Why can't we fill the balloon with the lightest substance available, i.e. vacuum?
Helium balloons explode if they go sufficiently high, presumedly from excess pressure?
It can't be as simple as to install a pressure relief valve..
Wonder how big a part radiation and subzero temperatures would play, say, 300 miles up?
The balloon would have to be rigid and withstand huge forces. With known materials, that would make it heavier than the air it displaces.
Could fill it with Hydrogen, which is lighter than Helium.
The balloon will not burst at the desired height because it is designed with enough surplus capacity to contain the gas after it expands.
Remember the Hindenburg?
They don't use helium because it's light. They use it because it's less dense than air at the same pressure.
If they use a vacuum what would the pressure differential be between inside and outside?
I like the concept of filling something with vacuum.
300 miles up, we have space with an extremely good vaccum (most satellites are there, flying for years without falling down). The density is way too low to use balloons there.
You can increase their maximal height with a valve, but that does not reduce the mass of the hull - at some point, air pressure is too low to generate the required lifting force.
Interestingly enough, what atmosphere does exist 300 miles up is actually extremely hot. That's near the upper end of the thermosphere, which can reach temperatures into the thousands of degrees during the day. The density is so low however that objects will not experience this temperature - the rate of heat transfer is simply too low.
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