SESKStuff Everyone Should Know
SESK: Stuff Everyone Should Know · sesk.info/aero-nozzle-throat-mach-1

In a choked rocket nozzle, how fast is exhaust moving at the narrow throat?

At the speed of sound, Mach 1. It goes supersonic after the throat.

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  1. Mach 1, the speed of sound ✔
  2. Almost stopped
  3. Its very top speed, faster than anywhere else

Why: Gas speeds up as it squeezes into the throat and reaches exactly Mach 1 there. In the widening section after it, the supersonic gas keeps accelerating, so the fastest flow leaves the nozzle exit.

Huh, didn't know that: Supersonic gas does the opposite of a garden hose: it speeds up when the passage gets wider.

The dad joke
Why did the exhaust gas break the sound barrier? It was tired of being heard and not seen.
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References

Answer: Mach 1, the speed of sound. Gas speeds up as it squeezes into the throat and reaches exactly Mach 1 there. In the widening section after it, the supersonic gas keeps accelerating, so the fastest flow leaves the nozzle exit.

  1. NASA Glenn, Nozzle Design www1.grc.nasa.gov/beginners-guide-to-aeronautics/nozzle-design
    The flow in the throat is sonic which means the Mach number is equal to one in the throat.
    Checked 2026-10-10.

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