The design, development, and operation of liquid rocket engines requires efficient stable burning of the propellants and the generation of a high-temperature, uniform gas that is the rocket's working fluid. In this chapter we treat the complex phenomena of the combustion processes in the combustion chamber of a liquid bipropellant thrust chamber. We describe in general terms the combustion behavior, the progress in analysis of combustion, the several types of combustion instability with its undesirable effects, and semiempirical remedies. The objective is to operate at very high combustion efficiencies and to prevent the occurrence of combustion instability. Thrust chambers should operate with stable combustion over a wide range of operating conditions. For a treatment of these subjects see Refs. 9-1 to 9-3.
The combustion of liquid propellants is very efficient in well-designed thrust chambers, precombustion chambers, or gas generators. Efficiencies of 95 to 99.5% are typical compared to turbojets or furnaces, which can range from 50 to 97%. This is due to the very high reaction rates at the high combustion temperatures and the thorough mixing of fuel and oxidizer reaction species by means of good injection distribution and gas turbulence. The losses are due to incomplete burning or inadequate mixing (nonuniform mixing ratio). For very small bipropellant thrust chambers, where the injector has very few injection orifices or elements, the combustion efficiency can be well below 95%.
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