here's a good anecdote to add to that: Remember the MiG-25? well, the russkies built that thing to counter the B-1B (back when it was supposed to be supersonic/ high altitude bomber) and also to counter the XB-70 (yea, the original Valkyrie! kinda weird hunh?). well the problem they had with it was, they could do Mach 3.2, but only for a little bit, and, here's the kicker: they melted the engines doing so! so therefore, they could only do Mach 2.5 for a high continuous afterburner on that plane, another problem they had with those engines was, they were hugely fuel hungry, like the VF-0.