Metal "permeability"

flatmaster

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?

Mapes

Yes, everything is permeable to everything, generally speaking. But the time it would take a significant amount of even the smallest gas atom/molecule (i.e., He) to permeate through 1 mm of metal is probably on the order of years (see Figure 10 here). A more important leak path is through connections (which may not be completely gas-tight) and through welds (which can be porous if not made correctly).

minger

I've typically referred to this as porosity. The experience I've had with it has been cast parts. At Honda one problem they had was block porosity, where if the part wasn't cast correctly, high pressure oil lines could cause seemingly phantom oil leaks through the block.

I also just went to a plant that is casting some of our turbine blades, they mentioned how porosity is something that they check for to ensure they're doing a good job.