# Balloon aerodynamics

In this recent anime that I have seen, there is a part where an atomic bomb is sent to the stratosphere in a box, by a helium balloon.

As you can see in this picture of the set-up, there is a tube attached to the bottom of the helium balloon as it is ascending. I wanted to know what this tube is called, and what function is has for the aerodynamics of the balloon.

Basically, why is it there?

• Well, it's anime, so why do you assume it has any physical meaning. Commented May 23, 2015 at 1:59
• @Jimmy360 It must have a physical meaning, otherwise there is no point of having it.
– BDLL
Commented May 23, 2015 at 2:04
• I do have a theory of what it might be, but I am not sure. It could be to let air in as it descends, inflating it further. Eventually, however, when it slows down enough the air will come out of the tube, making it go faster. This goes back and forth until it finds the ideal velocity. Commented May 23, 2015 at 2:06
• I don't think so, it most likely has something to do with air pressure.
– BDLL
Commented May 23, 2015 at 2:12
• The tube is most likely for filling the balloon and has no aerodynamic function. After all, a balloon drifts with the wind and has no aerodynamic forces to speak of acting on it. Commented May 24, 2015 at 5:53

If the balloon was completely isolated from the outside atmosphere and contained a certain gas (in this case helium), then the helium will exert a pressure outwards, attempting to expand the balloon. The atmosphere outside will also exert a pressure inwards, attempting to keep the balloon in a small shape. These two forces compete, and the balloon will always form a shape that exactly balances these two pressures, i.e. the net pressure acting upon the balloon surface is 0.

The only problem is that atmospheric pressure (the pressure outside the balloon), decreases with altitude, as shown in this graph supplied from Wikipedia:

Because the atmospheric pressure DECREASES as altitude increases, that means the pressure pushing the balloon inwards decreases, in other words as the balloon rises, the balloon will begin to expand outwards. Eventually, the balloon will expand until the expansion tears a hole in the balloon, and the bomb falls back to earth (not good). Therefore, there must be some regulatory system attached to the balloon, who's primary purpose is to regulate the pressure difference between the helium inside the balloon and the atmosphere outside (the exact mechanism can vary), so that the balloon won't explode, and can float away (though not indefinitely).

Most high-altitude balloons are only partially inflated at launch-off, so that they can have a lot of room to expand before popping:

here http://qph.is.quoracdn.net/main-qimg-738042f4a60fbb716c868f98c8329d0a?convert_to_webp=true

Out of curiosity, what anime?

• Terror in Resonance is the anime, and great answer thanks!
– BDLL
Commented May 23, 2015 at 14:11
• You're basically saying the balloon is a bubble, and as it goes up, it expands (just as it would if it only contained air, not helium). That doesn't explain the extra tube hanging off the gas bag. I don't know its purpose either, though I can guess it's for filling, as @PeterKämpf said. Commented May 24, 2015 at 12:25