Hydroxy-terminated polybutadiene (HTPB), a butadiene, is a stable and easily stored synthetic rubber, often used in tire manufacturing. HTPB is sold as a two part thermoset elastomer which is mixed prior to molding.
HTPB is used to bind the fuel and oxidizer into a solid mass for solid rocket motors. It is also used as a hybrid rocket fuel. Together with N2O as the oxidizer, it is used to power the SpaceShipOne rocket motor.