A solid rocket motor (SRM) contains propellant in which fuel and oxidizer are pre-mixed into a solid grain. Once ignited, the motor burns until the propellant is exhausted and generally cannot be throttled or shut down. SRMs offer very high thrust-to-weight ratios, extreme simplicity, and long shelf life, making them widely used as strap-on boosters (such as on ULA's Atlas V and Ariane 6) and in military missile applications.
The tradeoff is lower specific impulse compared to liquid engines (typically 240-280 seconds) and an inability to restart or precisely control thrust. Despite these limitations, SRMs remain critical for several small launch vehicles, including Northrop Grumman's Pegasus, and are used as kick stages to inject payloads into final orbits after separation from the primary vehicle.