Bjorn’s Corner: New engine development. Part 6. Open Rotor

By Bjorn Fehrm

May 3, 2024, ©. Leeham News: We do an article series about engine development. The aim is to understand why engine development now has longer timelines than airframe development and carries larger risks of product maturity problems.

To understand why engine development has become a challenging task, we need to understand engine fundamentals and the technologies used for these fundamentals.

We have in previous Corners discussed geared versus direct-drive turbofans. Now the time has come to discuss Open Rotor engines.

Figure 1. The GE 36 Open Rotor engine, the father to today’s CFM RISE. Source: GE Aviation.

Open Rotor history

We started the series by explaining that all aircraft engines work by accelerating the air passing the engines backward, thus inducing an Overspeed of the air compared with the air passing the aircraft.

The first method to induce this Overspeed was the propeller, with the blades forming a profile like an aircraft wing, cutting through the air and accelerating it backward.

As aircraft flew faster, the speed of the air hitting the propeller combined with the rotational speed of the propeller blades meant that the tips reached supersonic speed.

Supersonic flow is very different from subsonic flow. In subsonic flow, the air molecules are prewarned by the pressure wave that a propeller blade is coming, and they move due to the pressure to align themselves with the airflow around the propeller blade.

Subsonic air molecules are easy to move, and the flow around the blade is smoothly following the blade profile.

Supersonic flow is NOT smooth. Now, there is no pre-warning that the blade is coming (the pressure wave is traveling at the same speed as the blade and never leaves the blade’s surface). The collision of the air molecules and the blade’s leading edge is violent, like when a pool ball hits another ball (I call supersonic aerodynamics “Pool aerodynamics”).

The violent collisions create razor-thin shock waves traveling at the speed of sound away from the blades, generating a very high noise (noise is pressure waves). The collisions and shock waves also eat a lot of energy from the blade, thus, the efficiency of the propeller tanks.

Propeller planes could not fly through the sound barrier because of efficiency losses. The flow’s overspeed wasn’t sufficient, and the loss of blade efficiency increased the closer you flew to the speed of sound as more of the propeller blade got supersonic flow.

The solution was the jet engine. Instead of capturing a lot of air and giving it modest Overspeed, the jet engine captured little air and gave it a very high Overspeed. With the help of the air intake of the fuselage/nacelle, the air could be slowed down to about Mach 0.5 even when flying supersonically (through a widening diffusor section). Thus, the blades of the compressor could work subsonically even when the aircraft flew supersonically. The last part of the engine, the nozzle, then, due to a constrained circumference, accelerates the air to the needed Overspeed.

The initial jet engines had too high Overspeed and captured too little air. The propulsive efficiency was not good, at about 45%. The fix was the bypass turbofan engine, which lowered the average Overspeed and increased the massflow through the engine, thus increasing propulsive efficiency to 83% for the latest generation.

But we have now used this principle to where we have ByPass Ratios (BPRs) of 12 to 1 and project 15:1 or more for the next generation of engines (for example, the Rolls-Royce Ultrafan).

The fan sections of the engines and the nacelle with the desired inlet diffuser are getting so big that the idea goes back to a more compact version of the propeller, the Open Rotor or Open Fan.

Research into propellers that could fly faster without the tips entering supersonic flow intensified during the 1970s and 1980s, as shown in Figure 2.

Figure 2. The classical turboprop showed as Lockheed Electra versus projected high-speed props and Turbofans of the time. Source: NASA report from 1988.

To avoid premature supersonic flow, the same methods were used for propellers as for wings. The blades were swept, and their cord was increased (Figure 3).

Figure 3. Picture of High-speed propeller blades from NASA report. Source: NASA.

The propeller also got more blades, so more air was captured per swept area (the solidity ratio increased), Figure 4.

Figure 4. The NASA/Hamilton Standard SR-3 high-speed prop from 1982 in a supersonic wind tunnel. Source: NASA

The possible speed of the aircraft increased, and the drop-off in efficiency of the propeller was improved (Figure 2).

But there were still fundamental problems to solve; We will discuss these in the next Corner.

75 Comments on “Bjorn’s Corner: New engine development. Part 6. Open Rotor

  1. Props can in theory fly supersonic but it is very hard to design prop blades with high efficiency for supersonic flight. Most turbofans have parts of the fanblade flow normal to the l.e. going supersonic during climb (the buzz saw sound), the supersonic boundary used to be just outside the top mid span shroud. You can reduce the normal component of the flow triangle to the l.e. by shaping it like a boomerang as shown in Bjorns Fig 3 and 4. The blade induces a swirl component that for the RISE engine is straightened out by variable vanes making for a higher axial exit airspeed. The big advantage is the high bypass ratio, the disadvantage can be a hard time life limit on these blades for replacement as you do not have a containment case.

    • It would only take a couple of uncontained blade failures (kinda like the two-in-a-row “virtually impossible” Boeing MCAS failures) to nix the whole notion of “unducted fan”, or whatever it’s been
      re-named this week.

      “Better-faster-cheaper!” is a delusional notion: Never has happened, never will happen.

      • You have had carbon fibre props for a long time and their maintenance programs have been successful. Still if you are only allowed 10 000 – 20 000 cycles between replacements to a new set on UDF’s it gets expensive.

      • Uncontained blade failures in turbofan engines , while rare are probably 1 or 2 every 5 years .
        Anyway they are hardware failures , not related in any way to MCAS which was software

        We already have a tuboprop plane that can fly up to 400kts or 740km/hr , or what turbo fans fly at
        https://www.raf.mod.uk/aircraft/atlas-c1-a400m/

        • Vincent digs at things that are not connected, kind of like a dog chewing an old worn out bone.

          What he does not understand is its a dual issue and one can negate the other.

          Technically they may be able to make Open Rotor work. That issue is how much does it cost for the gain that jet engines already have and can get more, particular with use of GTF.

          Rear mounting was a have to and now at least per CFM, the claim is it can be wing mounted.

          In you can armor the aircraft hull where a blade would impact it. What is acceptable risk wise in a TP could well not get AHJ approval.

          But if the condition is armor, then that adds weight and more design impacts and you have to add that to the cost of the Open Rotor. Then putting it on the same hull as a Jet Engine becomes an issue of cost of one vs the other and why would you?

          So you spend more money to gain X efficiency that in theory may be X better and in total you wind up with -X overall or even break even and for what?

          And then there is public perception that props are only tolerated if no other choice.

          There is a reason there are not large single aisle aircraft with Turbo Props. In the US Turbo Prop aircraft are being replaced by E175s.

          And that gets into what airline is going to risk a prop job when they know people don’t like them?

          And that is from the perspective of someone who flew prop jobs all their life and have no issues with them.

          What I can say is once jets came along, way nicer. And that includes the Dash 8 with sound cancellation which was a fine aircraft but an E175 or a CRJ is a lot nicer.

          CFM is putting its eggs sort of in a basket because there is free money there. But they also are getting a core than can be put into a GTF as well as a gear box that can handle fan loads. Take the money and run.

          In the meantime the other two majors are going GTF all the way.

          I saw one diesel engine mfg go down the we don’t have to use EGR path and anyone that had a handle on it was shaking their heads.

          Well they had built up emissions credits and ran on those until it foundered on the failure of no EGR and then programs were killed, engines bought from other sources and buying EGR tech to get a few bottom line engines up to snuff as that was their bread and butter.

          so it goes

          • Yes, you hit the nail on the head! Government meddling is the cause of all of this nonsense. “Free” research money can distort almost any industry. Nobody would spend their own money on such old techbology. What’s next, radial engines?

            Do you think electric cars would be in such an awkward upply problem had the governments of the world not thrown so much money at them?

          • John:

            There is a difference in my view between basic research and as you said, free money with a direct order to make in this case RISE.

            Flip is the TTBW aircraft NASA and Boeing are working jointly on.

            The difference is NASA is not saying its a have to, but its a new concept there is no experience with vs a OR that we do have past experience with to have some assessment.

            EU said Open Rotor or bust.

            And agreed on electric cars, its only the Credits people get to buy one that has driven it (for the most part). I ran a lot of electric fork lifts and that was a really good application for battery.

            Ural Side Car company tested a battery side car, pretty slick setup but the range was only 120 miles. I think they could put double the batteries in it as they have the room but you give up carry capacity so…….

            That said my vision was to get one, a small diesel generator and install the generator in the side car and away you go. I suspect between downhill and stops you could go 300 miles and get fantastic MPG as the diesel at steady speed would be at its most efficient.

            But its a pretty silly notion cost wise. I would be happy to volunteer my Honda generator with the Eco Throttle, it works really well to power the house.

  2. Very interesting series. I’d like to hear more about how the nozzle works.

    • Hi Jan.

      I’m glad you find it interesting. In the series, we will cover how nozzles work when we examine the core of the engine.

      • I have the same question that I had when this subject was addressed in this forum years ago.

        Has the debilitating affect of the noise on ground staff been addressed?

        • Its not a improvement on the old configuration.
          Its radically different plus the use of high strength tailored composites for blades

          • While its radically different it is a (at least according to the PR) one where they have solved the issues.

            Now they don’t have a running example, but by golly they have solved all the issues.

            Having seen the whole OR thing morph continuously since proposed, this is another morph.

            It might even work as a test article. Put it into the real world and both the prop off reg issues as well as how public feels about props, well stay tuned.

            And regardless of what its called, its a Turbo Prop with fancy trimmings.

          • Well the Rise engine is going to be running in a test cell from 2025 and later on an Airbus A380.
            Seems to be a 2 yr lag from earlier testing timetable
            https://aviationweek.com/shownews/singapore-airshow/no-showstoppers-cfm-rise-so-far-tests
            “As well as the open fan, the program includes multiple areas of advanced technology ranging from a compact high-pressure core and hybrid-electric architecture to compatibility with alternative fuels and new materials.”

            Its wise to not just push the open fan with existing core engine tech but move the bar elsewhere including noise reduction using substantial wind tunnel work

          • Duke:

            Nothing I see says the RISE architecture is anything more than a state of the art core.

            Any novelty is the turning around of the engine with the variable pitch and the whisker thingies.

            That makes it a fancier Turbo Prop as they added a gear set to slow the props down.

  3. If RISE can have large prop without containment, can’t GTF fan slowed down more and have containment removed?

    • The RISE engine has and need variable pitch fanblades (like a DC-7 constant speed prop), that puts requirements of new components for its movement and control. In addition is has variable fan exit vanes as well.

    • I like the RISE concept.

      – it’s a simplification / weight reduction compared to the CROR’s of the late eighties
      – less noisy because puller prop iso pusher, half the number of bladetips & no shock waves hitting each other.

      A direction chosen to by Embraer on their new concepts too. Flight tests are planned for 2026 on an A380 test bed.

      • The A380 is a good segue.

        Like the OR, it always was just 10 years away from success!

        • Its taken 5 yrs just to put a new belly tank and single slot flaps on the A321. Certification hill has become a mountain range even for test programs

          • Boeing applied for TIA of its 777-9 back in 2021* and FAA still refused to grant it. What happened? What’s your explanation?? 🤔

    • I bet production RISE debuts with a cowling of some type.

      • Embraer put its next gen turboprop on hold due to no new powerplants. One has to know Embraer is looking at Rise with the WS NB rumors that Embraer has denied.

        • Embraer wanted something to fill the PR gap so they threw out a bunch of never to be stuff.

          As for a cowling, then its a GTF, the flow design goes out the window.

  4. Open rotors was looked at years ago and dropped because of blade off damage and noise. What has changed?

    • They changed many things to minimise those issues.
      The biggest one was moving the propeller-UDF to the front and use of composites
      The graphics for Rise and the 30 yr old GE36 show the differences

      • Yep, nothing changed other than the OR is always changing to see if they can make it work.

        Space X put a car into space, but while the car went 25,000+ MPH, its not what a car is intended for.

        • Don G:

          In theory they have solved the noise issue. Blade off damage no.

          Maybe like an interrupter gear on a WWI Biplane, you pick up the vibrations and blow the blade off at the right time to flip it to the open arch!

          They have a lot of problems to solve and the blade off is not mentioned. The AHJ (EASA in this case) will have their view and you want to be talking to them sooner than latter.

    • The power of the analytical tools has improved alot. So now they claim to be well within EASA/FAA noise limits as well achieving regular jet speeds. The 3D carbon fibre weaving technology developed for the LEAP engine fan blades are used as well. Still the variable pitch in this 25000-35000 shp fan drive system is not that easy, you cannot just double the size of the TP400M engine variable pitch system. GE will keep a close eye on this as it is part of Safran fan module in their work split.

      • I have been hopefully that computer technology would greatly advance aerospace progress. but I see little evidence. Airframe and engine issues
        seem to more frequent. Any theories? The 787, GTF, Max, RR Trent……. Seems like the good old slide rule and a drafting table worked pretty good.

        • John:

          I would call it low hanging fruit.

          The Wright brothers got their wonky airplane to fly, the wings were the key. The rest (tail in front, rear prop, wing warping) was not used much and wing warping not at all down the road.

          Once the tech settles, it was a tube and wing with tail in the rear.

          The next significant change was pylons and under slung jet engines

          Modern design can chip around edge improvements as well as materials but there simply is not escaping from the reality of air moves the way it always has and they understood it pretty well.

          The TTBW if it works is not huge but its the first real aerodynamics possibility out there in some time. BWB might be that as well for larger aircraft.

          Composites and being able to form a more efficient wing are an adder but again not huge, a more refined aspect of what they knew but were materialism limited on making better.

          Slowing down is one area that could be done but then you have the issues with Airlines and having to add more aircraft, crew and support to make up for the slow down on each route.

          • Me thinks bringing the Wright Brothers into this conversation is a bit
            unproductive. Let’s compare the B-777 to the 787. see my concern?
            PW JT9D to the GTF. Get my point? How about B747 to A380? As far as I can see the industry is in reverse. B777X delays are legendary.
            The Max is, well it’s the MAX! What can I say?

          • John:

            The point on the Wrights was they solved the flight issue, but the element that went forward was the wings.

            The other elements worked, but they were not the best solution.

            The tech rapidly changed and its matured and while you get refinements, there are no huge breakthroughs.

            Jet engines are the same, lot of refinements but no game changers (despite the over use of that term)

            TTBW may give a boost to single aisle and BWB to larger but then it stops again. Air flow refuses to change.

            All Embraer could do at best would be a duplicate of the A220. And the question is with the investment, ramp up times its 20 years before they see a return if it is a success.

            The A220 is state of the art with its composite wings and maximum ability to blend to the best airflow. The fuselage at that level is not worth composites though it pays a benefit in 787 and A350, but the big bonus was the engines and wings there.

            We see China failed to execute a current tech with the C919, they went with the same old 737/A320 tech.

            And you still have to break into a market with support and sustainment though Embraer certainly has that capability.

            Airbus has responses it can do, A220-500 and a new A321 wing. Them it gets into how much risk do you take for an Embraer that won’t be seen in numbers for 10 years?

          • @John

            Think about the shareholders’ return and CEO share options granted during that same period.

        • The powerful Finite Element tools like Ansys and advanced fluid codes really help the engineers determine life and performance. It makes them reduce margins for performance. Still you have areas where these tools have limitations, like friction and corrosion. The RR 787 issues were a mix of corrosion and allowing stresses “outside RR experience” and some flutter at max continuous thrust that could have been caught with the most advanced software tools, the GTF had many issues but burner liner clogging and ceramic tool residue in powder metal is not caught with the software above, “rotor bow” after shutdown could have been found, the 737MAX stick force could have been caught by analytical tools before Boeing went for a “software fix”

          • Claes:

            It really did not matter how the aerddyamics came to light and the issue is understood.

            It was more Boeing did not think it was an issue and the FAA did. And in fact, what then happened was to turn a non issue into an issue.

            Stick shaker and stick forward are well proven methods to avoid a stall. Airbus solution is also software.

            Its when they took one aspect, used cherry picked data and then used it for other parts of the mix turning MCAS into a monster. Jets have speed trim, so use of software to deal with the huge differences in ops from slow to top speed is not new.

            Boeing kept slapping adds to MCAS and the FAA was allowing it.

            Brazil did catch hints of the issue and that is a credit to them as they made it mandatory to be in the manuals. EASA and FAA etc clearly did not have liason with them (or ignored it).

            And regardless, you still have to build something and test it to see how good the software is in its validity.

  5. NASA, GE and Boeing spent millions proving fans are quieter, safer, less susceptible to FOD. Why is it still being pursued?

    • Free research money in this case which gets CFM a core and a gear box so they can do a GTF.

      Initially it was thought to be a way forward, but like a Wankle Rotor engine, it just never was as good efficiently and reliability as a combustion engine that is still going strong (battery cars are the latest rage they are finding out does not work)

      Hydrogen is the same, it has some appeal but the issues and cost defeat it.

  6. I remember sonic fatigue as an issue with the 7J7. To avoid metal fatigue on the airplane structure, the engines were moved back, so the fan was behind the tail cone.

    I started wearing ear plugs on airplanes, when a flight attendant on a DC-9 offered them to all of us seated near the back of the airplane.

    • Stan:

      In theory you can design around the noise, but it cost more and more to do all the work around design wise.

      Its not a stand alone tech, it has to be integrated into an airframe and it cost so much and regulatory agencies have not even weighted in as well as the public against it.

  7. Bjorn, This RISE engine is going nowhere, as: 1) it’s too complicated to be cost effective, 2) just too noisy for polite society & 3) furthermore, your and the RISE concept of thrust resulting from “action at a distance air flow” is fundamentally incorrect, as the only thrust possible is from asymmetrical surface pressure distributions on the blade surfaces themselves. Patience, Bjorn, a new thrust engine is coming soon to your neighborhood.

    • I am going to take exception to that statement.

      Bjorn is presenting the tech aspects. Its not HIS it is CFM and his job is to assess those tech aspects.

      While I think RISE is gong no where, that in no way makes Bjorn wrong – I have yet to see him wrong.

      He is presenting what is going on and a huge presence in the engine world (CFM) is working on it. It has to be taken serious despite the reservations on it.

  8. Sonic Fatigue? Imagine what the fare paying passengers heard.

  9. ‘But we have now used this principle to where we have ByPass Ratios (BPRs) of 12 to 1 and project 15:1 or more for the next generation of engines (for example, the Rolls-Royce Ultrafan).’

    As I understand (and please correct me if I’m wrong here) the way to increase bypass ratio is to make a bigger bypass section. Hence, a larger engine. Larger means heavier.

    Have NB engines topped out?

    Are you just size/weight restricted? Perhaps you can make an aircraft with a taller landing gear to get more clearance, but at a certain point in time, there’s only so much space there.

    ———————————————————-

    So it looks like any UDF/Open Rotor is going to be a back of the aircraft type engine – like a DC-9. It makes for a nice clean wing, but as I understand it, one of the advantages of hanging engines on the wing is for better weight and balance.

    Not my forte – CG’s & all that, but what are the penalties involved with having to weight the nose down to get it balanced?

    • So they can deal with tire treads, bolts, ice, FOD, ETC. as well as being noisy? Sounds like another failed concept from the desk of NASA!

      • This is not a NASA project, its an EU project.

        The weight and balance aspects can be solved.

        While I am skeptical on the viability,
        the information Bjorn presents is valid.

        They may well have solved some of the issues but the issue do remain but the claim is they can so it has to be assessed on that basis.

        • The original UDF was a NASA project taken up by numerous contractors.
          It was a dismal failure.

          • Can’t argue with that.

            And like Wankel engine, they keep adjusting it and it still is 10 years away from success!

            It was an is an interesting concept but if the cost is even equal to a GTF, then the GTF wins. How much tech does it take to make it work and how much more does that cost.

            And then the instillation and possible penalties.

            The GTF has a lot of refinements to be made. P&W did not go with exotic heat materials like CFM did on the LEAP. CFM can only go so exotic, P&W has speed refinements to make yet as well as exotic materials (which is probably what RR has done)

    • You increase bypass ratio by (massively) pushing up OPR. 🙂
      (there is an RR diagram around that links available OPR to optimum bypass ratio. Lost the link, may be Bjorn has it )

      Increasing “overall pressure ratio” ( and thus efficiency ) is an ongoing process.
      You need an energy dense core to provide more energy offtake for the fan.

  10. Hi Bjorn,

    As always good piece. Appreciate the explanation of the way subsonic and supersonic air moves around the blades and why blades are shaped the way they are. I have basic understanding but this is very good.

    The SR-71 inlet spike design is a great study understanding how going from subsonic to maintaining Mach +3 shock wave constant speeds has on inlets….. let alone the airframe. 😉

    I do understand the open rotor designs but still not convinced it’s practical for commercial.
    The future will be interesting.
    Thank you.

  11. Yes, you hit the nail on the head! Government meddling is the cause of all of this nonsense. “Free” research money can distort almost any industry. Nobody would spend their own money on such old techbology. What’s next, radial engines?

    Do you think electric cars would be in such an awkward supply problem had the governments of the world not thrown so much money at them?

    • You havent read the informative LNA series on electric planes which concluded using batteries for small planes when looking at life cycle costs are untenable and its a typical ‘hype bubble’ which founders on realities are faced.

      Henry Ford worked on electric cars around 1914 as a venture with Edison , so that technology for cars is old too. It was an model T-EV !

      • And while Bjorn is presenting the tech facts as far as is known, OR has the same drawbacks and only claimed to have been solved.

        Even a test article proves nothing. Its what it cost when industrialized produion wise as well as the mounting and the blade off requirements.

        We had the A380 Qantas blow out that proved even jet engine assumptions can be wrong and almost took out the left wing.

        Not so long ago a Herc threw a blade and cut the aircraft open so severely the fuselage failed.

        But again, CFM is taking free money to develop a new core and the gearbox tech to give them a foot in the GTF world.

        P&W took the GTF core and turned it into a stand alone jet engine. It goes both ways.

      • Battery tech is improving. We need a Moore’s law for battery. Lol.

  12. Life cycle costs are just the tip of the iceberg. Range, speed and payload make
    Electric airplanes an impossibility. And yes, electric cars have failed before, when ICE cars got an electric starter.

    • Not impossible just harder to certify and give good operating costs, still issues like flying into known icing conditions and windshear with these AAM’s will require new sensors and procedures. The electrical aircrafts with turbogenerators will need good life of batteries and very seldom use of its turbogenerators to be cost effective.

  13. The Russians did make an open rotor and it worked though it never saw enough service to validate and the noise aspects were not reported on (I think it was Ukraine that acualy built the engine)

    Orignaly the OR was rear facing. Noise issue were huge and 10 years or so ago they adherents of OR claimed they had quieted it down.

    What they refused to acknowledge as it cost fusagle weight to mount in the rear and even if the OR met the claims, it was net negative.

    then it was the Unique installations required and no two OR were the same.

    Ergo, you had to build an overweight and specific airframe for each mfg OR. It was not going to happen.

    Now its morphed to, oh hey, turn it around, make it variable pitch with those whisker thingies and wallah, its solved.

    No, how exotic is the core and how much does it cost.

    And you probably can’t just hang it in place of a jet engine, forces are different.

    Solve that, the blade off and can you get the speed?

    And props have to come off as the variable pitch mechanism has to be overhauled a lot sooner than a jet engine that you pull and just replace.

  14. If you fill up the sidecar with a stinky generator and batteries, where is your passenger going to ride? 300 miles? I doubt it.

    • Well in my case its my camping gear as I don’t carry a passenger other than short local

      But yea, silly notion but appeals to my twisted mind

  15. Great time to talk about the GTF. 20 years of developement, and all it has done is bankrupt several airlines and do severe damage to its manufacturer. Maybe there are limits to efficiency. Maybe simplicity trumps radical innovation in airline travel.

    • John:

      I don’t know its bankrupted anyone but could be wrong.

      The GTF itself works. Its standard stuff like seals that have failed so that is a failure to execute the fiddly bits. It baffles me it would happen. Then the new seal was worse than the old one, time for a new seal design team!

      Its not like P&W has not been making jet engines and includes high performance ones for F-16/F-15 and the F-35.

      The most recent was blade materials had impurities in them and another, you gotta be kidding.

      Bur RR is hanging their hat on GTF and NASA studies all baseline a GTF engine.

      I don’t see any issues with GTF, but as RR has shown us as well, execution is tough.

      I would think the engine mfgs would have an engine on a stand running 24 x 7 with the typical cycles seen for that type. Hook them to an inverter and put power back into the grid while you are at it.

      Find the problems before your customers do

      • I’m wondering if the industry’s quest for fuel efficiency hasn’t outstripped our current materials ability to perform. Is it possible that there is a heat and pressure limit for turbine engines? If you remember, piston aircraft engines had become quite reliable until fitted with power recovery turbines and 4 banks of cylinders. Are we expecting too much from a 90 year old invention? Maybe it’s time to concentrate on reliability and be happy with our efficiency achievements.

        • John:

          My take in the case of prop jobs was not efficiency gains, it was power needed to get heavier and heavier aircraft up to speeds that cut down travel time.

          At the same time altitude gains were important and to maintain power in a prop engine, you have to pump more and more air into it. Fighters were the ultimate there but of course no passengers, it was all about 6 to 8 50 cal gun carry or 4 x 20 mm cannon.

          Jet engines offered speed and while tehcnialy incredibly complex, once in motion they had fewer moving parts with a lot higher density of power that got the boost they wanted.

          The only engine that could get a 707 size up and moving would have been Turbo Props and you are back to the complexity adding of props when fans are simpler. No gear box (for the prop) and no variable pitch mechanisms and a Turbo Prop has the same complexity of a jet engine.

          Clearly Turbo Props had a place but for large ops, they jumped right to jets for those reasons.

          We are at the limits of materials, LEAP was the final push there and after that its a balance of materials, costs vs a GTF that does it better without the higher costs .

          But all jet engines start with issues and they work them out.

          The beauty of a GTF is you have some low hanging fruit to harvest and you can still go hot in the core and get more out of it (as well as fan speeds, blade shapes)

          P&W failed in not giving the attention to the seals and combustors and letting impurities into the mfg process is simply weird. They all know better.

          Other than P&W failed on aspects of execution there is no report what was going on that lead to that. Its not like they don’t have jet engine experience and hopped up military jet engines at that. Baffling.

          Then they came out with a seal that was worse than the original one, a lot worse. That indicates there was a major engineering cpaiblity gap in that area but not why on the GTF when they did it successfully on other engines that were pushed harder.

          But none of it was on the exotic end, the gear box has been solid and while fan speeds were not perfect, they were good and did the job.

          • Trans,

            No doubt the recips were chasing horsepower in the 1940’s and not efficiency. With oil gushing out of the ground worldwide, it was considered an amazing natural resource which was to be used for the benefit of man. The concept of fuel efficiency had yet to be invented and exalted.

            I think it is time to put dispatch reliability back in first position over
            fuel efficiency. Not much point in selling the engine with the best SFC if it is sitting at the gate or grounded.

    • John:

      Outstanding find and post. Its now in the Bookmarks (a lot of those of that type I should have done at the time I found them)

      The only aspect missing is the early blowup with the C-200 and I relive that had something to do with a seal leak as well.

      That write up really consolidates a lot of reads into one good write up (or 2 but same source).

      It includes all the gory tech details which I love.

  16. Trans,

    Although the reduction gearing appears to be the least of the GTF’s problems, you got to wonder if is not contributory in some way. When a compressor blows apart on a revenue flight with only a couple of hundred hours on it, I would hope they are considering vibration very carefully. Also, when two seals of two different types fail, I would think it’s working conditions are suspect.

    Imagine where aviation would be if the the JT-3 on the 707 had been this problemmatic and time consuming? I mean, the GTF concept has been actively pursued for over 30 years now, with absolutely nothing to show for it,
    other than making the P&W R-4360 the second most unreliable engine to ever carry passengers.

  17. Do you know that jet fuel is 100 times more energy dense than lithium batteries? And as it’s consumed, the aircraft gains 2-3 percent performance increase per hour due to reduced gross weight.

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