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If the sonic boom is an issue, why don't we build airliners that are capable of flying just below Mach 1? Big airliners fly at Mach 0.85 at most, and the record for current sub-/transonic civilian aircraft is the Cessna Citation X+'s Mach 0.935. Why don't we build airliners that cruise at Mach 0.95? Also, what would happen if the Citation X+ tried to reach or surpass Mach 1, since it's anyway close? There it isn't answered.

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That is because as you approach the transonic regime, the drag on the airframe begins to rise and can reach up to ten times the subsonic drag. after you go supersonic, the drag falls down again- and this makes the transonic regime the worst place to operate a plane from a fuel burn standpoint. So you either stay subsonic, operating below the drag rise point, or supersonic, comfortably beyond that point.

Another consideration is this: Even if you are subsonic but nearly supersonic, there will be points on the airframe where the airflow is locally supersonic. You can actually see this in Boeing's long-distance planes, where a cabin seat at roughly mid-chord affords you a view out the window of the normal shock sticking up from the wing at high-speed cruise. If the flow goes supersonic at one of the control surfaces, it may upset the controllability of the plane, which would be undesirable- so better to stay comfortably out of the transonic regime for cruise.

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