re: Balloon Tanks

Hey all,

An interesting data point on the whole baloon tanks discussion is 
SpaceX's new tank structure
they're using on the Falcon I.  Apparently, it is mostly a baloon tank 
with some slight reinforcement
that is only strong enough to take handling loads, while the flight 
loads are mostly handled by the
pressurization.  Supposedly it's patent pending.

For pressure fed systems however, the pressure is usually several times 
higher than the tank
pressure in a pump fed like the old Atlases.  Once you get to a certain 
tank pressure, the shell
becomes thick enough to easily handle both sets of loads without needing 
any additional ribs
or stringers.  If I wasn't so busy with my thesis, I could run an 
analysis of a balloon tank structure
to prove the point, but I don't have time at the moment.

~Jon