The RD-859 engine was used as the backup engine for the Soviet LK manned lunar lander in the event the primary RD-858 engine failed during landing on the lunar surface or ascent and return to lunar orbit. The engine was capable of a restart within three seconds after shut down. Testing of the engine on the ground and in earth orbital tests confirmed the high reliability of the design.
The autonomous twin-chamber single-mode double-start pump-fed engine featured deep thrust throttling using hypergolic propellants and used an open generator scheme (using the main engine propellants). During engine startup the turbine's rotor was spun up by a pyrotechnic device. The inlet valves were opened using gas tapped from the manifold of the pyrotechnic device. The valves were operated by fuel pressure delivered from the pump's discharge branch. Engine shutdown was performed by pyrovalves under command of the control system.
The engine was designed for a total burn time of 400 seconds. This included as little as 3 seconds in full-thrust mode during the first burn; followed by up to 400 seconds of full thrust during the second firing. The full engine installation on the LK consisted of the RD-858 main engine and the RD-859 back-up engine:
Engine RD-858 RD-859 No of chambers: 1 2 Operating regime nominal throttled Thrust in vacuum 2050 kg 858 kg 2045 kg 20104 N 8414 N 20055 N spec. vac. impulse 315 s 285 s 312 s 3089 Ns/kg 2795 Ns/kg 3060 Ns/kg Mixture ratio O/F 2.03 1.6 2.0 Thrust variation +/- 9.8 % +/- 35 % +/- 9.8 % Mixture ratio deviation 3 % 3 % Chamber pressure 80 kg/cm^2 33.8 kg/cm^2 80 kg/cm^2 78.5 bar 33.15 bar 78.5 bar Nozzle exit pressure 0.07 kg/cm^2 0.021 kg/cm^2 0.07 kg/cm^2 Operation time 50 + 350 s 100 s 400 s Propellants N2O4 / UDMH N2O4 / UDMH Engine mass 53 kg 57 kg Propulsion system height 1090 mm Propulsion system width 1102 mm
Application: LK lunar lander reserve engine.
Chambers: 2. Engine: 57 kg (125 lb). Chamber Pressure: 78.50 bar. Thrust to Weight Ratio: 35.87. Oxidizer to Fuel Ratio: 2.
Unfuelled mass: 57 kg (125 lb).
Height: 1.09 m (3.57 ft).
Diameter: 1.10 m (3.62 ft).
Thrust: 20.05 kN (4,507 lbf).
Specific impulse: 312 s.
Burn time: 400 s.
First Launch: 1964-72.