David,
 
DW> I still think the effect of pressure, making a very thin-walled can
DW> effectively rigid, must enter into the economic calculations. It
DW> reminds me of some early space rockets, which would virtually collapse
DW> under their own weight, except for the pressure of the fuel in them
DW> keeping the metal stretched tight.
 
The chief-engineer of the Atlas-project back in the 50-ies, Charlie
Bossaerts (hey, a Belgian), had the rocket built in such a way it was
pressurized. Not just the fuel, but the whole construction consisted of
pressurized cells virtually piled on top of eachother.
 
Now an Atlas-rocket is not a small thing ...
 
 \%/{at}rd 

--- D'Bridge 2.72