A convergent-divergent nozzle, also known as a de Laval nozzle, is a nozzle design commonly used in rocket engines and supersonic aircraft to accelerate high-speed gases.
The mass flow rate through a convergent-divergent nozzle is the quantity of fluid passing through the nozzle per unit time. This nozzle design is particularly important for achieving supersonic flow, where the nozzle consists of a converging section followed by a diverging section. The converging part accelerates the flow to sonic speed, and the diverging section continues the acceleration to supersonic speeds.
The density () for an ideal gas is given by the ideal gas law: 
which gives us,
where,
- ṁ is the nozzle mass flow rate
- P is the pressure
- R is the gas constant for the working fluid
- T is the Temperature
- A is the nozzle throat are
- V is the velocity of the flow in terms of the local speed of sound