Subsonic gas flow (gas moving at less than the speed of sound in that gas) is expanded and accelerated by decreasing the flow area (e.g. a converging nozzle). As the rocket grain burns, the gas generated by combustion of the grain will start to flow through the nozzle once the pressure differential between the combustion chamber and the exhaust is sufficiently high. The velocity and mass flow of the gas exiting the nozzle will increase until the exhaust velocity is equal to the speed of sound. The minimum pressure ratio (exhaust pressure divided by combustion chamber pressure) across the nozzle that yields a sonic flow is called the critical pressure ratio. At pressure ratios above critical, the exhaust velocity will remain sonic and the mass flow rate can only be increased by an increase in combustion chamber pressure. The sonic velocity will always occur (if it occurs) in the minimum area section.

Supersonic gas flow is expanded and accelerated by increasing the flow area (e.g. diverging nozzle). By natural extension, connecting the inlet of a diverging nozzle to the exhaust of a converging nozzle yields a nozzle which can produce gas which can flow at supersonic velocities. These types of nozzles are known as De Laval nozzles. As gas flows through such a nozzle, sonic conditions are attained in the minimum diameter section of the nozzle. Downstream, a pressure will be reached at which the gas is completely expanded ideally within the nozzle and the exhaust velocity is supersonic. At lower downstream pressures, the exhaust velocity remains supersonic, but the gas is underexpanded in the divergent section of the nozzle; additional gas expansion takes place outside the nozzle.

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