Thank you again to both of you for your sound answers. This subject is a lot clearer now.
I would like to move on to another topic : the propulsive efficiency of turbojet engines. Here is a quote from Wikipedia, which basically says the same as my Oxford ATP books :
For all
airbreathing jet engines the
propulsive efficiency (essentially
energy efficiency) is highest when the engine emits an exhaust jet at a speed that is as close as possible to the vehicle velocity. The exact formula for air-breathing engines as given in the literature,
[2][3] is
η p = 2 1 + c v {\displaystyle \eta _{p}={\frac {2}{1+{\frac {c}{v}}}}}
where
c is the exhaust speed, and
v is the speed of the aircraft.
We can derive that the propulsive efficiency will be 1 if the speed of the accelerated air that exits the engine equals the speed of the aircraft, right ?
But how can a jet engine create thrust if it doesn't accelerate the air faster that the speed at which it is flying ? Intaking air at 450 kts and exhausting it at 450 kts won't create any action/reaction effect...