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# Perpetual rocket top

To "moor" the top of the space elevator
 (+2) [vote for, against]

Recently posted on this site was in adea for a "lite" space elevator, with a minimal superstructure buoyed up by balloons. Balloons cannot provide lift in very thin atmosphere, and so the area of the space elevator extending very high still needs a means of support.

I propose that the top of the elevator be held in place by a solar powered perpetual rocket running on compressed air. Large solar panels capture the solar energy which is abundant at high altitude. This is used to run a pump which draws air up a tube running the length of the space elevator. At the top, this air is accellerated with great force into one or more rocket nozzles pointing down and away from the elevator. These nozzles could also be used to steer the top of the elevator to meet spacecraft etc as well as resist high winds lower down.

The problem with propulsion in space is that energy is available, but there is nothing to throw in the opposite direction. This approach makes use of the connection to earth to get something to throw.

 — bungston, Jun 25 2003

Buoyant Space Elevator http://www.halfbake..._20space_20elevator
A nifty idea. [bungston, Oct 05 2004, last modified Oct 17 2004]

Space Elevator http://science.hows.../space-elevator.htm

 How could any amount of pumping draw the air up the tube, when there is already vacuum all around failing to draw up the rest of the atmosphere? Fundamental flaw, methinks.

(Now, it might work if you had pumps creating a high pressure in the tube at ground level. Someone else can do the math - there's always someone with a calculator.)
 — DrCurry, Jun 25 2003

I suppose this is a space elevator, and the whole point is to move things up from the ground. However the question of whether the topside pump would work is interesting. I am not sure it would.
 — bungston, Jun 25 2003

 DrCurry is right, it will work only if the pump is on the ground. Also, why not put the solar power on the ground so the rocket has less to (perpetually) lift? the power loss through the atmosphere is small, and the arrays would be much cheaper. Also, the air could be heated electrically to give a better specific impulse to the rockets.

 BUT what about the environmental impact? are we sure that this won't cause a lot of air to be lost to space?

 Maybe you guys haven't read 'The Fountains of Paradise' by Arthur Clarke. The dynamics of the space elevator is clearly explained. It's afixed structure with a station at geostationary orbit and a counter weight past that. And yes, the problem is that it requires materials several orders of magnitude stronger than any we have under development.

That's what I like about Clarke: he only breaks ONE law of physics at a time.
 — RusNash, Jun 25 2003

 The solar power station is weightless, in orbit. It is attached to the elevator by a tiny wafer thin wire. I put it up there because the suns rays pack more punch and you dont have to worry about dust, clouds etc.

Re: Mr Clarke. There's more than one way to skin a cat.
 — bungston, Jun 25 2003

 I'm afraid [DrCurry] is right. Suction works by creating a presure differential that is greater than the gravitational pull. The difference between a partial vacuum and more of a vacuum is unlikely to spirit up much air from ground level.

The geostationary orbit should be almost good enough to hold it in place. As for trimming, why not consider a different propellant medium, such as using an ion drive. The propellant could be shot up the lift and the energy stored at the top used to accelerate it.
 — FloridaManatee, Jun 25 2003

 //are we sure that this won't cause a lot of air to be lost to space?//

It's estimated that our atmosphere looses billions of cubic feet of air at the polar regions. Be we also recieve tons upon tons of raw material from space each day, like fragments from asteroids whos chemical composition includes oxygen. Or ice from comets which also includes oxygen. The earth is not a closed system. We don't have to worry about sucking the air off our planet any time within the next eon.
 — Anarch, Jun 26 2003

(What RusNash said) Build it according to the Arthur C Clarke's instructions and the whole thing's under tension anyway. I can't see the need for these downward pointing nozzles.
 — st3f, Jun 26 2003

Steering?
 — bungston, Jun 26 2003

But then they'd point out to the sides, surely?
 — st3f, Jun 26 2003

 // There's more than one way to skin a cat. //

I know a dozen or so, but I'm always willing to learn ......
 — 8th of 7, Jun 26 2003

The nozzles would point wherever you wanted. I envisioned mostly down, to keep the rest of the elevator from being pulled back to earth. But they could be adjusted to point sideways - which would be useful for bringing the top of the elevator to bear on the space station etc.
 — bungston, Jun 26 2003

(+) Of course the real joke is the elevator itself. But this add-on idea is a good one. You want to use water, of course, not air -- no momentum with air. And you don’t want to use solar power -- there’s no way to support all those solar cells at the top -- it won’t be weightless, you know. So you install a nuclear plant at the bottom, and a string of pumps along the elevator to pump the stuff up. The weight of the pipe and pumps and electrical cable will be enormous, of course, much greater than what you’d get from ejecting the water...so eventually you’ll have to resort to vacuum balloons, disguising them as pumps...
 — pluterday, Jun 26 2003

//it won't be weightless, you know//
Er... it will at the orbital anchor. That's really the point of a space elevator. (link to howstuffworks).
 — st3f, Jun 27 2003

you are pretty close ...
 — gutemine, Sep 09 2010

 Even if you could suck the air out of the tube from space, you'd need a very stiff tube to not collapse under the suction necessary to move that air column, like sucking a straw in a really thick milkshake, it would collapse.

 And using positive pressure with pumps on the ground, you've got to put a lot of force behind that air column to push it out of the atmosphere and into space. Air looses it's buoyancy once it's out in space away from the surrounding air and it's simply dead weight. The tube would need to be able to stand up to that pressure. Keep in mind, the gravity up there isn't absent. When you see astronauts floating in orbit, it's because they're in a perpetual "fall", where their velocity is acting against the gravity, not because there's not any gravity pulling on them. This tube and air column would just be standing their getting pulled down. It would be very very heavy. To get the force pulling upwards that A.C. Clark theorized, you'd need a heavy weight at the end to be out beyond geosynchronous orbit which is really, really far out there.

 So by the time you've put all the structural stiffness and associated weight into a tube that could do such a job, the amount of air you could push through it wouldn't do much to keep it upright.

That being said, you could theoretically build a structure going out into space. Question is whether or not it would be worth it.
 — doctorremulac3, Sep 10 2010

the altitude to which a gas may be pumped is limited by the pressure at which that gas liquefies. I haven't done the math but I suspect that your gas would need to be delivered at liquefying pressures especially should any reasonable rate of flow be expected. Sounds like a real challenge re. drag, size of tube and density. Suspect that the result is neg. sum. You might get by with a fuel/oxidizer setup but the atmospheric forces seem like a real tough sticking point (not to mention the weight of the tube itself, angels hair I presume. )
 — WcW, Sep 10 2010

 Right WcW, I had actually looked into this with pumping gaseous oxygen and fuel and even with that there's no way. The static pressure you'd have to overcome just to push the stuff through would require a thick enough tube to make this un- reasonable.

 Even if you could do it you'd be burning so much fuel to keep this tube upright it would be far less efficient than just loading up a rocket with fuel and a payload and being on your way NOT trailing a 200 mile long tube behind you.

 I had also thought of having a tube filled with hydrogen that a rocket would travel up, burning the hydrogen as it went, but when you ran out of atmosphere you'd run out of floating hydrogen sock. I also considered a string of free floating hydrogen balloons being gobbled up Pacman style but again, what's gained by doing that?

Like many neat ideas, the efficacy of the present method has to be surpassed to make something worth while and getting into space with a 60 foot metal tube filled with fuel and oxidizer does the job pretty well.
 — doctorremulac3, Sep 10 2010

 you are simply thinking too big.

 There is a nice saying:

 If you want to build something in a desert - use the SAND

And rockets are like dinosaurs - big, loud and had to die out to make place for mamals
 — gutemine, Sep 10 2010

Wow, heavy man.
 — doctorremulac3, Sep 11 2010

you'll do better to leave off the weight of the motors and solar panels and pump the air from the ground.
 — Voice, Aug 29 2012

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