evp@sri-unix (11/12/82)
The external tank is made of aluminum, as ecn-pa.lebold pointed out, but is covered with an orange colored foam insulating material. This material not only keeps the liquid H2 and O2 cold during launch preparations, but protects the tank from fairly intense heating caused by shock waves that form between the orbiter and the tank during ascent. The paint was thought to be necessary during initial design to help insulate the tank from sunlight and other heat sources, but is no longer considered necessary, and the quite significant mass savings directly translate into payload capacity (since the tank is taken almost into orbit). - Ed Post (... teklabs!iddic!evp)