**ρ(h) = ρ0 * exp(-curve * h/h_max) - ρ0 * exp(-curve)**

And this is what i found experimentally:

**v=53,7541*ρ^(-0,2457)**

v: velocity

ρ: air density

What this empirical formula says that if your rocket is at a point where air density equals to ρ, rocket will heat. For instance, lets say our rocket is at the height of 5 km. According to the air density formula, density at 5 km is 0,000944. We will use this density value in the empirical formula i found.

v=53,7541*0,000944^(-0,2457)=297,6m/s. So it means that my rocket will start heating at the velocity of 297,6m/s, if the rocket is 5 km above the ground level.