Originally Posted by
Bahamasflyer
Just watched the launch of SpaceX and it got me wondering? What altitude does one have to reach in order for mach # to become meaningless? Is it a sudden thing, or gradual?
Gradual, and it becomes meaningless when the atmosphere becomes so thin as to have negligible affect on vehicle performance.
On re-entry, the shuttle started measuring mach at M 25. Nope, didn't forget the decimal.