could be done fairly well on a large boat of some kind, it would be more efficient to have it on a large platform anchored into the ocean floor, directly on the equator. The reason for this is that presumably the counterweight would be in geostationary orbit, and that would put it directly above some point on the equator. as for the debris avoidance, most operating satellites have RCS jets that can be used to avoid the cable, as for other debris, they could be easily monitored and de-orbited by autonomous robots on the cable.

If the cable is severed, the half that was more towards earth would fall and burn up, but not technically on reentry. the half more towards the counterweight would stay with it in much the same orbit as the counterweight originally started in.

Third, for the construction, you couldn't just let the cord fall, as it would be in orbit with the counterweight. it would have to be carefully de-orbited, to prevent both return to orbit, and burning up while landing.

And lastly, a topic that was not discussed in this particular post, but within this thread, is that the days would get longer/shorter during use. This is true, but the effect would be miniscule, on the order of yocto-seconds. In fact, every reentry causes not only a change in day length, but also in axial tilt. I personally don't notice any change in either, do you? In reality, the object would get most of it's orbital energy from the counterweight, which would frequently need to be reboosted. Similarly, if the counterweight's orbit had any eccentricity, the lost tension resulting from lower altitude could mean unsafe conditions.

Enterprise

P.S.
:hail:

 

[Linguofreak]

This thread had already been dead for 5 months before your dredged it up...

 

[Hielor]

This thread had already been dead for 5 months before your dredged it up...

And there were two other perfectly good threads on space elevators in the last week!

 

[tori]

So while we're reviving...

Third, for the construction, you couldn't just let the cord fall, as it would be in orbit with the counterweight. it would have to be carefully de-orbited, to prevent both return to orbit, and burning up while landing.

Nope! Yes you could. No it wouldn't Gravity gradient torque will orient the cord down, and tidal forces will start pulling on it once it gets long enough. You'll have to do some creative maneuvering with the counterweight to push it higher and higher, in order to compensate for the apparent increase in gravitational acceleration (force of gravity augmented by tidal forces pulling on the cord), but that's all, no deorbiting needed. No burning up either - once it reaches the atmosphere it will be going roughly at the speed of local winds (geostationary).

 

[N_Molson]

Put in a few words, you'll need a material stronger than anything we know to make a cable thinner than a hair. Oh and it has to stay perfectly straight also, even when stretched over 36.000 kilometers...

I think that building vessels like the XR-5 is actually easier

 

[Southwell]

lol didnt NASA have a competition inviting people to share their ideas and designs for a space elevator. At the geostationary altitude the velocity is much much greater than the velocity on the surface of the earth. as angular velocity multiplied by the radius to the point is equal to the tangental velocity of an object at that point. so a radius of 42M compared to around 6.38M will give us much larger velocity.

we also must remember the temperature change along the elevators structure as it drops from the surface until space, this would create enormous stress' within the structure itself.

 

[Linguofreak]

Put in a few words, you'll need a material stronger than anything we know

Not quite. Carbon nanotubes are strong enough over short distances under laboratory conditions. It remains to be seen if they can be made as strong over long distances outside of the lab.

to make a cable thinner than a hair.

How thin the cable is depends on who you talk to. The original cable would probably be fairly thin so as to be easy to get into orbit, but would mostly be used for lifting material to enhance the cable. By the time you had it ready for general use, you'd probably have a pretty thick cable (or multiple cables) supporting a fair amount of infrastructure.

Oh and it has to stay perfectly straight also, even when stretched over 36.000 kilometers...

It doesn't really, but since it's under tension, that will keep it as straight as it needs to be.

 

[RisingFury]

Let's have a bit of fun.

Imagine we stick a straight, rigid line into the equator, extending far into space, far beyond geosynchronous orbit. The line has a uniform density D and a constant cross-section S. We wanna know how much this thing would weigh and how far into space it would have to extend to hold itself up.

Keep in mind, anything under geosynchronous altitude effectively acts as weight, because force of gravity is larger then "centrifugal" force. Anything over geosynchronous altitude acts like an anchor, because force of gravity is lower then centrifugal force.

First we need to figure out how much force we need to keep the line up, if it only extended from Earth's surface to geosynchronous altitude.

F marks force, m marks mass, M marks the mass of Earth, G is gravitational constant, w is angular velocity and r is radius.

dF = dFg - dFc
dF = dm * g[r] - dm * w^2 * r
dF = dm * (g[r] - w^2 * r)

dm = S * D * dr

g[r] = M * G / r^2

dF = S * D * dr * (M * G / r^2 - w^2 * r)
dF = S * D * (M * G * dr / r^2 - w^2 * r * dr)

Now we integrate from the surface of the planet, marked as R0, to some high altitude marked as R:

F = S * D * (M * G * Integrate[dr/r^2, {r, R0, R}] - w^2 * Integrate[r * dr, {r, R0, R}]

The result is a bit ugly, but don't worry...

F = -D * S * (M * G * (1/R - 1/Ro) + (w^2)/2 * (R^2 - Ro^2))

Now, in order for the line to stay up there, the net force has to be 0. If not, it'll go flying off into space or come crashing down into Earth - or at the very least, end up in an elliptical orbit...

So:

0 = -D * S * (M * G * (1/R - 1/Ro) + (w^2)/2 * (R^2 - Ro^2))

The first thing that's noticeable is that density and cross-section don't matter (in this case).

You turn the equation around and you get this:

R * (2 * M * G + w^2 * Ro^3) = Ro * (2 * M * G + w^2 * R^3)

The first solution is obviously R = R0. But that would mean the length of the line is 0.

Of the two remaining solutions, one is negative. The line could stretch into the planet for the equation to be true.

The one we're interested in is this:

R = (Sqrt[R0^3 * w^2 + 8 * G * M] / (2 * Sqrt[R0] * w)) - R0 / 2

Obviously the whole thing fails if the length of the line is 0, or if the planet doesn't rotate... but we don't need to worry about that..................

---------- Post added at 14:36 ---------- Previous post was at 14:25 ----------

And some values:

For Earth, such a line would need to extend to about 1.5*10^8 m out (that's radius measured from the center of Earth - but not that it makes much difference).

The Moon, for comparison is 3.6 * 10^8 m out at it's closest, so assuming I did the calculation correctly, the line for the space elevator would have to extend out to almost half the Moon's orbital radius. A bit more, if we wanna keep the line straight and haul up some weight.


I hope someone can go over my calculations and check for mistakes...

 

[Southwell]

very interesting idea, that if we build the tower tall enough it would effectively have no weight acting upon its supports on earth.

Rising Fury, Do you enjoy performing Kinematic Analysis all the time, I just finished them at uni and cant wait to leave them alone for a while lol

 

[RisingFury]

Well... enjoy is such a strong word! :lol:

 

[Linguofreak]

For Earth, such a line would need to extend to about 1.5*10^8 m out (that's radius measured from the center of Earth - but not that it makes much difference).

Checks out with the quoted figure I've heard of 144,000 km (not sure if that's altitude or radius).

 

[RisingFury]

Checks out with the quoted figure I've heard of 144,000 km (not sure if that's altitude or radius).

When I put in the figure for sidereal rotation and subtract Earth's radius, I get like 143 700 km...

 

[statickid]

the only problem with your calculations is that they have no real bearing on practical use. In an actual design scenario, the anchoring counterweight would NOT be just the cable extending further into space, it would be a large mass just beyond the elevator station, if it were not the elevator station itself. It still only has to be as "tall" as geosynchronous orbit.

---------- Post added at 09:28 AM ---------- Previous post was at 09:27 AM ----------

or maybe a little bit "taller" to take advantage of the mass you put up there, at some point there would be an effort/length balance, where making it longer would outweigh the benefits of just launching more mass as a counterweight.

 

[RisingFury]

the only problem with your calculations is that they have no real bearing on practical use. In an actual design scenario, the anchoring counterweight would NOT be just the cable extending further into space, it would be a large mass just beyond the elevator station, if it were not the elevator station itself. It still only has to be as "tall" as geosynchronous orbit.

But that won't stop me from having a bit of fun, will it?

But since you asked for it, here's the height of the line taking account the mass of the counterweight at the other end:

F = -D * S * (M*G*(1/R - 1/R0) + (w^2)/2 * (R^2 - R0^2)) + m * (G* M / R + w^2 * R)

Taking into account that F = 0 and solving for m:

m = (R * S * D * (2 * G * M * (R0 - R) + w^2(R0^3 * R + R0 * R^3))) / (2 * A * (G * M - R^3 * w^2))

R is the height of the line, m is the mass of the counterweight, considered as a point-mass (or at least small enough so it's dimensions don't matter). Assuming I did this correctly, the equation will give you the mass required for a line of height R.

 

[statickid]

COOL! now thats much more useful! :thumbup:

---------- Post added at 10:23 AM ---------- Previous post was at 10:20 AM ----------

i guess the first one might be a good equation for some kind of proof of concept construction project. maybe the first step would be to make such a structure as you have suggested, even one strand of uniform material stretching from the surface of the earth and halfway to the moon would be the first step in figuring out a real space elevator. if we couldn't do that, then could we really make a space elevator??

 

[Linguofreak]

the only problem with your calculations is that they have no real bearing on practical use. In an actual design scenario, the anchoring counterweight would NOT be just the cable extending further into space, it would be a large mass just beyond the elevator station, if it were not the elevator station itself. It still only has to be as "tall" as geosynchronous orbit.

It only *has* to be as tall as geosynchronous orbit, but there have been serious proposals made (or, at least, as serious as any proposal for a space elevator is at this point in time) to use an extended cable as a counterweight, because of the enormous potential that you get for whipping stuff across the solar system with the extended cable. It doesn't take all that much more effort than a "low counterweight" design, and has considerable benefits.

 

[statickid]

I think it would be better to just extend the station itself a little way, so it is it's own counterweight. i think it would need some kind of minor adjustment system, so you could have a small counterweight on an adjustable length cable that you could use to adjust for the mass of the station changing over time from ships docking and intake/use of resources

 

[RisingFury]

i guess the first one might be a good equation for some kind of proof of concept construction project. maybe the first step would be to make such a structure as you have suggested, even one strand of uniform material stretching from the surface of the earth and halfway to the moon would be the first step in figuring out a real space elevator. if we couldn't do that, then could we really make a space elevator??

Actually, I imagine construction of the cable to be more difficult and expensive then the counterweight and having to only reach a few thousand km higher then GSO would probably be easier and cheaper to construct.

Don't expect a space elevator proof-of-concept. Proof of concept would be making structures or finding applications for materials used, to be tested in conditions that would be similar to what the material may experience. For instance: The cable could serve as suspension wires for suspension bridges, electric motors could be testing in vehicles, pullers or other heavy duty machinery, power transfer could also be tested separately. Once we decide (if ever) to build a space elevator, it's do or die.


Oh and as far as calculations go, I'd expect any student who passed the first year of Math, Physics or some kind of engineering, to be able to do the same calculation.

 

[statickid]

maybe, but also seems like people value their own lives and money too much for do or die projects. the cable would cost less than a whole station.

however, the stages of building could be implemented as a kind of on the fly testing basis. If the first step is achievable, then it is time to move on to the second step. Also, proof of concept missions can find applications no matter what. In a way, satellite technology is the foundation of having a space station.

The whole project is so big though it seems like it would be better to just build better spacecraft and have orbiting space stations.