# Why would the ideal mass flow rate be less than that of real life?

Comparing the mass flow rate at the throat using the below theory, I find that the mass flow rate isn't accurate but isn't always larger than that of the engine specs (ideal assumptions ).

What factors can play a role? Having a list of assumptions of the theory, which assumptions could be affecting it? Here's the theoretical content of the theory.

• Those ideal assumptions yield, the smallest mass flow rate that achieves a given thrust level. But conditions are never ideal. – David Hammen Oct 15 '19 at 14:33

• Perfect and Calorically Ideal Gas: No gas in real life is perfect, but in the throat flow this is especially true. The gas is at high temperature and pressure (often 1000s of K, 100atm, but very engine dependent), and step changes (See the freeze-out section in this link) in the specific heat are common place. This changes the isentropic exponent $$\gamma$$ with temperature. Furthermore, as the gas does not obey the ideal gas law, the equations for mass flow at sonic conditions are inaccurate even disregarding changes in the isentropic exponent.
• Negligible Velocity in CC: Of course the velocity in the combustion chamber is non-zero. But its kinetic energy is extremely small compared to its thermal energy (Roughly a factor of $$10^6$$). Hence the $$\approx$$.