The cross-sectional area of the throat for a solid propellant rocket nozzle is a critical parameter in the design of the nozzle and plays a significant role in determining the performance of the rocket motor. Solid propellant rocket motors have a different design compared to liquid rocket engines, as the propellant is preloaded into the motor casing in a solid form.
The design of the throat involves considerations such as optimizing the flow of exhaust gases and ensuring proper combustion and expansion. The throat is where the flow velocity reaches its maximum value. The relationship between the cross-sectional areas and velocities at the throat and exit of a rocket nozzle can be described by the rocket 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:
- Â is the cross-sectional area of the nozzle at the throat,
- Â is the velocity of the exhaust gases at the throat,
- Â is the cross-sectional area of the nozzle at the exit,
- Â is the velocity of the exhaust gases at the nozzle exit.