Air does not get hot because it is hot. It gets hot because you stopped it. Drag the Mach number and watch what the skin has to survive.
Heating goes as the square of Mach number, so every increment of speed costs more than the last. The bands are what vehicles were actually built from, not certified limits — see the caveat at the bottom.
A thin skin equilibrates in seconds. The time constant is linear in thickness, so the thing that buys you a brief excursion is not the skin — it is the mass behind it.
parameter-free ambient, stagnation and recovery temperature follow from the US Standard Atmosphere 1976, γ = 1.4 and Pr = 0.71 — nothing fitted. illustrative the equilibrium and the time curves need a convective coefficient, taken as 120 W·m⁻²·K⁻¹ with emissivity 0.85 and Ti-6Al-4V bulk properties. The shapes and the scaling are the claims; the exact minutes are not. Material bands mark what the X-15 and the Blackbirds were made of, which is not the same as a design allowable — high-temperature strength depends on how long you hold the load, not only on the temperature.