The doppler effect: why is it that the first jet pilots to fly their planes couldnt break speed of sound?

Question

why did the first jet pilots to fly their planes near the speed of sound thought it might be impossible to break the speed of sound (approx. 760 mph)?

Answer 1

Mechanical instability.

The planes were poorly designed to operate at this speed, and as they got close the control surfaces became unstable and sometime unusable.  the planes would sometime go out of control, at least until they dropped back to slower speeds.

So at first they thought that it was a mechanical limitation.

BTW: whats that got to do with Doppler Effect?  Doppler effect is the apparent change in speed of a waveform (sound or electromagnetic) as you move towards or away from the source.