The throat of a rocket nozzle is a critical part of its design, representing the narrowest section through which the high-speed exhaust gases pass. The throat is where the flow velocity reaches its maximum value. The design of the throat is crucial for controlling the flow of exhaust gases and optimizing the expansion of the gases to achieve the desired exit velocity and thrust. The specific shape and size of the throat, as well as the overall nozzle design, are critical for efficient propulsion.
The relationship between the cross-sectional areas and velocities at the Throat and exit for a Rocket engine can be described by the continuity equation:
where:
-  and are the cross-sectional areas at the nozzle Throat and exit, respectively,
-  and are the velocities at the nozzle Throat and exit, respectively.
which gives
where,
In this equation:
- is the cross-sectional area of the nozzle at the throat,
- is the velocity of the exhaust gases at the throat,
- is the cross-sectional area of the nozzle at the exit,
- is the velocity of the exhaust gases at the nozzle exit.