Mig21 uses burner to take off and initial climb to altitude.
They fight and attack in Military.

Burner to get out, FAST, or to flee from a fight. => At altitude, nose down, and in burner they accelerate like hell on fire in a dive.

The ONLY aircraft that can outaccelerate them in a full burner dive is the good old F-104. (When clean, no external stores.)

Nothing else comes close.

Poser 1, 2, 3, 4, 5, 7, P8 and PPro2010, P9 and PP2012, P10 and PP2014 Game Dev
"Do not drive faster then your angel can fly"!