An innovative partition model of liquid rocket engine combustion chambers is developed in this paper

which divides combustion chambers into two regions.One region is named as combustion region in which propellant changes into combustion gas instantaneously after constant time lag

and the combustion gas instantaneously and uniformly mixes with the staying gas of the region.The other is flow region

in connection with nozzle being treated as ideal gas pipe and being modeled by employing the finite element state variable model of one dimensional ideal gas flow.An example using the model to calculate the start up process of a liquid rocket engine is given

and the calculated results show that the model describes the thrusters' transients well.The model is expressed in the form of state variable equations

therefore it has simple expressions and can be solved easily.It is suitable for the simulation and control of liquid rocket engines.