Remember, a jet is not a reaction motor such as a rocket; it has to push against something to function, (air) if that air is moving at the same speed as its efflux velocity then there is no effective thrust!
Paul
With the caveat that I am NOT an engineer, and with all due respect to Paul, I simply don't accept this premise. Could somebody help to enlighten (prove or disprove this premise) for my already over-extended brain? Thanks,