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UR-100 / SS-11 SEGO

The development of the massively deployed UR-100/SS-11 liquid propellant light ICBM was the centerpiece of a major Soviet effort to reach numerical strategic parity with the USA. The SS-ll was the Soviet counterpart of the US Minuteman system in quantity, size and purpose. Initially deployed with a single warhead [with a yield of 1.1 MT according to Russian sources, or 0.6 to 1.2 MT according to Western reports] and a low accuracy [a CEP of 1.4 km according to Russian sources], the missile could be used only against soft targets.

The UR-100 intercontinental ballistic missile is a two-stage, tandem, storable liquid-propellant missile. It is about 64 feet long and 8 feet in diameter. In both stages the oxidizer and fuel tanks had a common bottom which reduced overall dimensions and launch weight of the missile. The bottom of the oxidizer tank of the first stage was placed inside the tank like an inverted truncated cone. The nozzle of the sustainer of the second stage was included in the formed upper volume. The first stage used a new set of four closed-cycle single-chambered rocket motors, while the second stage incorporated a single-chambered sustainer and a four-chambered control motor. Asymmetrical dimethylhidrazine and nitrogen tetraoxide were used as propellants. The missile uses an inertial guidance system consisting of an autonomous guidance/control system with a gyro-stabilized platform of floating gyros. The command structure also provided an automatic checkout of all systems during flight and automatic preparation of launch.

The development of the UR-100 was approved by the government on 30 March 1963. The developer was NPO Mashinostroyenia (OKB-52). The missile was deployed in at least four variants, and was probably tested in several additional configurations. There is some confusion among these variants between recent published Russian sources, which focus on the physical configuration of the rocket, and contemporaneous Western sources, which were limited to intelligence derived from observing flight tests. In the middle of the 1970's the UR-100 was replaced by two modernized versions that received the designations UR-100K (15A20) and UR-100U (15A20U).

• SS-11 Mod-1 UR-100 The flight-design tests were conducted at the Baikonur cosmodrome between 19 April 1965 and 27 October 1966. The first silo-launch was conducted on 17 July 1965. The Mod 1 reentry vehicle had a ballistic coefficient of 310 lb per sq ft and a CEP assessed by Western intelligence at 1.0 nm. Western intelligence assessed that an initial operational capability was reached in early 1966. According to Russian sources the first three regiments with UR-100 missiles were put on alert on 24 November 1966, and operational employment began on 21 July 1967. According to Russian sources, initially the missile was equipped with two different types of warheads: a light one for intercontinental targets in North America, and a heavier one for medium range targets in Eurasia.

• SS-11 Mod-1 UR-100UTTh Upgrades to the UR-100 missile provided a different nose cone that allowed improved flight characteristics and a modified command structure that reduced the time for pre-launch operations. The modified version received the designation UR-100UTTh (8K84UTTh). It differed from former missiles by improved warhead technology, improved launching equipment, and an autonomous power supply system, ensuring extended storage in fueled conditions. The flight-design tests of the UR-100UTTh missile were conducted between 23 July 1969 and 15 March 1971.

• SS-11 Mod-2 UR-100K The development of the UR-100K (15A20) missile [known in the West as the SS-11 Mod-2] began in the middle of the 1960s. The main design changes concerned lengthening of the first stage to increase the amount of propellant and modifications to the propulsion systems of both stages. These measures allowed an increase of launching weight of 8 tons and a 60 percent increase of throw weight. The maximum range of the UR-100K was increased up to 12,000 km, and through the use of improved sensors the accuracy was increased by a factor of 1.5 to a CEP of 1 Km. The Mod 2 reentry vehicle had a ballistic coefficient of 900 lb per sq ft and a CEP assessed by Western intelligence at 0.6 nm. The missile also dispensed decoys at the end of the boost phase of the trajectory before the separation of the reentry vehicle. The reentry vehicle itself had radio-reflecting properties. The sophisticated control system allowed an increase of combat readiness of a missile through the use of boosted spin-ups of gyros. The silo had its own system of power supply allowing remote changes of its mission and launch. The flight-design tests of the UR-100K missile were conducted from 02 February 1971 through 24 November 1971. According to Russian sources the missile was initially deployed on 28 December 1971. Western intelligence assessed that an initial operational capability was reached in 1973.

• SS-11 Mod-3 UR-100U The UR-100U (15A20U) missile differed from the UR-100K missile by the number of warheads. Instead of a single warhead it carried three warheads, though with a reduced maximum range. The tests of the UP-100U were conducted from July 1971 through January 1973, and its deployment started on 26 September 1974 according to Russian sources. According to Western sources, the first Mod 3 flight test was on 12 September 1969, and Western intelligence assessed that an initial operational capability was reached in 1973. During development of this missile efforts centered on increasing its survivability. The silos were hardened and the shock-absorption of the transport-launch canister was improved. The silo consists of a monolithic ferro-concrete trunk with a steel hardware compartment, rigidly attached to it. The trunk was covered with an accident protection device in the form of a sliding roof.

• SS-11 Mod-4 The Mod-4 missile differed from the Mod-3 missile by the number of warheads -- instead of three warheads it carried six. This variant was tested but not deployed.

The missile was deployed in a silo launcher, with a design that was substantially simplified in comparison with earlier complexes. The silo could be closed for protection with the help of a pneumatic driven sliding roof. This was the first Soviet ICBM to be deployed with a pressurized transport launch canister in which the missile was delivered to the launch complex and from which it was fired. During the storage of the missile the engines were isolated from the propellant components by membrane-valves that provided their safety during extended times of being in a fueled condition.

The SS-11 deployment was assessed by Western intelligence to be similar in concept to the US Minuteman, where a large force was deployed in hardened silos requiring a minimum of support facilities. Silo and launch control center hardness was estimated at 700 and 400 psi overpressure, respectively, from a 1-MT weapon. The sites were deployed in groups of ten unmanned silos with a single launch control center for each group. Reaction time in the normal readiness condition was assessed by the West as 0.5 to 3.0 min. with an unlimited hold time in this alert condition.

The UR-100 missile was the most extensively deployed ICBM within the Strategic Rocket Forces. Between 1966 and 1972 a total of 990 of these missiles were deployed. Between 1973 and 1977 some 420 launchers of the UR-100K/UR-100U missiles were deployed while the UR-100 missiles were phased out. As of 1991 some 326 remained in service, while by the end of 1994 all but 10 of the UR-100 and UR-100U missiles had been removed from combat duty in compliance with the START-1 treaty. By the end of 1996 all SS-11 missiles had been dismantled.

Sources and Resources

• Russian Strategic Nuclear Weapons, Pavel Podvig, ed., IzdAT, Moscow, 1998, 492 pp. (in Russian). Authors: Oleg Bukharin, Timur Kadyshev, Eugene Miasnikov, Pavel Podvig, Igor Sutiagin, Maxim Tarasenko, Boris Zhelesov
• UR-100 @ Encyclopedia Astronautica by Mark Wade
• SS-11 Sego @ US Naval Institute Military Database
• The SALT Handbook, edited by Michael B. Donley
• "A History of Strategic Arms Competition 1945-1972" (U), Volume 3, A Handbook Of Selected Soviet Weapon and Space Systems, United States Air Force, June 1976. pgs 180-185

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