Mc2
Increase the mass (using compression i.e.take a mass of
air and compress into a small space) to increase power output.
In a jet engine airmass accelleration is caused by heating the air in
combustion chambers.This action (accelleration) causes an equal reaction
(thrust).
To use only ram air,which I think the questioner is thinking of, there'd be
a relatively low mass, hence low thrust. A hypersonic engine, on the other
hand, relies on ram air, but requires very high velocities to compress
before heating.
This is the way I remember and understand how it works.
