Bjorn’s Corner: New aircraft technologies. Part 10. Engine choice

By Bjorn Fehrm.

April 28, 2023, ©. Leeham News: This is a summary of the article New aircraft technologies. Part 10P. Engine choice. The article discusses the engine architecture choices that must be made when developing the next-generation airliners.

Figure 1. The Pratt & Whitney high bypass geared turbofan technologies. Source: Pratt & Whitney.

Engine alternatives

We concluded in earlier Corners the most important decision for the next generation airliners that today is called the single-aisle segment would be the fuselage cross-section based on the need to house more than 250 passengers over the life of the project.

A single aisle configuration is not certain; in fact, it would start life on the limit capacity-wise as a longer fuselage than 50 meters doesn’t fit in the present single aisle airport gates. Thus, instead of calling it a single aisle segment, we call it the Heart of the Market segment.

The second most important choice would be the propulsion technology. For an aircraft of 250 seats or more with a range of 3,000+nm, the only alternative propulsion would be hydrogen burn gas turbine engines. These engines are hydrogen versions of hydrocarbon-fueled engines, so not a big change, but for the fuel system and the airport infrastructure, it’s a major change.

We, therefore, don’t see the hydrogen aircraft in development (STC conversions or new designs) replacing the heart of the market aircraft for the 2035 generation. These aircraft will use an engine based on further developments of existing technology as described in the last Corners.

Turbofan or Open Rotor?

The big question will be if the next generation engine for the most produced airliner segment will be a high-bypass turbofan or an open-rotor engine?

A next-generation aircraft using a very high bypass turbofan is known technology. The risk level of such a project is low.

The most attractive open rotor design is the CFM RISE engine with a front-placed open rotor followed by a fixed second stage of vanes, Figure 2.

Figure 2. CFM RISE engine with its front-mounted fan stage, followed by a fixed second stage. Source: CFM.

We have compared the achievable efficiency improvements with the two engine types, where the open rotor comes out on top.

Will the increased efficiency motivate airframe designers to change to an open-rotor design? It will have installation consequences, Figure 3.

Figure 3. The CFM RISE engine installation alternatives. Source: CFM.

How large will these be, and how will they influence the overall efficiency of the airplanes?

We will cover this when we have looked at the airframe improvements that are achievable for the next generation of airliners.

53 Comments on “Bjorn’s Corner: New aircraft technologies. Part 10. Engine choice

  1. If the engines burn hydrogen it can still have a fuel cell APU and taxi with electrical power. One can wonder if part of the breaks can be developed into motor/generators for taxing and charging batteries. Having LH2 available can then open possibilities to enhance engine performance and reduce cost as certification requirements evolve. Airports connected to natural gas pipelines will be the first to locally produce LH2 either from natural gas or from H2 mixed into the natural gas.

    • Where possible/viable, I can imagine that AB will embrace the use of fuel-cell powered electric motors for propulsion. Although this brings considerable technical challenges with it — MOFCs, superconducting, etc. — it rids the OEM of dependence on external engine manufacturers (in addition to producing truly zero emissions). Perhaps not suitable for the “heart of the market” referred to in the article, but perfectly adequate for a huge segment of the shorthaul market.

      • More info:
        “Airbus unveils hydrogen fuel cell propulsion system”

        ““At scale, and if the technology targets were achieved, fuel cell engines may be able to power a one hundred passenger aircraft with a range of approximately 1,000 nautical miles.”,aircraft%20within%20the%20next%20five%20years%20it%20said.

        • ” *if* the technology targets were achieved,”

          They are just projecting the unknown unknowns out to 2035…… as if that will solve anything except provide greenwashing till 2030 when the targets will be ‘retargeted’

          • The engine is being tested as we speak — more than can be said for the RISE, for example.

          • Anything is possible at small scale
            ‘500,000 RPM matchbox-sized gas turbine produces 100 watts”

            Batteries can good numbers from bench test, but generally high energy storage and high short time output dont go together

            Bjorn covered the downsides in his series . Part 22 and 23 I think. For one a full installed *system* for aircraft use only had a very low efficiency compared to bench testing

            Airbus hasnt run any fuel cell engine, no matter what your Airbus break room chatter tells you, its all carefully crafted PR to cover a range of bench testing of separate parts
            As quoted from Airbus
            “a fuel cell system integration bench featuring a 200kW fuel-cell stack and a climatic chamber; a functional integration bench (FIB) for the engine control system to validate real interfaces and actuators; and a Propeller Integration Bench (PIB). The latter will explore propeller pitch actuation gearbox integration, gearbox endurance, and partial blade-release tests.”
            See how many time ‘bench’ is used.
            Some artwork of an A380 flying engine testbed doesnt a actual test make

            A 200kW fuel cell ? The Cessna Caravan small PT6 gives 500kW

          • Yes, Duke, we know you had difficulty grasping/accepting the end of Bjorn’s series on fuel-cell hydrogen propulsion.
            Well leave it to the competent people in the Airbus hydrogen collaboration to see what this yields. That collaboration includes some big names — you should take a look 😉

          • I have no problems for Bjorns series on hydrogen, I learned a lot like all his series.
            However you havent learnt a thing as he pours cold water on H2 fuel cells for main propulsion

            The real experts at Airbus know this as well, hence their well crafted PR (to fool the gullible ) and bench testing of small fuel cells which will suit APU !!

            ‘As for the battery-based electric or hybrid airliners, we find the promotors of fuel cell propulsion are *far off the reality*, even when we use their most optimistic data and leave all spiral consequences of the inefficiencies we find by the side. Why is this?

            I can only assume they are blinded by the fuel cells’ 60% efficiency and neglect that today’s gas turbine cores are at 55%. When we add up the losses and the weight/volume penalties, it’s clear the hydrogen gas turbine is the clear winner for delivering shaft power to propellers, propfans, or fans for hydrogen airliners, be these 165 seater jets or regional turboprops.


            Print it all and put up on the Airbus break room

          • Yes, Duke, I recall very well that you had difficulty with the topic at the time.
            Thanks for the Leeham quote from 2021.
            Now, here’s a Leeham piece from 2022 — which you seem to have “forgotten”:

            “An air cooled high temperature superconducting system brings masses, losses, and cooling problems to levels where there are solutions without the aircraft going from a projected 70-seater to a 60 or 50-seater.”

            “The needed system engineering work is daunting, but other steps in our air transport history had similar challenges. It will take time and require substantial investment, but on a physics level, everything is doable (“We don’t need changed physics to make hydrogen aircraft work”: Airbus CEO Guillaume Faury).

            At the end of a long road, we have true Zero-emission airliners with useful operational characteristics.”


          • As I see it the newer article merely confirms the high temp *and superconducting* hydrogen fuel cell is way ahead of the battery based electric aircraft.

            I dont see that as being as good as the hydrogen gas turbine, neither does Bjorn say it is

            Its still the same old same old ‘unavailable technologies’ are the way forward

            And I repeat you made complete falsehood that a workable system is being tested by Airbus.
            To quote _ “The *engine* is being tested as we speak “
            another from your usual orchestrated litany of lies.

          • Duke, the fuel cell option doesn’t have to be as good as the turbine option in terms of propulsive efficiency, because the fuel cell option offers lots of advantages relative to the turbine. Several of those advantages have already been set forth here — can you find them?

            And here’s a detailed article on the tests being done with AB’s fuel cell propulsion unit:

            See? No litany of lies from me — the problem is at your end. It all seems to boil down to reading comprehension…

          • Nope . Your falsehood was saying the engine was being tested (by Airbus) as we speak.

            I leave the various efficiencies of the systems to Bjorns excellent analyses.

            What really happened to the Airbus/Rolls e-zero-X HS146 test flights . That seemed promising as a technology demonstrator that could actually be ready to fly instead of being stuck on ‘benches’
            Cancelled and substituted by another set of acronyms which is pushed out into the future before it flies too.
            Theres a pattern starting to happen

            meanwhile DHC working with Pratt are doing a flying
            single converted engine demonstrator in an actual typical TP which could be this year

            let us know when a proper flying demonstrator of this type – with a single fuel cell engine- is scheduled .
            Just repeating the Airbus blurbs – which I quoted too- about various separate bench testing of components does not make it an ‘engine test’ as you claimed.

            You do know what an aviation engine test cell engine run is like do you?
            Its a ‘fully assembled engine system’ run of course , for various lengths of time, to validate the components before it gets into the air on a flying test bed.

            As an aside, found an image the other day of a Rolls Royce flying test bed of a VC-10 with a RB211 at the rear one side to replace 2 Conways. A different era but the process is the same

          • Looks like Duke didn’t pick up on the difference between “testing” and “flight testing”.
            Subtle differences like that seem to evade him a lot…

          • Bryce , I pointed out that test cell long runs occur before flight testing. Thats what ‘engine running’ means. The whole system compete ( without nacelle of course) and able to run continuously.
            Thats what happened when Frank Whittle was able to be the first to have a complete jet *engine running*
            he wasnt testing individual components in different ‘labs’

            What airbus is doing is bench testing components trying to understand the technology as it isnt mature enough to put it all together in a test cell.

            Like I keep saying, get back to us when Airbus has a test cell engine running and doing its 20 hr , 50 hr, 150 hr runs etc

            Even the computer Im using came in parts ( Intel Bare bones and I carried over the SSD from my previous computer) it wasnt a computer till it was all up and running . Yes it needed some fully assembled testing as SSD boot sector was out of date. ( another story)

          • Ah, so now that Duke has gone back and accurately read what I wrote, the melodramatic “litany of lies” syntax has been dropped.
            Mind you, it took long enough 😏

          • The claim of Airbus ‘engine running as we speak’ was a lie…. from you.

            Airbus doesnt make that claim which you refuse to admit you got wrong when advised of the exact wording used by Airbus.
            They dont even have a 200kW engine running just a bench test fuel cell- fuel cells have been doing this for some time- let alone a 2MW one required for the future proposed engine
            Its all part of your MO.
            I read recently too that all airlines in China that had Max delivered are operating some or all of their Boeings.

          • @ Duke
            Reading skills (again)!

            I didn’t say “engine running” — I said “engine being tested”.

            There’s a difference.

      • This is an interesting point. Engines are about half the value of an aircraft and a change to new technology gives the OEMs and others an opportunity to capture some of this.

        • More importantly: it reduces the aircraft OEM’s dependence on engine manufacturers — and on “temperamental” turbine technology.
          Just look at the misery that is still being inflicted upon aircraft OEMs by the ongoing dramas with certain RR, PW and CFM products.

          The price of a motor should also be *much* lower than that of a turbine — even factoring in the new tech involved in (MOFC) power generation and (superconducting) power transmission. It should also be a lot easier to maintain, and a lot quieter into the bargain.

          • Todays fuel cells are neither cheap or light weight in the 100’s kW class.
            Going to High Temperature Proton Exchange Membrane fuel cells (HT-PEMFC) can open possibilites to use its heat to generate thrust. Locks like turboprop and APU applications first. Getting LH2 available at airports will take decades as they initially has to take the investment cost. H2 for fuel cells will probably come from Linde and AirLiquid in trucks in the EU.

    • You don’t need to use hydrogen or a fuel cell to taxi electrically. See wheeltug.

      • The option to integrate it into the breaks was the novelety and thus save mass using the rotors/staors in the breaks as motor-generator rotors/stastors.

  2. There was an interesting conversation on Anet about Embraer tabling their next gen turboprop because there is no next gen turboprop on the horizon. Then someone mentioned maybe a smaller RISE engine for the project. The RISE engine truly is blurring the lines between turbofan and turboprop. Curious how CFM fixed the NVH concerns.

    • In theory , with the prop in front and geared as well as blade profile.

  3. What about the open rotor blades themselves ? Have we seen toroidal blade technology coming to life for TP or Open Rotor applications ?

    • Doesnt feathering of TP blades limit that sort of design direction. Fan blades are much smaller diameter and cant be feathered to reduce drag.
      But maybe there is a way ?

      • Adding complexity and weight and its an excellent point, RISE does not address the engine out aspects.

  4. A single aisle configuration is not certain; in fact, it would start life on the limit capacity-wise as a longer fuselage than 50 meters doesn’t fit in the present single aisle airport gates.

    I’m glad that airport limitations on airplane length are being addressed, as normally we only hear about the wingspan limits (aside from speculation about A350/A380/B777 stretches exceeding 80 meters). But is there actually a regulatory or practical 50-meter limit? From the past, the Douglas DC-8 and Boeing B757-300 (which still in service IIRC) are both longer than 54 meters.

    • New to me as well, look forward to discussion.

      I do know some gates have issues with the length as it impeded the Taxiway and aircraft on each side.

  5. Considering that the A321XLR will already have a range of 4,700 nautical miles using existing engines, the next generation of single-aisle or HotM planes will become range monsters when you factor in another double-digit reduction in SFC. I wonder how much damage that would do to the widebody sector.

    • Not really . Wingspan restrictions to allow them to fit most single aisle airport terminal gates will limit fuel load.
      The A321XLR has gone to extra ordinary lengths to get around its internal fuel capacity problems and included full pass baggage load that longer haul travellers require. 5000 nm seems to be what Boeing is aiming for for the best weight efficiency without sacrificing hold baggage /freight and still fit in the
      allocated apron space ( with clearances) for a standard terminal gate

      • Don’t discount folding wings ends ala the 777X though that may be an issue with the long wing designs needed for maximum drag reduction.

        • Bjorn makes the point in his series on airframe that the induced drag component is dependent on span and pressure distribution not aspect ratio ( or long wings)
          The 777X is a special case of wing , not a generic one for say under 5000nm

          • Not sure what the disagreement is here.

            Properly designed long and thin wings are better economics than short and broad. Airbus was the first jet mfg to start further down that path, The A330 set the trend.

            But Sailplanes and gliders have done that for a long time (yes they are low speed).

            A TBW is going to have a longer wing. Ergo, the gate width issue. Also ergo, they know how to design said wing but it will be longer regardless. A folding tip was the 777X solution to that, but that is also some weight aspects as well as control surface avoidance.

            Gates are packed now and Seattle (SEATAC) is fighting with their construction contractor on a major miss for clearance. Its only going to get worse.

            I have no answers but I do understand the issue

  6. What about the noise emissions of open rotor designs? A lot of airports are noise restricted or charge planes for noise levels.
    CO2 is one thing but noise will generate more direct opposition by airport neighbors and would explain why open rotor gets so little actual traction. A demonstrator would be easy to setup though…

    • How much noise is generated by the turbine of a turboprop vs that from the propeller? And how much for an electric motor vs the propeller? I’d expect electric motors to be a lot quieter, even with open rotors but am I being too simplistic?

      • RISE is either Jet or Hydrogen fueled.

        Electric motors are ruled out with the issue of conversions of say a fuel cell into a usable form of electrical (voltage and frequency matching) to drive the prop. DC is not viable, AC is.

        • Roger:

          So called open rotor has a noise factor. That is one reason it keeps morphing around while they work for a solution.

          Solutions to noise reduction tend to cause efficiency problems so they take a lot of design work around.

          RISE claims they have solved that with front facing single rotor and the guide vanes on the case. I will believe it if they actually make one and prove it.

          But no, electric motors are not a solution as they add vastly too much weight with their generators (turbine or fuel cell) and or the batteries. It simply does not work.

          Hydrogen has the same issue, a large space taken up with the fuel tank and the added weight as you can’t use the wing skin per a jet fuel tank.

    • >What about the noise emissions of open rotor designs?

      Subs have gone the other way. One can gauge an answer to that in looking at trends in submarine propulsion. If you consider the evolution from Los Angeles class to the current Virginia class, they’ve moved from a open propeller to a shrouded propulsor (a shrouded fan). One of the reasons why is that it’s more quiet, more of the time. Interestingly, a USN submarine’s basic design is now pretty much a clone of Royal Navy submarine design from the 1980s, apart from the tea urn…

      My understanding is that, whilst a propeller can be quiet and efficient, it can do so only if the prop speed / water flow rate are well matched; the prop blades can dig everso gently and quietly into the water to provide thrust, but try too hard and it starts to get noisy. Whereas with the shrouded propulsor, it’s always quiet. Plus, the shroud provides quite a lot of physical protection to the rotor, which is no doubt useful if a sub is getting up to some sneaky beaky schennanigans.

      As it is with sea water, so it probably is with air.

      Given all the acoustic tricks one can play with a shroud around a fan that one cannot with an open rotor, I can’t see how an open rotor can be noise competitive unless it’s really, really big. And, if one does anything to accommodate a big open rotor on an aircraft, one would then have to consider the advantages gained by then putting a shroud round it.

      Ok, there is an argument that shrouds can get infeasibly big and heavy. However, if there is no blade-off containment requirement on an open rotor (there’s nothing to do any containing), a shroud around that same rotor could become a lot lighter too by not having to be armoured against blade escape.

      The re-examination of open rotor designs is also a re-examination of the blade containment requirements for turbofan engines, in the light of developments such as P&W’s GTF, RR’s Ultrafan. Logically, if an open rotor desgin can get certified, a shrouded fan design with the fan operating basically under the same conditions can probably also get certified, with no specific blade-off containment pack.

      The blade containment requirement is probably something that the regulators and industry should have a good, long chat about, because to me it feels like it’s now driving design in directions that are possibly undesirable and risking ending up with some illogical situations (e.g. it’s alright to not have blade containment if you have none at all). We all know that there’s more to come from the conventional turbofan engine architecture, if the weight and width of the blade containment pack can be dispensed with. Dispensing with it likely permits blades to be spun even slower. The less stressed the blades, the more strength margin can be built into them. That’s a double mitigation of the risk of a blade coming off in the first place.

      Possibly, that discussion has already been had, and I’m just an untutored behind the times casual observer!

      • There is drag from the nacelle itself and you can easily get interference drag between the big nacele and the fuselage. The most advanced codes can help design UDF’s that meet noise reglulations, still we do not know the FAA/EASA design/inspection requirements on UDF’s for blade-out.

        • Some things to keep in mind. Sub run at 45 mph max and most of the time they lurk quietly. So you don’t care if you make noise at Max, you do care on the lurk end (transit fast where its safe and go to lurk slow)

          Also while both water and air are fluid dynamics, the density is unfathomable comparison (pun intended)

          A jet engine is shrouded by its nature and the trade off in that containment is energy from a fan blade (QF32 anyone), not only individual passengers at risk but entire aircraft.

          Its one thing to refine a technology you know works (GTF) its another for a whole concept change and get it to work.

          You also loose engine choice due to the vastly different nature of an open rotor and a jet engine and who is going to risk a whole aircraft on a questionable and in this case unproven tech?

          New aircraft are a huge risk.

          So now you add in open rotor and props! I don’t think so.

          • Fan blades or propellers breaking off arent the real problem, its the turbine discs/blades that are the worry as they have far higher energy and speed.
            You should check your facts, the Qantas A380 engine failure was the hot section (IPT) , as I said the turbines are the big issue not the forward fans , props or propfans ( TP planes have a reinforced strip on the fuselage)
            Prop fans will also have their hot sections contained in a nacelle and are other wise no different to ordinary engines in that respect.

  7. Keep in mind, RISE only exists in the imagination.

    Its never proved any of the claims and at best its 12 years away with an open rotor history of changes before any production because the concept does not work.

    In the meantime I have submitted a link into what P&W has in the quiver for a GTF that provides a significant SFC improvement NOW and they will have even more in 12 years.

    • Rise 12 years away ?
      The open rotors were proved 25 years ago and one type went into limited production for the military An-70
      Rise is just a refinement of existing tech ( inc carbon fibre blades) , better layout and the slow pace is because of the funding

      • Duke:

        P&W took (15 years) to get the GTF to work, and they had a lot of smaller GTF operating. CFM can accelerate it, they are noit funding limited but they want the free money and have zero confidence in RISE.

        What the Russian Military did with or to the AN-70 is not relevant for civilian operations . There is a reason they were buying or leasing all Western aircraft before the Sanctions.

        The so called open rotor keeps morphing because they can’t get it to work.

        The D-27 engine was never proven and is an incredibly complex. Dual prop, 3 spool. Few were built and there was no non military testing. It never saw any real testing service.

        Very little if any tech from it applies to RISE.

        And that is ignoring people and prop dislike.

        • The plane , An70 failed because of the split up of the Soviet Union and Antonov was on the wrong side.
          Whats complex about 3 spools ? and dual props were at the time what prop fans were about- for obvious reasons they were forward of the core rather than at rear.

          • Duke:

            With all due respect you don’t know that. Nothing was proven.

            And yes, 3 spool is more complex and costly. One prototype crashed.

            It was never proven in long term real world use.

          • So what a prototype crashed- a mid air collision- at least it wasnt engine problems like the A400M. Thats not all that unusual.
            The aircraft never went into production because the plane and its engine maker were from Ukraine not Russia after the split up of the Soviet Union.
            At least all the engines were prop fans not the single test engine on others , D27 was tested on an IL-76 test bed plane and made visits to some shows in Europe and a similar smaller engine (D236) tested on the Yak42 3 rear engine tri jet
            3 spool engines werent new for Soviet Union either as it seems the core came from an existing fan engine.

            It would have never have got as far as it did if the whole system of a multi engine prop fan powered wasnt workable. Just an accident of timing and politics killed the project

  8. Interesting:

    “Safran To Lead Open Fan Studies for EU’s OFELIA Project”

    “Safran Aircraft Engines will coordinate the demonstration of new open fan engine technologies within the framework of the EU-led Clean Aviation Joint Undertaking project Open Fan for Environmental Low Impact of Aviation (OFELIA) the French engine maker confirmed Friday. Safran will work with 26 industry European key partners, including Airbus, Avio Aero, and GKN Aerospace, as well as research labs such as the French national aerospace research center ONERA and academics from several countries across Europe. The OFELIA consortium expects to receive €100 million in European funding from Clean Aviation.

    “OFELIA’s plans to demonstrate the efficiency benefits of an open fan architecture to address the needs of future short- and medium-range aircraft by about 2035. The program partners aim for a 20 percent reduction of fuel consumption and CO2 emissions compared with conventional narrowbodies. Under Clean Aviation’s OFELIA initiative, Safran Aircraft Engines and its partners expect to develop a set of technologies to TRL 5 for low-pressure systems, high-pressure core, and advanced systems including hybridization for ground and flight test demonstrations with an Airbus A380 powered by an open fan by the middle of this decade. The consortium also aims to ensure full compatibility of the architecture with sustainable aviation fuels (SAF) and hydrogen.”

    • Looks like Safran may be prepared to wear more than one hat when it comes to open rotor tech…

      • …..or in front of the technical risk or seeing AB/ BA not accepting RISE for their new project, dilute its R&T effort through a European project which remains with 100MEuro very little endowed : my guess:
        Keep also in mind that AB has its own path ( H2 tank +fuel cell+ electric motor+ efficient propeller) for a ” around 100pax)” regional a/c. Such solution compete with a small ( H2 or SAF )RISE which would be IMO the first realistic application.

  9. All: just attended a presentation about this tool
    Which was created by to investigate all contributions to climate gas emissions in alternate aviation fuels. The tool is being used in the UK to formulate a pathway to sustainable aviation. It compares the other fuels with the current fossil fuel. Note that it includes uncertainties. Considering the complete chain of production, SAF may be worse than regular fossil fuel ( indicated in the red color of the output dot).
    In an other presentation, it was stressed, that SAF are not all the same. There is no standard describing their composition. Some SAF have the same composition like kerosene, others miss many important chemical components (e.g., aromatics) that makes the combustion in these in a turbine difficult. Burning all types of SAF in the same engine may be impossible. A common standard for SAF is definitely needed.

    • In reality its like a hangover. Fossil fuels get a bad rap because of pollution back in the day.

      Coal is vastly worse but more unseen. Huge pits to dig, large bucks to transport, hard to eliminate the emissions and then you have a huge pile of coal ash to put in a pit that is a concentration of heavy metals.

      Now they are attacking Natural gas. This has just become nuts. Replace all coal with natural gas and XX % of the pollution goes away (and how much environmental impact overall?)

      Oil and gas comes from neat holes in the ground. All impacts are easily mitigated (vs the rest).

      And the wondrous electricity that now powers everything? Yep, natural gas and coal plants.

      Its a system and you can unbalance it badly and see it go off the rails.

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