Carbon Nanotube Pipeline

[tanstaafl76]

Much has been discussed about using Carbon Nanotubes to create a space elevator. Today I had a related and somewhat hare-brained idea, but interesting enough that I am curious if it is possible.

Imagine a small pipe or tube made up of carbon nanotubes that, like the Space Elevator, stretched from the surface into geostationary orbit. If this tube were open to the vacuum of space at the top, would the suction be sufficient to actually act as a pump and bring up a liquid (setting aside temperature/freezing issues) to orbit? In other words, like those old fashioned vacuum systems they used to use at drive-thru banking windows.

 

[Shpaget]

No, it wouldn't.
If it was, all our atmosphere would be sucked away many years ago (or more probably it would never have formed).

You could probably get some difference in altitude, but as soon as the atmospheric pressure above the liquid equalizes with outer pressure, it would stop, and probably fall back down.
Would also have atmosphere coming in from the top end (there is no absolute vacuum) as well through the walls of the tube itself (I believe no material is 100% non porous for small gas molecules).

Stick a tube in a bucket of water. At the bottom of the bucket, where the end of the straw, is the pressure is larger than above the surface, but you don't see water rising up.

 

[wtrix]

Vacuum would pump the water up to 9-10 meter heigth. No more. That is basic hydraulics. The mass of the water will then be equal to the difference of pressure between the vacuum and normal air pressure. That is why no water pump in deep wells pulls the fluid up, but rather pushes. Same applies to air, but the maximum heigth in tube is then equal to the thickness of the atmosphere.

 

[tanstaafl76]

Makes sense, thanks to you both. Could any capillary action be of assistance in the tube itself? And even if a substantial pump were needed I am wondering if it would be more efficient than transporting a container of liquid up the elevator itself?

 

[Shpaget]

Capillary action is pretty weak and it can even go "backwards" depending on material of the tube and the liquid, for example if a glass tube is inserted in mercury, mercury in the tube will drop below the level of the rest of the pool.

You also need a very narrow tube to get capillary action, so you would need lots of tiny, fragile tubes that extend hundred miles up.
Friction between the liquid and the walls of the tube would be extremely high (considering the narrow tubes).
The pressure at the bottom of the tube (where the pump is) would be enormous (10 000 atmospheres with 100 km altitude), so you would need many substations with pumps to help do the pumping (every few hundred meters). I'm not sure anything humans ever made could produce or even contain such pressures.

 

[wtrix]

The pressure at the bottom of the tube (where the pump is) would be enormous (10 000 atmospheres with 100 km altitude), so you would need many substations with pumps to help do the pumping (every few hundred meters). I'm not sure anything humans ever made could produce or even contain such pressures.

That woud be around 150 000 psi if I'm correct. At the moment of explosion pressures inside gun barrels reach somewhere around 50 000 psi. If they go up to 80 000, metal starts to absorb the pressure wave via compression failure and thus gun barrel life is limited.

 

[tanstaafl76]

Well I admittedly did not think through the pressure situation very well. What about lighter than air gasses? I don't know if there would be any reason to have them in orbit per say, but it would help with the pressure situation to some degree, wouldn't it?

 

[Shpaget]

To lift 1 kg you need around 1000 liters of helium (at the ground level). But you can't use balloons forever. At some point balloons hit the ceiling, and you still need pumps. Current world altitude record for balloons is 53 km, but you have to consider the fact that it wasn't a cargo balloon but a striped down almost balloon only thing (39.77kg in total, including the balloon material).
Even if you get that high, you're only half way up.

Putting lighter than air gas in the tube mixed with whatever you pump wouldn't work. It would be compressed by the pressure of liquid making it useless.

The largest pressure I was able to find in a gun barrel is 432 MPa (50 cal), but that is peak pressure It lasts only a fraction of entire high pressure period which itself lasts just a tiny fraction of a second.
That is 62656 psi or 4264 atm, which is only half of expected pressures which are constant, not temporary.

 

[wtrix]

The largest pressure I was able to find in a gun barrel is 432 MPa (50 cal), but that is peak pressure It lasts only a fraction of entire high pressure period which itself lasts just a tiny fraction of a second.
That is 62656 psi or 4264 atm, which is only half of expected pressures which are constant, not temporary.

To my knowledge, higher pressures are obtined in different light gas guns. Tests with oxygen/hydrogen fueled combustion light gas gun reached up to 70 000 psi for example.

 

[tanstaafl76]

What about pure helium? Perhaps we decide we want to pump a whole bunch of it into orbital containers so that they can be bombarded by cosmic rays and create helium-3 for fusion reactors (as a hypothetical example). Would the lower pressure of the helium make such a pipeline more feasible? I wouldn't have the first clue as to where to start on the maths, so thanks for indulging my random ideas!

 

[Shpaget]

I'm pretty sure it's cheaper to bombard it down here with anything you want in conditions you can fine tune, on a scale you want than transporting it up there, and than back down.
Note that to get to place where cosmic radiation exists you must go beyond the protection of Earths magnetosphere which is not practical.

It's much cheaper to produce isotopes here.

 

[tanstaafl76]

That may be, but I wasn't really trying to launch into the specifics of helium 3 creation, but rather of whether the transportation of a lighter-than-air gas to orbit via a pipeline was remotely possible.

 

[Shpaget]

Like I said in the balloon example, you can't get it all the way up. At some point gravity will start to be a problem.
Besides, lighter than air gas will rise only if there is actually air around it to push it upwards. If there is no air, like in the case of a tube filled only with for example helium, helium would still produce some pressure on the bottom.
Building such a pipeline (or any kind of structure) to such great heights is issue of its own.

If you want to use something like this in a SciFi story, you can, but it's not feasible or even practical for real world application.

I would have to say it's not possible with our technology or the technology that is likely to come in the next 50-100 years.

 

[tanstaafl76]

Right, I realize now it won't make it up on it's own, my question was whether the pressure created by the stack of helium would be lower to the point where it would be feasible to pump it.

 

[wtrix]

I don't see any point in this. I mean - why bother with specialized space pipeline (even if it's possible). It is far easier to lift the tanks full of needed liquid with ribbon-based space elevator. It's even easier to exploit icy asteroids for this purporse.

Well, the primary idea would have been fantastic, had it worked (do the pipe and nature takes care of the rest). But now you have wonderesd so far from the basic simple solution, that it's lost all sense. Helium is somewhere around 7 tumes lighter than air. Thus it applies probably maximum 7 times less pressure in your application. In those pressures, it's a neglible difference because you can't build space pipeline with weapon grade tube wall thickness anyway. So it's becoming a useless mental gymnastics now.

 

[tanstaafl76]

Well I would have conceeded that it is useless mental gymnastics to begin with, but thank you for indulging me nonetheless