Missile UR-100 (SS-11 Sego)

The UR-100 (Universal Rocket, GRAU index 8K84, NATO reporting name SS-11 Sego) was a Soviet intercontinental ballistic missile developed as a lightweight, mass-produced counterpart to the American LGM-30 Minuteman. Development was approved by the Soviet government on 30 March 1963, and flight testing ran at Baikonur from April 1965 to October 1966, the first silo launch taking place in July 1965. The first three regiments were placed on alert on 24 November 1966 and the missile was formally accepted for service on 21 July 1967. The similar designation UR-100MR belongs to a different weapon, the MR-UR-100 Sotka.

The UR-100 is a two-stage, tandem, storable liquid-propellant missile burning unsymmetrical dimethylhydrazine and nitrogen tetroxide. In both stages the oxidiser and fuel tanks share a common bulkhead, which cut overall length and launch weight. The first stage uses four closed-cycle single-chamber engines; the second combines a single-chamber sustainer with a four-chamber vernier motor. Guidance is inertial, built around an autonomous control system with a gyro-stabilised platform on floating gyros. It was the first Soviet ICBM stored and fired from a pressurised transport-launch canister, its engines isolated from the propellant by membrane valves so that it could stand fuelled for extended periods. Baseline versions carried a single warhead, Soviet sources giving yields of 0.5 Mt for the light intercontinental warhead and 1.0 to 1.1 Mt for the heavier one used against targets in Eurasia. Accuracy was low enough that the missile was usable only against soft targets.

The 15P784 silo complex was simplified relative to earlier Soviet systems: groups of ten hardened, unstaffed silos under a single launch control centre, each silo closed by a pneumatically driven sliding roof.

Four configurations followed the original 8K84. The 8K84UTTKh introduced a new nose cone, revised launch equipment and an autonomous power supply; the 8K84M variant, in service from 3 October 1971, was the first Soviet ICBM fitted with the "Palma" penetration-aid package. The UR-100K (15A20, SS-11 Mod 2) lengthened the first stage for more propellant, raising throw weight by about 60 percent and improving accuracy by roughly half; it dispensed decoys at the end of boost phase and was deployed from 28 December 1972. The UR-100U (15A20U, SS-11 Mod 3) replaced the single warhead with three multiple re-entry vehicles at reduced range and added shock isolation to a hardened silo, entering deployment on 26 September 1974. A six-warhead Mod 4 was tested but never fielded. The enlarged UR-100N was a separate design rather than a variant. The Strela and Rokot carrier rockets are derived from the family.

The Strategic Rocket Forces of the Soviet Union were the only operator, and no UR-100 was ever fired in anger. It was deployed in larger numbers than any other missile in the force: 990 were fielded between 1966 and 1972, followed by some 420 UR-100K and UR-100U launchers between 1973 and 1977 as the original version was withdrawn. Of the 326 that remained in 1991, all but ten had left combat duty by the end of 1994 under START I, and the last were dismantled by the end of 1996.