The relation between pressure and area ratios

The dependence of the pressure and ratio upon the area ratio is a function only of the ratio of the specific heats. Since, for a given propellant combination, the specific heat ratio does not vary greatly during the expansion process, the relation between the area and pressure expansion ratios takes a particularly simple form. 

The expansion ratio may be expressed, therefore, either in terms of the area ratio or the pressure ratio. It should be noted, however, that the pressure ratio rather than the area ratio determines the performance. 

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