Vacuum dirigibles don't work as much better than hydrogen or helium
ones as you would think. The lift of a balloon is proportional to the
difference in the molecular weight of the internal gas vs air. Air is
mostly N2 which has a molecular weight of 28. Helium has a molecular
weight of 4 and H2 weighs 2.
This means that the lift from a helium balloon would be proportional
to 28-4=24, the lift from a hydrogen balloon would be proportional
to 28-2=26, and the lift from a vacuum ballon would be proportional
to 28-0=28. So in going from helium to vacuum you only gain about 15%
more performance, not a huge improvement for normal use.
And of course the problem is that the total force on the outside of a
vacuum balloon would be enormous. Sea level air pressure is about one
ton per square foot. Considering the hundreds of thousands of square
feet in the balloon of a large dirigible, the total force is tremendous.
So you need a very strong material to withstand that, which will tend to
be heavy. And the energy release from a catastrophic implosion failure
would be far too dangerous, so you'll need internal compartments, making
the balloon even heavier.
Overall it doesn't seem like a very practical substitute for hydrogen
and helium.
Hal
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