Theory behind RPA

RPA is a program which calculates chemical equilibrium product concentrations from specified set of propellant components, determines thermodynamic properties for the product mixture, and calculates theoretical as well as estimated test (actual) nozzle performance.

The method used for obtaining equilibrium compositions is the minimization of Gibbs free energy. Applied to the combustion chamber, the method allows:

• to determine the equilibrium product concentrations from adiabatic, isenthalpic combustion of two or more reactants
• to determine the equilibrium product concentrations from monopropellant decomposition
• to calculate the isentropic quasi-one-dimensional nozzle flow for both shifting and frozen equilibrium flow models

To obtain the rocket performance, the tool calculates conditions at several sections of the chamber. It always includes the calculation of combustion (injector section) and nozzle throat parameters, as well as nozzle exit parameters, defined by either nozzle exit pressure pe, expansion pressure ratio pt/pe, or expansion area ratio Ae/At.

The user can force the program to calculate the performance with respect of pressure drop between injector and nozzle inlet, defining such parameters as a chamber mass flux or a nozzle inlet contraction area ratio Ac/At.

Default nozzle flow model is a shifting equilibrium: combustion products continue to react and reach chemical equilibrium at each temperature and pressure conditions along the nozzle.

The user can trigger the "freezing" of nozzle flow composition downstream of the throat. In this case, it is assumed that composition is "frozen" (infinitely slow reaction rates) during expansion along the nozzle. The location of "freezing" nozzle section is defined by either pressure ratio pt/pfr, or area ratio Afr/At.

In case of overxpanded nozzle flow, the tool calculates the performance with respect of flow separation.

The complete equation set as well as iteration procedure described in paper [1] which is based on work done by Gordon, S. and McBride, B.J. [2].

References

1. RPA: Design Tool for Liquid Rocket Engine Analysis  (Preview)
2. Gordon, S. and McBride, B.J. Computer Program for Calculation of Complex Chemical Equilibrium Compositions and Applications I. Analysis  (Preview)
   NASA Reference Publication 1311, Oct. 1994.

Last modified: May 9, 2012