For everyone, who mixing the MiG-21MF and the bis model's afterburner flames:
The MF's R-13-300 engine has longer but narrower app 9-10 shockwave rings, while the bis model's R-25-300 engine has a wider flames, 6-7 shockwaves in max AB
But the R-25-300 has the emergency, or "ChR" regime, when the flame lenght is going bigger.
Here, the soloist take off with ChR (which is lit on automatickly, after when the plane reach a specific speed), the flame reached its maximum shape right before the plane take off, and we cane see it in another scene, when the plane tooke a tight turne.
The three planes tooke off by max AB only.