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, George Paul, and Oscar Biblarz. 2006. Rocket Propulsion Elements. Norwich, NY: Knovel.