Making a bouyant high altitude balloon?

[jasonhtml]

Ok, I'm sure all of you have heard about or even built high altitude balloons. They often travel to about 80,000 - 100,000 feet (about 30 km or so). I was reading about a lot of them lately and I noticed that all designs had a parachute so that when the balloon burst, the payload could safely fall back to earth. Then I wondered, could you possibly put in less helium (as that is the gas commonly used) so that the balloon will not burst? Would it stay buoyant in the air or not? It seems like someone would have tried this to elongate their mission time. Also, how much longer do you think it would stay aloft if it did stay buoyant in the air?

btw, I hope this is the right forum. I'm new here :)

 

[jasonhtml]

i was just reading an article about high altitude blimps that the US government just recently contracted. why wouldn't these burst? it said in the article that the blimps could be up for months at a time...

sorry for the double post, the forum apparently doesn't allow edits after a period of time

 

[Mech_Engineer]

My senior design project had to do with weather balloons. Our design to have a balloon payload float at a goal altitude for an extended amount of time was a two-balloon system, where one balloon is released at the goal altitude to achieve neutral buoyancy, and the second balloon was released after an hour or so, dropping the payload back to Earth with a parachute.

It's possible to make a balloon neutrally buoyant at a specific altitude by calculating the pressure and tempertuare at your goal altitude using standard atmospheric models, but there are a couple of problems with it:

1) generally weather balloons are launched with excess helium to make them rise quickly through the jetstream and minimize drift in the wind. But, if a single balloon is filled to be buoyant at a specific altitude, it will have a very slow rise time. A balloon with the correct amount of helium for neutral buoyancy at altitude will only be very slightly positively buoyant at low altitude. This will tend to maximize float time in the jet stream as its rising, making the baloon fly very far away, out of range of radio receivers. Whatever is launched on the balloon will probably never be found again.

2) Helium leaks out of the baloons pretty fast. The balloons are thin latex, and helium is very diffusive, so the baloon might be neutrally buoyant for a while, but in a few hours begin to fall.

3) The temperature in the atmosphere drops until about 50 or 60,000 feet, and then begins rising again. Therefore it wouldn't really be possible to design a balloon to be neutrally buoyant higher than this temperature gradient unless you vent helium or use some other system to reduce/control buoyancy.

 

[jasonhtml]

ok, for 1) maybe you could have some sort of system that restricts the size of the balloon, so that at any time, you can "set" its density by shrinking or or letting the balloon expand a little?

2) are their any balloons that have a thin layer of aluminum coating? maybe this could keep the helium from diffusing out?

3) same idea as 1, i guess.

also, you mentioned the jet streams and rising above them... so, is there relatively little wind above those jet streams?

 

[Mech_Engineer]

ok, for 1) maybe you could have some sort of system that restricts the size of the balloon, so that at any time, you can "set" its density by shrinking or or letting the balloon expand a little?

How do you suggest doing that? Doesn't sound possible to me... Remember whatever you design has to be very light!

You could perhaps try carrying a little extra helium in some small pressure cylinders to add helium to the balloon, and then a valve to vent the helium. We thought about a design like that but it ended up being prohibitively heavy as well as complex. Plus depending on how much weight you need to lift, the balloons get VERY big. Lifting a 6 lb load or so to 100,000 ft requires a 1200g or 1500g latex baloon, which expands to about 30-40ft across at the goal altitude. Filling the balloon on the ground takes a long time, and a couple of large tanks of helium.

2) are their any balloons that have a thin layer of aluminum coating? maybe this could keep the helium from diffusing out?

There are mylar balloons that have slower leak rates than natural latex, but in the end helium evertually leaks out of everything, even storage tanks that are half inch thick steel. You can't perpatually float a baloon without a reserve supply of helium to refill it.

also, you mentioned the jet streams and rising above them... so, is there relatively little wind above those jet streams?

Yes, above the jet stream there is very little wind. Balloons that fly to 100,000ft are relatively stationary at altitude, and tend to fly farthest when going through the jet stream.

 

[jasonhtml]

ok, i thought about changing the altitude by changing the balloon's size, and i have to admit that it would probably be way to heavy... so, i had another idea: what if you had a smaller secondary balloon for changing the altitude? it could be in a small box that could be compressed to lower the payload, and expanded to rise. [see attachment]

i also saw another high altitude balloon on the internet, which had ham radio, and the payload was only 16 grams! how could you possibly fit ham radio on something so light (and small)?