Combustion Instability

If the process of rocket combustion is not controlled (by proper design), then combustion instabilities can occur which can very quickly cause excessive pressure vibration forces (which may break engine parts) or excessive heat transfer (which may melt thrust chamber parts). The aim is to prevent occurrence of this instability and to maintain reliable operation (see Ref. 9-8). Although much progress has been made in understanding and avoiding combustion instability, new rocket engines can still be plagued by it.

Table 9-2 lists the principal types of combustion vibrations encountered in liquid rocket thrust chambers (see Refs. 9-3 and 9-9). Admittedly, combustion in a liquid rocket is never perfectly smooth; some fluctuations of pressure, temperature, and velocity are always present. When these fluctuations interact with the natural frequencies of the propellant feed system (with and without vehicle structure) or the chamber acoustics, periodic superimposed oscillations, recognized as instability, occur. In normal rocket practice smooth combustion occurs when pressure fluctuations during steady operation do not exceed about ±5% of the mean chamber pressure. Combustion that gives greater pressure fluctuations at a chamber wall location which occur at completely random intervals is called rough combustion. Unstable combustion, or combustion instability, displays organized oscillations occurring at well-defined intervals with a pressure peak that may be maintained, may increase, or may die out. These periodic peaks, representing fairly large concentrations

TABLE 9-2. Principal Types of Combustion Instability

Type and Word Description

Frequency Range (Hz)

Cause Relationship

Low frequency, called chugging or feed system instability

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