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myles
12-16-2003, 08:11 PM
In 2002, in Manitoba, NASA launched a large unmanned helium balloon into the outer reaches of Earth's atmosphere, reaching at one point 161,000 feet (49 kilometres).

My question is this.

What is the theoretically physically possible absolute upper limit in space reachable by balloon (lighter than air preferably)?

And what sets that limit?

If balloons can reach 50 kilometers, can they reach higher? How much higher?

Enola Straight
12-16-2003, 09:28 PM
Try a baloon filled with the light isotope of hydrogen known as Protium...THE lightest gas there is.

Whats that? Hindenburgh, you say? Hydrogen is explosive and flammable?

Yeah, so is the gasoline in my car.

Rabid_Squirrel
12-16-2003, 09:40 PM
What you need to to is find a formula that equates altitude to air pressure, and another one equation relating density of hydrogen (or protium) to air pressure. Find an altitude where the density of the air equals the density of the gas, and that will give you the theoretical maximum.

I can't be arsed though. Anyone else has the equations on hand?

spingears
12-16-2003, 09:46 PM
Just a bit OT.

Originally posted by Enola Straight
Try a baloon filled with the light isotope of hydrogen known as Protium...THE lightest gas there is.

Whats that? Hindenburgh, you say? Hydrogen is explosive and flammable?

Yeah, so is the gasoline in my car.

A relatively recent investigation of the Hindenburg disaster exonerated hydrogen as the culprit. The hydrogen bags burned AFTER the skin on the ourer frame burned almost explosively. The skin was coated with a thick film of (airplane dope as a carrier sealant ?) an oxidizer such as iron oxide, and aluminum flake or powder to reflect the suns heat. The skin burned very rapidly with a thermit type reaction.

If you have further interrest try the web search engine to find the details.

________________
"Beware of the Cog"

Rabid_Squirrel
12-16-2003, 09:47 PM
Crap, I've just realised a problem with my method. :smack:

In space the pressure is ~0bar, so the density of the gas would be ~0. This means the theoretical atitude would be infinity. Of course it's false since I assumed an balloon with zero mass.

It probably means you would have to include a fixed volume of gas and some sort of estimate for the weight of the balloon in your calculations.

MikeS
12-16-2003, 09:49 PM
The maximum height a balloon can reach is determined by when it's "neutrally buoyant", which means that the mass of the balloon divided by its volume is equal to the density (mass/volume) of the gas outside. So theoretically, if we had a balloon made from some space-age material that we could make arbitrarily light, and we could pump all the air out of this balloon and not have it collapse under the 16 lbs/in2 that the atmosphere exerts on it down here, then it would get arbitrarily high.

If you insist that the balloon be filled with some kind of gas, though, then Enola Straight's suggestion of using hydrogen is your best bet (I think). Figuring out exactly how high such a balloon can go is an interesting thermodynamics problem, which I may try to solve later tonight. I'll let you know if I find anything interesting.

scr4
12-17-2003, 12:27 AM
The current record is 52.0 km, I believe, set in May 2002 by a Japanese balloon. I was fortunate enough to see the launch. This was a helium balloon.

The altitude is determined by the thickness of the balloon material (usually polyethylene), volume and payload weight. Lift is proportional to volume, which scales as the cube of balloon diameter, while weight is proportional to surface area which scales as square of diameter. So the bigger the balloon, the higher the altitude. The maximum altitude depends on the technology for making thin lightweight film, so there's no definite theoretical limit.

Valgard
12-17-2003, 01:19 AM
Well I made a bit of a start on this one:

*Note 1 - Hideous rounding and significant figure liberties taken.
*Note 2 - Made assumptions to keep formulas simple. I'm good with spheres, not teardrops.

Found a formula for air density as a function of altitude:

d = 1.21 * exp(-height/8000)

Where d is in kg/m^3 and height is in meters. Dunno how accurate this is or how high it's valid but it's all I got.

This gives d for various altitudes as:

0.002337 kg/m^3 at 50km
0.0006698 kg/m^3 at 60km
0.0001919 kg/m^3 at 70km
0.00005499 kg/m^3 at 80km

The largest balloon on record that I could find mention of was 70 million cubic feet capacity built by Wintzen Research (they also launched a balloon to 49km back in 1972 so they seem to know their stuff).

Let's assume for the sake of argument that at extremely high altitudes the balloon will be spherical (from photos I've seen this may be wrong but again it's all I got and math for spheres is easy).

70 million cubic feet is a touch under 2 million cubic meters.

To "float", the balloon will need to have (mass/volume) = air density at that altitude.

This gives critical masses of:

4600kg at 50km
1329kg at 60km
380kg at 70km
109kg at 80km

Let's ignore the weight of the actual gas in the balloon and any non-envelope stuff (instrument package, gondola for those who want to ride along :-o, etc).

A sphere with a volume of 2 million cubic meters has a surface area of 76,356 square meters.

Our balloon envelope will have a mass/area found by dividing the critical mass (above) by the area (76,356 m^2), and then we can check this against known materials to see if we've got a chance.

I get the following:

1.43 grams/m^2 at 80km
4.98 grams/m^2 at 70km
17.4 grams/m^2 at 60km
60.2 grams/m^2 at 50km

I couldn't find any definitive listing of masses for a good thin film polymer like Dupont Mylar, but some kite enthusiasts have a discussion board and mentioned that it's commonly available in 20, 40 and 60 grams/m^2.

So, back of the envelope figuring, I'd say that 60km is probably achievable, although pretty darn challenging. Dunno about higher, it looks like the material will be hard to find - incredibly light yet rugged enough to survive the temperature and mechanical stresses.

Anyone care to sharpen this up a bit? Does anyone have better info on materials used?

scr4
12-17-2003, 03:09 AM
Originally posted by Valgard
Anyone care to sharpen this up a bit?
I think I can. Using density information from the US Standard Atmospheres, I get

50km: 1.0e-3 kg/m3
60km: 3.1e-4 kg/m3
70km: 8.3e-5 kg/m3

Your numbers aren't too far off. The Japanese ultra high altitude balloons use 3.4-micron polyethylene for the balloon envelope. I can't find the number for NASA balloons but I think it's similar. That would make it approximately 3 g/m2. At 1 million cubic meters and no payload, it should go up to 64.8 km. With a 20kg payload it drops down to 63.8 km, still very respectable. The 53km record (my first post was wrong, sorry) was set by a 3.5-micron 80,000 m2 balloon.

It doesn't mean it's worth doing a 64km balloon flight. There isn't a lot you can do with a 20kg payload. A million cubic meter balloon is expensive to manufacture and launch, and it's only good for one flight lasting a couple of days. Most people who do balloon experiments are more interested in longer duration flights and larger payloads. Longer flights are possible if you use "super-pressure" balloons, i.e. sealed balloons that don't lose any helium due to day/night cycles.

CalMeacham
12-17-2003, 07:43 AM
Heck, according to the noted Hans Phfall, you can go all the way to the moon!

Shade
12-17-2003, 07:54 AM
Couldn't you get arbitrarily high if your material could withstand arbitrarily hot gas as well?

Mangetout
12-17-2003, 07:55 AM
Originally posted by MikeS
The maximum height a balloon can reach is determined by when it's "neutrally buoyant", which means that the mass of the balloon divided by its volume is equal to the density (mass/volume) of the gas outside. So theoretically, if we had a balloon made from some space-age material that we could make arbitrarily light, and we could pump all the air out of this balloon and not have it collapse under the 16 lbs/in2 that the atmosphere exerts on it down here, then it would get arbitrarily high. How about having some space-age rigid material that isn't strong enough to support the pressure of all that air, but filling it with hydrogen, gradually pumping the hydrogen out as it rises and the external pressure decreases?

toadspittle
12-17-2003, 11:19 AM
What's the usual fate of such balloons? Do they regularly reach neutral buoyancy, then leak and eventually come down? Does mission control activate a remote depressurization to drop the balloon? Or is it more likely (as I would imagine) that the balloon would eventually reach an altitude where it bursts?

Satyagrahi
12-17-2003, 01:31 PM
Some years back--or decades, maybe--I read a science fiction story that suggested flying communities held aloft by vast, rigid spherical floating structures.

That is, you had a massive spherical shell of reinforced concrete, a mile or two in diameter, enclosing a hard vacuum. The shell would have to be strong enough to support the pressure differential. The idea is that the larger the sphere, the larger would become the ratio between weightless volume (which increases as a power of 3) and weight of the shell (which increases as a power of 2).

Is such a thing feasible? Do we have materials strong enough to achieve it? What's the minimum size necessary to achieve neutral bouyancy?

Just a little more for you to chew on.... :)

toadspittle
12-17-2003, 01:50 PM
We've discussed hard vaccum a few times; basically, it's impossible to make a container strong and light enough.

Sunspace
12-17-2003, 02:05 PM
Would it be possible to take a hybrid approach for a really large balloon? Imagine a spherical shell made of helium-inflated structural members and panels, surrounding a really large volume at a lesser pressure but a much lower density--warm hydrogen, perhaps, heated by the sun. If it was a kilometre or so in diameter, would solar heating suffice to provide negative buoyancy?

Bytegeist
12-17-2003, 03:35 PM
Originally posted by toadspittle
What's the usual fate of such balloons?
After they retire, they're sent to the Village for security work (http://www.retroweb.com/pris_splashpage.html).

Be seeing you. ;)

toadspittle
12-17-2003, 05:25 PM
they're sent to the Village

Being from NYC, this gave me a rather strange first impression. But then I remembered Number Six.

Enola Straight
12-17-2003, 05:56 PM
Perhaps a black baloon will heat up in the sun to provide a partial effect like that of hot air balloons.

GreyWanderer
12-17-2003, 06:37 PM
http://web.sysrq.no/mith/bilder.skeptiker.no/albums/andoya/DSCN1019.sized.jpg

This is from And°ya rocket range in Norway. The balloon that went up to 40km was launched by NASA, we managed to reach 30km as part of a student project.

http://web.sysrq.no/mith/bilder.skeptiker.no/albums/andoya/DSCN1003.sized.jpg

Here you can see me ready to launch the balloon.

What's the usual fate of such balloons? Do they regularly reach neutral buoyancy, then leak and eventually come down? Does mission control activate a remote depressurization to drop the balloon? Or is it more likely (as I would imagine) that the balloon would eventually reach an altitude where it bursts?

Zero-pressure balloons can float for weeks. There is usually one altitude at day and one at night, because the heated air makes it go higher. Sometimes weights are used to make it stay at a constant height. Obviously that can only work for a limited period of time. (Every nightfall a weight is dropped, every morning gas is released because the volume of the gas increases). When you have midnight sun this problem conveniently disappears (:

Balloons that are made to withstand pressure can float longer than zero-pressure balloons, but they don't go as high because of the increased mass. You also have pressure-ballons made from lighter materials that are destroyed when they reach a certain height.

Theoretical maximum height is limited first by the pressure of the gas used, but also by the weight of the balloon. As it reaches higher heights in the mesosphere temperature drops and that can make lightweight materials to be destroyed.

Temperature drops to about -50 degrees celcius at 10km then it gets warmer, before it drops to about -90 degrees celcius at 85km.

Getting through the 10km point (tropospause) is a critical moment for most balloons. I think temperature is a pretty big limiting factor.

scr4
12-17-2003, 06:45 PM
Originally posted by toadspittle
What's the usual fate of such balloons? Do they regularly reach neutral buoyancy, then leak and eventually come down? Does mission control activate a remote depressurization to drop the balloon? Or is it more likely (as I would imagine) that the balloon would eventually reach an altitude where it bursts?
Most high altitude balloons are "zero pressure" meaning the pressure inside is exactly the same as the outside. There's a hole in the bottom, just like a hot air balloon. Since the balloon envelope doesn't need to withstand a pressure difference, this allows you to use an extremely thin material, like the 3.5-micron film I mentioned. The balloon is only partially inflated at launch, but as the external pressure decreases it expands. When it's fully inflated the gas starts escaping from the bottom, that's the maximum altitude for that balloon. Temperature shifts also cause them to lose gas, so they can only fly for a couple of day/night cycles. One way to extend flight time is to take it to Antarctica or the Arctic where it can remain in the sun for over a week. This is done routinely.

These balloons are usually equipped with a device for ripping up the balloon. The payload is cut off first, and it lands by parachute. The balloon itself is then destroyed; they try to recover it too, but only because otherwise they'd get complaints about littering. Both the balloon and payload are equipped with radio transmitters so they can be easily found. If you don't destroy the balloon remotely, I expect it'll slowly come down within a few days.

A more recent development is the "superpressure" balloon which are strong enough to withstand a moderate pressure difference. If designed correctly, pressure inside will be slightly higher than the outside at neutral buoyancy altitude. At night it will go down slightly, but in the morning it will come back up to the same altitude because there is no loss of gas. Google for "ULDB" (ultra-long duration balloon) for more info.