the3dintern Posted November 27, 2014 Posted November 27, 2014 (edited) I have seen discussions on here and online about vacuum buoyancy and had 3 questions: 1. Can a vacuumed out cavity be created on earth or just less of what was vacuumed out? I ask because I am thinking that if you created a container underwater with water in it, if you try to evacuate all water from the container with a vacuum (not by pumping in air), what would happen to the water inside? 2. If a structure meant for vacuum buoyancy cannot withstand atmospheric pressure near sea level when a vacuum is attempted to be created, what would happen if that structure (while open) was taken up in a weather balloon where atmospheric pressure can be far less http://www.altitude.org/air_pressure.phpand then sealed and then a vacuum attempted again? What would happen once it is dropped? 3. What would happen if a structure with a large enough cavity (to dispace more air [N/O/CO2/H2O/etc] than it weighs) were 3d printed in space with a seal so no gases can enter and then descended into earth's atmosphere? Will it float, collapse, or something else? Edited November 27, 2014 by the3dintern
swansont Posted November 27, 2014 Posted November 27, 2014 You can create a vacuum — people do it all the time. But be careful in discussion, because there are multiple definitions of vacuums. The definition used in most cases is that it's a region that's at significantly lower pressure than atmosphere. Some people interpret the word to mean a perfect vacuum (while not using the modifier), i.e. a region completely devoid of anything, which is not achievable. The trick is in creating the vacuum in a chamber that can withstand the pressure difference, and if you want this to be buoyant, the structural material has to be lighter than the mass of air that was evacuated. That's hard. Issues of vacuums include the fact that the structure itself outgases; you generally bake metals hard to encourage these materials to diffuse and get pumped out. A sealed container will lose quality of vacuum owing to outgassing, and if in atmosphere, diffusion of Helium (and some Hydrogen) into the chamber. On often puts a passive pumping in the chamber (non-evaporable getter pumps), but these don't pump noble gases.
Enthalpy Posted November 29, 2014 Posted November 29, 2014 Vacuum vessels are a common thing, but building one light enough to buoy in the atmosphere hasn't been made up to now, to my knowledge. I believe it is possible (not easy!) and give hints there http://saposjoint.net/Forum/viewtopic.php?f=66&t=2520 in short: graphite composite is strong and light enough for the purpose (very few metals too, but they're more difficult to use). The difficulty is to avoid buckling, because the compressed structure is very hollow. I suggest to hae a very heterogenous structure, with a few beams (possibly as an icosahedron) holding reinforced foils that resist traction only.
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