Hi there,

I wonder if someone could help to consolidate my knowledge regarding how a jet engine creates propulsion.

Every piece of text that I have read on the subject regurgitates Newton's second and third laws as explanations for the production of thrust ('f = ma' and 'every action has an equal and opposite reaction'). In the simplest terms, shooting air rearwards imparts a force (thrust) on the engine which is equal and opposite to the force that shot the air backwards in the first instance.

This explanation is fine for a propeller or the outer part of a turbofan disc that propels bypass air whereby something hard is physically exerting a force on the air to accelerate it. My problem is that the core of a jet engine essentially accelerates air through combustion and expansion - I'm having difficulty in defining the 'action' and locating the (presumably co-incident) points of action and reaction within the gas generator.


D P Davis' 'Handling the Big Jets' states that "the thrust of an aircraft propulsive system is the reaction to the force required to accelerate a mass of air through the system, and is manifested as pressure forces . . . . . on all internal surfaces in the case of a jet engine". This is the only reference that I've seen which actually says that components within the engine experience a physical propulsive force. This helps my understanding to a degree but it also raises two questions:
Why does force on all internal components equate to thrust? Shouldn't only the forces that act on the internal surfaces that are perpendicular to the engine's longitudinal axis be of significance?
Shouldn't only the components that provide the mystery force that accelerates the air experience the reaction force? If so, we go back to the conundrum of defining the 'action' and relating this to combustion and expansion.
I'd be very grateful if somebody could shed some light on this for me.

Many thanks and best regards to you all.

This isn't definitive but it is my take on the process:

Imagine a sealed cylinder containg a fuel air mix at ambient pressure. The mixture is ignited and the contents of the cylinder are now at much higher pressure. This pressure is exerted evenly on the inside of the container in every direction. So no net thrust.

Now imagine one end of the cylinder is suddenly removed. In the instant it takes the pressure to equalise, the pressure on the opposite (remaining) end of the cylinder has no balancing force at the other (now missing) end. Therefore the cylinder has net thrust in one direction.

A jet engine manages to maintain this state of a pressurised container that is open at one end. It does this by virtue of having a compressor instead of the closed end.

Logically the thrust-force must be applied at each rotor/stator stage because it is across these stages that the pressure differential is. So ultimately, in a sense, similar to propeller or fan thrust.

This view satisfies my own "Newtonian" instincts. No doubt it will have aerodynamycists screaming in outrage at their computer screen, and thermodynamycists will sob quietly whilst banging their foreheads gently on the desk.

But for the rest of us - perfect! Thank you.

Thanks for the reply DHU.

Your analogy of the jet engine being a pressurised cylinder with an opening at one end was helpful.

Your analogy of the jet engine being a pressurised cylinder with an opening at one end was helpful.

Well, in fact René Lorin did just that in 1908. He had imagined a row of several piston engines with special exhaust nozzles, as you can see on the right page:

Lorin's article in the journal "L'Aérophile" (http://www.archive.org/stream/larophile16besa#page/n547/mode/2up)

bravobravo74
In case you are still on the lookout for any more info the book 'The Jet Engine' by Rolls-Royce, has a chapter on thrust distribution and shows that the thrust on each component, eg compressor, combuster, final nozzle, etc, comes from 2 main components, a pressure force due to static pressure difference and a momentum force due to change in axial momentum.
All the contributions feed either through the rotor thrust bearings or directly into the fixed structure.
The calculation above does a whole component as one, eg a complete LP turbine, rotor blades and fixed nozzles, whereas as far as the engine is concerned the loads are split up and the blade thrust is netted with the fan thrust before going through the thrust bearing.
Even when there is no energy transfer there is still a thrust as in ducts and final nozzles of course. With ducts their function is to slow down the air to reduce pressure losses so there is still a difference in static pressure force and momentum between entry and exit.