ORIGINAL: broke_n_bummin
Well, actually, if you look at the illustrations of the build, the inner balloon holds the fuel and the outer balloon is pressurized by the exhaust. i know they weren't meant to last forever, but eventually, the outer balloon would get easier and easier to inflate. It seems to me this would decrease the pressure on the inner balloon unless the whole cell was in some kind of container.
Broke,
That is plain wrong! Any enclosed volume will have the same pressure as its source.
The muffler has an average of 0.5 psi and the outer tank is made of of poly-propylene, of poly-ethylene, which are semi-rigid plastics - not an 'external balloon'...
The pressure in the bladder will be the same as in the external tank, or just marginally lower (-0.01 psi), as fuel is constantly drawn from it.