RD-864

The RD-864 (, GRAU index: 15D177) is a Soviet liquid-fuel rocket engine burning a hypergolic mixture of unsymmetrical dimethylhydrazine (UDMH) fuel with dinitrogen tetroxide () oxidizer 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 index: 15A18) and Dnepr. For the R-36M2 (GRAU index: 15A18M), an improved version, the RD-869 (GRAU index: 15D300) was developed.

==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.