Unstart

showing unstart of axisymmetric intake at Mach 2. Unstarted shock structure evident on left, started intake on right. In supersonic aerodynamics, an unstart refers to a generally violent breakdown of the supersonic airflow. The phenomenon occurs when mass flow rate changes significantly within a duct. 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.

showing unstart of axisymmetric intake at Mach 2. Unstarted shock structure evident on left, started intake on right.]] In supersonic aerodynamics, an unstart refers to a generally violent breakdown of the supersonic airflow. The phenomenon occurs when mass flow rate changes significantly within a duct. 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.

Intentional

When an unstart occurred on the SR-71, a very large amount of drag from the unstarted nacelle caused extreme rolling/yawing. The aircraft had an automatic restart procedure which balanced the drag by unstarting the other intake. This intake had its own tremendous amount of drag, with the spike fully forward to capture the shock wave in front of the intake.

Avoidance

Decelerating from Mach 3 required a reduction of thrust which could unstart an intake with the reduced engine airflow. The SR-71 descent procedure used bypass flows to give unstart margin as the engine flow was reduced. Thrust reduction on the XB-70 was achieved by keeping the engine flow stable at 100% rpm even with idle selected with the throttle. This was known as "rpm lock-up" and thrust was reduced by increasing the nozzle area. The compressor speed was maintained until the aircraft had slowed to Mach 1.5.

Theoretical basis

Using a more theoretical definition, unstart is the supersonic choking phenomenon that occurs in ducts with an upstream mass flow greater than the downstream mass flow. Unsteady flow results as the mismatch in massflow cannot gradually propagate upstream in contrast to subsonic flow. Instead, in supersonic flow, the mismatch is carried forward behind a 'normal' or terminal shock wave that abruptly causes the gas flow to become subsonic. The resulting normal shock wave then propagates upstream at an effective acoustic velocity until the flow mismatch reaches equilibrium. There are other ways of conceptualizing unstart which can be helpful. Unstart can be alternatively thought of in terms of a decreasing stagnation pressure inside of a supersonic duct; whereby the upstream stagnation pressure is greater than the downstream stagnation pressure. Unstart is also the result of a decreasing throat size in supersonic ducts. That is the entrance throat is larger than the diffusing throat. This change in throat size gives rise to the decreasing mass flow which defines unstart. The choking reaction of unstart results in the formation of a shock wave inside of the duct.

Shock instability or buzz

Under certain conditions, the shock wave in front or within a duct may be unstable, and oscillate upstream and downstream especially when interacting with the boundary layer. This phenomenon is known as buzz. 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.

References

Text derived from https://en.wikipedia.org/w/index.php?oldid=1330899682 under CC BY-SA 4.0; modifications have been made.