A wall of a cryogenic propellant tank includes inner and outer skins supported parallel to, and at a distance from, one another to define between them a zone into which pressurized gas is introduced. The zone houses a reinforcing structure, such as honeycomb or corrugated material, to create a sandwich structure. The gas is introduced into the zone using conduits in the reinforcing structure. The gas preferably is an inert gas, most preferably helium.