I know most of you probably already know the answer to this, but it escapes me, so could somebody please enlighten me. I understand what after burning is, its the dumping of raw fuel directly into the hot tail pipe, the raw fuel ignites and almost doubles the thrust of the engine, but how exactly does the igniting of the raw fuel create so much extra thrust? is it the displacement of volume from the expanding gas, or is it because the rise in temperature is so great that it sucks air into the front of the engine faster, or both, or something else? I'm trying to wrap my mind around the actual physics of it...

Well, it will be custom here on this board to start with "I am no expert".

Now that that has been done.

I tell you that everything about propulsion is to accelerate ***mass*** to create enough of a counter-reacting force in the other direction. This can be achieved even in outer-space! You have a tube, and you do something to cause some mass to accelerate out of the pipe/tube. And that is the basics ...

After-burner can only improve thrust if it can do that BEFORE the gases leave the pipe. My view is null or near-null effect is caused by any fuel or burning of fuel done AFTER the gases leave the pipe.

And so that is why the AB is not outside the pipe.

There are several stages at physical distances inside the pipe. and the AB gradually heats up the gases more and more. And so without AB just a weak shimmering of light. With full AB a-l-o-t of light. light sorta being an indicator of temperature/energy.

full AB sorta heats up the surrounding metal if the engine pipe and definitely the tail-pipe .... not good if you have heatseekers after you ... long distance heatseekers you might have time to cool the pipe down but short range .... no no.

If i remember right initial ignition is slightly slightly slightly aft of the mid-section of the engine but I guess it all depends on how long the compressor is .... but after that there is a long section where you can have those ..... AB-rings .... don't know what they are called in english ..... and I don't remember again .... but 3 - 5 of them .... I guess ....

hmm ... your question .... the hotter the gases the more they expand .... but the "walls" of the engine "confine them" except in one direction ..... the opening of the tail pipe .... so .... since the air is hotter and "more furious" it accelerates faster out of the pipe since it is sooo damn HOT, faster than if it was just heated less .....

5 Kg accelerated in one direction at 10 m/s can accelerate 10 Kg at 5 m/s in the opposite direction if I am not mistaken, long time since I went to school ... but the basic principle of accelerating rockets in space or jet aircraft is to partially set some mass in motion in one direction and the rest of the aircraft moving in the other ... but i am wondering if there isn't more to it than that inside an atmosphere ... in outer-space it is clean and simple ... but in the atmosphere .... sorry .... starting to be unsure .....

Nope. There really isn't any difference inside the atmosphere. The only difference lies in how the aircrafts powerplant gets the air and exhaust moving.

So then it (is ) as simple as I (kinda) said
It's the displacement of volume from the expanding gas.
I guess the Brit's put it best when they refereed to it as (re-heat) because its essentially a second combustion chamber, an (external) combustion chamber if you will. Its inside the tail pipe, but outside the engine performing exactly the same function except the air mixing with the fresh fuel to be burned is not pre-compressed, although the unburnt gas exiting the engine is still in the process of expanding.

Thanx Solo

the compressor blades on civilian airliners since decades are so huge/long that all air that is compressed does not go through the engine but pass-by on outside to the fan today works like a propeller almost, at least at low speeds, but the basics are there, accelerate one mass aft. But still I am unsure, there is a theretical maximum speed any propelller can propell a vehicle and I think it is somewhere 700 maybe 800 kph. So question is if a civilian airliner can do 900kph ... I think they can ... so that type of engine definitely needs an atmosphere to grab/hoist/accelerate backwards. OK OK I google:

Boeing 777 max speed: 930-940 kph

Maximum theoretical speed for propeller: 845 kph (Wiki)

So that confuses me how the fan turbine can be efficient at that speed.

Searching for IAS seems to point at 0.84 Mach cruising speed at 35000 ft .... but is that IAS? Why add altitude if it is IAS?

Seems it is IAS and max speed is 0.86 Mach.......that's fast ... so looking at some huge fans some aircraft have you wonder why they are not "in the way" when you are up at cruising altitude and speed .... seems a venturi tunnel is needed in the bypass section since 0.84 Mach = 890 kph, 0.86 Mach = 910 kph according to a pilot is max, on another board they say it is 0.88 = 930 kph which fits better with Boeings own data above.

Alt: 0m Speed Of Sound: 1225 kph
Alt: 11000m Speed Of Sound: 1060 kph
Alt: 30000m Speed Of Sound: 1085 kph

seems the fans should have trouble being efficient at all and that the engine more sort of collides head on with the air and scoops it up from wider to narrower. But on the backside a very wide engine there should be a lot of drag ....

hmmm .... I am having trouble finding any info on the compressor itself ... whats behind the compressor is not so complex to understand .... I would like speed figures on the compressor ... which blades are angled for operation at high rpm / the cruise speed at altitude, but having problem seeing how they can be helpfull at that speed ...

if the turbine is a compressor and similar to a propeller then I imagine it could work like this that the air pass by at 1000 kph, the compressor is only able to swallow/transport air at 800 kph, the heatup of the air and the shape of the bypass tunnels cause the air to leave at higher speed than 1000 kph, so you get a positive net effect even if the compressor is slow.

But still I would like figures if they are out there on the net .... continue to search ....

military jet engine compressors in the area ~ 20:1 ratio. so thrust can be provided but the question I have that remains is at what air-speed the first stage can swallow air .....

they seems to have an rpm of 10000 - 15000 ... isn't that high? must be the high compressor stages then ... seems there are 10+ stages ... 1 low compressor + 5+ medium compressor + 5+ high compressor stages, seems there are also very small jet engines running at 100000 rpm and above ....

no the question remains does the jet engine swallow air at a lower speed than the speed of the passing air at high speeds .... I guess it does .... someone please help me with this ...

( also I picked up that yes .... it is different inside the atmosphere, a slow moving aircraft jet engine dissipates/looses energy in the exhaust, and it looses less at cruise speed, has something to do with the differnce between the airspeed of the gases coming out of the pipe and the air passing by on the outside, at cruise speed something about gases streaming out better but I didn't understand how it worked )

Well this is the reason I posted the question in the first place, it's all very confusing. I'm a fairly intelligent person, but I don't have a PHD in physics either, and the reason I was confused about how the after burner works (physics wise) is because it seems to me, that although the benefits of the after burner are obviously higher than the penalties, the biggest penalty being fuel consumption, it would seem to me that all that stuff going on directly at the exit of the engines fan blades, would slightly degrade the efficiency of the engine itself. To put it another way, if you don't look at the thrust gain, but rather just the performance of the engine itself, it probably runs better with out the after burner engaged. The after burner is a rather crude and thirsty way of gaining lots of thrust. I know with the SR-71 Blackbird, the engine itself got in the way at higher speeds, so they bypassed the engine with large tubes and dumped the high speed air directly into the tail pipe and mixed and burned there, more or less a (Ram-Jet) This also eliminates the need to slow the incoming air to subsonic speed before entering the jet engine at high mach, they simply bypass the engine altogether. Thanks for the feed back, even if I am still a little confused.

my last post was me adding questions .... I know the SR71 has a different type of engine the way you describe. It is an interesting subject. The airframe + IAS combination can be simplified. It is the engines and their performance curve that dominates the entire equation. I got some ISBN numbers, there seems to be a Shaw-type book for jet engines if I understood it right. I think I am hooked now, I read about the Su-27 airframe and others about airliners, but being a sailor and shaping the sails and when you build hulls and race and avoiding turbulence in the wake the air flowing around an airplanes lifting areas was not hard to understand since I've been racing with sailboats since age 12. I think it is time for me now to do something about this black hole knowledge wise when it comes to hwo jet engines exactly works ...

Why use AB. Well, any ace/hunter will tell u if both are aces then it is the small differences in hardware that counts. You don't build a fighter without an AB cause the enemy will have one with an AB. As you say the AB is thirsty. I am not checking any book now (which I ought to) but AB don't give you 100% more thrust but maybe 30% (unsure)(gain being important tactics wise but waste is huge) and fuel consumption I guess is several hundreds of percent higher (just quick thinking full AB you may last a couple of minutes, Mil power you can last for hours .... what does that make in percent ).

And also at extreme altitudes the AB might be just that that makes it possible to operate at those altitudes and speeds.

So here we have a good link:

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

and one can read about several types and there are links pointing further in different directions.

Still it is weird on airliners there is no inlet almost the air hits the first stage directly. Still they fly at 900 kph.

I'm sorry, maybe my first post wasn't as clear as it could have been. I fully understand the necessity of an after burner, no modern fighter jet could hope to engage and maneuver against another fighter unless it had the added thrust of an after burner. I was just not clear on all the physical interactions between the engine and after burner (the physics aspect of it all) It's kind of like person A asking the question "what makes a plane fly?
and person B saying "the wings" and although that's a correct answer, the answer I was looking for would be - if you looked at the cross section of a wing, you would see that the upper surface has much more curvature than the lower surface, this means that the air flowing over the upper surface must accelerate faster than the air passing under the wing in order to cover the same distance in the same amount of time, this creates low pressure over the wing, or (lift) and the physics of this lift sucks the wing & plane up into the low pressure area created by the shape of the wing. I think I'm OK with the whole after burner stuff now, thanks for your time and responses. One of the questions you brought up in a previous response was, is the air going into the engine the same as the outside air going over the aircraft. The engine takes what it needs, that's why lots of jets have axillary blow in doors to help feed the engine when its on the ground and not moving fast enough to force it down the inlet. The engine acts like a huge vacuum cleaner in the front, sucking in air much faster the the jet is moving while its on the ground - I'm sure you've seen footage on TV where people get sucked into an engine air intake. but after the jet passes mach 1, the air must be slowed down to subsonic speed before entering the engine or else it will flame out (stall). That's one of the reasons earlier fighters like the F-4 had variable geometry inlets, to reposition the incoming air to slow it down as well as redirect some of it for air bleed and cooling. More modern jets like the F-16 and F-18 have fixed inlets, but the design was a result of extensive studies to make sure it worked properly. The pro's to fixed geometry inlets are that they are (after the initial design phase) simple , light weight, cheap, nothing to go wrong or fix, but the con's are that they will only allow a limited amount of air in - ever. In fact the F -16 had its inlets redesigned to allow more air flow for the bigger engines, if you look up information on the F-16 you will see that they are refereed to the (small mouth) and (big mouth). When the old F-14 got the new, much more powerful engines, all they had to do is recalibrate the inlet ramps inside the air intakes to allow the correct amount of air flow to the new engines. Even the mach 3.3+ SR-71 Blackbird's engines had to ingest subsonic air.

p.s.
The reason the air must be subsonic before entering the engine is because of the (would be) supersonic shock wave, its the shock wave that would snuff out the engine.

Well, the AB is simple chemistry and physics and it is the same physics for gases either they are heated/created in the engine or traveling around the wing. Atoms in gases are wild annd bounce around and collide with each other and take new directions all the time.

And there is an infinite number of rectionary forces being created that take out each other. In a still gas the sum of a collection of atoms and the individual atoms direction changes as a result of collisions over time .... the sum is zero in a still gas.

In high pressure there are more atoms per volume of gas, more collitions per time unit, usually greater temperature. In low pressure there are fewer atoms per volume.

if we have a center with pressure 5, and a north weast and south point also with pressure 5, but then in the east a presure zone with a pressure of only 2.

Low pressure mean fewer atoms per volume so there a fewer of them to collide with, some of the atoms in the center will still collide with the few in the east but many atoms will experience fewer collisions in the east compred to north west and south, annd the sum effect of that is the they will start to move east.

And the sum sum of all sums is now that we have a little guy traveling east, a small mass, summed up with all the other small guys we now have a big mass miving east.

Same thing with a wing. Becase of inertia the atoms have trouble following the more curved upper side of the wing the same way you cannot drive around a corner of a building at too high speed, but that is exactly what we are forcing them to do and the atoms can not cope but get dispered. That means fewer atoms per volume of air in the absolute vicinity of the wing upper surface. Atoms in gases bounces around and is bouncing around hitting each other. So with low pressure above the wing and high pressure underneath and solid material between these two zones we get a situation with 800 atoms banging on the underside of the wing and 50 atoms banging on the upper side, guess which team wins, and so the high presssure zone pushes the wing upwards.

in hot gases the atoms have higher speed and therefore hits each other harder. If an atom is accelerated harder more energy is needed and the more counter reactive force will it produce as a result of the collision whatever the atom collided with has to go in the opposite direction. That could be a solid atom being part of the engine wall, or an atom part of a high pressure zone but the net effect being an atom on the other side of that high pressure zone eventually will hit some solid that is part of the aircraft and thus press the aircraft forward.

The total number of collisions per time unit is a function of speed of the atoms and distance between the atoms. We can either increse the pressure and squeze the atoms tighter together so they don't have to travel so far to hit a neighbour. Or we can heat them up so they travel faster.

The air in the jet engine travels forward and in a short time frame it will have passed the engine.

Burning ... if there is such a thing, is like divorce and marrying a younger person. Atom type-A might be bound to Atom type-B and it is a tense binding, along comes atom type-C and atom type-A releases the binding with B and enters a more realaxed binding with C and the excess energy is released and travels away in a direction. Sometimes A might need to get hit by energy first to become unstable enough to get free, but once it settles down again it will find the less tense binding.

So causing high pressure by sqeezing atoms together inside the combustion chamber is like students at a party, the more crowded the greater the chance you hit somone which you can react with. And when you do energy is released, not always in the visible spectrum, this energy heats up the gas and so the atoms travels faster and more reactions than before occurs and yo get a snowball effect and it becomes an ongoing process as long as you provide new raw material at one end you will keep the burning going.

Now , your question maybe might have been is there enough raw material left when we reach the AB can the burning continue? why not spray in enough fuel immediately?

I guess it would become too hot too fast. So you burn and let the gases expand and drive the turbine and expand more then you ignite again and again heating up more and more, like an apollo rocket with stage one stage two and so on.

If there weren't enough oxygene back there they could just have provided new raw material via ducts from the compressor, and I don't care to check since it is obvious that that would be the thing to have to be done.

In the end we have the same basic principle of high and low pressure gases, they hit the walls of the can, where the atoms of the solid material are in arm-hook with each other screaming "one for all and all for one", tough luck there for that lone gas atom, there is only one way in the end, out the hole back there of the aircraft.

And the hotter the atom is the faster it is moving, and the bigger the reactive force, annd that, that is what the creator races called homo sapiens intended from the beginning.

This I find not complex at all.

In a high enough pressure and perhaps also temperature, you can maintain a burning process at say ..... MACH 48 .... or MACH 132 ... so speed is not the problem.

i do find the compressor is the most interesting and the flow mechanics there with high and low presure zones.

The air is not slowed down ..... it simply WILL slow down cause if it is too fast the fan blades will start operate a negative alpha annd the winf will start driving the compressore instead. Most of the air atoms wil just crash into the fan blades. Instead of being sucked in it will now press against the engine.

I thin it is all about flow mmechanics and get an even airflow so that the compressor can do an even job.

I mean I guess the shock waves wwould apear anyway ..... so why not design the intakes so we place them as we like. No supersonic air being able to hit the engine directly and also coming towards the engine slower, and in order.

It all really is only about one thing, the collision of atoms.


I find the flow mec

Couldn't have said it better myself, even if I wanted to! Thanks Maj_Solo

Maj-Solo - Great explanation!

I've been to parties like that when I was a student

Yeah u guys, What I ijn the post above called a tense binding between A and B and a relaxed binding between A and C, actually, I think the chemists think of it as a relaxed (sloppy binding) between A and B and a more ( hard binding) between A and C.

Someone please step in and help me!!!!


Carbon based fuels of different types ( I only remember they had different "cracking" values ) are carbon chains ... so maybe carbon chains C8 C16 oe whatever has something to do with O2 or O3 which might have something to do with CO2.

If we are going to talk about chemistry please, of anything on earth don't refer to me, all I remember from school 20 years ago is molecules can react to each other forming more hard bindings and if it is a harder binding than previous then energuy is released.

I wish Simon King was here, he was a Dr. in chemistry but worked as a software engineer at my job cause there were not enough open places for chemists in the world so he took whaever joob payed him enough. He could have helped us out here.