I'm trying to do a computational fluid dynamics test on this rocket engine chamber and nozzle, but for some reason whatever I set my outlet environmental pressure to be, it makes the whole chamber that pressure, which is not true with an inlet mass flow of 4.1lb/s and an initial condition of 450PSI and 6766 DEG Rankine. I don't think it's relevant to helping me, but the working fluids are 70% Liquid Oxygen and 30% Liquid Propane.

Initial Fluid parameters

Inlet Fluid Parameters

Outlet Fluid Parameters

Flow Pressure Results

Any help would be greatly appreciated, I've followed many tutorials to the tee yet I haven't been able to produce sensible results.