When I say "Why don't we just remove the nozzle?" I don't mean "why not get rid of the exit hole". If we consider a typical jet pipe I mean "why not just leave it parallel"?
We have already established the fact that the thrust is caused by the increased pressure acting on the aft facing surfaces of the chamber.
The forward thrust is the Newton3 reaction to the rearward force that we have applied to the air in the jet pipe. It is the increased pressure in the jet pipe that is producing both the rearward acceleration of the air and the forward thrust. The increased pressure is tending to accelerate the air rearwards and accelerate the engine forwards.
The fact that the air gets accelerated rearward is (in a sense) just a by-product of the process.
The fact that we can use the eqaution T = mA to calculate the magnitude of the thrust does not mean that the acceleration actually causes the thrust. We could just as easily use the same equation to calculate the acceleration, but that does not mean that the thrust caused the acceleration, nor that the thrust and acceleration caused the mass.
I am not suggesting that getting rid of the nozzle would increase the thrust. It would not.
What I am doing is inviting readers to think about why we need the nozzle.
If
A. The thrust force acts on the front of the chamber (in a simple rocket).
In a jet pipe it acts on the exhaust unit (which is just behind the rear
turbine), and on the rear face of the rear turbine.
B. The force on the nozzle acts rearwards.
Logically we can say that the resultant forward thrust is A minus B
This suggests that having the nozzle actually reduces the resultant thrust.
Which brings us to the real question.
Why don't we just remove the nozzle and get rid of the rearward acting force?
The above results suggest that this will give us more resultant thrust.