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The US military is testing stratospheric balloons that never have to come down

technologyreview.com

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Re: The US military is testing stratospheric balloons that never have to come down

#21

Post by thrower123 »

sehugg wrote: post #10
kylehotchkiss wrote: post #8

Don't balloons expand in the day and contact at night a bit due the sun heating the gas inside? Couldn't this stretch/release weaken the material a little and result in potential gas leaks? To counteract this, I recall reading about people trying to send balloons across the Atlantic and needing to use ballast to stabilize the altitude and handle the gas losses. I wonder how DARPA can go without this requirement, as H…

Maybe the balloon could condense water in its environment and electrolyze it to refill itself with the produced hydrogen. (Though I'm pretty sure you also need salt.)

If a balloon is relatively small, and designed never to come down, would it be such a bad idea to use hydrogen as the lifting gas? We're understandably a little gunshy about using hydrogen after the Hindenburg, but surely we could come up with some safer designs now.

Re: The US military is testing stratospheric balloons that never have to come down

#22

Post by FlyMoreRockets »

thrower123 wrote: post #21
sehugg wrote: post #10

Earlier quoted context omitted.

Maybe the balloon could condense water in its environment and electrolyze it to refill itself with the produced hydrogen. (Though I'm pretty sure you also need salt.)

If a balloon is relatively small, and designed never to come down, would it be such a bad idea to use hydrogen as the lifting gas? We're understandably a little gunshy about using hydrogen after the Hindenburg, but surely we could come up with some safer designs now.

Hydrogen is a great lifting gas, but because the molecule is so small, it leaks out easily.

Re: The US military is testing stratospheric balloons that never have to come down

#23

Post by ProblemFactory »

Tepix wrote: post #6
rmason wrote: post #4

Earlier quoted context omitted.

Do you know what materials they use to make these balloons? Weather balloons can go as high as 100,000 feet and then they'd blow up.

AFAIK they only blow up because they're designed that way.

Bursting is not a goal in itself - it is just a side-effect of using cheap and simple latex or rubber balloons.

You fill the balloon so it has slightly positive buoyancy. As it rises, it encounters lower outside air pressure, and expands to match it. Eventually it will rise to an altitude where the pressure is so low, that the latex is stretched beyond its limits and bursts. Typically at about 40km and about 100x expansion in volume.

To prevent bursting, you need a more complex material that will expand many times at first (to allow the balloon to rise to operating height), but will then stop expanding without breaking.

Re: The US military is testing stratospheric balloons that never have to come down

#24

Post by 14 »

ProblemFactory wrote: post #23
Tepix wrote: post #6

Earlier quoted context omitted.

AFAIK they only blow up because they're designed that way.

Bursting is not a goal in itself - it is just a side-effect of using cheap and simple latex or rubber balloons. You fill the balloon so it has slightly positive buoyancy. As it rises, it encounters lower outside air pressure, and expands to match it. Eventually it will rise to an altitude where the pressure is so low, that the latex is stretched beyond its limits and bursts. Typically at about 40km and about 100x exp…

Or perhaps some sort of venting system that engages at a certain altitude or pressure preventing the ballon from over expanding. Like a hot water tank has a pressure release it the tank gets too hot will vent releasing pressure.

Re: The US military is testing stratospheric balloons that never have to come down

#25

Post by FlyMoreRockets »

14 wrote: post #24
ProblemFactory wrote: post #23

Earlier quoted context omitted.

Bursting is not a goal in itself - it is just a side-effect of using cheap and simple latex or rubber balloons. You fill the balloon so it has slightly positive buoyancy. As it rises, it encounters lower outside air pressure, and expands to match it. Eventually it will rise to an altitude where the pressure is so low, that the latex is stretched beyond its limits and bursts. Typically at about 40km and about 100x exp…

Or perhaps some sort of venting system that engages at a certain altitude or pressure preventing the ballon from over expanding. Like a hot water tank has a pressure release it the tank gets too hot will vent releasing pressure.

Or just use zero pressure balloons.

https://sites.wff.nasa.gov/code820/spb_differences_between_z...

Re: The US military is testing stratospheric balloons that never have to come down

#26

Post by londons_explore »

PhasmaFelis wrote: post #16
7j wrote: post #15

Earlier quoted context omitted.

So the "never have to come down" is ~100 days record which is held by project Loon?

~6 months is the current best, actually. [1] This is a development project; there's no reason to assume that the current record is a hard limit. [1] https://en.m.wikipedia.org/wiki/Loon_(company)#Timeline

Loon balloons are helium, and helium gas is famously hard to contain, so eventually leaks out and the balloon comes down.

I don't see any way to make a balloon stay up forever unless you have a particle accelerator to manufacture new helium to make up for leakage...

Re: The US military is testing stratospheric balloons that never have to come down

#27

Post by londons_explore »

DoctorOetker wrote: post #19
bfirsh wrote: post #3

The article fails to mention these are called "superpressure" balloons in case you're interested in reading more about them: https://en.m.wikipedia.org/wiki/Superpressure_balloon There are some fantastic amateur projects doing similar things, such as this 11 gram payload (!) that circumnavigated the earth 6 times: http://leobodnar.com/balloons/B-64/index.html

my interest was piqued, but the wikipedia page is actually quite substandard compared to other articles on wikipedia: while it does explain the problem of non SPB and the requirement to have a SPB, it does not state how this is arranged for: are they literally using a much higher pressure? apart from weight issues of stronger sking, so assuming the same weight for a stronger skin, doesn't this cause the balloon to be…

Yes - they have a much lower ceiling. But if you want controllable altitude, that's what you need.

Re: The US military is testing stratospheric balloons that never have to come down

#28

Post by londons_explore »

14 wrote: post #24
ProblemFactory wrote: post #23

Earlier quoted context omitted.

Bursting is not a goal in itself - it is just a side-effect of using cheap and simple latex or rubber balloons. You fill the balloon so it has slightly positive buoyancy. As it rises, it encounters lower outside air pressure, and expands to match it. Eventually it will rise to an altitude where the pressure is so low, that the latex is stretched beyond its limits and bursts. Typically at about 40km and about 100x exp…

Or perhaps some sort of venting system that engages at a certain altitude or pressure preventing the ballon from over expanding. Like a hot water tank has a pressure release it the tank gets too hot will vent releasing pressure.

Venting does not work. For a given altitude, there is no amount of helium which will make your craft stable unless the volume of your craft is non-linear with pressure (usually done by having something that stretches to a limit, and then cannot stretch further)

Re: The US military is testing stratospheric balloons that never have to come down

#29

Post by londons_explore »

sehugg wrote: post #10
kylehotchkiss wrote: post #8

Don't balloons expand in the day and contact at night a bit due the sun heating the gas inside? Couldn't this stretch/release weaken the material a little and result in potential gas leaks? To counteract this, I recall reading about people trying to send balloons across the Atlantic and needing to use ballast to stabilize the altitude and handle the gas losses. I wonder how DARPA can go without this requirement, as H…

Maybe the balloon could condense water in its environment and electrolyze it to refill itself with the produced hydrogen. (Though I'm pretty sure you also need salt.)

There is very little water in the air at those altitudes. Also, electrolysis uses a lot of energy.

Re: The US military is testing stratospheric balloons that never have to come down

#30

Post by londons_explore »

bradleysmith wrote: post #12
Tepix wrote: post #5

How do these balloons change their altitude? Is there a pump that can move gas between the balloon and a pressurized container?

At least at loon, the balloon has a ballonet that takes on atmosphere as ballast for descending.

A balloonet is a small balloon inside the big balloon. By filling it with air, you are adding mass to the craft without changing the crafts volume.
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