What altitude does mach# become meaningless?
 

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?

I’d say once you leave the atmosphere completely. Sound doesn’t travel in the vacuum of space so there would be no Mach number.

Meaningless in what respect? In terms of using external control surfaces? More a function of air density than actual Mach number. The higher you go the more irrelevant Mach is.

The NF-104 had rocket thrusters because it went high enough the rudder and ailerons ceased to work.

https://youtu.be/TwpJ7fIDmCI

It was used when you went over 75,000 feet IIRC. Without them it was kind of like running out of speed in a rolling uphill scissors only more so. You could move the controls but the aircraft was basically in ballistic flight because the density of the air flowing over the control surfaces was inadequate to provide any meaningful control.

The RCS starts about 45 seconds in to the video.

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.

So does this not happen until reaching the Karman line (300K I think)? Or is it where the practical end is, perhaps at around 100K?

I'd love to know what IAS M25 is during reentry!

About 17,500 mph typically for low earth orbit.

I meant indicated airspeed, not true.

Calibrated airspeed is about 8500kts at 100,000 feet. That was the closest I could come. Any higher was out of range for the calculator. Barometric pressure at that altitude is down to 0.329 inHg. Indicated airspeed outside the atmosphere would be zero. Nothing coming in the pitot tube.

https://i.ibb.co/93fPSDX/A6-C426-DC-...-CC3-A1-CD.jpg

Max Q is probably the most import factor.
The max q condition is the point when an aerospace vehicle's atmospheric flight reaches maximum dynamic pressure. This is a significant factor in the design of such vehicles because the aerodynamic structural load on them is proportional to dynamic pressure. This may impose limits on the vehicle's flight envelope.

Pretty sure once you’ve gone plaid, no more need for a Mach meter.