Solid Rocket Engine / Motor / Booster (SRB)

"A solid rocket motor burns solid fuel like Hydroxyl-terminated polybutadiene (HTPB) with a solid oxidizer like ammonium perchlorate. Both propellants are typically mixed and loaded into a casing prior to vehicle assembly. Once ignited, a solid rocket motor cannot be deactivated." (Sutton, Paul, and Biblarz 2006)