Ah yes. Some years ago I worked some preliminary design stuff on such an engine - Low-bypass fan, twin spools, variable shutters to gradually close down core airflow and divert more to the afterburner, so the machine migrated toward a ramjet system. Similar in principle to what
OldBUFFkeeper describes.
Very complex, but should have performed well past Mach 3. It could even be used as a reusable orbital-insertion booster vehicle with fly-home capability. NASA might be interested if they only had some $$$.
