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Two or Three Spools?

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Two or Three Spools?

Old 7th Apr 2021, 20:17
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Two or Three Spools?

A question here for those in the know after a debate on another forum. Is Rolls-Royce new UltraFan design considered a two spool or a three spool engine? On their homepage RR describes that the engine uses Advance 3 core design, but as I see it the UltraFan only have two turbine sections and only two concentric axels, with the inner axel driving the LP compressor as well as the fan section through a reduction gearbox, kind of the same way as on Pratt & Whitneys GTF design.

Any thoughts?
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Old 7th Apr 2021, 20:38
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Looking at the mobile page (so who knows how the hell it relates to the normal page) as well as the Wiki, it seems that the Advance is a 3 spool engine like the rest of the 211/Trent lineage, and the Ultrafan is based on that but goes to 2 spool/geared fan
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Old 7th Apr 2021, 21:18
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well if you took away the fan and just ran it as a pure jet, the question becomes how many stages of compressor and small dia turbines do you need to achiever an optimum pressure/fuel burn.

This sounds like a Tdtracer analysis is needed (keep the nacelle drag low and the weight low and at the same time improve the fuel burn enough to fill a niche)
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Old 7th Apr 2021, 22:10
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When PW announced the GTF concept I remember a spokesperson from Rolls being a bit sniffy saying its like having a "two and half spool engine".

Looks like PW are on to something after all... as long as they get the bowed rotor problem sorted. 😁
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Old 8th Apr 2021, 11:16
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I agree.
at time 30 secs says "we have removed the low speed turbine" and the diagram only shows 2 spools.
The IP turbine drives the fan through the new clever gearbox - but it is still marketed as having 3 shafts.

Last edited by Goldenrivett; 8th Apr 2021 at 11:22. Reason: extra info
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Old 23rd Apr 2021, 05:23
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The reduction gearbox allows the fan to be driven directly from the IPT with both at optimal RPM. So LPT (running at a lower, fan optimised RPM) is no longer required. It's still essentially a three shaft engine it's just recovering the fan drive energy mechanically, directly from the core, instead of via LPT

As I understand it, by running each stage closer to optimal RPM, the gearbox allows high OPR over fewer, smaller diameter stages, so the core is more efficient, has less parts and is lighter. However the forces on each compressor blade are much greater because the pressure ratio of each individual stage is then higher.

This puts RR in a very good position to develop an efficient large GTF because triple spool core architecture shares similar characteristics (i.e. less stages, high pressure ratio per stage.) so their previous experience with blade materials/cooling/geometry etc. will be applicable.

I think P&W did well to develop their GTF and it's certainly been a commercial success. But they will need to work on the core to scale it up. RR have the advantage with core tech but they are still working on the fan and gearbox. So essentially the two manufacturers are pursuing the same goal from opposite directions.

Last edited by Sonikt; 24th Apr 2021 at 00:52.
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Old 23rd Apr 2021, 12:42
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As the chief engineer says at 1:33 in the video it's a two spool engine, but it's a three shaft, the gearbox/fan counting as a separate shaft is my take away, spool and shaft having different meanings.
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Old 24th Apr 2021, 00:25
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Yes I agree. The "shaft" driving the fan, not being in any way part of the core, is not a "spool" in the classical definition. But the average observer won't spot the subtle difference here.

​​​​​​Manufacturers often use different terminology to highlight their own products. I guess in the case of RR we could say it's just marketing bluff to call their GTF "three shaft", due to the similarly of the expression with "three spool", for which RR decades of experience is renowned.

Maybe it's a bit misleading but as per my previous, the comparison does still have some basic technical merit, so IMO it's "honestly misleading", if such is possible! Unfortunately for RR most people are somewhat cynical about manufacturer claims these days, hence this thread.

Perhaps more amusing, is that RR persist with the expression "intermediate pressure spool" when there is seemingly no longer any low pressure section. No doubt will become something for the old hands to explain to the young uns' why this anomaly exists!

Last edited by Sonikt; 24th Apr 2021 at 00:40.
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Old 24th Apr 2021, 22:51
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LOL you can have that one if you like 🙂

But seriously, "refinement" is not really the issue IMO, flexibility is. A key characteristic of triple spool, is that fan RPM is fully independent of core RPM, because it's on a separate shaft. This has long been commercially advantageous to RR, due to much lower development costs for simple derivatives.

But as you rightly pointed out, RR don't say to customers "triple spool is best because it's good for RR" they have to find another way to say it's better 🙂

With triple spool, a common high/intermediate pressure core (i.e. the difficult/expensive bit) can be easily (and therefore cheaply) scaled up or down, without significant modification, just by throttle push. Only the (separate, lower cost) LPT need be tweaked to achieve optimal fan RPM at a range of throttle settings/BPR.

GTF can achieve the same characteristic, but via gearbox setting, instead of via LPT setting. It's no coincidence, that RR are designing their gearbox, to work at a range of different ratios.

Last edited by Sonikt; 25th Apr 2021 at 00:15.
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