Mach 6 is over 4400 miles per hour. This is the Lockheed Martin SR-72, which is still in the conceptual stage.
It is intended to replace the SR-71 Blackbird, the famous spy plane that was a workhorse for the intelligence community until satellites took over the load. The SR-71 could outrun missiles and none was ever shot down. The SR-71 could go 2200 miles per hour.
This aircraft will need new kinds of sensors and weapons to operate at that speed.
Best informed people say the concept will test fly in 2025.
I want one.