The Castor 120 is a medium-size, high-performance solid propellant motor developed in 1989 to fill the gap between motors in the 30,000-pound range and large motors like the Space Shuttle reusable solid rocket motors and the Titan IV solid rocket boosters.  The first flight of a Castor 120 occurred on August 22, 1997 with the launch of Lockheed Martin's LMLV-1 (Athena) as the first-stage motor.

Thiokol developed the Castor 120 from the Castor series of solid rocket motors making it a very versatile motor.  It can be used as a first stage, second stage or a strap-on booster.  The thrust vectoring system provides nozzle vector angles of up to five degrees.  It has a graphite epoxy case produced using a continuous winding process and it has a tailorable thrust profile.  The Castor 120's low cost was achieved through advanced design features such as a reduced part count, a reduction in cycle time and a modified propellant grain machining process.