Isolator Unstart
Unstart is a phenomenon that can affect all air-breathing engines. As a generic term, it means that the airflow into the engine is partially blocked or restricted in some other way. The main type of unstart that I'm interested in happens when the isolator shock train is longer than the isolator and prevents some of the air from entering the engine.
When operating in ram mode, a pressure rise occurs in the isolator to balance the back pressure caused by the subsonic combustion. As the amount of fuel burned increases, the shock train gets longer. Eventually the unstart margin (β) hits zero, and unstart occurs.
Ram-mode condition with weak shock train
Ram-mode condition with stronger shock train
Ram-mode condition demonstrating isolator unstart (β<0)
Isolator unstart has been known about for some time, but the details remain a little fuzzy. Of particular interest is the ability to determine the flight conditions at which it will occur.
Unstart margin plotted on flight corridor map for steady, level flight.
