Uncovering the Mechanics of Thrust in a Turbojet Engine

[tsimon]

Hi! I have a question about how thrust is made from a turbojet engine.

The most common explanation goes something like "As the fluid gets accelerated there is a net force". I don't think this explanation is either good nor rigorous.

The force that accelerates the fluid is, of course, the same force that is the thrust but where does that force come from and where does it operate?

Is the thrust equal to the static pressure after the turbine minus the lower static pressure before the compressor? I don't think this is right, but it is my only guess.

Cheers, Simon.

 

[AlephZero]

Hi! I have a question about how thrust is made from a turbojet engine.

The most common explanation goes something like "As the fluid gets accelerated there is a net force". I don't think this explanation is either good nor rigorous.

You might argue about whether it is good English, but the basic phyiscs is right.

Force = rate of change of momentum. The air has a higher momentum when it leaves the engine than when it enters it. In other words, since the mass of the air doesn't change, its veloicty is higher when it leaves.

To increase the air velocity you have to supply energy, which comes from burning the fuel.

In principle you can calculate the thrust by adding up the priessure distribution over the whole surface of the engine casing (taking account of the different cross section areas of different parts, of course), but that is not the CAUSE of the thrust, it is just the way that the thrust is transmitted from the air into the structure of the engine, and then (through the engine mountings) into the plane.

 

[tsimon]

It is definitely not wrong, altough it doesn't explain where the pressures that gives the thrust are "operating" or how the pressure is created.