THE RD-0120 ENGINE

The RD-0120 (also labelled 11D122), developed at the Chemical Automatics Design Bureau (KBKhA) in Voronezh, was a LOX/LH2 engine with a vacuum thrust of 190 tons and a vacuum specific impulse of 454 s. It was a staged combustion cycle engine in which the gases from the gas generator are cycled back into the main combustion chamber for complete combustion. The propellants first passed through low-pressure turbopumps (“boost pumps’’) that boosted the pressure significantly to prevent cavitation of the main turbopump assembly. The low-pressure hydrogen pump used a gas turbine driven by gaseous hydrogen from the main chamber cooling loop. The low-pressure oxygen pump had a hydraulic turbine powered by liquid oxygen.

Each RD-0120 had a single-shaft turbopump consisting of a two-stage axial turbine, a three-stage hydrogen pump, and two oxygen pumps. One of the oxidizer pumps was intended to feed the main combustion chamber and the other to feed the gas generator and the hydraulic turbine of the low-pressure oxygen pump. The 32,500 rpm turbopump was driven by a single fuel-rich preburner operating at 530°C.

During ascent the RD-0120 could be gimbaled plus or minus 11 degrees in pitch and yaw to help steer the rocket. Each engine was gimbaled with the help of two hydraulic servoactuators developed by KB Saturn (the “Lyulka bureau”), with hydraulic pressure being provided by pumps driven by high-pressure hydrogen gas from the engine itself. The engine could be throttled over a range of 45 to 100 percent, a significantly higher throttlability than the Space Shuttle Main Engine (SSME) (67-104%). The pneumatic control system included pressure helium bottles, pneumatic and electro-pneumatic valves, and system piping.

Nominal burn duration was between 450 and 500 seconds, although this could be significantly extended in case one engine had to compensate for the loss of another. The RD-0120 engines for the first three flightworthy Energiya rockets (6SL, 1L, and 2L) were certified for a total burn time of 1,670 seconds (230 seconds for test firings, 480 seconds for the launch, and 960 seconds back-up capability). This was increased to 2,000 seconds for flight vehicle 3L. Theoretically, this meant the engine could be reused for about three to four missions, although they were of course destroyed on re-entry together with the core stage. However, there were plans to certify the engines for 10 to 20 missions for reusable versions of Energiya and for possible use on foreign reusable launch vehicles. It was also planned to gradually uprate the engine, increas­ing the vacuum thrust to 230 tons and the vacuum specific impulse to 460.5 s. One of the modifications would have been the inclusion of an extendable nozzle to prevent loss of specific impulse in vacuum conditions.

Although the RD-0120 was built to the same overall performance specifications as the SSME, it certainly was not a copy of the SSME, differing from it in several important aspects. Also, the Russians could draw on their extensive experience with staged-combustion cycle engines used on the Proton rocket and various inter­continental missiles. While the RD-0120 had a single turbopump assembly both for liquid oxygen and hydrogen, the SSME has separate turbopumps for each propellant. Soviet engineers did consider a similar scheme, but opted for the single turbopump system because it simplified the computer control system and the ignition sequence. The RD-0120 had a channel-wall nozzle with fewer parts and welds than the SSME nozzle and was therefore easier to manufacture. In the late 1990s NASA even considered building a similar nozzle for the SSME, eliminating the tubular construction as a potential source of nozzle leaks. The new nozzle was also expected to have a higher degree of reusability [2].