At the risk of sounding too broad, I want to better understand the complications of jet engines. This time, I'm asking about extra "stuff", which unfortunately I have no better name for since I don't know what it is.

I'm familiar with the basic cycle of jets. Compressor stages draw in air and pressurize it, combustion heats and expands the air, and turbine stages extract some torque from the jet to drive the engine.

enter image description here

It all seems so simple. However, then I see pictures like these:

enter image description here

It's a modern turbofan. What is all that extra stuff? Can it really all be fuel and air lines? I'm especially scratching my head because the basic design has only a handful of moving parts (the stages/blades) all driven by the turbine and its shaft, so I can't imagine those are mostly hydraulic or electric lines either.

I found a simpler example here (Tumansky RD-10):

enter image description here

It is a turbojet and much older.

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    $\begingroup$ If life were only as simple as the drawings made them out to be :) $\endgroup$ – Ron Beyer Sep 19 '16 at 21:08
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    $\begingroup$ Haven't you ever opened the hood of your car? The basic principles of the internal combustion engine are simple. That doesn't mean there aren't about 10 000 different components in a modern car engine. $\endgroup$ – user16965 Sep 20 '16 at 0:28
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    $\begingroup$ @FighterJet And I never said it should be. I was comparing pictures and asking what all the extra stuff is, not denying its existence. $\endgroup$ – DrZ214 Sep 20 '16 at 1:01
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    $\begingroup$ To make a rather obvious remark, your photographs don't show anything that is drawn in your diagram, because everything in the diagram is hidden inside the engine casings! This is a cut-away display of a real engine - but note the static blades are missing from the cut-away section, because they are fixed to the casing that has been removed. howthingsfly.si.edu/media/jet-engine $\endgroup$ – alephzero Sep 20 '16 at 1:17
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    $\begingroup$ @FighterJet Np, my OP proly came across as a lament about simple concepts being really complex IRL. I just hope no one confuses that for surprise at the complexity, or outright skepticism of it. $\endgroup$ – DrZ214 Sep 20 '16 at 5:39

Of course the basic concept of a turbine engine is relatively simple, but the devil is in the details. To make an engine efficient, reliable, powerful, and safe, a lot more components are needed.

Fuel. Obviously an engine needs fuel to operate. The fuel is burned in combustors located around the circumference of the engine. This requires separate fuel lines to each of the combustors around this section of the engine. The fuel must be connected to the main fuel supply, and to the throttle/engine controls to meter the fuel flow.

Oil. All the rotating machinery inside of a turbine means that oil is needed to keep everything moving smoothly. There is an oil tank, and tubes to move it to where it is needed.

Main bleed air. Bleed air is taken from certain sections of the engine. This serves the air conditioning and anti-ice system and can be used to start other engines. This bleed air is taken from multiple places around the circumference, and from multiple stages provide the required pressures at different engine speeds.

Other bleed air. The basic design of a turbine engine has air entering the front and leaving the back. However, in a real engine, a lot more movement of air is going on. Engines have to operate in a huge range of conditions. Sometimes air needs to be bled from a certain section to manage the pressures and maintain stable operation. Sections of the turbine are also cooled by bleed air. It takes more tubing to move all this air around.

Sensors. Modern engines measure a lot of information to remain stable and efficient. This includes temperatures and pressures at multiple places all along the engine, as well as fire detectors and the speeds of the different rotors. These are all hooked back to the engine controls.

Generator. This is connected to one of the engine spools, and provides power to the aircraft, including all of the electronics on the engine itself.

Starter. Usually separate from the generator, also connected to an engine spool. This uses bleed air to turn the rotors in order to start the engine.

Control. Usually on the outside of the fan case (at the front of the engine, left side of the RR image in the question), modern engines run with a lot of electronics, referred to as Engine Electronic Control (EEC) or Full Authority Digital Engine Control (FADEC). All of the items above require wiring for control and feedback. Each tube will need different valves, connected back to the electronics, to control the operation. Liquids will need pumps and filters. There are also additional movable components inside the engine that can be adjusted by the engine control system.

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    $\begingroup$ Suggest you add that there are pumps required for both fuel and oil. $\endgroup$ – SomeoneSomewhereSupportsMonica Sep 22 '16 at 8:24
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    $\begingroup$ Under sensors, you mention fire detection. There is also plumbing to bring the fire suppression foam into the engine core should a fire start, and some sort of canister in or near the nacelle to hold the foam when not in use. $\endgroup$ – FreeMan Jul 12 at 16:56

There are many extra components, generally known as "accessories".

Fuel pumps, the ignition and starter circuits and control units, turbo pumps, hydraulic pumps, stator vane angle actuators, fuel flow controls, FADEC computers, fuel metering unit, fuel filters, fuel return valves, bleed air pickups and ducts, generators, gearboxes to drive the pumps and generators, nozzle guide vane actuators etc, plus all of the piping and wiring to connect the above to and from the aircraft.

All of the above make it possible for the bits in your first image work in modern, fuel efficient, high thrust turbofans.

The "simpler" example you give is a 1950s technology turbojet which is simpler by definition than a turbofan and will not have variable stators and vanes and a much simpler fuel system plus a whole host of the modern accessories on a modern turbofan.

Contrast 1950s car engines with contemporary ones. It's a similar story.

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    $\begingroup$ Fuel return valves? How would fuel be returned to the tanks after being injected into the combustion chamber? Also, what are nozzle guide vane actuators? You already listed stator vane angle actuators, which I presume allow variable blade pitch. $\endgroup$ – DrZ214 Sep 20 '16 at 0:12
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    $\begingroup$ @DrZ214 Fuel gets used for more than just combustion - for example there may be a heat exchanger that cools the engine oil by dumping the heat into the fuel, instead of the water-cooling system in a car engine. Also, the usual terminology is that the static blades in the compressor section of the engine are called "stators", but those in the turbine section are called "(nozzle) guide vanes." $\endgroup$ – alephzero Sep 20 '16 at 1:09
  • $\begingroup$ @alephzero Okay. So both stators and rotating vanes can have variable pitch. I thought nozzle might mean something like the flexible iris nozzle on some fighter jets. $\endgroup$ – DrZ214 Sep 20 '16 at 1:22
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    $\begingroup$ @DrZ214 Even many cars have fuel return lines. $\endgroup$ – sweber Sep 20 '16 at 7:02
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    $\begingroup$ @DrZ214, returning the fuel that was used for cooling oil back to tanks keeps the fuel from freezing as the ambient temperature at cruise level can often fall well below the -47°C freezing point of Jet A1. $\endgroup$ – Jan Hudec Sep 20 '16 at 18:06

Science of everything, once well established, is far easier than its Engineering. That's the difference between those 2 pictures.

I'm a scientist I can easily understand the cycle of most of the machines I get to use in day to day life but that does not mean that with those concepts I can build those machines as easily as the simplistic pictures depict.

Consider the term Rocket Science

enter image description here NASA

So easy! Now look at its Engineering

enter image description here

WikiMedia Commons

So none of that stuff is extra, its just an engineered version of the basic concept, for today's engines

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    $\begingroup$ Everything you said is true, but does not answer the question. This would be better posted as a comment, not an answer. $\endgroup$ – DrZ214 Sep 20 '16 at 6:20
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    $\begingroup$ Imo it does, no answer to such a broad question can do justice to it. Even the existing answers try to over simplify and only provide details for some general parts that are there. No answer to this question can be complete imo $\endgroup$ – Hanky Panky Sep 20 '16 at 6:23
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    $\begingroup$ I agree with @DrZ214 here. I see it as: "How birds fly?" "Because spaceships fly even higher!" $\endgroup$ – Agent_L Sep 20 '16 at 7:52
  • $\begingroup$ My question was, what is some of the extra stuff, and people have answered with a list of stuff such as oil lines, bleed air lines, etc. My question did not ask for detailed blueprints and operational procedure of all the stuff, in which case the existing answers would indeed be oversimplifications. $\endgroup$ – DrZ214 Sep 20 '16 at 8:08
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    $\begingroup$ @HankyPanky Scientific question: "How does it work?" Engineering question: "How to make it work?" $\endgroup$ – Crowley Sep 20 '16 at 15:11

A modern turbofan core will have lots of plumbings. Chiefly pneumatic, oil & fuel. For pneumatic, you have high stage & low stage bleed air tapping for your aircraft's bleed air requirements. [aircon, anti-ice etc] You also have pneumatic ducts for turbine case cooling, nacelle anti-ice, core compartment ventilation etc.

For fuel, you have lines from the fuel pump for combustion, servo fuel [fuel muscle pressure for actuators [variable stator vanes, fuel/oil cooler valve etc], servo fuel heater [heat exchanger between oil&fuel] etc.

For oil, you have oil lines, oil sump breather lines, air/oil cooler etc.

  • $\begingroup$ On the fan case the dark grey rectangular box is the Electronic Engine Control. The 2 small light grey boxes below are the ignition exciters. $\endgroup$ – Noob Sep 20 '16 at 17:52
  • $\begingroup$ These are good details. You can edit your answer rather than adding a comment. $\endgroup$ – fooot Sep 20 '16 at 17:53
  • $\begingroup$ The front fan case is shrouded by kevlar [yellow part] to arrest any fan blade liberation. $\endgroup$ – Noob Sep 20 '16 at 17:54
  • $\begingroup$ Sorry this is my first time here, really a Noob $\endgroup$ – Noob Sep 20 '16 at 17:54

I've always found this Canadian fellow to be quite good at demystifying jet engines. In this video he describes a few visible components of a GE J79 engine from an F4 Phantom. It's a single-spool turbojet from the 50s, but I think it will still help explain what you're seeing. (ignore the afterburner/variable exhaust) Basically, even a single simple component can involve at least two tubes, hoses, cables, wires, brackets, linkages, sensors, control lines, inspection ports, drains, heatshields or clamps. And not a single thing that doesn't absolutely have to be there.


Let's compare modern car engine bay and the old one. They are, in the principle, very same, four pot, internal combustion engine, two wheel drive, manual gearbox.

But the old one is normally aspirated, but the new is turbocharged. The old one has mechanical distributor, the new one has ECU. The old one has carburetor, the new one uses direct injection. The new one has cruise control, ABS, ESP, air conditioning, sat-nav, catalyser, filter, parking sensors, etc. The old one has one lightbulb for hadlights, one lightbulb to indicate low oil level and one lightbulb for indicators, mechanical speedometer, liquid thermometer and fuel gauge. Nothing more.

Back to your question, the extra stuff can be sensors and their circuits, more complex fuel distribution, regulation of stator blades, vents and air supply to prevent/assist recovery from compressor stall.


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