Voluminous results of these machine calculations are available and only a few samples are indicated here to illustrate typical effects of the variations of various parameters. In general, high specific impulse or high values of c* can be obtained if the average molecular weight of the reaction products is low (usually this implies a formulation rich in hydrogen) or if the available chemical energy (heat of reaction) is large, which means high combustion temperatures (see Eq. 3-16).
Values of calculated specific impulse will be higher than those obtained from firing actual propellants in rocket units. In practice it has been found that the experimental values are, in general, 3 to 12% lower than those calculated by the method explained in this chapter. Because the nozzle inefficiencies explained in Chapter 3 must be considered, only a portion of this correction (perhaps 1 to 4%) is due to combustion inefficiencies.
Figures 5-1 to 5-6 indicate the results of performance calculations for the liquid propellant combination, liquid oxygen-RP-1. These data are taken from
chamber to throat area ratio A\/A, = 1.
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