From the ideal gas law, pressure within the chamber may be expressed as
Introducing the term G relating the fraction of igniter charge burned at any time t, and considering that the initial chamber pressure is ambient
letting ("effective force") gives
which may be rewritten as
letting (loading fraction) gives
multiply numerator and denominator by r (charge density)
since , this gives
where:
P = pressure in combustion chamber at time, t lbf/in2
r= density of charge material lbm/in3
D = loading density = C / V lbm/in3 C = original mass of charge lbm V = free volume of combustion chamber at time, t in3
R = universal gas constant in-lbf /° R-lbm
M = effective molecular weight of combustion products (system mass divided by number of moles of gas) lbm / mole
T = adiabatic flame temperature ° R
G = fraction of original charge mass consumed by time, t dimensionless
Pa = atmospheric pressure lbf/in2
Note that the assumption is made that combustion of the igniter charge occurs rapidly with no pressure loss due to gas seepage through the nozzle opening.