The solid-propellant motor is the oldest and simplest of all forms of rocketry, dating back to the ancient Chinese. Solids require no turbopumps or complex propellant-feed systems. A simple squib device at the top of the motor directs a high-temperature flame along the surface of the propellant grain, igniting it instantaneously. Solid propellants are stable and easily storable. Once ignited, it will burn until all the propellant is exhausted.
Solids have a variety of uses for space operations.
Small solids often power the final stage of a launch vehicle, or attach to payload elements to boost satellites and spacecraft to higher orbits. The PAM-DII provides a boost for Delta and Space Shuttle payloads. The IUS goes on the Space Shuttle and the Titan III and Titan IV class of launch vehicles. Only one of the nation's launch vehicles, the Scout, uses solids exclusively. This four-stage rocket launches small satellites to orbit.