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From what I understand the practical limit of high altitude
balloons has to do with the fact that as they rise they
expand to the point of popping. Why not squeeze extra
altitude out of them by releasing that extra gas generating
thrust? The other benefit will be that instead of popping
they
will deflate gracefully and will be fully reusable upon
recovery.
If the amount of thrust is too small to be of any use, why
not fill them with hydrogen or something that can double
as rocket fuel? At the altitudes that we're talking about,
air resistance is virtually non existent, so the poor
aerodynamic properties of a huge balloon won't matter.
Displacement Buoyancy & The Art Of Not Crashing Your Satellite
Displacement_20Buoy..._20Your_20Satellite
[Skewed, Jul 08 2014]
[link]
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Very high altitude balloons are zero-pressure,
meaning that they are open at the bottom. They
start out as a huge mushroom of plastic, filled
with enough hydrogen (or helium) to provide lift.
As they rise, the gas expands, filling them out.
When the expanding gas fills the entire envelope,
the surplus vents from the neck at the bottom. |
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So, high-altitude balloons already vent the surplus
gas, but the thrust it provides is negligible. |
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The altitude limit is determined by the weight of
the envelope. A given mass of hydrogen gas will
provide a given amount of lift at any altitude, but
for an envelope that weighs less than the lift, you
can only contain a given volume - and hence can
only go to a certain altitude. |
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Because of the square/cube law, a bigger balloon
can reach a higher altitude before envelope
weight exceeds lift. With a big enough balloon
you could go arbitrarily high. |
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Using the surplus hydrogen as rocket fuel has been
suggested (here, by an N-Prize entrant). The
problem is that you have to compress the gas in
order to feed it into the combustion chamber,
and the weight of the compressor (plus chamber)
is a big penalty. |
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//With a big enough balloon you could go arbitrarily high// |
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To a point somewhere a little below where the gas if free of any envelope would settle in the atmosphere? |
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The question (for me) then is what altitude is that. |
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//To a point somewhere a little below where the
gas if free of any envelope would settle in the
atmosphere? // Exactly, and well put. That
limit would be reached if you had an infinitely
large balloon or, alternatively, a balloon with an
envelope mass of zero. |
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As to what altitude that would be... well, the
Earth's atmosphere doesn't have a boundary, and I
believe that it's mainly nitrogen and oxygen all
the way up (i.e. there isn't a distinct "helium
layer" or whatever). |
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So, in theory I suppose it would be an altitude
where other forces dominated (for instance, the
gravitational pull of the moon, or the force of the
solar wind). But, in theory, it would be well above
the arbitary "border of space" at 100km. |
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However, this would not allow for any payload (or,
indeed, any envelope weight). |
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There's also the fact that free hydrogen and
helium tend to be lost from Earth's atmosphere
completely. However, that's because the hottest
atoms/molecules, if heading away from Earth,
achieve escape velocity. In an envelope, that
wouldn't happen. |
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So, long story short: theoretical upper limit is very
high; possibly even higher than that. Definitely
higher than you'd want to fall from. |
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Looks like you found some of the same links I did. |
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I read somewhere that high altitude balloons reach altitudes of up to 37km / weather balloons I think, & I'd think their payloads should be be pretty light, so that may be close to the maximum? |
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Yes, that's about the practical maximum, but only
because of envelope (and payload) weights. |
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I would guess that a much larger balloon would be
vulnerable to damage from wind-shear and suchlike. |
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Another limiting factor might be space debris.
Would that kick in before or after difficulties with
the solar wind? Wouldn't the solar wind just push
the balloon around to the dark side of the earth? |
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I think I have some of your annos over here <pointing at Displacement Buoyancy & The Art Of Not Crashing Your Satellite> <link>. |
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And I think this is yours too <waves & proffers recently acquired patent for Space Blimp>. |
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You could make the envelope OF HELIUM ITSELF!! |
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Anyway, hands off those whales, they need that high altitude baleen... |
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Would someone please correct the spelling in the
Title? It's driving me crazy like fingernails dragging on
a chalkboard. Please HHHEEELLPPP! (Keeps seeing
Baboon.) |
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"Baloon" is the British English spelling. |
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The British should model idiosyncratic spelling approaches after "gaol". If you want to spell something your own funky way, go big. |
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that said I like this approach to typos. Claim to have invented the word and with it, spelling rights. |
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Heh. (You can't spell it one way in the title and another way in the description. That's not right. That's just wrong. WRONG!!!) |
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Fixed the title. But now I regret fixing it. (after seeing
high altitude baboons) |
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