As others have already pointed out Paul, a jet engine is a reaction engine. It does not work by pushing the exhaust against the air outside the back of the engine. The greatest part of the thrust is reacted against the combustion chamber front wall as reaction to the increase in momentum imparted by the heating of the air, the second greatest part of thrust is created by the compressor. Together they create several times more thrust than the net thrust of the engine. The airflow aft of the combustion chamber does not produce thrust except in a small amount if you have the correctly designed divergent pipe, otherwise all flow aft of the combsution chamber front wall produces a rearward force that deducts from the forward force produced in the compressor and combustion chambers. Even an afterburner does not create a forward force as such, its effect is to reduce the rearward force in the jet pipe.
H.