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?
Speed limits are derived from q-bar limits. In the case of the Space Shuttle, it had a max limit of 819 psf, which equated to a maximum structural speed of 492 knots equivalent airspeed (KEAS) the mach transition equates to around Mach 1.5, at this point KEAS decreases as Mach increases.
The flight planning and thrust reductions kept the maximum speed on ascent below 475 KEAS again to minimize the maximum structural pressure to remain below 819 psf.
On descent, the maximum airspeed (KEAS) was set at 321 knots or a maximum pressure of 350 psf. "Redline" in that context was established at 333 KEAS for the Space Shuttle.
Minimum controllable airspeed (Speed that flight controls remain effective) is 163 KEAS and less than MACH 5.0.
L/D for the Shuttle occurred around 190-220 depending on MACH at the time. They always flew faster to retain energy reserves to extend the glide if needed.