Triple Spool vs Double Spool

[Andrew804]

Hi guys,

I recently had an interview where I was asked the difference between British made Rolls Royce RB211 and American made Pratt&Whitney 4000's, now I answered by saying the Rolls is a triple spool design where as the PW is a double spool design and this was right. I was then asked if I knew why Rolls decided to go with the triple spool design and PW didn't. I wasn't sure of that answer. So....
I've done a bit of reading and found it maybe had somthing to do with allowing each spool to spin closer to its optimum speed, therefore reducing fuel burn.
Is this correct and is this the only reason or could there be something more?? If someone knows of a book or website I can look at that would be good aswell.

Thanks alot!
Andrew

[Marant]

Is that a realistic interview question?
Can you do anything about it even if you know your engines are triple or double!
Good for you Andrew that you had an answer, but, really, isn't there a thousand other questions out there more aligned with what really matters!

OK...I'm ready for the flak now.

[lomapaseo]

Your book is correct and gives a simple enough answer. Your interview answer was also correct. A TV interviewer would have broached a technical question to you beforehand to avoid a stuttering live answer on film.

An opposing lawyer would have nowhere else to go with your answer.

Nobody should expect immediate answers to every question except on PPRune

[411A]

I've done a bit of reading and found it maybe had somthing to do with allowing each spool to spin closer to its optimum speed, therefore reducing fuel burn.

Another benefit...quiet ops.
The L1011 was (is) stage three at all weights...the early B747's and DC10's were not...

[Papa2Charlie]

RB211 Series

Hi Andrew,

The primary reason for the three shaft architecture is to have the compressor / turbine stages running nearer to their ideal speeds. Aside from reduced fuel burn, the three shaft design means there are less variable stages in the compressor which reduces complexity and weight. The RB211 is significantly lighter than its competitors which translates into additional aircraft capability. There is also a school of thought which says that the RB211 has better surge margin than the competition due to its three shaft architecture as the IP and HP compressors are not as highly loaded as a HPC in a PW4000 for example.

Another claim that Rolls-Royce make is that the three shaft design results in a stiffer core (less bending) meaning the performance retention is superior to a two shaft design. From a line maintenance perspective, the RB211 is alot easier to work on than its competitors. By mounting the gearbox on the fan case, it frees up alot of room around the core engine meaning access is easier. The fan case mounted gearbox also means it is possible to change the gearbox on-wing rather than having to remove the engine.

If you want additional information, RR sell a book called "The Jet Engine" which is very well written and explains most of this stuff in considerable detail.

All the best,

P2C

[Dual ground]

And if you ever do throttle slams on engine runs you certainly notice that they spool up faster.

[XPMorten]

The RB211 is significantly lighter than its competitors which translates into additional aircraft capability.

According to the FAA certificates, the RB211-524 is significantly HEAVIER then the PW40XX...

PW40XX 9,420 lbs

RB211 12,731 lbs

Which makes sense from an engineering point of view since a more complex design usually is heavier than a simple one...

M

[barit1]

The published weights probably don't tell the whole story, because one engine type may not include certain QEC gear that needs fitted before the airplane is ready to fly. I don't have details handy, perhaps someone can fill the void so we are comparing apples to apples.

Offhand I'd expect the three-shaft engine to be a bit heavier (D'OH! One more heavy shaft...) plus more bearings, oil system, etc. but this could quickly be repaid in reduced fuel burn.

[Flap Track 6]

Papa2Charlie, your answer was very complete except for the bit about weight. The three spool architecture should have produced a lighter weight engine, but the RB211 was the heaviest engine on the B747. However, the 3 spool Trent is the lightest engine on the B777.

The thread starter could have answered the interview question with the answer 'The Rolls engine rotates the opposite way to the Pratt one'.

[NSEU]

The three spool architecture should have produced a lighter weight engine, but the RB211 was the heaviest engine on the B747.

Someone once told me that it was only heavier because the opposition didn't include, in their basic weight specs, engine parts that RR considered standard (possibly cowling, reversers, etc). GE cowling, I believe, hangs off the strut, rather than the engine.

The RB211 may be quicker to spool up (once running), but it takes significantly longer to ground start

[Junkflyer]

But it sounds great when it lights off.

[aterpster]

NSEU:

The RB211 may be quicker to spool up (once running), but it takes significantly longer to ground start

Didn't bother us at all on the 1011. Once it was running it was a gem.

[Bullethead]

My experience with the RR RB211-524H v GE CF6-80C2B6 engines, on two different aircraft types, is that the RRs are heavier, take longer to start, burn more fuel and produce less thrust.

The triple spool theory sounds good but the reality, with these two engines at least, is different.

Regards,
BH.

[spannersatcx]

The PW appears to take longer to start than the RR, the SFC of the RR is better than the PW, the RR includes the reverser the PW doesn't which gives the weight difference, crews say the PW is noisier in cruise!

Was the interview with CX by chance as we operate both?

[rmm]

I once checked the 744 maintenance manual about the weight figures. It gave figures for both the CF6 & RB211. The total weight for each engine included thrust rev's, inlet cowl, fan and core cowls. Essentially it's all the weight hanging from the pylon. The Roller was 500kg heavier. No info on the PW4000 though.

[MrBernoulli]

Another possible advantage of the 3-spool over the 2-spool is physical size. Having flown B777 with GE90-76B and -85B engines, and also RR Trent 895 engines, the triple-spool Trent has significantly less diameter around the engine cowling than the GE (around 24 inches less!). This gives greater ground clearance below the Trent, when compared to a GE hanging on the same aircraft type. Not of huge significance on the B777, I know, but it may be on other aircraft where ground clearnce is more critical?

I guess the already-mentioned efficiency (running closer to optimum RPM) of the 3-spools allows reduced fan diameter. In the case of the Trent 895, it also produces more thrust than both marks of the GE90 mentioned, but still has slightly better fuel economy (3-spool efficiency again?).

[XPMorten]

According to Airport Planning Data, these are the Operating Empty Weights;

B744
CF 394.088 lbs
PW 394.660 lbs
RR 396.284 lbs

B772
GE 298.900 lbs
PW 296.600 lbs
RR 293.400 lbs

B757
RR 134.090 lbs
PW 128.380 lbs

[Papa2Charlie]

Hi again,

Thanks for the corrections. To be honest, the only comparative figures I had seen were for the Trent 800 so my apologies for the error in my original post.

All the best,

P2C

[barit1]

Junkflyer:

But it sounds great when it lights off.

30 years ago I was conducting a flt ops survey on newly-delivered 747s - a flock of mechs were on a fam tour of the cockpit as the a/c wasn't scheduled to go for an hour or so. So rather than add to the confusion I took a pax seat in the upper deck, enjoying a bit of breeze from an open hatch.

Then I heard a sustained foghorn-like sound. At first I thought "What ARE those guys doing that makes that noise? - hydraulics, or what"??

After a bit I noted a TriStar 1/2 km down the ramp blowing a BIIIG cloud of fuel vapor out #2 exhaust. When the vapor finally stopped, so did the noise.

[no-hoper]

For maintenance the Trent 900 compared with the GP 7200 is much easier and faster to handle.Most action on the Trent requires only one cowl to be opened-the GP needs all four and special steps/lifter to work on it.
But the GP is 222kg less heavy and
sounds much better during startup.To my mind the future will show
geared fans in all classes and no more electric driven reverser like A380.