Editing Compressible flow (section)

===Achieving supersonic flow===
As previously mentioned, in order for a flow to become supersonic, it must pass through a duct with a minimum area, or sonic throat. Additionally, an overall pressure ratio, P<sub>b</sub>/P<sub>t</sub>, of approximately 2 is needed to attain Mach 1. Once it has reached Mach 1, the flow at the throat is said to be ''[[choked flow|choked]]''. Because changes downstream can only move upstream at sonic speed, the mass flow through the nozzle cannot be affected by changes in downstream conditions after the flow is choked.