The Castor motor family was originally developed in 1959 to support the NASA Scout and Little Joe vehicles and was designed to provide inexpensive, reliable boosters for small payload launch vehicles.  The Castor IV solid rocket motor was developed to provide first stage propulsion for the Athena H and was later adapted as a strap-on booster for the Delta launch vehicle.  The Castor IV has been replaced by the IVA and the IVB.  There is also an extended length version of the IVA motor called the IVA-XL.

The Castor IVA Delta strap-on booster motor was qualified by NASA in 1988.  On December 15, 1993, four Castor IVA motors were used on an Atlas IIAS launch, increasing the payload capacity by 1500 pounds. The IVA uses a steel case and nozzle.  It was designed with multiple launch vehicle applications in mind.  It can be built with a straight nozzle or one canted either seven or eleven degrees.

The Castor IVB solid rocket motor is a thrust vector controlled version of the IVA used for first or second stage or strap-on boost applications. Using similar construction to the IVA the IVB can also be used on several vehicles.  The omni-axial thrust vector contol / thrust vector actuation system includes a hydraulic blowdown system, ± 6 degrees deflection, 15 degrees/second slew rate, and 250 degrees capability.

The Castor IVA-XL Delta strap-on booster motor is an eight foot extended length version of the Castor IVA.  The larger motor provides greater launch capability and 30% increase in performance.