RD-107 / RD-108 engines

Design Background

In 1952–1954, Glushko developed the single-chamber RD-105 (Stage 1) and RD-106 (Stage 2) for the R-7, each targeting 68 tons of thrust, but could not overcome combustion instability in large chambers. In 1954 he adopted a more conservative approach: a cluster design in which a single turbopump feeds four smaller combustion chambers (each 43 cm in diameter). The solution combined Glushko's own multi-chamber experiments from the 1930s with the technical legacy of the German V-2.

Technical Features

The RD-107 and RD-108 use a gas-generator cycle, with a steam turbine driven by the catalytic decomposition of hydrogen peroxide (H₂O₂) over a solid catalyst. The propellants are refined kerosene (RG-1 or T-1) and liquid oxygen. Each engine has four fixed main combustion chambers; the RD-107 has two vernier chambers for attitude control, the RD-108 has four for full vector control of the core stage. Kerosene regeneratively cools the nozzle walls before entering the injector plate. Chamber pressure is 60 atmospheres and combustion temperature reaches approximately 3,250°C.

Innovation: Variable Mixture Ratio

A key innovation was the active mixture-ratio control system. Manufacturing variations between engines meant that without active control, each booster could deplete propellant at a different rate, leaving tonnes of unused fuel or oxidizer and creating severe mass imbalance. The system ensured simultaneous propellant depletion across all four boosters.

Operational History

First flight: May 15, 1957, on the R-7 Semyorka ICBM. Subsequently used on virtually every R-7-derived launch vehicle: Sputnik (1957), Luna, Vostok (carrying Yuri Gagarin in 1961), Voskhod, Molniya, and Soyuz. Successive upgrades include the RD-107K (8D74K), RD-107MM (8D728), RD-117 (11D511), and RD-107A (14D22). The RD-107A/108A, introduced in 2001, replaced 260 two-component centrifugal injectors with over 1,000 single-propellant injectors, increasing specific impulse by about 5 seconds. As of 2024, chemically ignited 14D22KhZ/14D21KhZ variants power Soyuz-2.1a and Soyuz-2.1b. With over 1,800 launches, this is the most-flown liquid rocket engine family in history.

Production

Manufactured at JSC Kuznetsov plant in Samara (formerly Factory No. 24) under the supervision of the Privolzhsky branch of NPO Energomash. In 2019, the first RD-107A/RD-108A engines using the new naphthyl fuel successfully completed testing.