The RD-864 (Russian: Ракетный Двигатель-864, romanized: Raketnyy Dvigatel-864, lit.
'Rocket Engine 864', GRAU index: 15D177) is a Soviet liquid-fuel rocket engine burning a hypergolic mixture of unsymmetrical dimethylhydrazine (UDMH) fuel with dinitrogen tetroxide (N2O4) oxidizer in a gas generator combustion cycle.
[1] It has a four combustion chambers that provide thrust vector control by gimbaling each nozzle in a single axis ±55°.
[12] So, while the production of the RD-864 has long since been finished, the engine is still to this day operational.
It had improved efficiency, restart capability and burn life over the RD-864.