Mach number represents a flow condition.
Stall speed represents a dynamic pressure condition + geometry effect.
The flow condition is based on the compressibility which in turn is based on the ability for gas molecules to get out of the way which is based on how fast they are going which is based, in part, on the temperature of the gas. The compressibility is notable when going more than 80% of the local speed of sound. The lower the temp, the lower that speed is.
Dynamic pressure is based on the square of the speed and the density, usually by how deep one is in the ocean of atmosphere, but also from humidity and temperature.
To give a feel for this, at the pressure and temperature on the surface of Mars one might hardly notice a breeze blowing by at Mach 1. The actual speed is low so Mach 1 is low. But the density is so low that even at Mach 1, the dynamic pressure is low. It might not push an empty paper cup from a picnic table, were NASA able to put both there, hence the problem of getting fine dust off the solar panels.
The main intersection is that dynamic pressure is from the square of the speed times the density. When a shock wave forms the speed and density change. Per conservation of mass, density times velocity will be a constant. But notice the dynamic pressure is related to the square - if I am getting this right, the dynamic pressure drops as it passes through a shock wave. This means that whatever balances were in place before reaching Mach 1 will be out of balance when the shockwave forms.
This is not a stall problem but part of the function of a wing is to produce downwash; if the shockwave change of the dynamic pressure reduces that effect the horizontal stabilizer sees less downwash, produces less down force, and the nose tucks under, increasing descent rate, increasing speed, increasing the strength of the shockwave and increasing the chance of being aluminum parade confetti if the condition isn't dealt with.
It appears to me the coffin corner is the intersection between having too little airspeed to produce enough dynamic pressure and getting too much airspeed and having a drastic change in controllability. Look at the NASA Space Shuttle that re-enters at around Mach 40 (flow condition) and nearly no dynamic pressure, it can be a surmountable problem - as long as the rest of the maneuverability envelope becomes that of a bread loaf.
Last edited by MechEngr; 7th October 2023 at 22:21.