Metal "permeability" I went to the Von Braun planetarium the other day and was surprised to find out that their planetarium dome was a prototype for the Saturn V's Fuel tank!! As a result, it wasn't exactly hemi-spherical which threw off come constellations, but that's another story. I bumped into my friend who works on Aries. He said the prototype was most likely made to measure the leakage rate through the steel. LOX or H2 will seep through steel? Yes, everything leaks. This, among other reasons, is why there is a vented space between fuel tanks. I believe it was "porosity" or "permeability" of the material that he spoke of. However, I've only been able to find information on the geological contexts of these words. Am I looking for the right word? Was he just joshing me?