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The Dream:
A cylinder with an affixed spiral daVinci'esque screw running inside. Halfway, the screw changes direction. Thus when spun (in the proper direction), the screw will push out the air while centripugal force <link> works, against radial compressive pressure, to retain the shape of the cylinder.
Working
Design:
While the basic design carries the thrust of the idea, there will be forces pushing in against the ends(axially) as well as the cylinder walls(radially). In order to keep our nice cylinder from turning into a wrinkly donut it's constructed as a sealed double-wall: if the inner and outer walls are linked to each other in the "crawlspace" it won't prolapse and will support the hefty longitudinal pressure exerted on it by the edge of the joined Archimedes screw. Picture an inflated blood-pressure cuff with a piece of rigatoni inside.
A design [bungston] which avoids adding the propellor pressures to the walls along its length would be to forego the integral screw and simply pump the air out using a regular air pump.
Such a design requires the ends be capped. Concave hemispheres transmit the longitudinal pressures directly onto the termini of the walls without trying to pull them inwards radially.
Practical Considerations:
To all this we should probably add a "windbreak": a stationary outer envelope which keeps the wind from doing interesting things with a large lightweight cylinder spinning at high speed. The proposed construction material is Kevlar or the like: something relatively lightweight with great tensile strength.
Synopsis: spin cylinder, get en"lightened"
.
.
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(I felt I needed a vacuum blimp in my portfolio)
Note: the idea of using centrifugal force on a cylinder was proposed by [caspian] in "Centrifugal vacuum balloon" 3 years ago.
And the idea of using an inflated shell, for a non-spinning vacuum blimp was broached by [pashute] over 10 years ago in "Inflated Shell For Vacuum Balloon"
[relevant links provided]
Great minds, etc. :-)
Centripugal force
Centripugal_20Force
[MaxwellBuchanan, Jan 03 2011]
Magnus Effect
http://en.wikipedia.../wiki/Magnus_effect
Something tells us we've been here before ... [8th of 7, Jan 04 2011]
Well...
Centrifugal_20vacuum_20balloon
same basic idea: use centrifugal force to keep the walls tensed. [FlyingToaster, Mar 03 2011, last modified Nov 06 2012]
more Well . . .
Inflated_20Shell_20for_20Vacuum_20Balloon
usage of a pressure cuff as a shell for a vacuum balloon [FlyingToaster, Dec 12 2012]
[link]
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This is not as stupid an idea as I had expected. |
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A pair of these spinning in opposite directions would be good, to allow a helicopter like base below to remain stable. One could retrofit those big double-rotor helicopters with big blimps of this sort. |
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In fact, just put the rotor inside the bottom of the cylindrical blimp and close the top. |
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Now thinking scale: warm air lifts because it is less dense than air and one needs a big balloon to lift a basket with occupant. Assuming low pressure air of comparable lift, how much bigger will balloon be to accomodate pump and strong walls? |
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The double-shell needs to be robust and airtight enough to contain it's air while supporting the longitudinal "weight" of: disk(x2) area x atmospheric pressure. So the envelope will weigh more per sq.in. than a comparable hot-air or gas envelope. You might even want to add a third, stationary, non-loadbearing envelope around it to keep from potentially interesting reactions caused by a large spinning surface and windgusts. |
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What you need, surely, is anisotropic air? Then, it could be
kept in compression longitudinally to support the tube, and
put under vacuum transversely by the centripugal force. |
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Even though I think vacuum balloons are pointless, I
love the ingenuity of this idea. I think a better shape
would be like 2 circus tents attached base to base,
with the central pole taking all of the compressive
load. |
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Wikipedia says that hot air balloons operate at up to 120°C.
If my physics is right, that means the air density is about 75%
of air density at 20°C. So, that should be equivalent to a
vacuum of about 200mmHg or 4psi. |
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Why not do the math ... ? It's not rocket science. Well, it's buoyancy/aviation science, but it's definitely NOT rocket science. |
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//circus tents// I can't say as I've thought through the ends of the cylinders yet, but bear in mind as you get closer to the axis you get closer to a full 14psi differential pressure so, to my way of thinking, you want to make that area as small as possible, ie: a plain cylinder with flat disks at the ends... well okay, not flat: a flattish parabola so as not to put inwards radial stress on the cylinder. |
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I think you'll find that, if the longitudinal section is elliptical,
all the forces will balance everywhere. |
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// the forces will balance everywhere // |
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"You refer to the prophecy of the one who will bring balance to the Force......" |
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Actually it's a job-share. |
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//if the longitudinal section is eliptical// Hmm... I don't think so.... |
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Let's use, as an example, a scale model of this thing: a sealed, double-walled sleeve where the inner and outer walls are reinforced to each other within the "crawlspace" to keep it from simply turning into a donut under gravity. |
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Place the model upright on a table. You should be able to put vertical pressure on it without it collapsing or distending in any way. |
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But if it's barrel-shaped then once vertical pressure is put on it, the endpoints will try to crumple towards the axis. |
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In this design, the weight of the double-sleeve resists air pressure due to the spin. But *only* at a normal to the axis and only in proportion to the radius. A barrel shape would be more sturdy at the center and less sturdy towards the ends due to overage/underage of centripugal force. |
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As soon as the curve deviates from parallel to the axis, longitudinal forces come into play against the surface. And the closer you get to the axis, the less centrifugal force is there to mitigate the inward radial force. |
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That being said, curved endpoints might be useful in some respects of force balancing... |
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// the endpoints will try to crumple towards the axis // |
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That depends on the rotation - if it spins sufficiently fast, the centipugal forces will act to prevent any reduction in the axial length. |
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However, there may be undesirable gyroscopic effects, and the drag on the outer surface will be considerable; it will shed turbulence vortices, and the Magnus (Flettner) forces acting on the rotating cylinder may have unintended consequences. Heh heh heh. |
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//Magnus effect// I've edited the post to include a flyweight, non-spun envelope around the whole thing for such eventualities. Admittedly precession + wind shear could prove amusing. |
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However [8/7], I think you're wrong on the centripugal bit: the faster you spin an ellipsoid, the more radial loading difference there's going to be between the middle and endpoints. Unless you were just saying in general, in which case the same is true of a straight cylinder. |
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If you scale this thing up a bit, the best arrangement
may be to have a ring of connected cylinders. Each
cylinder rotates and the ring rotates. It would look
awesome as it spins up to speed and expands. |
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The rotating ring will force the axis of the cylinders
to rotate, so could the whole thing be constructed
like a Powerball? |
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Hmmm... while using more than one cylinder solves the reverse-rotation problem (as would a helicopter-like tailrotor), and eases the weight pressing on the individual endcaps (because they'd be smaller), it would also expose more rotating-cylinder-wall to the outside... sounds like it might be a fair tradeoff though. § x1 |
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Another thought, could this not just be something
(nearly) spherical spinning about 2 axis?
or are gyroscopic forces prohibitive? |
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<aha!> The endcaps are hemispherical envelopes (give or take adjustment for centripugal force) which bulge inwards. The rim of a hemisphere transfers all the force to the bottom of the sleeve. |
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I like it, and not just because of the title change either. |
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well somebody didn't like it. first bone. Makes me want to post "Jewish Ninja Throwing Stars of David" in retaliation for over-PC-ness. |
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//"Jewish Ninja Throwing Stars of David// Made of matzoh,
to evade airport screening procedures. |
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Have a [+] to balance the fishbone. |
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[+] here is a croissant that you can spin around as you bite it and get enlightenment! |
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Everybody knows the croissant is just something to focus the frizzy bits of the brain on while enlightenment comes from within, but yummy is still yummy. Thanks (even though somebody added another bone in the meantime). |
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[sqeaketh] yes, but this idea actually works (though granted, apart from the "spinning cylinder" concept, the working details are in the annos, until I get around to editing the post) |
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Quick recap (I suppose I should clean the post up a bit)... |
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A double-walled cylinder has air trapped between the walls, making it a nice springy shock absorbing open-ended sleeve able to withstand great pressure longitudinally (ie: from the ends). The inner wall is attached to the outer wall at quite a few points/lines to keep it from turning into a donut. So it's like a big blood-pressure cuff. Even when it's not spinning it's going to be sturdy: it has to be in order to support (longitudinally) atmospheric pressure of each of the end disk areas against an internal vacuum. |
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The end caps are convex hemispheres. This bit of geometry means that as a vacuum is established inside the cylinder, the rim of each cap will pull directly inwards, parallel with the axis. Since the rim of each cap is attached to the end of the sleeve, all the forces will be going where they should be. |
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Now let's spin it up. As we spin it up we'll remove air from inside the now closed cylinder. In order to successfully match the outside pressure, the cylinder has to be spun up until the centrifugal weight of (the outside wall + air in between + the inside wall) is at least equal to the outside pressure per square inch. |
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The endcaps deliver their pressure nice and neatly longitudinally from the rims of the endcaps to the rims of the cylinder. This force gets distributed by evenly trying to push the outer wall out and the inner wall in. |
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...And of course hordes of minutiae like we haven't mentioned how the thing is going to spin, or where the pump is connected to evacuate the inside air or fill the gap between the walls, or the static non-loadbearing outer-outer envelope that's just there to keep gusts of wind from sending the contraption into conniptions from errant wind gusts. |
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[caspian]'s post <link>, which I anno'd in 3 years ago rather freely but don't remember, has the basic principle: centrifugal force to keep a rotating cylinder expanded radially. However, this idea also covers working solutions for:
- evacuation of the cylinder
- stopping longitudinal compression
- endcap design |
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(I will however continue to claim that I thought it up independently prior to that, with propellers at the ends of the cylinders: spin the entire thing and evacuate the cylinder) |
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[post revised to include increased elegantishness, credit, and cause bone-to-bun conversion miracles] |
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Credit happily accepted. I like the idea of a double-walled
shell to try and support a vacuum, maybe it could work
with solid struts. |
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I analysed the cylindrical case of trying to use pressurised
gas to support a vacuum, and it doesn't work. |
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Say you've got a cylinder with 100 square inch cross
section, at 14psi, same as the atmosphere, and it's
neutrally buoyant, ignoring the cylinder wall weight.
It has 1400 pounds force at each end, balancing
atmospheric pressure force. |
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Now make that 90 square inches cross section of vacuum
and 10 square inches cross section of pressurised air. |
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You still need 1400 pounds force at each end, so the gas
needs to be at 140 psi. To get 10 times the pressure you
use 10 times the density. |
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So you've increased the density of air by a factor of ten,
but decreased the volume by a factor of ten, giving you the
same mass of air as if it was evenly distributed. It is still
neutrally buoyant, ignoring the weight of the cylinder
walls. |
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This suggests a metric for vacuum balloon pressurisation
usefulness. Compressive force times length divided by
mass. High pressure water is good, but limited by how
much pressure the cylinder walls can take. Not sure how
much lighter you can get with liquids. |
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What about a volatile hydrocarbon above its critcal pressure, like propane ? Cheap, widely available, less dense than water. |
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For my next trick, levitation using a simple set of bootlaces. Anyways... |
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H2 would be too embarassing. A gas which is supposed to liquify under its own weight ? hmm... |
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Balsa would need 40:1, not 10:1 ... aerogel perhaps. At which point it stops becoming a pressure-sleeve and is simply a structure which can withstand the axial pressure. |
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Bun for mating Buddhist sensibility to Halbakerist, er,
insensibility. [+] |
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How come the space fountain is on wikipedia while
other much better halfbakery ideas are not? Not fair! |
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Much as I would get a kick out of seeing a Wikipedia entry with "as proposed by Flying Toaster of the HalfBakery" in the text, on many levels, [caspian] got here first with the basic principal in "Centrifugal vacuum balloon". |
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The nift of this design lies in using a reflexed Archimedes screw to power it; that and a plausible method of avoiding axial collapse. |
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and [pashute], you really should have mentioned your pressure-shell idea as well. (yet again I anno'd but didn't recall doing so. . . I'm going to have a word with my ego) |
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However I still remain smug that I put both ideas together to make something that (theoretically, with one eye closed and three sheets to the wind) could make a workable LTA vacuum balloon. |
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