Would a sealed hot air balloon get more lifting at 20km+ of altitude?

Question

So, the record altitude for a manned helium balloon is 40 km/25 miles/131234 feet, while the record altitude for a manned hot air balloon is 21 km/13 miles/68897,6 feet.

My guess is since the hot air balloon is open and it can only heat the air around its altitude, the rarefied atmosphere doesn't allow for a bigger difference in pressure between the inside of the balloon and the outside (not to mention the surrounding temperature).

So, I was wondering: would a sealed hot air balloon would make any difference?

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I would suggest to heat the air inside of it electrically instead of using methane, but the question isn't about how it gets hot.

If the envelope was elastic or not and if this would make any difference, I don't know.

Answer 1

A sealed hot air balloon doesn't work at all.

Beyond a certain point, hot air balloon envelopes don't expand any more as the air inside is heated up. Instead, as the air heats up inside, some of the air is actually forced out of the balloon. This makes the balloon lighter and gives it its buoyancy.

If you seal a hot air balloon and try to heat it up, the volume of the balloon will stay mostly the same and so will the density. Instead, the pressure will increase, which doesn't contribute to buoyancy at all.

Answer 2

There are several factors to consider. The buoyancy of a hot air balloon is achieved by having the air inside it hotter than ambient, and so of lower density. As the balloon ascends it will expand and so the surface area from which heat is lost goes up. In any case, the air needs to be heated in flight otherwise it will quickly lose buoyancy. The ambient temperature initially drops as altitude increases, but at higher altitude this no longer holds. In general, if the balloon is capable of expanding without resistance and the ratio of absolute temperatures of ambient and internal air is maintained then the buoyancy will be constant.