Unstart

Avoiding unstarts is a key objective in the design of the engine air intakes of supersonic aircraft that cruise at speeds in excess of Mach 2.2.

[1] The shock waves that develop during the starting or unstart process may be visualized with schlieren or shadowgraph optical techniques.

Other aircraft that have flown with internal compression include the Vought F-8 Crusader III, the SSM-N-9 Regulus II cruise missile[8] and the B-1 Lancer.

[9] Partial internal compression was considered for the Concorde (the Supersonic Transport Aircraft Committee, in 1959, had recommended an SST to cruise at Mach 2.2[10]) but an "external configuration was chosen for the inherent stability of its shock system, it had no unstart mode".

[13] Early in the development of the B-1 Lancer its mixed external/internal intake was changed to an external one, technically safer but with a small compromise in cruise speed.

Upstream or downstream disturbances, such as gusts/atmospheric temperature gradients and engine airflow changes, both intentional and unintentional(from surging), tend to cause the shock to be expelled almost instantaneously.

For example, the North American F-107 during flight at high speed experienced an engine surge which bent the intake ramps.

Thrust reduction on the XB-70 was achieved by keeping the engine flow stable at 100% rpm even with idle selected with the throttle.

The resulting normal shock wave then propagates upstream at an effective acoustic velocity until the flow mismatch reaches equilibrium.

This change in throat size gives rise to the decreasing mass flow which defines unstart.

[25] Stronger shock waves interacting with low momentum fluid or boundary layer tend to be unsteady and cause buzz.

Buzz conditions can cause structural dynamics-induced failure if adequate margins are not incorporated into design.