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I've been thinking about doing a presentation about how jet engines work. I've watched some videos and everything seems understandable to me, but I have one question. What originally starts spinning the intake fan? The intake fan is powered by the turbines at the back of the engine as far as I understand, so what powers the fan before there's air in the engine to spin the turbines? Thank you for any help.

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  • $\begingroup$ I may have misunderstood your question. Can you clarify: are you asking about how the starter motor works (which is what niels answered) or how the fan starts spinning (which is what I answered)? $\endgroup$
    – Bianfable
    Commented Oct 2, 2022 at 18:10
  • $\begingroup$ I’m interested in the entire process of the engine start, which is why both answers are useful to me; the more I know the better. $\endgroup$ Commented Oct 2, 2022 at 18:17
  • $\begingroup$ OK, got it, then I'll leave the answer as it is... $\endgroup$
    – Bianfable
    Commented Oct 2, 2022 at 18:20
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    $\begingroup$ This is an excellent video about starting a CFM56 engine. I think it will help answer your question:youtube.com/watch?v=0OgEbs3ovOw $\endgroup$
    – user22445
    Commented Oct 2, 2022 at 18:25
  • $\begingroup$ Thank you for the video. It helped me visualize the answer to my question. $\endgroup$ Commented Oct 2, 2022 at 18:35

3 Answers 3

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Most turbofans have a 2 spool design:

  • The fan and low pressure compressor (LPC) are powered by the low pressure turbine (LPT) and rotate at a speed called N1.
  • The high pressure compressor (HPC) is powered by the high pressure turbine (HPT) and rotates at a speed called N2.

Only the N2 stage is connected via an accessory gearbox to the starter motor (this starter motor could be powered by different things, see niels nielsen's answer for details). Therefore, the N1 stage will not be moving initially during engine starting. Only the N2 stage starts to accelerate, powered by the starter motor. Note that air can move through the fan and LPT, even when they are not rotating.

The faster N2 rotates, the more air is now pushed through the engine. The air exiting the HPT still has enough speed to accelerate the LPT as it moves towards the exhaust. This is what starts to accelerate the N1 stage. Typically, you'll see N1 starting to rotate a few seconds after N2.

The N1 speed will remain relatively low during the starting process, until fuel is added. With the added energy from the combustion, a lot more pressure is available in the turbines. This will now increase the N1 speed significantly as more energy is extracted in the LPT.

Eventually, the starter motor can be disconnected as the HPT supports the HPC without external help and the engine becomes self sustaining.

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There are several ways this is done. You can use an electric motor running off a battery to spin up the jet engine, or you can use a big fan to blow air into the air inlet of the engine, or do the same with compressed air pipes that feed compressed air into a pneumatic motor that is geared to the engine.

The energy source that runs each of these things can be either built into the airplane, or rolled out to the plane on a cart (called a start cart) and plugged into the engine temporarily.

In some cases you can put the starter motor into the start cart and connect it to the jet engine with a rotating shaft to spin up the engine. The SR-71 used a start cart with two Buick Wildcat V-8 engines in it to start its engines.

It is also possible to use a big brick of solid rocket fuel or a really big shotgun-shell-like cartridge in a cannister that feeds the hot gas from the burning fuel straight into the turbine section of the engine to spin it up. This lets you start jet engines in remote places that don't have start carts available.

BTW shotgun or Coffman starters were also used on piston-engine aircraft and farm tractors, where the hot gas was fed to a pneumatic motor with a piston in it that was geared to the crankshaft.

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  • $\begingroup$ Alright, this definitely helped, I appreciate the help. $\endgroup$ Commented Oct 2, 2022 at 18:08
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    $\begingroup$ Don't forget shotgun cartridges. The English Electric Canberra and Hawker Hunter fired shotgun cartridges to start the turbine spinning. IIRC they had several redundant shots so they have several attempts they can make before the mechanics need to reload the engine. $\endgroup$
    – slebetman
    Commented Oct 3, 2022 at 13:25
  • $\begingroup$ @slebetman Flight of the Phoenix shows a shotgun start. $\endgroup$
    – Schwern
    Commented Oct 3, 2022 at 21:38
  • $\begingroup$ @Schwern, i remember that movie, will edit!!! $\endgroup$ Commented Oct 3, 2022 at 23:34
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    $\begingroup$ If I remember my book on Chuck Yeager's life, he apparently was part of the team that figured out that you could "blow start" a jet, blowing air into it via the exhaust of a previously-started aircraft. I consider it to be one of the more amusing variants to the "blow air into the air inlet of the engine" approaches you mention! $\endgroup$
    – Cort Ammon
    Commented Oct 4, 2022 at 4:02
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What originally spins the intake fan [...]?

There is no "intake fan" term used when talking about turbine engines. There's a fan, there's compressor, and there's an intake. But you can't mix those terms: they all mean different things!

A fan is used to blow air that bypasses the core of the engine. Turbofan engines have fans. Turbojet engines don't.

A compressor is used to provide the cooling and working air for the combustors and the turbine.

An intake is generally the system used to route air to the engine. Typically it has no rotating parts, although it may have parts that translate (move back and forth as the flight regime changes).

The compressor in jet engines - whether turbojet or turbofan - is usually axial, and has several stages. In the simplest of the engines, the compressor and the turbine are on a single shaft. This trades off performance for simplicity. More complex engines may have two independent compressor systems, each powered by its own dedicated turbine system. The shafts for those systems are coaxial. The first (low pressure) compressor is powered by the last (low pressure) turbine. The 2nd (high pressure) compressor is powered by the first (high pressure) turbine then.

The fan may be powered by the same turbine as that of the compressor. That would only work for very low bypass ratio turbofans. At higher bypass ratios, the fan is too heavy of a load during the engine startup, and cannot be coupled to the compressor shaft. Instead, the fan then has its own, free power turbine located behind the core of the engine. The word free refers to the fact that this turbine's rotation is completely independent from the rotation of the core of the engine. In many cases you could mechanically brake the free power turbine during engine startup, and then release it later, although it's a rather unnecessary exercise typically.

As for how the engine starts: there is a source of mechanical power that spins up the engine core - it is called a starter. Starters may be electric, pressurized air turbines, hydraulic, or even pyrotechnic (so called cartridge starters where a slow burning rocket fuel is the energy source).

First, the ignition (spark) source is usually turned on, then the starter spins up the core.

Eventually the core is spinning about as fast as the starter can make it go - it's called motoring, and modern engine management systems may display the MOTORING flag in the engine speed readout. The fuel is then turned on to the engine, and it ignites in the combustor.

The core then accelerates. The starter energy source may need to be left on for some time to help the core go up in speed faster. The startup process is, perhaps counterinitutively, a very hard operating regime for the engine. That's because the turbine engines use the air from the compressor to cool the combustors and, most importantly, the turbine blades. When the engine is initially ignited, the compressor is providing well over an order of magnitude smaller mass air flow, and the cooling of the hot section of the engine is marginal. If the engine is not accelerating quickly enough during a start, it may overheat - especially if the engine is fed with too much fuel. This requires immediate shutdown and usually continued motoring to cool down the engine before another start attempt.

As a rough idea to where the energy goes in a jet engine: about 2/3rds of the power available for extraction by the turbine(s) is used to drive the compressor. The remaining 1/3rd is used either as a thrust source via the jet nozzle, or powers the free power turbine that then drives the fan, or a reduction gearbox that drives a propeller, a rotor, an electric generator (typical in peaker electric plants), or a turbopump (typical in natural gas applications).

Now you can get a hint as to why the engines not get spinning faster during startup before introducing fuel, to reduce the turbine temperature during startup. It takes an enormous amount of power to get the compressor operating at any significant fraction of the idle speed where turbine cooling is sufficient. This speed would be at least ground idle, but sometimes higher than that, up to flight idle speed. To keep starter size and power demands manageable, the starters bring the engine up to a speed that only just allows startup without overheating and not much more than that.

Another reason for low startup speeds is the airflow limit for reliable fuel ignition. Once the fuel is burning, the engine can be brought up to full air flow. But when the airflow is too high, it's almost impossible to ignite the fuel, so there's an upper engine speed limit for reliable ignition and development of stable flame in the combustor.

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    $\begingroup$ "At higher bypass ratios, the fan [...] cannot be coupled to the compressor shaft. Instead, the fan then has its own, free power turbine [...]" -> this is true for Rolls-Royce Trent engines with a 3 spool design, but other high-bypass turbofans have the fan connected to the compressor shaft (LPC and LPT shaft), sometimes with a gearbox in between (e.g. PW1000G). $\endgroup$
    – Bianfable
    Commented Oct 4, 2022 at 6:40

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