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.-- ixnaum, Jul 03 2014 Displacement Buoyancy & The Art Of Not Crashing Your Satellite Displacement_20Buoy..._20Your_20Satellite [Skewed, Jul 08 2014] 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.
So, high-altitude balloons already vent the surplus gas, but the thrust it provides is negligible.
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.
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.
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.-- MaxwellBuchanan, Jul 03 2014 //With a big enough balloon you could go arbitrarily high//
To a point somewhere a little below where the gas if free of any envelope would settle in the atmosphere?
The question (for me) then is what altitude is that.-- Skewed, Jul 06 2014 //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.
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).
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.
However, this would not allow for any payload (or, indeed, any envelope weight).
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.
So, long story short: theoretical upper limit is very high; possibly even higher than that. Definitely higher than you'd want to fall from.-- MaxwellBuchanan, Jul 06 2014 Looks like you found some of the same links I did.
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?-- Skewed, Jul 06 2014 Yes, that's about the practical maximum, but only because of envelope (and payload) weights.
I would guess that a much larger balloon would be vulnerable to damage from wind-shear and suchlike.-- MaxwellBuchanan, Jul 06 2014 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?-- scad mientist, Jul 07 2014 Hey [ixnaum].
I think I have some of your annos over here <pointing at Displacement Buoyancy & The Art Of Not Crashing Your Satellite> <link>.
And I think this is yours too <waves & proffers recently acquired patent for Space Blimp>.-- Skewed, Jul 08 2014 You could make the envelope OF HELIUM ITSELF!!-- bungston, Jul 08 2014 Anyway, hands off those whales, they need that high altitude baleen...-- not_morrison_rm, Jul 09 2014 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.)-- blissmiss, Jul 09 2014 "Baloon" is the British English spelling.-- MaxwellBuchanan, Jul 09 2014 The British should model idiosyncratic spelling approaches after "gaol". If you want to spell something your own funky way, go big.
that said I like this approach to typos. Claim to have invented the word and with it, spelling rights.-- bungston, Jul 09 2014 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!!!)-- blissmiss, Jul 09 2014 Fixed the title. But now I regret fixing it. (after seeing high altitude baboons)-- ixnaum, Jul 16 2014 random, halfbakery