RD-864

The RD-864 (GRAU: 15D177) is a Soviet liquid propellant rocket engine burning UDMH and nitrogen tetroxide in a gas generator combustion cycle. It has a four combustion chambers that provide thrust vector control by gimbaling each nozzle in a single axis ±55°. It is used on the third stage of the R-36M UTTKh (GRAU: 15A18) and Dnepr. For the R-36M2 (GRAU: 15A18M), an improved version, the RD-869 (GRAU: 15D300) was developed. The MEA is an evolution of the RD-869 used on the Vega C.

History
When the Soviet military developed an improved version of the R-36M ICBM, Yangel's OKB-586 developed a new engine for the third stage, the RD-864. Developed between 1976 and 1978 it flew for the first time on October 31, 1977. With the START I and START II the some 150 R-36M and R-36M UTTKh were retired and to be destroyed by 2007. So, a civilian application was looked for and during the 1990s, Yuzhnoe Design Bureau (the R-36M designer) successfully developed the Dnepr launch vehicle. It flew for the first time on April 21, 1999 and as of June 2016 it is still operational. So, while the production of the RD-864 has long since been finished, the engine is still to this day operational.

The RD-869 was an improved version for the most powerful Soviet ICBM ever, the R-36M2 (15A18M). It had improved efficiency, restart capability and burn life over the RD-864. As of January 2016 there are still 46 operational R-36M2 (RS-20V, SS-18) and thus the RD-869 is still in service, if out of production.

Versions
There are two versions of this engine:
 * RD-864 (GRAU Index: 15D177): First developed as the third stage engine for the R-36M UTTKh (15A18) ICBM and, by extension, on the Dnepr launch vehicle.
 * RD-869 (GRAU Index: 15D300): An improved version of the RD-864. It has improved efficiency, restart capability and burn life. It is used on the R-36M2 (15A18M).