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Leeham News and Analysis
- CFM gets FAA and EASA certification for a more robust LEAP-1A turbine. December 10, 2024
- Global instability poses questions for aerospace industry December 9, 2024
- How Trump tariffs affected, and could affect, Airbus, Boeing and Embraer December 6, 2024
- Start-up hybrid plane OEM MAEVE teams with MHIRJ for sales, more December 5, 2024
- Production increase delays hurt Airbus costs December 2, 2024
Bjorn’s Corner: New engine development. Part 22. High Turbine technologies.
By Bjorn Fehrm
August 30, 2024, ©. Leeham News: We do an article series about engine development and why it has longer timelines than airframe development. It also carries larger risks of product maturity problems when it enters service than the airframe of an airliner.
We reached the turbine part on our way through the engine, where we last looked at high-pressure turbine temperatures. It’s the most stressed part of the engine and, in most cases, decides its durability. To understand why, we look closer at turbine technologies.
Figure 1. Our example engine, the LEAP-1A, is in cross-section with booster to compressor bleed valve area marked with a red circle. Source: CFM.
The High Pressure Turbine
Last week, we looked at the diagram in Figure 2. It shows the development of the high-pressure turbine inlet temperature T41 for long-range engines. T41 is the temperature at the entry into the rotating part of the first high-pressure turbine.
The graph is a Rolls-Royce graph from before 2010. I have extended the T41 development until today when we are around 2,000K for long-range engines and about 150K below for shorter-range (higher flight cycle) engines.
Figure 2. The development of T41 over time. Source: Rolls-Royce and Leeham Co.
The graph shows the different cast techniques used for turbine blades and how this has improved the possible T41s, together with cooling and coatings. One development that did not progress as fast as indicated in the graph is the introduction of CMC into the hot parts of airliner engines.
The CFM LEAP, which entered service in 2016, was the first airliner engine to incorporate CMC components, then in the outer shroud of the first high-pressure turbine stator (Figure 3).
Figure 3. The use of CMC to reduce cooling flow for the LEAP. Source: CFM.
CMCs’ major advantage is that they reduce the cooling flow needed by the parts. CMCs have been used for many years in ground-based gas turbines.
Introducing new technology for airborne engines is tricky. If there is any issue, taking the engines off the wing and into the overhaul shop is an expensive operation, and if the problem is epidemic (hitting many engines), spare engine availability becomes problematic.
Therefore, the introduction of new technology into commercial engines is carefully prepared. The parts have been proven in the labs, test stands, test aircraft, and ground-based gas turbines for many years. However, not all effects can be simulated. For the LEAP CMC, there were problems with the coating of the shroud blocks wearing off too fast. A change in the coating application fixed it before it became a major problem. The next GE engine, the GE9X, increases the use of CMC but, once again, only to stator parts.
Improved cooling and coatings
Instead of widespread use of CMC, lots of mileage was left in improved cooling and better coatings. Figure 4 shows how GE Aviation developed the high turbine cooling from the CFM56 to the LEAP.
Figure 4. The improved cooling of the LEAP turbine blades compared to CFM56. Source: CFM.
Through an improved cooling flow, the metal temperatures could be similar between the CFM generations despite an increase of the max. T41 from around 1650K for the CFM56 to 1850K for the LEAP. The result is a similar Low Cycle Fatigue (LCF) life.
The improved blade cooling requires more intricate internal blade flows. To prevent the cooling paths from being clogged with dirt and dust, the engine uses particle separation techniques to prevent particles from entering the engine’s core.
Figure 5. The particle separation of the LEAP. Source: CFM.
The fan will centrifuge most particles in the inlet flow into the bypass duct (1). For heavier particles in the center flow, the spinner shape will kick them into the bypass flow as well (2). Lighter particles will follow the booster flow into the engine core, where many will swirl against the outer wall after the booster. These will be ejected into the bypass stream by inward-opening Variable Bleed Valve (VBV) doors (3 and red ring area in Figure 1).
Still, light dust particles can enter the engine compressor and thus the cooling flow. GE had to develop a second generation of high turbine blades with adapted cooling channels for the LEAP, as in extremely dusty conditions, the blade cooling was clogged. GE developed special dust injection rigs for their test stands to verify the effects of these fine particles on the new generation LEAP blades.
Variable cooling flow
The need for turbine cooling varies depending on the airliner’s mission phases. Therefore, the last generation of engines implements cooling flow control to the turbines. It’s regulated by the FADEC, where maximum flow is used during takeoff and the initial climb. Then, the cooling is cut for the climb and even more for the cruise, thus improving engine fuel consumption.
Rolls-Royce was the first to introduce cooling regulation for the Trent XWB turbines. Pratt & Whitney and CFM introduced turbine cooling regulation for the GTF and LEAP engine generation.
The trade of performance versus durability
We have discussed different techniques to enhance the performance of the new generations of engines. As we can see from the above, these techniques improve engine performance and thus lower fuel burn and emissions. At the same time, the new techniques introduce durability challenges. We will spend the next Corner diving into this topic.
if you are curious enough, you can check the following GE public patent:
Variable cooling: EP3153661 A1
HPT dust seperator from GE9X: US 20180209284 A1
no talk about about HPT active clearance system (HPTACC)? i’m sad, it’s clearly the top tier device to improve efficiency during cruise compare to variable cooling…
We talked about the active clearance control in a previous Part when we described the turbine functions.
Normally at very high steady power you want the engine bleed valves closed, hence at low power taxing it works fine but I would be surprised it they are open and Max Take-Off thrust. You need service air thru the high stage valve to pre-cooler and Air Cycle Machines and anti-ice bleed valve air. The old PW4000 had HPT shrouds of ceramic on I guess cobalt alloy shrouds, they had to refine the manufacturing process to make it stick when the shroud deformed thermally from the hpt gas hot gases. Don’t know if the LEAP engines 30 years later had the same problem?
I gather the VBV ports are only opened the slightest as the particles slide the outside wall and only for takeoffs and initial climb in dusty environments. These bleed ports don’t serve the airframe/engine bleed system, the pressure gain is only about 3-4 times ambient pressure. These VBVs are for booster stall line control and particle separation.
This series has been very informative and well presented – thanks for putting it all together.
Y’know, though, it’s my impression that engine development has always been complex and subject to delays and maturity problems.
That doesn’t address the question of overall airplane development going from 4 years, for design on mylar with ink with physical prototypes, and new models coming out regularly, … to 7 or more years for computer-based design with 3-D modeling, and leadership becoming gun-shy and unwilling to take on technical risk.
Every airplane program I am familiar with encountered issues with design, testing, manufacturing, and entry into service.
The difference I see now, is that the 777 and prior programs had strong project management – starting with good initial planning, very strong communication and coordination, anticipating problems, moving quickly to recognize unanticipated problems, and reallocating resources to solve the problems. …. Strong problem solving culture enables strong project management.
What I see from the 787 program and later is shockingly poor project management – relatively poor planning, very poor coordination and communication, getting caught flat-footed by problems, then paralysis and failure to respond to the problems, compounded by siloed organizational behavior and beleaguered supply chains, rigid corporate boundaries, and conflicting incentives that encourage sub-optimizing around individual organizations instead of optimizing around the project overall.
Every effort spent on advanced engine design and operation will be well worth it. Relieving those technical bottlenecks would just move the spotlight to the tougher underlying social and organizational issues.
Thanks, Stan, for the insightful entry.
I agree. I studied the 777 project, which had a culture that was very close to what is seen today as Agile. Competent teams that communicated in an atmosphere of mutual trust and dedication to what’s best for the project. This was all torn apart by what I call the KPI management style, numbers to reach to be a performer. If the KPIs served the project or not was less important, reaching them served you.
Bjorn –
I’ve tried to understand the thought process coming from the financial community. One financial analyst told me, ” What I’m hearing from you [engineers] is that you’re saying you are different. No one is different. This [cost-cutting] strategy works for everyone and it will work for you.”
Over the years, I’ve thought back to that conversation. That financial analyst had no lived experience in aerospace or manufacturing of any kind, so he could easily imagine all manufacturing is the same. Of course, cost-cutting failed in lots of industries, so, maybe he should have just shown more humility.
That said, some financial analysts do have lived experience in aerospace – Ron Epstein, George Shapiro, and Peter Jacobs come to mind. They had no trouble being skeptical of the cost-cutting strategy for complex heavily engineered products.
McNamerra comes to mind!
But it should also be kept in mind, while all involved in the 787 had some issues, Boeing had the worst by far.
RR had issues but it took some years to show all of them (no sooner was one sort of resolved when the next came up)
GE had some but sorted the coating problem before test was done, the icing problem was resolved fast once it showed up.
When the 787 came along Boeing was well into the tear down period. What amazed me was they saved the program.
And how much right there was despite jumps in airframe and the elect rial system tech.
Fingers crossed that Ortberg can at least begin to correct things and mid term start them on a path to capable again.
The 777 project blew the budget and that made impact on the 787 project to try a “risk and revenue” type of finacial risk reduction by letting suppliers take the financial risk and letting them design their parts. That was a huge misstake by Boeing management not having their engineers control and double check everything they did carefully, a few suppliers were skilled enough but many failed.
” a few suppliers were skilled enough but many failed.”
If your customer does not know what he should want
any supplier is out on a limb. neither the (outsourced) functional module will work as necessary nor the interfacing.
If you add a “blood barrier” via dividing up into different corporate entities that pushes communication into MBA upper echelon levels ( i.e. people that have no grasp of the required transinformation talking to each other … ) failure is assured.
GE P&W has been succesful with risk&revenue suppliers but needed tons of engineering oversight.
Claes:
You are really wrong on what occurred.
Nothing was going to satisfy McNenearny (yea sp), he did not want to do an aircraft.
What happened was play right out of the MD book, lie, change things up and then shrug when they failed.
There were 3 major parts of the program.
1. Composite (airframe)
2. Much more electrical (almost to the point of all electric and no bleed)
3. Outsource everything and have it halfway across the planet.
I know some will disagree, but 1/2 actually did far better than would be expected. Yes there were problems and the -8 still stands alone as its own build (with some rationalization to the -9/10 in the back). Wing join was another one but restoring cut structure was relatively easy enough.
What was the dagger was believing the BS (or presenting it in the first place) as you could get something for nothing.
Any of the problems in 1 and 2 were fixable. The debacle of a non coordinated world wide mfg system, that is what broke it. The battery issue was a result of item 3. Scattered in 4 different places with no cohesive management (and the insane confirmation of it all being a nail driven into the battery). The fix was quality control (with adding enclosure but stopping failures was at the core of the fix, enclosure was added insurance to make up for the debacle of the rest.
I can’t begin to describe the insanity of not having a Boeing team at each site to ensure standards and quality were met and the fasteners were a cascade of that as each entity that had them hoarded them because they got paid for what they made not work flow.
The 787 problems were a a management failure that was all about stock prices and the ill gotten gains for keeping the stock juiced up.
That is what has finally been acknowledged by the board and why Calhoun is out (a total looser) the ouster of the Chairman of the Board is outed and and Ortberg in as the CEO.
Boeing engineering wanted a close monitoring of many “partners” engineering/manufacturing but did not get the OK for the cost/resources. Then you got the product/quality and schedule delays.
I do not know what Boeing enginering got but the something for nothing meant no teams.
When they realized the magnitude of the debacle, they formed a whole bunch of multi discipline teams including finance. It took a full on team to assess, figure out where the problems were and how to fix them.
Going form memory it was 20 teams. They went from one problem mfg to the next. Again from memory it was 10 or 20 per team.
No one that is serious would have done a program that way. But that was the early phase of liquidating Boeing.
GE : all very well described devices to eliminate dust at HPT level should be very relevant for Emirates hard environment operation on GE90X. Does RR use the same devices for Trent 97K on A350K?
Great question. Add in the GenX and Trent 1000/7000 for same.
GP7000 does not seem to have issues in the ME
GP7000 is an extremely conservative “brewery horse” design.
Trent1000, GenX were the upcoming racehorses.
And AFAICS GE shone due to massive media protection.
( AF GP7000 “loses its head” and a shitstrom is released against RR , absolutely amusing! )
Reputation mgmt worths big bucks.
A bit off track but the GP7000 was more than a match for the RR900, even the RR900 Super Duper. Now the SD was purely a work of TC imagination (well spin as he said it was 5% better than a GP7000 which was absurd – but he was trying to leverage Airbus and RR into A380-900 and was desperate to keep it going even if he had to lie.
The GP7000 had a 2 to 3% better SFC than the RR900 did and by lack or reports, its dessert performance was fine. I don’t know the RR900 had any dessert problems either. Call them even there.
But, a 3 spool costs more and it cost more to overhaul. So unless you get better SFC and recover those two costs, a 3 spool winds up a net negative.
Prior to the GP7000, RR had it covered. GE was close on SFC but not even or ahead. Engine buy costs were less as was overhaul. The fiddly parts like fuel pumps and such are a wash as those are not more complex in a 3 spool.
The 3 spool may have still been better SFC, I am not remotely an expert though I have followed the fortunes of PW/GE/RR as they are engines and engines have always been of interest (though all the work other than some maint a a Solar Generator plant was piston engines).
If the 3 Spool could have been with better design? Maybe there was a tech level that once achieved a two spool was better than a 3 spool.
All GE (and its partner on it PW) engine from the GP7000 have been better SFC. The Trent 10 was an effort to match the GenX and it did not succeed.
What happened within RR I have no clue. But the reality is various issues with the 900/1000/Ten and the XWEB -97 did.
RR clearly has determined that 3 spool is not the future so maybe they hung onto it too long.
PW stole a march on all with the GTF (then suffered severe teething issues). Still they got two exclusives on airframes that will sell (are selling) so they get an offset having nothing on the MAX.
Some have determined that I don’t like RR. I am fine with RR. Clearly they have struggled and to gloss over that would be the same as glossing over Boeing issues.
you are fertilizing the wrong tree.
Engine Alliance built the GP7000 and involved more partners than the major ones of GE and Pratt.
Safran 17.5% was aligned with GE side and MTU 22.5% with both the Pratt and GE.
As for SFC
“According to one source, EK determined the RR Trent 900 was determined to be up to 4% more efficient than EA”
https://leehamnews.com/2015/04/16/rolls-royce-displaces-engine-alliance-for-emirates-a380-order/
Duke:
EK determined, based on being behind on SFC and suddenly you move the needle to 4% ahead?
Without a new engine that is absurd.
It all came from TC and he was trying to get the A380-900 as well as an all new engine. He thought he could lure RR into a trap by switching to the 900 Super Duper as a stop gap.
The Trent 10 which is virtually an all new engine finally met the spcs RR had promised, but in the meantime GE has pipped the GenX as well as tech insertions and stays ahead.
RR kept the core of the 1000 only and that is where the blad issues were.
The 900 SD may be ok, it may have moved up to even (reports of blade issues on that) but it sure did not jump 6%.
You missed a huge upside of the three spool … it’s much lighter than the two spool, from memory something like 4 tons a shipset on the 777-200 and 200 ER. So even at comparable SFC (and I believe the Trent is better) the payload / range equation is signifiant. GE’s major advantages were better ground handling and, more significantly, the massive purchasing power of their leasing company. As the Trent continues the three spool architecture I’m not sure what you mean by ‘clearly decided’ it’s not the way forward. RR does continue with 2 spool on smaller engines but the weight, stiffness, lack of the variable guide vane complexity and performance advantages payoff in large engines.
The Ultrafan technology is a geared front fan instead of being 3rd shaft driven.
The other features are OPR is 70 to one, and BPR is 14 to 1
The 3 shaft arrangement allows to run more of the LP(C/T) section at higher rpm and smaller diameters. Only the turbine stages that drive the fan have low rpm and thus require a large diameter.
The gear reduction between Fan and LPC provides for the same effect. : GTF supersedes 3 Shaft layout.
Curious to know whether it is feasible or desirable to contemplate at least a 2-speed transmission setup that would allow the engine to run at a higher gear ratio at cruise altitude
No idea if it makes sense at all.
beyond that:
I don’t see a solution that would not incur a major weight penalty ( a second epicyclic gear set and the clutch arrangement for switching.)
WC:
I had forgotten the weight aspect. It still cost more due to its complexity and it cost more to overhaul.
It has to make up that cost difference and exceed it to be competitive.
What we do know is that the Trent TEN, it is still below the GenX by 2%. That was almost an all new engine (anything over 50% difference is considered new).
As RR has gone with the GTF, my take is that 3 Spool reaches its limits and the added cost becomes a hindrance.
I believe they used to get 2-3% better than two spool and a lot of airlines went with RR.
Now you see Airlines dropping RR. NZ and ANA being two major ones but I believe another order went in with a switch recently.
Found the link I was looking for, Thai and LATAM dropped the RR Ten as well (there is only the Ten for new aircraft, the 1000 is supported and has upgrades but not the engine offered)
For a fleet that is full invested in an engine to shift means that its in negative territory by a lot. Airlines invest a lot in engines just like commonality of cockpit.
Its bad when someone drops your engine, really bad. When top Airlines are doing it, they have given up on trying to get ahead and understand its not going to get better and moved to GE.
Complexity?.
That doesnt add up essentially there is only one extra turbine stage for XWB compared to GE90–110 but the downside for the GE is is that it has 2 stages of blades in the very critical HP turbine compared to 1 stage for XWB HP ( plus 2 IP and 6LP turbine)
RR also uses variable stators in the IP compressor since the Trent 1000 and after – the IP compressor design manufacture is done by Kawasaki
Duke:
More parts, more bearings.
Its worth it if it pays off in longevity and SFC. If it does not, then more costs and negati8ve return.
There is a reason the Trent 700 became the dominant engine on the A330 (before NEO)
There is also a reason GenX has at least 66% or the market on the 787 and that keeps changing to the worst for RR.
Remember the argument was the 3 Spool paid off on long distances?
Now its, well it wins if the distance is short. Bjorn did an article on that.
Back in the days the RB211 got popular due to time on wing and keeping its performance (EGT) while the CF6-80 and PW4000 lost more performance with time (mainly due to blades rubbing different seals). That kind of changed with the GE90 structural design that was copied to GP7000; GEnX, GE9X and LEAP-1 that keep its performance better (still some have other problems right now). RR now going for 2 turbines and spools with a geared fan on the Ultrafan getting more like a GE9X with a geared fan.
Large high BPR turbofans are already highly complex beasts.
A few extra bearings doesnt change much in the parts count…. have you ever bought a car because its got *less* engine bearings than a rival car…didnt think so.
Not really comparable but doesnt illustrate your line of thinking
The bearings in a jet engine are core design elements
coming with a wide problems domain.
The latest stock movements on Boeing IMO indicate a growing awareness about the current market situation of Boeing.
The door that fell off, sometimes seems just a perception decoy. Solving that issue doesn’t change the total situation.