•To the right is a very simplistic diagram for a solid fuel motor. •Ignition is usually a series of pyro explosions. •O-Rings separate each section of the engine. •Once ignited, conventional solid rocket engines can not be turned off or re-ignited. •Solid propellant in the Shuttle SRB’s was a mix of  ammonium perchlorate (oxidizer, 69.6% by weight), aluminum (fuel, 16%), iron oxide (a catalyst, 0.4%).