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RD-858

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RD-858
Credit: via Dietrich Haeseler

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Status: Hardware. Date: 1964-72. Thrust: 20.10 kN (4,519 lbf). Unfuelled mass: 53 kg (116 lb). Specific impulse: 315 s. Burn time: 470 s. Height: 1.09 m (3.57 ft). Diameter: 1.10 m (3.62 ft).

The RD-858 was the primary engine for the Soviet LK manned lunar lander. The propulsion cluster provided the variable thrust needed for a soft landing onto the lunar surface. The same engine was then restarted for lift-off from the moon and injection of the LK into a lunar elliptical orbit. Testing of the engine on the ground and in earth orbital tests confirmed the high reliability of the design.

The autonomous single-chamber two-mode double-start pump-fed engine featured deep thrust throttling using hypergolic propellants and used a gas generator scheme (using the main engine propellants, without generator gas afterburning). During the first run of the engine (lunar landing), maximum thrust was followed by deep thrust throttling. The engine restart for ascent used the full thrust mode without throttling. During 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 commands of the control system. The engine featured control systems for thrust and mixture ratio regulation. Transducers provided the data to control chamber pressure according to the operating mode.

The engine was designed for a total burn time of 470 seconds. This included up to 50 seconds in full-thrust mode during descent to the surface; followed by up to 100 seconds of deep-throttled firing during the soft landing; and then 350 seconds at full thrust during the ascent back to lunar orbit. 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

Throttled thrust(vac): 8.414 kN (1,892 lbf). Engine: 53 kg (116 lb). Chamber Pressure: 78.50 bar. Thrust to Weight Ratio: 38.67. Oxidizer to Fuel Ratio: 2.03.