The gas enthalpy change of the hot gas in the combustion chamber is numerically equal to the heat of formation. Using data from the table,
This is not identical to the 246 kJ/mol obtained previously, and therefore a lower temperature is to be tried. After one or two iterations the final combustion temperature of 2470 K will be found where the heat of reaction balances the enthalpy rise. The above-mentioned composition will be approximately the same at the new temperature. The molecular weight can then be obtained from Eq. 5-5:
_ E ntm _ 28 x 0.5 + 18 x 0.836 + 2 x 0.664 + 28 x 0.836 + 44 x 0.164
The specific heat varies with temperature, and average specific heat values cp can be obtained from each species by integrating p2470 - _ J298 Cpal LP — r2470 ,T J298 al
Values of cp can be obtained from tables in Ref. 5-7 and, if not done by computer, the integration can be done graphically. The result is cp= 41,440 kJ/K-kg-mol/20.3 = 2040 kJ/kg-K
The specific heat ratio is, from Eq. 5-7,
Cp-R' 41440 -8314
With 9Ji, k, and Tx now determined, the ideal performance of a nitromethane rocket engine can be established from Eqs. 3-16, 3-30, and 3-32 for />!=69atm and Pi = 1.0 atm. The results are c* = 1525m/sec CF = 1.57 (from Fig. 3-6) c= 1.57 x 1525 = 2394m/sec Is = 2394/9.80 = 244 sec
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