R-7 Semyorka

The R-7 Semyorka (Р-7 Семёрка), officially the GRAU index 8K71, was a Soviet missile developed during the Cold War, and the world's first intercontinental ballistic missile. The R-7 made 28 launches between 1957 and 1961. A derivative, the R-7A, was operational from 1960 to 1968. To the West it was unknown until its launch (later it would get the NATO reporting name SS-6 Sapwood). In modified form, it launched Sputnik 1, the first artificial satellite, into orbit, and became the basis for the R-7 family which includes Sputnik, Luna, Molniya, Vostok, and Voskhod space launchers, as well as later Soyuz variants. Various modifications are still in use and it has become the world’s most reliable space launcher.

Description
The R-7 was 34 m long, 10.3 m in diameter and weighed 280 MT; it had a single stage with four strap on boosters powered by rocket engines using liquid oxygen (LOX) and kerosene and capable of delivering its payload up to 8000 km, with an accuracy (CEP) of around 5 km. A single thermonuclear warhead was carried with a nominal yield of 3 megatons of TNT. The launch was boosted by four strap-on liquid rocket boosters with a central 'sustainer' engine powering the central core. Each strap-on booster included two vernier thrusters and the core stage included four. The guidance system was inertial with radio control of the vernier thrusters.

The widely used nickname for the R-7 launcher, "Semyorka", is a rough translation of "old number 7" in Russian.

Development
Design work began in 1953 at OKB-1 in Kaliningrad in Moscow Oblast (presently Korolyov, Moscow Oblast) and other divisions with the requirement for a missile with a launch mass of 170 to 200 tons, range of 8,500 km and carrying a 3000 kg nuclear warhead, powerful enough to launch a nuclear warhead against the United States. In late 1953 the warhead’s mass was increased to 5.5 to 6 tons to accommodate the then planned theromonuclear bomb. On 20 May 1954 the USSR Council of Ministers approved the development of the R-7.

The principle of a staged missile, also known as a “rocket packet”, was first proposed by Mikhail Tikhonravov at NII-4 in 1947. Korolev became aware of this proposal in 1948 and supported further base studies at NII-4 in 1949–50. This was further refined by Dmitry Okhotsimsky’s Department of Applied Mathematics in 1951 and expanded by Korolev’s OKB-1 in 1952–53, which concluded that a core and four strap on boosters as the preferred model, which the R-7 used.

To lift the 5.5 ton payload required a redesign of the existing RD-105 and RD-106 engines. Valentin Glushko’s OKB-456 combined four combustion chambers using a single turbo pump, which provided a cumulative higher thrust than a single engine. Other advantages included an overall lower engine weight and simpler design, test and construction, via standardisation. The main engines for the central core and strap on boosters all used the four combustion chamber configuration. The four strap on propulsion engines were powered by the RD-107 engine providing a sea level thrust of 83 tons, each with two Vernier engines to assist with steering. The central core’s RD-108 engine provided sea level thrust of 75 tons and included four Vernier engines utilized for steering.

The rocket had some key features to it that made it unique. Instead of using jet vanes for control, which increased resistance generated at the engine nozzle exhaust outlet, the R-7 used special control engines for steering. These same engines served as the last stage's vernier thrusters. Because of clustered design, each booster had its own propellant tanks. The design team had to develop a system to regulate the propellant component consumption ratio and to synchronize the consumption between the boosters.

Instead of a free-standing missile which was launched from a horizontal pad, it turned out that assembling a cluster of a central core and four boosters on the pad is almost impossible without it falling apart. Also, a wind gust could knock the unfuelled missile off of the pad. The solution was to eliminate the pad and to suspend the entire rocket in the trusses that bear both vertical weight load as well as horizontal wind forces. The launch system simulated flight conditions with strap-on boosters pushing the central core forward.

Testing
The existing testing site at Kapustin Yar was inadequate for the testing required for the R-7. Therefore a new site in Kazakhstan was approved on the 12 February 1955, that would eventually become the Baikonur Cosmodrome. The first series of test commenced when a flight-ready vehicle was delivered on 1 May 1957, and flown on 15 May. A fire broke out in one of the strap-on boosters almost immediately at liftoff. The missile broke away from the booster at T+88 seconds, which crashed 400 km (248 miles) downrange. The next attempt on 11 June an electrical short caused the missile to start rolling uncontrollably and disintegrate 33 seconds after liftoff.

The first successful long flight, of 6000 km, was made on 21 August 1957 with the missile reaching the target at Kamchatka, however the dummy warhead disintegrated in the upper atmosphere. Five days later, TASS announced that the Soviet Union had successfully tested the worlds’s first intercontinental ballistic missile. A second successful test took place on the 7 September 1957, with the missile travelling 6,000 km. However again the dummy warhead disintegrated in the upper atmosphere. The first series of tests concluded on the 10 July 1958.

The second test series incorporating improvements commenced on the 24 December 1958 and concluded on the 27 December 1959. Seven of the eight test launches were successful. Problems were resolved during these intensive tests and the missile was declared operational on the 20 January 1960.

Operational history
A modified version of the missile (8K71PS) launched the world’s first satellite into orbit when Sputnik 1 lifted off from Baikonur on 4 October 1957. Sputnik 2 followed on 3 November 1957. Sputnik (8A91) subsequently launched Sputnik 3 on 15 May 1958.

The first strategic-missile unit went on alert status on 15 December 1959 at Plesetsk in the north-west of the USSR. An improved version, the R-7A with a lighter warhead, all inertial guidance system and a range of 12,000 km, became the standard version once it reached operational status on the 12 September 1960.

The costs of the system were high, mostly due to the difficulty of constructing in remote areas the large launch sites required. Besides the cost, the missile system faced other operational challenges. With the U-2 overflights, the huge R-7 launch complexes could not be hidden and therefore could be expected to be destroyed quickly in any nuclear war. Also, the R-7 took almost twenty hours to prepare for launching, and it could not be left on alert for more than a day due to its cryogenic fuel system. Therefore, the Soviet force could not be kept on permanent alert and could have been subject to an air strike before launching. These issues meant that the original planned fifty launch complexes were reduced to six, five for strategic forces, Site 31 at Baikonur and Sites 16, 41 and 43 (2 pads) at Plesetsk and one for space launches at Site 1, Baikonur.

The limitations of the R-7 pushed the Soviet Union into rapidly developing second-generation missiles which would be more viable weapons systems, particularly the R-16. The R-7 was phased out of military service by mid 1968.

While the R-7 turned out to be impractical as a weapon, it became the basis for a series of Soviet expendable space launch vehicles, including Vostok family of launchers, Molniya and Soyuz family of launchers. As of 2018, in modified versions (Soyuz-U, Soyuz-FG, and the Soyuz-2 (including the boosterless 2.1v variant), the vehicle is still in service, having launched over 1,840 times. The R-7 is also a record holder in terms of longevity, with more than 50 years of service with its various modifications and has become the world’s most reliable space launcher.

Variants

 * SS-6 Sapwood: NATO reporting name for all versions of the R-7, variants identified by suffix letter on the name portion (e.g. Sapwood-A).
 * R-7 Semyorka: First launch 15 May 1957, last launch 27 February 1961.
 * R-7A Semyorka: First launch 23 December 1959, last launch 25 July 1967.
 * 8K71:The GRAU designation for the R-7 Semyorka missile (GRAU 8K: Missiles 71: model number)
 * 8K74:The GRAU designation for the R-7A Semyorka missile (GRAU 8K: Missiles 74: model number)
 * 8K71PS: Sputnik 1 launcher

Note: Some variants of the R-7 are still used:
 * Soyuz-2.1a	(14A14A)
 * Soyuz-2.1b	(14A14B)
 * Soyuz-2-1v (14A15)

Operators

 * The Strategic Missile Troops was the only operator of the Semyorka.