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R-6: a missing link in the Soviet rocketry
A quarter of a century after opening to the world, the Soviet rocket genealogy still had no entry for the R-6 designation, even though the previous R-5 and subsequent R-7 designs were well documented. Nevertheless, multiple sources that have emerged since the 1990s enable us to add an important missing piece of the puzzle illustrating the history of space launchers carrying cosmonauts and astronauts into space until this day.
A scale model likely depicting the R-6 rocket. One-chamber engines shown on the model probably represent RD-105/106 designs (left) or their derivatives. According to Russian sources, this version of the rocket would have a liftoff mass of 170 tons and have five (or four) individual launch platforms for each booster stage. (527) Copyright © 2011 Anatoly Zak
Roots of R-7
The considerable progress in the development of long-range ballistic missiles achieved during the decade after World War II demonstrated to the Soviet leadership the potential advantage of rockets over aviation as carriers of atomic bombs. For the USSR, rockets increasingly looked as a potential counterbalance to the overwhelming advantage of American air power in the Cold War. Moreover, if the range of rockets could be increased, the US territory would end up within reach of Soviet nuclear weapons for the first time. By 1953, even leaders of the aviation industry admitted that "the possibility exists that in the next 7-10 years... rockets of all types will become the main type of weapon and will be able largely replace fighter planes, bombers and long-range artillery." (473)
That same year, the Kremlin gave the go ahead to what would become an intercontinental ballistic missile project, ironically, in one of the last acts of Stalin's cabinet. The Soviet of Ministers' Decree No. 443-213, signed by Stalin on February 13, authorized "theoretical and experimental studies providing the development of a guided two-stage ballistic missile..." No designation of the missile was mentioned in the document, but it listed following technical specifications for the future rocket:
According to the decree, the project would have to be completed with the production of seven experimental rockets in the first quarter of 1955, followed by their flight tests during the same year. Not surprisingly, the document assigned the prime development of the project to NII-88 in Podlipki near Moscow, which had previously built the R-1, R-2 and R-5 missiles and had also worked on the R-3. The document reconfirmed Sergei Korolev as the chief designer, Vasily Mishin and Konstantin Bushuev as his deputies and Mikhail Yangel as the director of NII-88.
The same decree also gave NII-88 an assignment to develop a two-stage cruise missile with the same capabilities, something which had also been previously studied by the organization. That part of the work would have to result in the production of eight scaled-down experimental multistage rockets with a launch mass of six or seven tons and a flight range of 700 kilometers, in the first quarter of 1954 and their flight tests in the second quarter of 1954. (473)
The new assignment escalated the previous theoretical studies at NII-88 and other centers on long-range missiles into a full-scale development effort. Since 1950, the OKB-1 design bureau at NII-88 had been conducting a study code-named N-3, which had looked at possible designs of long-range rockets capable of carrying warheads with masses from 1 to 10 tons to distances from 5,000 to 10,000 kilometers.
Following the February 1953 decree, OKB-1 superseded the N-3 study with a development project code-named T-1, from "Tema" (theme) No. 1. Reflecting the expansion of the work on the ballistic missile, several new departments were formed within OKB-1 to focus on the task. To meet the required specifications, the intercontinental rocket was expected to have a launch mass of 170 tons. (52) For comparison, the largest existing Soviet ballistic missile at the time -- the R-5 -- weighed just under 29 tons! Thus, the proposed new rocket represented a major technological leap.
From the outset, the intercontinental missile was conceived as a two-stage rocket because it was the only practical way to achieve the required flight range. However, the ignition of the second stage during flight presented a serious technical obstacle at the time. After considering up to 60 designs during 1953, engineers chose a "parallel" architecture, clustering short boosters of the first stage around a much taller core booster. (18) Some called it a one-and-a-half-stage rocket, because all five boosters would ignite on the pad, but the second stage (also known as a sustainer stage) would continue firing, after the four boosters of the first stage had separated.
According to original plans, the first-stage boosters of the huge missile would be equipped with a "classic" rocket engine -- RD-105 (8D56) -- featuring a single combustion chamber and developing a thrust of around 60 tons. A similar engine on the second stage, designated RD-106 (8D60), would have a longer nozzle for better performance in vacuum. (145)
The OKB-456 design bureau led by Valentin Glushko started work on RD-105/106 engines in 1952. Within the bureau, Vladimir Kremenetsky, who had just become de-facto head of the design team, oversaw the project. For the first time in the Soviet rocket industry, an engine would burn a mix of liquid oxygen and kerosene fuel, finally replacing the alcohol fuel inherited from the German A-4 (V-2) missile. The engine also employed new materials such as chromed bronze. (424)
In October 1953, Vechaslav Malyshev, who oversaw the Soviet atomic project, asked OKB-1 to re-tailor the already oversized rocket to accommodate a three-ton hydrogen charge, bringing the total mass of its warhead to 5.5 tons. At the same time, the rocket was still required to fly its full distance to reach the US. However, with the heavier payload the flight range of the proposed missile fell to 5,500 kilometers. (52) Ironically, for Korolev, it was likely a welcome demand, because he got an excuse to build an even more powerful space booster. The question remained how to do it.
One solution was to further scale up the rocket's engines. However, propulsion engineers were facing seemingly insurmountable problems in controlling the combustion process inside such a large volume due to high-frequency pulsations. Alternatively, it was possible to split the engine into several smaller combustion chambers while keeping a common main turbopump for all chambers. Korolev had already singled out a four-chamber engine as promising for the ICBM project back in August 1952, when he had signed his conclusions for the N-3 study. (84) Moreover, a propulsion design bureau led by Aleksei Isaev had successfully developed a multi-chamber rocket engine reaching a thrust of 40 tons. (173)
In January 1954, key leaders in the ICBM project, including Sergei Korolev, Vladimir Barmin and Valentin Glushko, met to discuss the situation. It was decided to switch to a four-chamber engine on the first ICBM. The combined thrust of the new propulsion system would grow from 60 to 75 tons. Although the number of combustion chambers on the rocket had multiplied from five to 20, Korolev's team endorsed the solution and agreed to accommodate the much more complex engine design. (424) Still, the biggest challenge was facing propulsion engineers at OKB-456, who had to ensure that all multiple chambers and engines could work according to a carefully choreographed sequence. (750)
As an added benefit, the engine's combustion chamber and the nozzle became shorter, thus allowing to cut the length and mass of the entire rocket. Apparently, because the overall design of the future rocket was affected, it received a new designation -- the R-7. (749) Given the fact that the previous Soviet long-range missile was designated R-5, it is possible to assume that the original designation was the R-6.
Read much more about the history of the Russian space program in a richly illustrated, large-format glossy edition:
The RD-105 engine originally intended for the first ICBM. Copyright © 2002 Anatoly Zak
An early version of the Soviet ICBM, possibly designated R-6, represents a "missing link" in the Soviet rocket genealogy. Copyright © 2011 Anatoly Zak