If the foam is sprayed on to the tank during assembly, then maybe a
sub-surface Kevlar netting that is integrated into the application of
the foam would help. The netting would be below the surface of the
foam layer (thus eliminating any aerodynamic issues), and it would be
able to help 'hold together' the foam.

I'm not sure if the thermal coefficients would match up between the
materials, or if they could be engineered so that they do, but I think
the concept might be worth exploring. I'm also not sure if the added
weight would be prohibitive (I read elsewhere that even the weight of
paint on the external tank was too much, so they discontinued using
it).

-Randy