World Surveillance Group Inc (fka WSGI)

[JackedOnCaffeine]

I don't believe expansion capability of the material is an issue. The Helium will expand to fill more volume at higher altitudes as the airship rises. The ballonets containing the Helium only need to be flexible, not stretchable. The ballonets will only be partially inflated at ground level, allowing for expansion as the airship rises in altitude.

The typical mylar balloon doesn't really stretch, although it does expand during filling, becoming very firm and tight as more gas is pumped in, compared to your typical rubber balloon which expands from a smaller to larger size because the material stretches.