SS-20 "Saber": Theater-based Ballistic Missile The RT-21M / SS-20 is a solid-fuel, two-stage, theater-based ballistic missile based on the first and second stages of the SS-16. The SS-20 was probably intended to replace or augment the SS-4s and SS-5s deployed in the Soviet Eastern European theater, thus providing approximately three times the number of warheads of the older force. The SS-20 also has the capability to be reloaded and refired.