The largest and heaviest (when loaded) element of the space shuttle, the external tank (ET) supplies the liquid hydrogen fuel and liquid oxygen oxidizer under pressure to the three main engines during lift-off and ascent. When the SSMEs are shut down, the ET is jettisoned, enters the Earth's atmosphere, breaks up, and impacts in a remote ocean area.

To prevent the super-cold fuels from forming ice on the outside surfaces of the ET, a multi-layered thermal protection coating approximately one inch thick is applied. The insulation allows the ET to withstand the extreme internal and external temperatures generated during prelaunch, launch and flight.

The entire outer surface of the external tank is insulated with a half inch thick cork/epoxy layer covered with 1 to 2 inches of spray-on foam, as can be seen in the cross section ET segment pictured above.