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And no wind tunnel can produce Mach 3 flows. Test pilots had to slowly expand the flight envelope of prototype aircraft.

Check this out https://www1.grc.nasa.gov/facilities/htf/ Back in the day they could test up to Mach 7. Not it is a test bed for hybrid electric planes.

Supersonic wind tunnels have existed since the 1920s and Mach 3+ versions were built in Germany during World War 2 for the V2 project. See https://en.wikipedia.org/wiki/Supersonic_wind_tunnel