UR-100MR / SS-17 SPANKER

The UR-100MR / SS-17 intercontinental ballistic missile, the first Soviet ICBM to carry multiple independently targetable reentry vehicles (MIRV) as well as the first to employ a cold launch system. It was composed of a two-stage, tandem, and storable liquid-propellant missile. It was initially intended to replace the light UR-100 SS-11 missile. It was assessed by US intelligence was being capable of delivering a throw-weight of 6,000 lb to a range of 5,500 nm. The throw-weight consists of a post-boost vehicle and between one to four reentry vehicles. It was a competing design with the SS-19 Stiletto, though in fact both were deployed.

The SS-17 was designed for launch from existing SS-11 silos. As thus, the overall dimensions of the SS-17 were determined by the dimensions of the SS-11 UR-100 silos. The diameter of both stages was increased to 2.25 m for the first and 2.1m for the second stage in relative to the previous UR-100. The UR-100MR used asymmetrical dimethylhydrazine and nitrogen tetraoxide propellants. The first stage used a closed-cycle single-chambered sustainer engine and a four-chambered open-cycle control motor. The second stage was equipped with a single-chamber open-cycle sustainer that was placed inside the lower part of the fuel tank. The missile used deflecting control motor chambers for first stage flight control; solid propellant retrorockets separated the stages. Finally, the second stage used gas injected into the diverging part of the nozzles for its flight control.

The rocket MR UR-100 was placed in previously hardened SS-11 UR-100 silos in their transport-launch canister with the use of two shock-absorption belts. The existing SS-11 facilities did not exactly fit SS-17. The SS-17 needed a sabot cold launch or pop-up launch system that required modification to the SS-11 launch sites.

• SS-17 variant-1 - This initial version of the SS-17 carried a 4 MIRV warhead with an individual yield of 0.3-0.75 Mt each. It had an onboard instrument module containing a command structure while a solid-propellant rocket motor comprised the post-boost vehicle. According to Western estimates each RV weighed about 900 lbs. In 1975 estimates indicated that the missile's inertial guidance system had an estimated CEP of 0.34 nm and a potential CEP of 0.28 nm in 1980. The program was approved in September 1970 and its development was conducted by KB Yuzhnoye (OKB-586) which was headed by V. F. Utkin. Pop-up tests of the launch system, which improved the mortar launch technique, began in May 1971. The full scale flight-design tests of variant 1 and 2 were conducted at the Baikonur cosmodrome test site from December 1971 through December1974. Flight testing was first detected by the West on September 1972. The first regiment with MR UR-100 missiles was put on alert in May 1975 and its deployment began in December 1975.
• SS-17 variant -2 - The Mod-2 missile carries only a single warhead with a yield of 4-6 MT.
• SS-17 variant -3 - On 16 August 1976 a governmental order to improve upon the performance characteristics of the UR-100MR. The preliminary design of this MR UR-100UTTh missile known as SS-17 Variant 3 was completed by KB Yuzhnoye (OKB-586) in December 1976, receiving the industrial index number 15A16. Like the previous SS-17 variant -1 it carried four MIRV warheads, but unlike variant-1, it incorporated enhanced survivability in addition to being equipped with an improved commando system and modernized nose cone. Flight-design tests began in October 1977 and were finished 2 years later in December 1979. The variant -3 reached its IOC in 1978 and its deployment started along with the R-36MUTTHon December 1980.

In 1979, 130 missiles were deployed in two missile fields surrounding Yedrovo and Kostroma. From 1979 through 1980 all single warheaded missiles were replace by MIRVed missile systems. Between 1982 and 1983 all MR UR-100 missiles, as well as 20 UR-100 missiles, were replaced by the MR UR-100UTTh missiles. The system reached its maximum operational inventory of 150 in 1983. By 1991, when START-1 was signed, the Soviet Union had 47 launchers for the MR UR-100UTTh which were all subject to dismantling.