Encyclopedia Astronautica

Rocketdyne MON/UDMH rocket engine. 5.635 kN. Satellite maneuvering motor. Developed 2005. Isp=292s. New technology motor with improved thrust/weight ratio and use of mixed oxides of nitrogen oxidiser with a much lower freezing point than N2O4.

New technology motor with a quantum leap in thrust/weight ratio and use of mixed oxides of nitrogen (MON) oxidiser which has a much lower freezing point than standard nitrogen tetroxide (MON-25, 75% N2O4 and 25% nitric acid, had a freezing point of -50 deg C versus -12 deg C for N2O4 alone). The engine has a thrust to weight of 290, compared to 40 for the Marquardt R-40A in the same class. A key development problem was getting MON-25 to burn without catastrophic combustion instability. The engine uses a fuel-rich mixture ratio and is film and radiatively cooled. The engine can be throttle from 50 to 115% of design thrust. The engine also had excellent ramp-up and minimum pulse characteristics, important for military applications. A version of the motor using a composite nozzle and thrust chamber was built, demonstrating a T/W ratio of 600, but it was found to be too fragile for typical applications.

Application: Satellite maneuvering motor.


Throttled thrust(vac): 250.000 kN (56,200 lbf). Engine: 2.00 kg (4.40 lb). Chamber Pressure: 34.00 bar. Area Ratio: 12. Propellant Formulation: MON-25/MMH. Thrust to Weight Ratio: 290. Restarts: 1000.

AKA: Multi-Use Thruster.
Status: Developed 2005.
Unfuelled mass: 2.00 kg (4.40 lb).
Height: 0.22 m (0.71 ft).
Diameter: 0.12 m (0.39 ft).
Thrust: 5.64 kN (1,267 lbf).
Specific impulse: 292 s.
First Launch: 2005.

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Associated Propellants
  • MON/UDMH Mixed Oxides of Nitrogen - Nitric oxide (NO) is a low-boiling cryogenic gas. Both the liquid and the solid are blue. Solutions of NO in nitrogen tetroxide sharply depress the freezing point of the high-melting oxidiser. The mechanism of depression is believed to involve the formation of N2O3, which is soluble in nitrogen tetroxide. Solutions are called mixed oxides of nitrogen (MON), and have been used as oxidisers for liquid-rocket engines. Various concentrations have been considered. However, the high vapour pressure of MON limits the concentration of NO in N2O4 to about 30 per cent. Aside from the high vapour pressure of MON, the material is quite similar to nitrogen tetroxide. Unsymmetrical Dimethylhydrazine ((CH3)2NNH2) became the storable liquid fuel of choice by the mid-1950's. Development of UDMH in the Soviet Union began in 1949. It is used in virtually all storable liquid rocket engines except for some orbital manoeuvring engines in the United States, where MMH has been preferred due to a slightly higher density and performance. More...

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