Considering the formula for thrust under "choked" nozzle conditions: Thrust =(P-P0 )A + WVj /g
Where: P = Pressure P = Ambient Pressure A = Area W = Mass Flow V = Velocity
It can be seen that the thrust can be further affected by a change in the mass flow rate of air through the engine and by a change in jet velocity. An increase in mass airflow may be obtained by using water injection to cool the air and increases in jet velocity by using after-burning.
Changes in ambient pressure and temperature considerably influence the thrust of the engine. This is because of the way they affect the air density and hence the mass of air entering the engine for a given engine rotational speed.
Thrust Correction - Turbojet
To enable the performance of similar engines to be compared when operating under different climatic conditions, or at different altitudes, correction factors must be applied to the calculations to return the observed values to those which would be found under I.S.A. conditions. For example, the thrust correction for a turbo-jet engine is:
Thrust (lb) (corrected) = thrust (lb) (observed) x 30/Po • Where P0 = atmospheric pressure in inches of mercury (in Hg) (observed)
30 = I.S.A. standard sea level pressure (in Hg)