Materials for insulation of cryogenic fuels

[StevieC]

What sorts of insulative material, other than the orange foam we've seen on the space-shuttle's external-tank, would make for attractive choices to thermally isolate cryogenic fuels?

Would some sort of vacuum-flask type of containing-vessel serve to help insulate the fuel to prevent boil-off? Is this method already used in existing cryo tanks?

Also, would silica aerogel be an effective insulating material for cryo-tanks? It's both extremely lightweight, which would be appealing for use in a rocket, yes?

[PB666]

http://www.thermon.com/us/products.aspx?prodid=13