Testing anti-missile systems requires a target. That is, an incoming ballistic missile. This can be expensive. So the United States has developed the MRT (Medium Range Target) to provide a realistic incoming warhead, without the cost of a complete ballistic missile. This is done by modifying a Castor IVB rocket motor. Normally, the Castor IVB is used as a strap on booster for larger satellite lifters like the Delta. The Castor IVB is 29 feet long and contains ten tons of solid fuel propellant. To turn it into an MRT, a warhead and guidance package is added. The MRT can be launched from the deck of an aircraft carrier, from land, or from the rear of a C-17 in flight (using a parachute to pull it out.) The air launch enables the warhead to go higher, and thus come down faster, representing a longer ranged missile.