Rocket Motors burn
solid grain propellant to generate large amounts of thrust.
Launch vehicle designers typically have to decide between using
liquid rocket engines versus solid rocket motors. Rocket
motors have more thrust, but are less efficient than liquid rocket
engines. Furthermore, the rocket motor's dense solid
propellant make them more compact. As a result, rocket
motors are typically used as strap-on boosters for first stages,
such as the Space Shuttle, and for applications that require
simple compact designs.