SR-71---ok, technically it does fly for hours on burner. But its afterburners are completely unique, as are the engines. The burner itself acts like the engine---it has its own air supply and compression, every other afterburner out there just dumps fuel into the turbine's exhaust. An SR-71's burner at Mach 3 is basically the ramjet itself, with the turbine section practically idling along just to keep the accessories going with most of its air supply bypassed to the burner. Blackbird burners at speed are doing far, far more than what your typical burner does--they're not even really operating on exhaust. They operate as normal burners sub-sonically, but once the engine starts acting like a ramjet---they're the main component.