hitower3
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GTF vs. triple spool engines

Mon Dec 03, 2018 9:41 am

Dear experts,

When I read through a few articles related to the PW1000G GTF engine, the reason-to-be of the gear box is to allow the fan to rotate at its optimum speed (4-5krpm), as well as the LP turbine (12-15krpm), with a 3:1 gear box in between these two systems.

While all of this makes perfect sense to me, I remember another - already existing - solution to this very same problem: the triple spool engine. This technology, first introduced by RR in the RB203 engine in 1967 and becoming popular in the widespread RB211 family since the 1970s (even the latest Trent XWB and Trent 1000TEN are referenced under the RB211 family).

My question is: does'nt the addition of the third spool provide an easier and proven reliable solution to the problem of this very same problem?
- Fan & LP turbine at 4krpm on spool 1
- IP compressor and IP turbine at 12krpm on spool 2
- HP compressor and turbine at 20+krpm on spool 3

I would greatly appreciate your kind input.

Hendric
 
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Francoflier
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Re: GTF vs. triple spool engines

Mon Dec 03, 2018 10:00 am

3 spools add weight, complexity, and do not change the fact that the LP turbine is going to have to spin slower than it wants to, which is less than efficient. The main benefit is that you're now running the whole IP and HP compressor assemblies at optimum speed.

It is still a very elegant design, especially since the technology to make fan gearboxes didn't exist back then.
I'll do my own airline. With Blackjack. And hookers. In fact, forget the airline.
 
strfyr51
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Re: GTF vs. triple spool engines

Mon Dec 03, 2018 10:10 am

hitower3 wrote:
Dear experts,

When I read through a few articles related to the PW1000G GTF engine, the reason-to-be of the gear box is to allow the fan to rotate at its optimum speed (4-5krpm), as well as the LP turbine (12-15krpm), with a 3:1 gear box in between these two systems.

While all of this makes perfect sense to me, I remember another - already existing - solution to this very same problem: the triple spool engine. This technology, first introduced by RR in the RB203 engine in 1967 and becoming popular in the widespread RB211 family since the 1970s (even the latest Trent XWB and Trent 1000TEN are referenced under the RB211 family).

My question is: does'nt the addition of the third spool provide an easier and proven reliable solution to the problem of this very same problem?
- Fan & LP turbine at 4krpm on spool 1
- IP compressor and IP turbine at 12krpm on spool 2
- HP compressor and turbine at 20+krpm on spool 3

I would greatly appreciate your kind input.

Hendric

actually? No, The GTF is driven off the N1 spool and geared down via a planetary gear box. On the RB211 and Trent there are 3 spools all independent turning at different speeds. the LP, and Fan have their sets of bearings, the IP has it's set of bearings and the HP has It's set of Bearings The Problem with the 3 spool design is that the intermediate spool could have a bearing problem and take out the rest of the engine via vibration. the problem has been Minimized in the Trent series I've heard But it is a concern. I have yet to see the GTF's real problems, though I know they're there. I would be concerned with the GTF Fan Gearbox oil cooling and reliability.
 
parapente
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Re: GTF vs. triple spool engines

Mon Dec 03, 2018 11:26 am

Obviously nobody knows more about the triple spool than RR.It has been a brilliant and faithful idea.But with the advent of planetary gearing the fan it's time has come and gone-RR recognise this ( although they call their new engine a two and a half spool!).It will of course remain with us for at least an other decade but after that it will fade away with the advent of large geared turbofans.RR have informed us that they have a gearbox reliably running at 70klbs plus.So we know what aircaft types they are looking at for such an application! You can bet that GE are too!
 
Tristarsteve
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Re: GTF vs. triple spool engines

Mon Dec 03, 2018 5:25 pm

I believe that Rolls Royce patented the triple spool when they designed the original RB211-22. PW and GE countered by very complex compressor bleed systems to achieve the performance.
 
WPvsMW
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Re: GTF vs. triple spool engines

Mon Dec 03, 2018 8:10 pm

Is the third spool the reason for the RB211's very distinctive sound ("whine"), esp. during start?
 
stratclub
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Re: GTF vs. triple spool engines

Mon Dec 03, 2018 9:01 pm

You mean that unmistakable rumble right after you put fuel to it? I would have to say yes because the twin spool Prat's and GE's sound different than a RR triple spool.
 
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rjsampson
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Re: GTF vs. triple spool engines

Mon Dec 03, 2018 10:46 pm

Francoflier wrote:
It is still a very elegant design, especially since the technology to make fan gearboxes didn't exist back then.


What about the GTF on the Lycoming ALF 502 engines on the BAe 146? Seems like those came out around the same time as the RB211?
"..your eyes will be forever turned skyward, for there.." yeah we know the DaVinci quote. But GA is so dang expensive these days! :(
 
Wacker1000
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Re: GTF vs. triple spool engines

Mon Dec 03, 2018 11:09 pm

rjsampson wrote:
Francoflier wrote:
It is still a very elegant design, especially since the technology to make fan gearboxes didn't exist back then.


What about the GTF on the Lycoming ALF 502 engines on the BAe 146? Seems like those came out around the same time as the RB211?


As the BAE-146 was best known for its 3 engine variant, the ALF 502 makes a strong argument for the 3rd spool.
 
GalaxyFlyer
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Re: GTF vs. triple spool engines

Mon Dec 03, 2018 11:19 pm

The ALF 502 makes a strong argument that tank engines shouldn’t be used in aviation. Can’t speak for the 146, but they were hand grenades waiting to explode on the CL-600
 
flipdewaf
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Re: GTF vs. triple spool engines

Tue Dec 04, 2018 1:55 am

Francoflier wrote:
3 spools add weight, complexity, and do not change the fact that the LP turbine is going to have to spin slower than it wants to, which is less than efficient. The main benefit is that you're now running the whole IP and HP compressor assemblies at optimum speed.

It is still a very elegant design, especially since the technology to make fan gearboxes didn't exist back then.

3 spools actually reduce weight and complexity. More independent and speed optimised shafts reduce the requirement for variable stator blades and give an increased surge margin meaning that fewer stages are required. Because fewer stages are required the engines are shorter and stiffer ultimately meaning more time on wing.

They have tended to be better during the climb phase than their twin spool counterparts but not so good (efficient) during cruise. Some of this deficiency could be down to only having 1ip turbine, this has been increased to 2 on the Trent XWB.

Fred


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WIederling
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Re: GTF vs. triple spool engines

Tue Dec 04, 2018 8:57 am

rjsampson wrote:
Francoflier wrote:
It is still a very elegant design, especially since the technology to make fan gearboxes didn't exist back then.


What about the GTF on the Lycoming ALF 502 engines on the BAe 146? Seems like those came out around the same time as the RB211?

Compare the power rating.
All the (rather)small early GTF, turboshaft/turboprop engines ( those too have gearboxes) are small fry in
relation to the energy trashed in a 50++ klbs Turbofan.
Murphy is an optimist
 
parapente
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Re: GTF vs. triple spool engines

Tue Dec 04, 2018 2:07 pm

Sticking my neck out but the GE9x maybe the last new direct drive turbofan.
 
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rjsampson
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Re: GTF vs. triple spool engines

Tue Dec 04, 2018 9:43 pm

GalaxyFlyer wrote:
The ALF 502 makes a strong argument that tank engines shouldn’t be used in aviation. Can’t speak for the 146, but they were hand grenades waiting to explode on the CL-600


Was the ALF Gas Generator ever used on a tank? I know there were variants used as turboshafts for Choppers and turboprops, but didn't see any tank applications (although didn't one power a train?)

I'm sure the BAe 146 / Avros were produced in an order of magnitude greater than the CL-600 so I imagine if it were the engine itself, two more hand grenades per airframe would have given it a well-known reputation.

I'd love for you to elaborate on your funny "hand grenade" comment about its application on the CL-600 (my uncle used to fly those). Was there something unique about the ALF 502s hanging on the back of a CL-600 that made them particularly dangerous?
"..your eyes will be forever turned skyward, for there.." yeah we know the DaVinci quote. But GA is so dang expensive these days! :(
 
WPvsMW
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Re: GTF vs. triple spool engines

Tue Dec 04, 2018 10:11 pm

There's reverse precedent... the Sherman tank used a radial aircraft engine... famous for fouling spark plugs at low RPM.
 
LH707330
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Re: GTF vs. triple spool engines

Wed Dec 05, 2018 6:01 pm

WPvsMW wrote:
There's reverse precedent... the Sherman tank used a radial aircraft engine... famous for fouling spark plugs at low RPM.

Also famous for going ablaze when hit by anything, which is why the British crews called them "Ronsons" after the lighter.
 
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lightsaber
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Re: GTF vs. triple spool engines

Wed Dec 05, 2018 8:33 pm

parapente wrote:
Sticking my neck out but the GE9x maybe the last new direct drive turbofan.

If you qualify excluding business jet engines, the Passport (possible RJ application), and the CFM 797 engine, I agree.
You only have the first amendment with the 2nd. If you're not going to offend someone with what you say, you don't have the 1st.
 
GalaxyFlyer
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Re: GTF vs. triple spool engines

Wed Dec 05, 2018 10:38 pm

The ALF powers the M1 Abrams, then adapted to aviation use. Early ones hand grenaded in early Challengers fairly often. Often enough the truck driver delivering spares had more Marriott points than me. A friend said it was the only plane he truly each take-off had a good chance of engine failure.
 
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jetmech
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Re: GTF vs. triple spool engines

Thu Dec 06, 2018 3:19 am

hitower3 wrote:
- Fan & LP turbine at 4krpm on spool 1
- IP compressor and IP turbine at 12krpm on spool 2
- HP compressor and turbine at 20+krpm on spool 3


Hmm, might be a tad high with the N2 and N3 RPM's :scratchchin:. They would be closer to 7,000 RPM for IP and 10,600 RPM for HP.

https://www.easa.europa.eu/sites/defaul ... e%2002.pdf

Regards, JetMech
JetMech split the back of his pants. He can feel the wind in his hair :shock: .
 
WIederling
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Re: GTF vs. triple spool engines

Thu Dec 06, 2018 9:00 am

GalaxyFlyer wrote:
The ALF powers the M1 Abrams, then adapted to aviation use. Early ones hand grenaded in early Challengers fairly often. Often enough the truck driver delivering spares had more Marriott points than me. A friend said it was the only plane he truly each take-off had a good chance of engine failure.


tall story.
is a gas turbine in a MBT not in itself a "hand grenade"~ less than bright idea? :-=

The ALF 502 seems to be a derivative of the T55 turboshaft engine.
The T55 has a 3600 kW takeoff sized core

The M1 has the Honeywell AGT1500 which as the name says has ~~1200 kW takeoff.
( sizes don't match, really. though the M1 was said to consume fuel commensurate for the engine you attribute.
A fuel hog if there ever was one. :-)
Murphy is an optimist
 
parapente
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Re: GTF vs. triple spool engines

Thu Dec 06, 2018 4:13 pm

I agree with your view Lightsaber.imho if the 797 comes to fruition then it has to be built fast and cheaply.A single engine supplier is quite likely and if so it would be Cfm.We have already read in press that the 50klbs barrier would be waived if found necessary.Thus ( surely) it has to be a scaled LEAP.And that's turning out to be a very good engine.It won't be a long range aircaft so the economies of gearing have a lesser effect.Certainly got to be simpler/cheaper to maintain.
Could turn out to be good news for Cfm as if (if) Airbus chose to respond ( say carbon winged A310) then they would use the same engine.
 
strfyr51
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Re: GTF vs. triple spool engines

Fri Dec 07, 2018 8:29 am

Tristarsteve wrote:
I believe that Rolls patented the triple spool when they designed the original RB211-22. PW and GE countered by very complex compressor bleed systems to achieve the performance.

the bleed system for the GE engines originally came from the J85 and the J79 turbojet engines. GE used articulating compressor vanes for their bleed control while the PW J60 and J52 used the Poppet type bleed valves. The J52 later became the JT8D and now PWA uses both articulating Compressor vanes and Poppet type Bleed Valves mainly for their 5th and 10th stage Bleeds. Rolls a combination of bleed control methods refined from their Military turbojet engines through their Pegasus engines. I don't think any of them are better than any other, just different though the Variable stator vanes and the variable bleeds are a pain to rig on the line and if there's a problem with them it might be easier to replace the engine for a shop visit and a run in the test cell rather than re-rig on the airplane, though I've seen it done on the CF6. The EEC's on later engines took most of the Guess work out of the problem on later engines which made the CFM-56 and the GE90 a pleasure to work with.. Though I think the PW 4077 and -4090 are just as easy to work on..
 
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Matt6461
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Re: GTF vs. triple spool engines

Fri Dec 07, 2018 10:17 am

Doesn't three spools mean more internal friction loss vs. GTF?
Sure there's some friction loss via gearing but it's gotta be less than a whole additional spool.
 
parapente
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Re: GTF vs. triple spool engines

Fri Dec 07, 2018 11:54 am

Interesting point Matt ,could be.
The fact is 'something' has changed for RR to drop the triple spool concept after all these years.Although they do call their advance 2.5 spool.But if I am being honest I don't understand what that means ( love to know).
But ( I think) that the internal triple spool arrangement has nothing to do with planetary gearing ie you could have them both if you wanted.
Is it because the planetary gearing allows ( in itself) for optimal rotation of the key elements that the triple spool concept becomes ( sort of) redundant?
Love to know.
 
PerVG
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Re: GTF vs. triple spool engines

Fri Dec 07, 2018 12:37 pm

I would think that a power transferring gear set (one which, I assume, must have at least a thrust bearing component itself) would lose more energy than a couple of extra bearings. Am I wrong?
 
flipdewaf
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Re: GTF vs. triple spool engines

Fri Dec 07, 2018 1:29 pm

PerVG wrote:
I would think that a power transferring gear set (one which, I assume, must have at least a thrust bearing component itself) would lose more energy than a couple of extra bearings. Am I wrong?

It sounds right, my understanding is that the heat dissipation required for the GTF is in the order of low 100s of kilowatts, easy on a test stand, less easy at 45c and 95% humidity.

Fred


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WIederling
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Re: GTF vs. triple spool engines

Fri Dec 07, 2018 1:36 pm

parapente wrote:
Interesting point Matt ,could be.
The fact is 'something' has changed for RR to drop the triple spool concept after all these years.Although they do call their advance 2.5 spool.But if I am being honest I don't understand what that means ( love to know).
But ( I think) that the internal triple spool arrangement has nothing to do with planetary gearing ie you could have them both if you wanted.
Is it because the planetary gearing allows ( in itself) for optimal rotation of the key elements that the triple spool concept becomes ( sort of) redundant?
Love to know.

Triple spool allows matching (small) sets of fan/compressor mated to a set of turbine stages.
on every spool the turbine diameter is linked to the largest compress ( or fan) disk.
the longest compressor blade tips on a common spool going supersonic limit rpm for that spool.
limited rpm requires a larger turbine section with more surfaces.

The fan really is the biggest obstacle forcing low rpm on its spool.
if you can fit the IP compressor on its own spool that one can turn faster and decrease in size on both ends.
voila . weight saved.
Murphy is an optimist
 
sciing
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Re: GTF vs. triple spool engines

Fri Dec 07, 2018 1:48 pm

jetmech wrote:
hitower3 wrote:
- Fan & LP turbine at 4krpm on spool 1
- IP compressor and IP turbine at 12krpm on spool 2
- HP compressor and turbine at 20+krpm on spool 3


Hmm, might be a tad high with the N2 and N3 RPM's :scratchchin:. They would be closer to 7,000 RPM for IP and 10,600 RPM for HP.

https://www.easa.europa.eu/sites/defaul ... e%2002.pdf

Regards, JetMech


Trent 1000 is little bit higher.

https://www.easa.europa.eu/sites/defaul ... e%2011.pdf
Rotor
HP
IP
LP
Reference speeds, 100% rpm
13391
8937
2683

A PW1100G is much faster, near the range of what the OP wrote
https://www.easa.europa.eu/sites/defaul ... e%2005.pdf

Low Pressure Rotor (N1) rpm
High Pressure Rotor (N2) rpm
10047
22300
 
Aerospice
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Re: GTF vs. triple spool engines

Fri Dec 07, 2018 10:03 pm

Hi, the Advance engine is a triple spool, the Ultrafan is the 2.5 spool, 2.5 spool refers to the two HP and IP spools plus a stubby LP spool between the fan and gearbox. The reasons why RR are dropping the triple spool are described about 1/3rd the way down this:
https://www.aerosociety.com/news/pioneers-of-power/
The fan can increase in diameter and spin more efficiently while the turbine can spin faster, more efficiently and is significantly lighter. This makes the overall engine lighter even though there is a new heavy power gearbox module in the engine.
Regards,

parapente wrote:
Interesting point Matt ,could be.
The fact is 'something' has changed for RR to drop the triple spool concept after all these years.Although they do call their advance 2.5 spool.But if I am being honest I don't understand what that means ( love to know).
But ( I think) that the internal triple spool arrangement has nothing to do with planetary gearing ie you could have them both if you wanted.
Is it because the planetary gearing allows ( in itself) for optimal rotation of the key elements that the triple spool concept becomes ( sort of) redundant?
Love to know.
 
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rjsampson
Posts: 297
Joined: Sat Jul 31, 2010 12:00 am

Re: GTF vs. triple spool engines

Sun Dec 09, 2018 8:10 am

Francoflier wrote:
It is still a very elegant design, especially since the technology to make fan gearboxes didn't exist back then.


I thinking asked this on another thread: I fail to understand why the N1 GTF is now considered a “revolutionary’ advance. From turboprops to turnoshafts: Reduction gears from the LPT are nothing new. Is this just marketing? Or has there truly been some breakthrough in this particular PW engine’s gearbox (besides the obvious idea that its driving more air out the business end of the engine)... What is so special about this implementation of a gearbox?
"..your eyes will be forever turned skyward, for there.." yeah we know the DaVinci quote. But GA is so dang expensive these days! :(
 
WIederling
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Re: GTF vs. triple spool engines

Sun Dec 09, 2018 9:37 am

rjsampson wrote:
... What is so special about this implementation of a gearbox?


4..6 times the industry established power capability. predictive health detection. highly efficient ..

run of the mill turboprops handle 2..5MW
biggest turboprop handles 8 MW ( TP400D6, the NK12 @ 11..12MW has two power path.)

The P&W GTF PW1000G family gearbox seems to max out @ 25 MW .
Murphy is an optimist
 
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rjsampson
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Re: GTF vs. triple spool engines

Sun Dec 09, 2018 9:44 am

rjsampson wrote:
Francoflier wrote:
It is still a very elegant design, especially since the technology to make fan gearboxes didn't exist back then.


What about the GTF on the Lycoming ALF 502 engines on the BAe 146? Seems like those came out around the same time as the RB211?
"..your eyes will be forever turned skyward, for there.." yeah we know the DaVinci quote. But GA is so dang expensive these days! :(
 
WIederling
Posts: 7142
Joined: Sun Sep 13, 2015 2:15 pm

Re: GTF vs. triple spool engines

Sun Dec 09, 2018 10:08 am

rjsampson wrote:
rjsampson wrote:
Francoflier wrote:
It is still a very elegant design, especially since the technology to make fan gearboxes didn't exist back then.


What about the GTF on the Lycoming ALF 502 engines on the BAe 146? Seems like those came out around the same time as the RB211?

The turboshaft core of the ALF502 is a 3.6MW engine.
The RB211 should push ~21..25MW over the fan shaft
( more thrust than the GTF but also much lower BPR
super primitive assay via tsfc, full power thrust and guessed efficiencies.)

What just about worked for the ALF would have lingered in rapid disassemble for the RB211. :-)
Murphy is an optimist
 
WIederling
Posts: 7142
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Re: GTF vs. triple spool engines

Sun Dec 09, 2018 10:41 am

found along the road:
https://levyy.net.technion.ac.il/files/ ... gn-1-1.pdf
at piece on the how and why and some limits.
Murphy is an optimist

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