mike - I'm not sure whether you took my advice to 'search' but there are so many threads where this has been discussed it would take forever to cut and paste the links! I would, however, endorse a look at one thread,
http://www.pprune.org/forums/showthr...ighlight=mcrit which goes some way to answer your initial query and in which our friend Old Smokey gives you an Mcrit (for the DC9). Unfortunately his helpful drag-rise graphs will not display now, but I'm sure we could encourage him to 'refresh' the link?
I think that describing Tornado as a 'supersonic aircraft' in the context of this discussion is misleading, unless of course its 'econ cruise' is supersonic, which I doubt. I know only of the Lightning where our 'econ cruise' was 0.9M, and was, I suspect, close to Mcrit if not slightly above. Cruising above Mach 1 is very 'fuel intensive', but there is often another supersonic 'window' for cruise where acceptable drag rise is traded against operational needs - M1.3 I seem to recall?
As you will see from the discussions on the various posts/threads on this topic, 'MCrit' is of little relevance in modern airliners and is more a concern for performance guys/girls in calculating econ cruise and in any failure cases of stab augmentation devices. It merely represents the point at which shockwave formation SOMEWHERE on the aircraft begins and there may well be subsequent effects on tail controls/pitch or roll trim and drag rise depending on where the shock/s form/s. Drag certainly increases above it on 'normal' aircraft, but modern aircraft design delays the critical drag rise (which is another definition).