The RD-120 (GRAU Index 11D123) is a liquid upper stage rocket engine burning RG-1 (refined kerosene) and LOX in an oxidizer rich staged combustion cycle with an O/F ratio of 2.6. It is used in the second stage of the Zenit family of launch vehicles. It has a single, fixed combustion chamber and thus on the Zenit it is paired with the RD-8 vernier engine. The engine has been developed from 1976 to 1985 by NPO Energomash with V.P. Radovsky leading the development. It is manufactured by, among others, Yuzhmash in Ukraine. It should not be confused with the RD-0120, which is a discontinued LOX/hydrogen rocket engine that was used in the Soviet Energia launch system. History During the Buran programme initial development of the 11D77 —the launch vehicle later known as Zenit—, KBKhA had been assigned the development of the second stage engine, as they had done for the Proton and Soyuz vehicles. But given the difficulties for NPO Energomash in developing the RD-123 (which would later be known as the RD-170), they ceded the hydrogen / oxygen sustainer engine development to KBKhA. This project, the analog of the SSME, was project RD-130 within NPO Energomash. But when KBKhA tackled the development, named it RD-0120, a name that is always a source of confusion with the engine of the current article. In exchange for them tackling the difficult development of the cryogenic propellant engine, Energomash assumed the responsibility of developing the second stage engine of the 11D77, which would eventually be known as the RD-120. The fact that the RD-120 and the RD-0120 had this intertwined conception, within the same program, and with a swap of designer bureaus, does not help to avoid the confusion. On March 16, 1976 the Government passed a resolution for the development of Zenit, the RD-171 and RD-120. By April 1976, Yuzhnoye supplied NPO Energomash with the final requirements for the 11D77 first and second stage propulsion. One of the desirable effects of consolidating the first and second stage propulsion on the same designer, was that they could learn their lessons on staged combustion engines on the smaller and simpler upper stage engine, and then apply them to the bigger and more innovative first stage RD-170. NPO Energomash, had already worked on a prototype kerosene / oxygen staged combustion engine in that range, based on the RD-268 hypergolic engine, which was already under serial