Misusing “Mach 1”

November 2019

I enjoy Aerospace America each month for all the interesting articles, but this month’s cover [October 2019] shocked me with your statement about “Mach 1” passenger jets. Whoever wrote that ought to get a brief education on supersonic flight.

No aircraft company would build a jet designed to cruise right at Mach 1. In the transonic speed range (from roughly the high Mach 0.8s to around 1.1), there are many areas of mixed subsonic and supersonic air flow over the airframe, and following the localized supersonic flow there are shock waves. These shocks tend to be unsteady because of the shock/boundary layer interactions, and they tend to dance around over the control surfaces. If you stay at this speed, it can be a bumpy ride, so in supersonic flight operations, the aircraft prefers to punch through the transonic regime, and the ride gets smoother and the drag decreases once you’re fully supersonic. I’m sure you’ve heard that the supersonic aircraft in development right now are all designed to cruise at Mach 1.4 or above. If you’re going to go through the trouble of making an aircraft have supersonic capability, one would not stay at Mach 1. I would have just called them supersonic passenger jets on your cover.

Don Durston
AIAA senior member
Sunnyvale, California

Editor’s note: Our economy of words failed us in this case. We indeed understand that supersonic passenger jets would fly faster than Mach 1.