h a l f b a k e r y"This may be bollocks, but it's lovely bollocks."
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According to Wikipedia, which is probably rubbish, Kevlar® has a breaking length
of about a quarter of a megametre. This takes it up into official outer space. It
needn't be Kevlar® if that doesn't work out or there's a problem with it (too
floppy?) but surely there could at least be a hollow
tube made of something
which is open at the top and closed at the bottom stretching up to the point
where pressure was quite a bit lower.
Place a light object at the bottom of the tube, which fits neatly in the diameter
and seals it. Open some shutters at the bottom. The force pressing on the
bottom of the object is then about one kilogramme per square centimetre of its
surface. The force pressing on the top, assuming it's open to "space", is
effectively zero. Put some fans in the openings made by the shutters.
The air will push through the shutters, applying an upward force proportionate to
the area of the lower surface of the object, against which there is no resistance
on the upper surface, though gravity is of course a factor. Electricity is generated
by the fans. The object rushes up the tube until it reaches a certain velocity, not
sure what. At some point, it fires a rocket whose fuel is oxidised by the air under
it, thereby foregoing the need for oxygen tanks and therefore extra weight.
Eventually, the object emerges at the top end of the tube at considerable speed,
perhaps orbital velocity, and an airlock closes the top of the tube off. The weight
of the airlock, which is not fixed in place, starts to push it back down. The
electricity generated by the fans then _helps_ (does not provide all the power -
this is not a perpetual motion machine) to re-evacuate the tube down to ground
level.
It would probably need to slant sideways to make orbit possible.
Is this feasible or not? I realise there may be a problem with the tube collapsing
laterally under pressure, but maybe it could be made of something more rigid.
I'm not attached to the material or even completely closed to the idea that it
would just push the object up a few kilometres and would then need to be closed
with an airlock, but what am i missing?
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Hang on. Before you go any further, are you saying: |
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"The tube is open to space at the top, so it contains only a
vacuum." |
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or am I misunderestimating you? |
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You're actively pumping out this tube at the beginning,
aintcha? |
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Well, all well and good, but you can't build a self-
supporting tower out of Kevlar (or anything else, afaik)
which will reach anywhere near the necessary altitude. If
you could, you'd have a "space tower", which would be
kewel. |
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However, there is a baker lurking here in the background,
who does know how to build something not so very
different from this. |
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The first thing that occurs to me is kevlar is not particularly strong in compression. |
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Second, the structure required to hold it open against the vacumm and compensating for wind is going to be non-load bearing weight, so it shortens your tower. |
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A practical space elevator has always envisioned working in from geostationary, not up from ground level. This is where the material limits come in. Kevlar allows a 16000 to one taper from geosynch, or 16 meters for every mm at ground level. Also that works out to about 2000000000 tonnes of the stuff for that 1mm at ground level. Your shorter tower would require less if it could be held under tension, admittedly. |
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OK, interesting, on here at the moment or not? |
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I do think the material concerned doesn't exist, but what can exist is a hollow
evacuated tower which pushes things up, at least a bit, depending on its width.
If not into space, then at least as a way of launching a glider or something. |
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Yes, but it's not necessarily a very efficient way of launching
gliders. All the energy you put into evacuating the tower
could be more profitably used to arm a trebuchet, for
instance. |
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//OK, interesting, on here at the moment or not? // |
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at least for a few more days :-) |
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But let's say it this way - it is not really a material strength problem, or a fuel problem, or a vacuum problem ... |
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So if Kevlar is used, then it would be floppy and not stand up. Perhaps the solution is to fill it with helium to slightly above atmospheric pressure. This should , keep it tight and provide lift, greatly decreasing the strength required. |
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To improve lift, very hot hydrogen would be a bit better. |
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Nobody has yet pointed out that, while the tube itself maintains a relative velocity (slower at the bottom higher at the top, like a record) the object to be propelled with have to have this additional velocity added to it which will require not only a substantial addition of energy but also that the space tube be able to resist the counter force. No material currently imagined even approaches this level of strength. A tube attached to the moon would be far less challenging. |
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every rotating object knows how to fix the problem of different speed at different radius - ask them :-) |
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You have to accelerate the top once (similar to an orbiting object, but with much less speed difference) and then you are done besides keeping up the (air) friction slowing it down. |
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This is what is called same angular speed |
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I lean towards the idea that the best way to explore space is with intelligent
glitter. |
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Actually now you are really close (but there is no need for the glitter beeing intelligent) |
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PS: Tinker Bell would know the solution in this case |
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My thought a while back on this was to use miniature
devices with a very large scale integration, which were
highly reflective, and included a camera, a transmitter and
a means of manoeuvring themselves, perhaps by changing
the angle of reflection of sunlight, so it gets up there fairly
easily in terms of energy expenditure, then does its stuff,
and maybe even returns to Earth having done it.
Alternatively, a cloud of these glitter-like devices which have
various functions which communicate with each other and
have a combined mass of however many grammes it is,
perhaps assembling in orbit once they reach it (difficult bit
that). |
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This idea tends to babel on... |
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Well, gas would be the ultimate 'glitter' but the law of thermodynamics are aginst you - eg the re-assembling would mean a lot of energy - entropy is your enemy in this case. |
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And the original idea of using a pipe is not that bad - it just needs some fixing. |
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well the inflatable space tower people have done the math. |
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If you want to hold for example 30t of mass (which is more then the payload of a space shuttle) with an inflatable cigar of 3m diameter this needs: |
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30.000*9,81/(3²Pi)=10000Pa which is a handsome pressure of 0,1bar |
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So there is no real need for a rocket plume, every household vacuum cleaner can produce this. If you add another 30t for the cigar you need 0,2bar. Which is a structure we can build without kevlar, or nano tubes,... |
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And the surplus air pressure will be always 0,2 bar - from bottom up to the vacuum surrounded top ! |
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BTW I think you can even buy pneumatic car-jacks working that way. |
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This is an interesting idea, but I have a few reservations... |
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In essence, it seems that it is a long slender piston. The motive force is the change in pressure between the atmospheric gas at ground level, and the artificial vacuum induced in the tube. (The Second Law of Thermodynamics says that it will cost more energy to evacuate the tube, than you'll get from that difference.) |
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But, how can that atmospheric pressure push the piston any higher than the atmosphere? It would simply neutralize and reach equilibrium, in my drug-addled mind... |
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You are lucky I calculated this just a few days ago to find out if my own idea would be feasable, so it is just cut and paste. |
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pV=nRT or simplier:pV/T=constant |
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Bottom: T=293K (20 degree C) p=10000Pa
Top at 100km: T=183K (-90 degree C) p=100Pa |
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If we move 1m³ gas from top to bottom you have to compres to end up with 1 bar atmospheric pressure at the bottom |
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100*1/183=100000*Vbottom/293 |
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gives for Vbottom=0,0016m³ That means you have to compress 625x =feasable but expensive. If you use the piston weight to create an isobaric situation the calculation is even simplier with the above formular. |
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So the problem is not the piston, it is the cylinder (weight) :-( |
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[Wily], i'm sacrificing some of the speed through the friction or drag on the fans, but generating power with it. There would have to be some loss there of course, or we'd be off in cloud-cuckoo land wouldn't we? |
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The evacuation of the tube is a means of storing energy. One advantage of storing it that way is that the choice of how to generate the extra energy needed to do it is open. It could be fossil fuels, solar, nuclear, pedal-powered. It needn't be electrical either, and come to think of it maybe the fans could just crank weights up somewhere nearby rather than convert to electric power. |
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I agree it would probably equalise, but it does give a head start and it could be used as a conventional lift inside a building. |
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Come to think of it, there's a more effective solution: immerse the tube in water. The tube is strong, still evacuated, but open at the bottom of the ocean. The pressure differential pushes it harder. How about that? |
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[Gutemine], hello! That's neat. I'm wondering now about seawater, MHD generators and the incompressibility of liquids now. |
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Ah, you have read the Space Hose approach |
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Hopefully you now understand the mistake in the vacuum or pumping fuel approach. |
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If you blow simple air upwards you get only positive effects. A medium to be blown out at the top (which will be already pretty fast due to the expansion to vacuum - add a de Laval nozze and you can do magic as with the perpetual rocket top idea) and the friction when pumping upwards can even help (!) holding the weight. And storing energy with a pressure surplus is much easier and stable then with a vacuum. |
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Once you figure out how to support it, you have to figure out how to cool it. |
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It could be an energy source, who knows? |
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Hmm. The weight at the top pressurises what's below, heating it, but there's also a breeze as it fills, cooling it. It could maybe be quite dramatically star-shaped, increasing the tower's surface area for that purpose, but that would also increase the friction between the sides of the tower at the object inside it. |
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in an ideal gas there is an equilibrium of pressure, temperature and volume. It is that easy - ideal gas law even gives you the relation (what goes up when what goes down, etc). And no, you cann't build an energy source this way, becasue this would violate another thermodynamic law. |
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In this case here the weight of the head would be simply balanced by the pressure, that's all |
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But I did the math already further up, holding the 30tons station with a 3m diameter tower underneath needs only 0,1 bar. It is still quite some effort to fill such a long tower with 0,1 bar, but you have plenty of time and can increase pressure and top weight step by step. |
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So there real problem is how to hold the tower's weight - that is where the friction part comes into motion. |
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And cooling the tower is pretty easy - do you know how big the cooling surface of such a huge tower is ? And there is no real need for cooling, because you are fanning air on the inside, worst case the gas heats up and you get some more chimney effect (blowing even more upwards) |
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So, you feed the cats in at the bottom ...? |
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Yes i thought so. Details of the maths are not something i've gotten into though. It's along the lines of "what if the Moon were made of platinum". |
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yes, feeding from bottom is much easier - the cats live there :-) |
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Exactly. They live at the bottom and die at the top. |
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does this mean we have to open an idea thread on cat (space) suit design ? |
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