Note 1
Potassium Nitrate - Xylitol (KNXY) propellant
65/35 O/F ratio @ 1000 psia chamber pressure
From PROPEP results, for 100 grams mixture:
The effective Molecular Weight is given by dividing the number GAS moles into the system mass. Since the system mass is 100 grams:
The mass fraction of condensed phase is given by the mass of the condensed phase (K2CO3) divided by the system mass
The MW of K2CO3 = 138.21 g/mole, thus
Note 2
KNXY 65/35 O/F ratio @ 1000 psia chamber pressure
Mole fractions and mass fractions for each combustion product are calculated in the table below:
The table below shows the computation of k, the ratio of specific heats:
The values for Cp and Cs are taken from the JANAF Thermochemical Tables and NIST Chemistry WebBook. Units of Cp and Cs are J/mol-K.
The Cp for the gas only products and mixture (gas+condensed) is given by
where ni is the number of moles of gas component i , ns the number of moles of condensed component, n the total number of gas moles. The ratio of specific heats for the mixture, for the gas-only, and for two-phase flow is given by
where 
= 8.314 J/mol-K (universal gas constant).
where y = X /(1-X).
Note that k for two-phase (gas+condensed) flow is a modified form of the gas-only k'. This is the correct form of k to use in the thermodynamic equations involving products with a significant fraction of condensed-phase particles. The value of k given in the PROPEP output (Cp/Cv) is for the mixture.
with
To = 1623 K
M = 39.3 kg/kmol
k = 1.138 Note: k for the mixture is the proper value to use, as c* represents a static condition
= 8314 J/kmol-K
this gives c* = 921 m/s (3022 ft/s).
The propellant specific impulse is given by the effective exhaust velocity divided by g.
with
To = 1623 K
M = 39.3 kg/kmol
k = 1.043 Note: k for 2-phase flow is the proper value to use, as Isp involves two-phase flow.
Thus, Isp = 166 sec.
for standard conditions of Po = 68 atm. (1000 psia) and Pe = 1 atm., and g = 9.806 m/s
(maximum theoretical, assumes frozen equilibrium, and no particle velocity lag or thermal lag).