- #36
Baluncore
Science Advisor
2023 Award
- 15,619
- 9,367
There are some changes that need to happen, to take the thought experiment towards a real experiment.
Using a sphere of the lightest weight film would weigh less and still limit the volume. Also, the top of the envelope needs thicker film to withstand greater differential pressure and surface tension, than at the bottom, where thinner film can be used. That film mass distribution will tend to capsize the balloon, unless there is a suspended payload that defines the base of the envelope.James Demers said:a hypothetical envelope of volume V and mass M that is perfectly rigid and thus maintains constant volume.
A film envelope will lift with only about 10% hydrogen or helium. Why waste the limited helium resource, by 100% filling a rigid envelope, when a spherical film envelope is easier to transport and costs 90% less to partially fill?James Demers said:Fill this vehicle with one atm. of helium at sea level, and turn it loose.
As it rises, a film envelope will expand with the lift gas, until finally, the base of the film envelope becomes spherical. Then the mode of operation changes. I would put a one-way flap valve at the base to release the excess lift gas, so there is zero differential pressure pushing the base of the envelope downwards from inside.James Demers said:As it rises, the external pressure drops. Releasing helium will indeed reduce the mass of the vehicle, and reduce its density, allowing it to rise higher.
Bursting is used to aid recovery of the payload, and to clear the controlled airspace. By continuously venting excess lift gas from an opening in the base, a greater height than the burst height would be reached.James Demers said:Scientific high-altitude balloons will get to this height before bursting, where V is the mechanical limit on the volume of the envelope.