Take the 2-minute tour ×
Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It's 100% free.

As the pressure ratio increases in an ideal turbojet (fixed flight Mach number), the specific thrust, $\frac{F}{\dot{m_{air}}}$ rises, reaches a peak for small pressure ratios and then starts to decrease with increasing pressure ratios in the compressor.

It seems counter-intuitive to assume that the specific thrust should decrease with increasing compressor pressure ratio since we're generating a greater pressure gradient, therefore should be able to generate more thrust.

share|improve this question

1 Answer 1

up vote 2 down vote accepted

Increased compressor pressure ratio means increased temperature in inlet of the turbine.
Turbine has limits $T_{max}$ on inlet temperature (turbine could melt). To keep temperature in operational limits the fuel/air mass ratio $f$ is lowered at high pressure ratios.

Lowered fuel/air mass ratio $f$ means that less chemical energy is released per mass unit of the air.

share|improve this answer
follow up: Do you know why specific fuel consumption has a minimum if losses are considered? –  l3win Nov 20 '13 at 6:05

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.