Its not rocket science...or is it?
 

We all once sat in a classroom and learned that a rocket engine is a reaction motor, relying on the reaction of expelled exhaust mass to create an opposite reaction force (Newton 2). We also learned that a turbine creates thrust in exactly the same way (reaction from exhaust action). Simple.
We then sat in another classroom, learning about propellors and helo rotors. These things use rhos, vee squareds and angles of attack to create lift and thus "pull" the aircraft forward/upwards. Great.
Then along came high bypass turbofans - ah, still a reaction motor. Then someone took the bypass ducting off - so is it a prop or a reaction motor....? How do those air molecules know which equations to follow? (obviously, they follow both)
Is it not so that all of these should be regarded as reaction motors, with a propellor just being a type of unducted fan?
Sorry, this may be an old chestnut? Simple comment from the propulsion experts out there would be appreciated. Thanks

Rocket motor?...... Isn't that a bit like eating a can of beans?? Sorry, better ask an expert. :8

Aha, now apply NASA's lift theory stuff to the fan or prop and realise how little you really do know about propulsion - just when you were coming to terms with realising you had no idea about lift!

Yep, it IS rocket science...

Is it science or nature? Where did the idea of 'lift' or for an airfoil/wing come from.....

Everything comes from observing nature in the end, we just don't undertand how nature 'works'. :)

Not quite. The wikipedia definition of rocket engine is helpful.

Rocket engines take their reaction mass from within the vehicle and form it into a high speed jet, obtaining thrust in accordance with Newton's third law. (ny bold)

A turbojet or turbofan starts by sucking in a great deal of the reaction mass it pushes out the back. Thus it's different to a rocket.

If only is were rocket science, I might stand a chance then.

I await the day that most irritating of rhetorical questions is changed to "Well, it isn't exactly lift science, so what's the problem?"

Thrust is achieved by accelerating a mass.

If you were in a boat with a water tank, you could propel it analogous to a rocket by filling a bucket with water only from the tank and pitching the water over the stern. When you fill your bucket instead from the water outside the boat, the analogy becomes an air breathing engine.

The thrust does not come from the turbine; in fact, the turbine extracts energy from the hot air to power the compressor and optionally a fan, propeller or rotor which produces the majority of thrust.

I don't think the OP was suggesting that the turbine as such produces thrust, merely that a turbine engine works by reaction.

Not quite true for helicopters: they don't generate lift but they are so ugly that the ground repels them.

http://en.wikipedia.org/wiki/Jet_engine

Or try reading The Jet Engine by Rolls-Royce.

Slightly off thread but... we were watching one of the space shuttle landings when the Station Engineer was heard to say, "Yeah, but it isn't rocket science is it?", at which point we all collapsed in a heap!!

Anything (and/or anybody for that matter) that (who) moves in the universe does so as a reaction to an action, be it a spaceship, a submarine, an helicopter, a row boat, a car or you walking. Indeed, walking consist of propelling the planet earth backwards with the feet, the reaction to it being that one moves forward. Obviously, the differences in masses makes the resulting movement of the earth imperceptible. More practically, for a movement to happen, there must be a balance of energy between the two masses in opposite directions: Mass x Velocity = Mass' x Velovity' (Velocity being speed AND direction as it is a vector).
Whether using a "turbo" jet (fanned or not), a rocket or a propeller engine, all three systems of propulsion "propell" a mass of air backwards (and downwards for choppers and VTOLs) and move forward as a reacting result.
Propellers move a huge mass at relatively low speed, turbo jets a small mass a very high speed, big fans are in between, and rockets carry their own mass of gas to propell.
The basic equation is the same for all modes of propulsion, however insufficient for operational and efficient use; hence, for equal amount of air mass propoelled, the propeller engines have a (more or less) constant "power" at all speeds, and the jet engines a (really more or less) constant "thrust" at all speeds.

For nitpicking physicists, the baxic equation is more like
dM/dT.dV2/dT/2 = dM'/dT.dV'2/dT/2 (half the vectorial product of differential of mass by differential of velocity squared, in relation to time), but we are not here to discourage young aviators from learning about what they are doing, no?
One can notice that dV2/T is dV.V/T, and V/T is an acceleration. In space, as long as there is thrust, there is acceleration, and the velocity increases indefinitely, until reaching the speed of light, at which point the mass is totally transformed in energy. Inside the planet, the acceleration stops when counteracted by the drag of water (boats, subs) or gas (aircraft), and the velocity becomes constant for a constant amount of thrust.

There was a thread a year or more ago re whether Bernoulli's theorem, or "equal & opposite reaction" of downwash (or propwash) were responsible for airfoil lift.

The answer is: you can't have one without the other. :8

Turbine or piston, jet, fan or prop; the idea is still the same: you move air one way, you go the other. It's just a matter of how to move the air, and that portion I think you already know. Sometimes things get harder the more we think about them :)

I was just about to start a new thread with a question that has interested me for some time, when I thought I had better do a search first :)

So this seems to be the perfect place to ask my question.

In a turbojet, which parts of the engine does the thrust reaction force act on and in roughly what percentage?

To put it another way, which parts of the engine transfer the thrust reaction force to the engine mountings and thence to the airframe?

The bits that go round and round push the bits that don't.

For example, if you look at NASA's nice picture on engine pressures (thanks Bernd on the BA038 thread) you'll see there are different pressures on either side of the bits that go round and round and where there are different pressures the thingie in between those different pressures gets pushed.

PBL

Well, to be fair he was correct - no rocket motors are used in the landing phase of a space shuttle - it's all "gliding science" at this stage.

sorry - couldn't help myself.

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Simple ... but the things that get pushed are not all in the forward direction and that's why most folks only talk about the exhaust when in fact it's the summation of the forward and the aft pushies :)

As PBL said the spinny bits of the compressor are 'pushing' air backwards and according to newtons third law, the compressor blades want to go forward. It's then a case of the knee bone is connected to the shin bone.

Compressor(or fan/prop) blade pushes air back, blade goes forward, blade connected to disc(hub), disc connected to shaft, shaft connected to 'thrust' bearing, thrust bearing to bearing support, bearing support to engine case, engine case to engine mount, mount to aircraft, aircraft therefore goes forward.

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Exactly, compressors provide 'forward thrust' , turbines provide 'negative thrust' ie they are 'pulling back', exhaust cones provide forward thrust due to convergent duct. (let's not start on re-heat) As lomapaseo says, the 'sum' of these forces is the "thrust" of the engine.

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