h a l f b a k e r yI CAN HAZ CROISSANTZ?
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This is a way to launch into solar orbit with no fuel. The
first step is to make a sphere with vacuum inside that
is lighter than air, where the supports needed to keep
the
sphere from collapsing from air pressure are lighter than
the air they are keeping out. This hasn't been done, but
I
suspect it is doable. Next step is to make it a
hollow sphere, have the supports leave an open center.
Next step is to make it a hollow tube instead of a hollow
sphere.
Have an evacuated tube 2000km long, with the eastern
500km freefloating and supported/steered by kites and
its own buoyancy, rising
as high as possible, tens of km up. Stratospheric winds
above 30 degrees latitude consistently blow from west to
east. Have an opening at the eastern end of the tube,
and actively evacuate the tube as the air rushes
in.
Accelerate spaceship to beyond escape velocity via
railgun
through the tube. The kites need to hold the tube
steady
and straight enough that the spaceship do not hit the
sides,
and so that they go out the small opening at the eastern
end. There
would actually be two such tubes side by side, with
connecting tubes to dynamically switch from one to the
other in case of maintenance or accident.
Unlike space elevators, this can be built with existing
technology. It's just big and expensive. The ships it
would launch would be big,
around the size of jumbo jets. They would accelerate at
5G, so people could ride them. You could launch the
whole
human race into space with a few of these over the
course
of a few years if you could build ships fast enough.
Obligatory link
Cat_20food_20can_20shotgun
[normzone, Jul 05 2019]
NASA: recent NIAC proposal for vacuum balloons for Mars
https://www.nasa.go..._for_Mars_Missions/
Mentioned in my anno [notexactly, Jul 22 2019]
LONG Gauss gun
[xaviergisz, Aug 27 2019]
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Err...how about point the 2,000 km tube
vertically, then people would be able to slot
satellites in, without a ladder? |
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Well thinking of those "flailing man" tubes that are outside garages and car salesrooms, where the escaping air causes the tube to flail about wildly. Wouldn't the escaping vacuum cause similar flailing here? If you timed it right you could flip your sattelite out the end when the tube was flailing in the appropriate direction, aided by a puff of vacuum |
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Is it possible to have puff of vacuum? |
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// You could launch the whole human race into space // |
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Possible, but not necessary; the list of those who need to be relocated is actually quite short. |
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// two such tubes side by side // |
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If you used it to launch politicians, would it be a double-barreled shitgun ? |
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Obligatory link to cat food can shotgun ... |
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//Surface area grows with the square of radius while
volume grows with the cube, so this seems doable if it is big
enough.// |
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I think your thinking is wrong. Suppose you have a material
which is **just** strong enough to let you make a 1m-
diameter vacuum sphere with 1mm thick walls. |
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Now you want to make a 2m-diameter sphere. The
compressive force will be increased 2^2-fold, i.e. it will be
4x greater. But the material supporting that load is only 2x
greater. (If you have problems imagining it, think about a
thin band of material running around the sphere's equator;
it has to support the compressive force from the two half-
spheres.)
So, the sphere will fail. To make it work, you need to make
the material twice as thick. |
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So now your 2m sphere has 4x the surface area, and 2x the
wall thickness of the 1m sphere. Which means it needs 8x
as much wall material. And it also displaces 8x the volume.
So, you haven't gained anything. |
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Therefore, it follows (ipso calendro, ipsiur thesaurus) that if
you *can't* find a material strong enough to make a small
vacuum balloon, you *won't* be able to make it work just by
building it bigger. |
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This is all based on the assumption that the structure will
fail by crushing. You can also model it for failure by
oilcanning (which is more likely for an imperfect sphere
made of metal), but then it actually comes out worse as the
balloon gets bigger. |
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MaxwellBuchanan, I agree, the surface area vs internal
volume reasoning was
wrong, and you are right that scale doesn't matter. I was
thinking an internal scaffolding, sort of like a tree,
but your reasoning still holds: if a 1x1x1 cube can support
n
kg, then eight copies of that 1x1x1 cube (arranged in a
2x2x2 cube) can support 4n kg. |
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Atmosphere at sea level is 0.1MPa and weighs
0.001g/cm^3. Glass is about 3g/cm^3, so the scaffolding
has to be less than 1/3000th of the volume in order to
float. So it has to resist bending and not be crushed at
3000 atmsopheres (300Mpa). Glass has a compressive
strength of 1000Mpa, so it's 3x stronger than needed to
keep from crushing. Don't know about bending. Plain
glass breaks easily, too, which is not a useful property
here. |
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Pocmloc, no, the air rushing into the end will eventually
hit the insides of the tube (adding momentum), but it's
cancelled by an equal momentum should have hit the
outside of the tube but didn't due to the hole at the end.
And it's pretty high up where air pressure isn't much. |
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A worse problem is sonic booms (and air spontaneously
catching fire) due to the escape-velocity spaceship inside
the end of the tunnel going through the not-yet-
evacuated air. The fix for all that is a big bulb at the
end, to keep the walls further from the spaceship and to
spread out the air. |
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//Atmosphere at sea level is 0.1MPa ...// On the other
hand, at high altitudes the pressure is much much less, so a
vacuum balloon becomes more feasible (but then again, the
bouyancy is much less, so the balloon structure has to be
much much lighter). |
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//it's 3x stronger than needed to keep from crushing. Don't
know about bending.// Agreed. If you could make a
perfect sphere, glass would work. But any deviation from
sphericity, or any knock at one point, will cause a
deformation and then you get a sort of positive-feedback
collapse. It's a bit like trying to built a very tall, thin
column with a weight on top: as long as it's perfectly
vertical and not knocked, it will be OK; but if it bends even
slightly to one side, it'll buckle. |
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MaxwellBuchanan, lengthwise the tube is actually under
tension not compression, due to outside wind drag. I
expect the empty center of the tube to be about half the
diameter of the outside, with the remainder filled with a
lattice of supports. Buckling, the weight a column fixed at
both ends can
support without buckling is proportional to 1/length^2.
And it takes close to 0 force in the middle to keep it from
buckling. So, a light latticework of small triangles can
stabilize large supports so they crush before they buckle. |
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Do levitating magnets have that same crush weight, if stacked on top of each other? Can they be stacked?. If not, might make a good powered exoskeleton. |
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wjt, elaborate? I don't follow. |
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Make it a cylinder. Spin it to use centrifugal force to help maintain the shape against air pressure. The thicker your walls the more force you have. |
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But a spinning cylinder, if there's any incident wind, will act as a Flettner rotor and experience large forces normal to its primary axis ... |
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As it's extremely long, the leverage at the lower end will be enormous. |
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It will probably start to "snake", become unstable, and disintegrate. |
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Super conducting magnets will lock at a levitating position. Can they be stacked (Stargate like)?and , a lot of if's , if they can be made room temperature and toroidal, wouldn't that help form a rigid tube structure against gravity. |
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// Can they be stacked (Stargate like)? // |
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Yes, of course they can, but if you have the technology to do that, why not just use your network of Stargates ? |
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//network of Stargates// You don't get a discount for repeat uses. The fees mount up quickly. |
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It might be more practical to simply pump the
Earth's atmosphere into a large tank, thereby
removing the air drag out of the equation. |
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Over millennia, humans will evolve to hold their
breath come launch time. |
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On holding one's breath at lunch time, Is it in fact possible to breathe and swallow at the same time? |
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Anatomically speaking, no. |
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[8th] When extraterrestrial tech gets in human hands, whats the bet it won't be used how it was intended. Point in case, a magnetic structured ground escape tube. |
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By the way, the reason not to launch into earth orbit with no
fuel is you'll collide with the earth again within an orbit.
Solar would miss if the orbit isn't exactly a year. Also a year
is more time to adjust orbits by slow means. |
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"self-supporting" led me into thinking "self-
launching" which in turn led me into thinking about
recursive railguns. Since they're just a bunch-o-
magnets, could you construct a series of matroishka
rail-guns such that each launched the remaining
onion-like package inside of itself, up and up,
smaller and smaller, until you were out in space? |
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Great idea [zent]. If they were infinitessimally small, each would require zero energy and therefore would be very cheap to run. |
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[zent], that does reduce the relative speed of the projectile
vs the railgun. |
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That would be possible, and it would be equivalent to a
multi-stage rocket, sorta. For maximum efficiency, you'd
want to control the mass and launch speed of each stage
such that the recoil left the previous stage momentarily
motionless. (Because, otherwise, whatever velocity your
previous stage has is wasted kinetic energy.) But I suspect
it would be, overall, no more efficient than a gun launching
a gun which launches a gun... |
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^ Due to conservation of momentum and Galilean relativity, that doesn't make any
sense. |
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Re the idea, vacuum balloons are well considered infeasible on Earth, but they may
be non-infeasible on Mars. (Chrome was complaining about my use of "feasible"
there, so I fixed it.) But we don't yet know about the winds at altitude or latitude on
Mars (:-#). Anyway, if you somehow manage to do this on Earth (maybe with
centrifugally supported vacuum tubes with sheaths to prevent them acting as
Flettner rotors), at a latitude of 30°, then the minimum orbital inclination that you
can launch to is also 30°. Therefore, it would probably be a good idea to launch to a
parking orbit, reduce inclination, and then burn to escape, rather than launching
directly to escape, which would result in a minimum inclination to the ecliptic of
6.5° (I think). That is, of course, unless you want a high inclination to the ecliptic.
Then launching directly to escape is fine (orbital-mechanically at least, maybe not
for other reasons), and you can also get a higher inclination if you can aim the exit
end of the tube equatorward. |
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// they may be non-infeasible on Mars // |
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Why not just say "feasible" rather than employing a double negative ? |
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Buoyant flight using a vacuum lifting body is progressively less practical as the external pressure increases. On Mars, you have a thin atmosphere - lower density, lower pressure - but concomitantly less lift, which is directly proportional to the difference in mass between the envelope and the surrounding gas. |
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Gas filled buoyant balloons are typically at the same pressure at their lower end as the surrounding gas, indeed hot air balloons have a very large hole which has no effect on their lifting power. At the top, the force exerted is the "lift" of the envelope. |
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A balloon with 1kg of disposable lift in the Martian atmosphere will have to be proportionately bigger than an equivalent on your planet, and because of square/cube law the forces a vacuum balloon would be subjected to, given its much greater size, would be increased. |
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The best place to deploy a vacuum balloon would be on one of the gas giants - but it wouldn't be a "balloon", but a float. |
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Think of it this way; a thin steel sphere containing a vacuum would be buoyant in liquid water and could sustain an appreciable crush depth; but it ain't gonna fly ... |
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// Why not just say "feasible" rather than employing a
double negative ? // |
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Because Chrome wanted me to change it to "possible", and I
didn't feel like it. |
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See [link] re feasibility of vacuum balloons on Mars. |
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There have been similar proposals. "StarTram" supported
itself with magnetic repulsion
between a superconducting cable above the evacuated
tube with railgun and
superconducting cables on the ground. They had a
"plasma
window" to keep air from rushing in at the end. Also a
"space fountain", which had a
similar evacuated tube with things rushing in it, but it
slowed things down instead of
speeding them up and elevated itself off the stolen
momentum. They're all about 2000km long because that's
what it takes to accelerate people to escape velocity
safely. |
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There's that strange word again ... |
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In this context "safely" means "without killing them by
design". |
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Just as long as killing them by accident, carelessness, incompetence or indifference is still OK. |
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