SLA-561S
The Shuttle-ET uses a spray-on ablator called SLA-561S. It's a Lockheed-Martin proprietary TPS (thermal protection system) material and chemically similar to SLA-561V which was used on Viking, Mars Pathfinder, Mars Polar Lander and Mars Exploration Rover. The big difference between the two versions is SLA-561S is spray-on while SLA-561V is hand packed into a graphite honeycomb structure. An ablator TPS is required for the ET because the stagnation point temperature exceeds the melting point of the aluminum-lithium structure.
Gary