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Okay, I really have three questions, but I can only post once every 40 minutes and they’re all related.

  1. Say I have a balloon with the inside supported by a collapsible frame and made from a very strong substance. When you make it not in it’s collapsed position, it would become a vacuum inside. Since that’s lighter than air, would it float? (If it wouldn’t due to the frame being heavy, what if it was made of aerogel?)

  2. If it does float, would it gain a fast enough velocity on the way out of the atmosphere to escape earth’s gravity when it gets to space?

  3. If when it got to space a servo motor collapsed it again, making there no vacuum, and it was still within earth’s gravity, would it go back down now that there’s no vacuum holding it up?

Sorry for so many questions, and thanks for understanding.

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    $\begingroup$ I'll just mention that you don't see vacuum balloons in practice because of the immense pressure pushing in on the vacuum. A cubic meter of vacuum only displaces a 1.2kg of air, so your internal frame must weigh less than that. It's a significant engineering and materials challenge to build a frame of that size with sufficient strength, and keep it light enough to float. $\endgroup$ Jul 8, 2020 at 15:24
  • $\begingroup$ Since aerogel weighs 1.5 kg as well, it should work with it, because we won’t be filling up the whole balloon, so if you only used an eighth of that and if you made everything about 10 times larger than that, than you’d have over four pounds of payload weight available, And that’s all I want due to FAA regulations. $\endgroup$ Jul 8, 2020 at 15:35
  • $\begingroup$ Most aerogels have compressive strength in the range of tens of MPa, but the pressure exerted on a spherical shell of vacuum will be in the range of GPa. You can get creative with different shell designs to reduce the strength required, but I'm not sure aerogel will be strong enough to resist the immense crushing force. It might work with the right design and material, but it'll be tough. $\endgroup$ Jul 8, 2020 at 16:10
  • $\begingroup$ Helium at air pressure will gave a mass $\frac{1}{7}$ of air and hydrogen only $\frac{1}{14}$. Dropping that to $0$ by using vacuum won't add a lot of lift. Almost certainly, the small gain will be far exceeded by the mass of the extra strong container. For all of these, there will be an altitude at which they are no longer buoyant. I very much doubt that will have achieved enough velocity to get far beyond this point. Helium balloons don't launch themselves into space. I doubt that your vacuum balloon will either. $\endgroup$
    – badjohn
    Jul 8, 2020 at 16:13
  • $\begingroup$ I do realize that it wouldn’t add much lift, but it would get all the way up to space that way, although now I do realize that it would just be cheaper to have a pressure release valve. Does anybody here know of any good ones (possibly that are triggered by a lot of pressure)? $\endgroup$ Jul 8, 2020 at 16:56

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1: Assuming the frame is lighter than air of the same volume as the vacuum inside, yes. It would not have a balloon shape, though.

2: By experience with helium baloons, no way. This source (paywalled) gives ascent speeds of a couple meters per second throughout the entire flight, which is also dependent on the balloon expanding as it rises (which would not happen with a vacuum balloon). Earth's atmosphere around 500 km above ground is pretty much negligible (comparable to the vacuum inside your frame, since slight leaks will occur from the frame). Even if the baloon got to that height with some speed, escape velocity from 500 km above ground is still several kilometers per second, which it could not get to within the atmosphere due to drag. Your baloon will end up floating at some height determined by its density (frame mass over volume of vacuum).

Extremely theoretically, if you were to ignore drag and make your apparatus extremely light, it might leave the Earth gravity well. Effectively, it would gain the potential gravitational energy of air of its volume (at a height difference between ground and upper levels of the atmosphere), and transform it into both potential and kinetic energy. Earth escape velocity is about 11 $\frac{km}{s}$. The height difference might be about 10 kilometers - a precise calculation would have to take into account how air density varies with height. This corresponds with a velocity of about $400 \frac{m}{s}$ (from the free-fall formula, and assuming a constant gravitational strength, which at these scales is not a problem). That is a speed ratio of ~28, which then gets squared to get the energy ratio. So sufficient energy can be provided for a material of density about 800 times lower than air. Of course, making such a balloon, let alone making it zero-drag, is closely approximated by "impossible".

3: If it were light enough to cross the 100 km "space" boundary, then yes, losing volume would make it go down again. A similar thing happens if a helium balloon pops - it loses volume and falls back on the ground, creating plastic pollution (don't let go of your helium balloons!).

You may also want to read this explanation of the relative sizes of the Earth, its atmoshpere, and space. Basically, atmosphere forms a very thin shell around the Earth and getting out of the atmosphere is not enough to go into orbit or away from Earth's gravity influence.

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