# Atmospheric ship

SF
How big would a vacuum-filled chamber need to be (built with currently existing materials) in order for it to displace enough of the atmosphere so it starts floating?

Think about it, such contraptions would work like ships, except they don't float on water this time, but up high in the atmosphere :D

Harrrrr
http://img244.imageshack.us/img244/213/piratebaythevideobaypubud2.jpg [Broken]

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stewartcs
Don't think it's plausible. The material would have to be really strong yet extremely low in density.

In general the net force on the object would be:

$$F_{net} = mg -\rho Vg$$

Which implies that the buoyant force would have to be greater than the weight of the object (chamber) in order for it to float.

In the above equation, the net force would be negative if the object floated.

CS

DaveC426913
Gold Member
Well, air has a density of 1.2mg/cm^3, so 1 cubic metre would weigh 1.2kg. If you could enclose that volume with a material that weighs less than 1.2kg and yet can withstand an external pressure of 14.7lbs/in^2 then you've got yourself a floating object. You could scale that up or down (down is better) as you wish, but it's still the external pressure that'll be the showstopper.

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DaveC426913
Gold Member
The material would have to be really strong yet extremely low in density.
Not density, mass.

stewartcs
Not density, mass.

Sorry, I wrote density but meant mass (as implied by the equation given).

CS

actually the idea is wonderful, the problem is in maintaining a control volume around a vacuum. considering standard sea-level pressure of about 14.7 PSI, the structure needed to maintain the 'bubble' of vacuum would be irreconcilably heavy. hence the ideas of a dirigible using hydrogen or helium: they allow for the density affect you speak of, but they have equal pressures (more or less) inside and out. The only way I could see this being done would be a rigid frame with a film stretched over it. Maybe an air-tight carbon-fiber matrix could do it, but you'd still be talking about huge internal stresses, and a really inherently unstable system.

DaveC426913
Gold Member
This is why hydrogen is so great. It has the best of both worlds:
1] it is so light that it is effectively vacuum (the difference in bouyancy between hydrogen and vacuum is not very much) and
2] it creates it own internal pressure.

Seems to me, if there's any advances going to be made, it will be in rendering hydrogen less dangerous. That could take several forms:
1] mechanical - some sort of matrix that prevents spreading of fire
2] chemical - doping the gas in some way that discourages ignition