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Joined: Fri Jun 29, 2001 12:42 am

Twin Spool Engines

Fri Nov 09, 2001 12:42 pm

Can some one explain the exact difference between, and what exactly is a twin spool engine. I know that the RR Olympus 593Mk610 is a twin spool engine and so is the P&W F119, the F-22 and JSF engine. But other very powerful engines like the P&W J58 (SR-71) are single spool engines. So what is the difference and which one is considered to be better. Also, does this apply to both turbofans and turbojet's.
Posts: 23
Joined: Sat Oct 13, 2001 6:15 am

RE: Twin Spool Engines

Fri Nov 09, 2001 1:00 pm

Think of a "spool" as any compressor that is attached to a turbine. Whether it is single, twin, or triple spool simply indicates how many "sets" of these compressor/turbine units there are in a given engine. Note that I am not talking about how many individual compressor or turbine sections there are, as these will vary by engine, but instead talking about complete units or spools. A straight turbojet engine is a single spool engine. It has a compressor section, a combustion section and a turbine section. They are all connected by a single shaft and all sections turn at the same speed. Now, to build a twin spool engine, make that shaft hollow and put another shaft inside of it. On the front of the shaft put a large fan and at the rear put another turbine section. What was just added was another "spool". It turns independently of the first "spool" and at a different speed. In this example, this is also known as a turbofan engine and is what you see on most commercial airliners today.

I'll let someone else explain which is better and why, but those are the basics of a twin spool engine.

Hope that helps.
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RE: Twin Spool Engines

Fri Nov 09, 2001 5:38 pm

It has a compressor section, a combustion section and a turbine section. They are all connected by a single shaft and all sections turn at the same speed


I expect you didn't mean it to come out like that, but the above stement is misleading. the combustion section doesn't rotate as it is not connected to any shaft.

'Spool' just descibes the number of rotating asemblies in a jet engine. A rotating assy being made up of a compressor, its driving turbine and the connecting shaft.

A R.R. Avon was a single spool engine (1 comp/turbine)

JT9's & CF6's are twin spool (Low press Fan/Comp/Turb, and an HP Comp/Turb)

An RB211 is a 3 spool engine ( LP Fan/Comp/Turb, Intermediate comp/turb an an HP comp/turb)
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RE: Twin Spool Engines

Sat Nov 10, 2001 1:08 am

Yep, you're right. Thanks for the correction.

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RE: Twin Spool Engines

Tue Nov 13, 2001 6:55 pm

Multiple spools are used in engines so that each one may rotate at a different speed for optimum efficiency. In a twin spool turbofan for examlple, the Fan and LPT spool (N2) handles more air than the HPC and HPT spool (N1). This calls for the two spools to rotate at different speeds for maximum efficiency.
A/c train
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RE: Twin Spool Engines

Wed Nov 14, 2001 5:32 am

Were as in a single spool you have one Turbine driving the compressor, you now have a second spool with another Turbine driving another compressor,
LP Compressor ( N1) Driven by LP Turbine
HP Compressor ( N2) Driven by HP Turbine,
Think were the pressure is greatest??? At the arse end of the HP COMPRESSOR!!!! makes sense so they call it the HIGH PRESSURE COMPRESSOR!, most of the terminology is idiot proof so now you know you wont forget it, don't have a go at me for putting my two pence worth in forgod sake!!! LOL
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RE: Twin Spool Engines

Wed Nov 14, 2001 6:43 am

Hi B1C17L1011, Buzz here. Speedbird touched on better efficiency. As the air is compressed, it takes up a smaller volume, the blades and vanes get smaller, and the rotation should be faster. Why? Well people discovered that the engine accelerates faster, and tends to burp less. OK, so engine burps - compressor stalls are a nasty thing. At night you see the ball of orange fire in the inlet and it sounds like a cannon shot. And that's during a ground run with mechanics. You should see what it does to the attitude of pilots and flight attendants (grin).

Buzz Fuselsausage: Line Mechanic by night, DC-3 Crew Chief by choice.
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RE: Twin Spool Engines

Wed Nov 14, 2001 7:14 am

Just seen in the historical perspective, a two spool engine doesn't need to be a turboFAN engine. At least in earlier days it could also be a turbojet.

What comes into my mind right away is the P&W JT3C on early Boeing 707 and 720. I think it was a civil version of the military J57.

I'm really not sure, but I shouldn't wonder if exactly the same is the case with the JT4A (military J75?).

Regards, Preben Norholm
Always keep your number of landings equal to your number of take-offs
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RE: Twin Spool Engines

Wed Nov 14, 2001 8:04 am

If you opened up a modern twin-spool engine, you find two engines housed in one. Think of a spool of thread without the string around it. At one end is a bunch of compressor fans and the other has a few turbines. Now insert a pencil of straw into the spool's center. Imagine adding a giant fan at one end and more tubines at the other.

This is the idea behind twinspool, two independently rotating axial jets. The "inner spool" is running faster and hotter and louder which is reacting to the energy of the combustion chamber. This exhaust goes through the "outer spool's" tubines as it goes out the back, which acts like a crankshaft to the big fan in front. It provide a heck of a lot of thrust with reduced size.

I think modern turbofans on widebodied aircraft have triple-spools. Hope this helps.  Big thumbs up
The meaning of life is curiosity; we were put on this planet to explore opportunities.
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RE: Twin Spool Engines

Tue Nov 20, 2001 10:26 am

Do any jet engines have reduction gears?

What about turboprop engines? If I'm not mistaken, these have various designs including twin spools and reduction gear designs.

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Joined: Wed Nov 03, 1999 3:54 pm

RE: Twin Spool Engines

Tue Nov 20, 2001 10:37 am

Turboprop engines have two design philosophies on the same principle.
A fixed shaft turboprop engine is like a single spool turbojet. All rotating parts in the compressor and turbine section are linked directly. An example of this is the TPE331 fitted to Metroliners, MU-2s etc.
A free turbine turboprop runs on the twin spool principle, with two sets of turbines, the compressor turbine which turns the compressor and the power turbine which applies power for the propeller. An example of a free turbine is the PT-6 fitted to B1900s, Twin Otters etc.
Because the speed of the turbine is in the tens of thousands of rpm in both designs both utilise a reduction gearbox before linking the propeller, to allow the prop to turn at below the speed of sound!
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RE: Twin Spool Engines

Tue Nov 20, 2001 12:05 pm

>Do any jet engines have reduction gears?

I believe this is the "next big thing" engine manufacturers are researching: geared fans (that and variable cycle engines). This would allow bigger fan diameters to increase the thrust of engines even further.
Like a seagull on the MCG of life...
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RE: Twin Spool Engines

Fri Nov 23, 2001 3:50 pm

There has been a jet engine with "reduction gears" in production for quite some time....and it's called TFE-731, has a geared fan. Very popular with the biz-jet set.
aaron atp
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Joined: Sun Mar 19, 2000 1:17 pm


Fri Nov 23, 2001 4:40 pm

Ah yes, the reason hawkers sound like toy-jets at takeoff power...

note the gearbox behind the fan

these things have been around since at least the early 70's... I don't know why everyone thinks it's such a radical new idea.

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Joined: Tue Feb 06, 2001 5:56 pm

RE: Twin Spool Engines

Sat Nov 24, 2001 12:27 am

Aaron, yes those gearboxes for the fans have been tested, however, up to now, they have not been reliable enough to use them on commercial jetliners. Things will change in the next few years.

About the 'spooling'

Single - spool:
Most Turbojet engines.

Twin - spool:
Most of the non-RR turbofans, turboprops (all have reduction gears, since LP-turbines rotate at about 5000rpm or more) and helicopter engines, fighter turbofans.

Triple - spool:
some of the RR turbofans, some more fighter turbofans.

Quad - spool:
There is one active quad - spool engine, I know it was on a russian-built get fighter (either the MiG 27 or 29, maybe the Su-37, I forgot  Sad )


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