There's a long negative history of high-speed gearing. Jet engines have one big rotating unit, and that basic simplicity is good for reliability. Turboprops have a gearbox, and fragile, high-maintenance gearboxes are a long-standing problem with turboprops. Adding a gearbox without reducing reliability is a major achievement.
High-speed gearing is indeed very difficult, but jet engines already typically have two separate, concentric rotating units (spools: https://en.wikipedia.org/wiki/Turbofan#Basic_two_spool). Also, as the article mentions, turboprops (essentially jet engines connected to a propeller) already commonly use gearing. I guess you can think of it as a turboprop with a duct.
The reality is probably just that this is a solvable problem that just wasn't worth the work while there were easier efficiency gains to be had.
> High-speed gearing is indeed very difficult, but jet engines already typically have two separate, concentric rotating units [...]
Yes, everyone knows that, but the spools are not connected by a set of gears as they are in the P&W's new engine.
> Also, as the article mentions, turboprops (essentially jet engines connected to a propeller) already commonly use gearing. I guess you can think of it as a turboprop with a duct.
The new engine is ALSO unlike most installations of turboprop engines in that the fan is driven by the power spool, not a free turbine. The free turbine (in turboprops that have them), which powers the reduction gears for the propeller, runs at a much lower speed than the power turbine.
I wonder if instead of gearing a single large fan, you could just remove the fan from the turbine altogether, add a much larger alternator, and power several large electric external fans from each turbine? That would give you more fan area than you can get with a geared turbofan, optimal fan speed for efficiency, redundancy in case one of the fans suffered damage, allow cross-routing of power between turbines if one turbine failed, and not require quite such large fans so they'd fit under the wings more easily.
Airbus are working on more or less this concept with the E-Thrust.
One Jet engine providing power and thrust and six electric fans along the wings for additional thrust. They have also added batteries, so the single jet engine doesn't need to be big enough to provide full take off power + redundancy.
Currently jets require at least 2 engines, each large enough to complete the take off if the other fails. With this system you would need either the jet, or the batteries + fan to complete takeoff in the case of a failure in the other system.
Fascinating! Seeking out more information on E-Thrust led me to this article, which talks both about it, and where Airbus are now with electric aircraft:
I don't think you can. The large volume of airflow surrounding the turbine is what makes it a turbofan[1]. Without, it is a turbojet[2]. The airflow from the turbofan surrounds the hot exhaust gas and it is much quieter than the turbojet.
But, other than noise, the rest of your comment makes a lot of sense to me. But I know very little about this stuff.
You're exactly right. But as you suspect, there's reasons why you wouldn't want to do that in all cases.
Turbojets derive all of their thrust from the turbine exhaust. It's a small, compact engine, which means high power-to-weight ratio, and it's great for small jet fighters. Unfortunately, it also consumes a lot of fuel. Turboprops are at the other end, where the turbine drives the propeller at much faster speeds than a regular 4-stroke engine would. But the exhaust provides next to no thrust.
Turbofans sit in the middle. They provided much better fuel efficiency than a turbojet but greater power than a turbofan. It was very much a "we need a middle solution" type of engine.
As turbines became more efficient and compact, the turbofan became smaller as well. To the point that they could be places on a fighter, but with a smaller bypass ratio. This gives much better fuel efficiency (range), while still retaining high power.
For jetliners, it's all about efficiency, but still retaining a degree of speed. But for a really high-bypass turbofan (50% or more), it's going to be a huge engine. Fighters are between 15-20% now, but you won't see much more than that.
Turboprops are at the other end, where the turbine drives the propeller at much faster speeds than a regular 4-stroke engine would
You're mistaken. Prop speeds are comparable between piston and turbine engines. Bigger, slower moving props are preferable. When prop tip speeds approach the transonic region, on either turbine or piston power plants, efficiency goes south.
The curving of the blades I don't think is intended to support faster blade speeds at all, but rather scoop air more efficiently. Computer modeling has enabled designers to develop much more advanced blade designs.
And BuffloBagel is right, I got that part wrong. I think what I was trying to say is that a turboprop supports faster aircraft speeds than a traditional engine. But yes, the propeller remains subsonic.
Edit: So apparently the Tu-95 does have supersonic turboprops. But I think it's the only production aircraft with them. Also, it's loud as hell.
https://en.wikipedia.org/wiki/Tupolev_Tu-95
I can't rattle off the model numbers, but some commercial jets have a turbine in the tail section for auxiliary power. If there's something that looks like an exhaust pipe in the tail, it probably is.
I think the issue would be weight. You'd need an alternator capable of absorbing some 20000+ HP (~15000 kW). And then the electric motors and associated fans and housings and mount points. And then the cabling to handle distributing the current. And if you're going to cross route you'd need the switches and associated stuff to drive it.
Not to mention the wiring. How much wire and shielding do you need to keep that sort of current safely contained? Especially since the gas tanks are in the wings too?
I think you are off by 3 orders of magnitude unless you are planning on putting 1000 motors on each wing. I said 15000 kW. That's 15 MW. 300v @ 50A is only 15000 W or roughly 20 HP (actually a bit less when you consider the power factor).
They'll actually need some very high voltages to make this work. That's going to be an enormous challenge - even simple motor windings, what will you insulate them with?
15MW? That's some serious power, are you sure about that number?
There really isn't a good way to rate turbines to hp. I did find this article [1] that says a 747 uses some 87000 hp in cruise across all 4 engines. That's approx 20000 hp per. It also says that's only 1/4 of the rated thrust it's capable of. I have no idea if you can say each of those engines is capable of 87000 hp or not since it's entirely dependent on speed.
If you watch agentjayz on YouTube he rebuilds turbojets that have been converted to become gas generators for power plants and some of those are easily capable of generating 30000 hp using tech from the 60s.
It really is phenomenal how much power turbine engines can make. There is a thread on the Concorde [2] that goes through a q&a session with the people who operated it. In that thread it says that the engines at idle consumed some 5 metric tons of fuel per hour per engine.
Yes, but why is a high bypass turbofan quieter? If I understood correctly, it's because most of the thrust comes from the large slow moving fan, so you don't need to blast nearly so much air through the turbine as a turbojet to get the same thrust.
The whole purpose of decoupling the fans is to give you an even higher bypass ratio - two or three large electric fans driven from one turbine can have a larger swept area than a single large geared turbofan, so can run slower for the same thrust, and hence be quieter.
The noise comes from high speed air alongside slow moving air. A high bypass fan moves more air at slower speed to get the same push. The slower speed means less noise.
Actually would be a lot quieter, since the turbine wouldn't have a high velocity exhaust output. And the fans are fairly quiet.
One real advantage would be quick throttle response. Gas turbines are slow to ramp up output power which is a real problem when dealing with wind shear on takeoff and landing. A 30 knot wind shear turns your 140 knots air speed turns into 110 knots airspeed. An electric fan's response would be very fast, probably faster than a prop plane.
Love this: “It’s the antithesis of a Silicon Valley innovation,” says Alan Epstein, a retired MIT professor who is the company’s vice president for technology and the environment. “The Silicon Valley guys seem to have the attention span of 3-year-olds.”
Well, that's what you get with age discrimination. There are probably all manner of worthwhile problems that older people understand better due to experience in long-haul, delayed gratification efforts (raising kids to adulthood, financing a house, chronic illnesses) rather than scrambling for the latest gastropub or gallery opening.
It's easy to dismiss Silicon Valley companies as trivial, but if nothing else, SV stands for a culture of experimentation. This, by contrast, sounds like an engineer's nightmare:
> “In other years we hid him [McCune] behind the curtain and slipped him some sandwiches so management wouldn’t know what the investment was,” he jokes.
Maybe a decades-long attention span wouldn't have been required in the first place if the company in question gave their engineers better support.
Yeah, there are definitely people in Silicon Valley who are working on the same problems they were working on 20 years ago – it's just not very interesting to outsiders so you don't hear about it (until they have a breakthrough).
Could some knowledgable aircraft people answer these questions for me?
1. Can/are jet engines ever swapped out after a plane as been put into service? Or will these only ever be sold attached to new aircraft orders.
2. Any idea on what the engine costs? And what percentage of the overall airline cost is engine? I didn't see any references to costs of this engine vs existing models.
Engines are relatively easily swapped out for maintenance, with most airlines carrying spares, but these engines are highly unlikely to be retrofitted to older generation aircraft; too many engineering and certification issues.
The engines will be somewhere north of $10m each, but since replacement of parts at required intervals will also cost several million dollars the airlines will look at total cost of ownership to take into account differences in frequency and cost of replacement parts, probably with a health risk premium for the new technology. Comparing that with existing models is more complex, and of course fuel economy savings depend very much on the routes you intend to operate. The engines alone will be worth more than a typical 10-15 year old narrowbodied aircraft (with its engines, midway through maintenance cycle)
Yes, they're placed into a duct/shell and can be removed and replaced because the engines have a set number of hours they can run. Also if newer engines are developed they can be swapped (Thanks for /r/justrolledintotheshop) - I recall the cost to replace around $16 million, don't hold me to that though.
Also I believe airlines often lease backup engines while they service the original purchased ones. It's been a while since I was in the business but that's what my old company did. They leased spare engines and aircraft on demand if needed.
It isn't the mount that is at issue, it is stresses on the airframe. Different engines stress the airframe in different ways leading to different failures.
You actually could. It would take a very strong, very heavy airframe that could handle any stresses generated by the intended selection of engines. It would just be overbuilt and get terribly fuel hungry for most engines, while being optimum for one. This isn't financially acceptable for an industry where operating costs are razor thin (all driven by the fact that a $10 difference in airfare can mean losing a passenger).
Aircraft safety margins are 1.5 (built to handle 1.5x the maximum expected load), while bridges are 5+. The latter aren't constrained by weight, so you might as well go to town with a cheap, heavy design. A lower safety margin is only acceptable though when the expected loading is extremely well defined with very low uncertainty. This takes extensive testing (years for an aircraft), basically reducing the sigma by increasing the sample size.
I upvoted cause not dumb question. Yes they do to an extent. One of the major reasons the engines are mounted on pylons is exactly to make maintenance and replacement easy.
Yeah, it's a fair question. Most are unused to weight optimization. It's why everything in a commercial jet down to the forks served with meals is purpose designed for aviation.
Consider that the engines for the Me-262 turned out to be heavier than anticipated, throwing the center of gravity off. The easiest fix was to sweep the wings back, inadvertently discovering the advantages of swept wings for high speed flight.
It'd be interesting to know what exactly they are doing in the gearbox, apart from the flexible mount to absorb excess torque. New materials? Surface treatments? Special lubrication?
The gearbox is described as the epicyclic type with the compressor driving the sun gear and the carrier of the five planet gears driving the fan. The article doesn't seem to identify any extra special sauce. It points out that geared turbofans have been used on an airliner in the past, and identifies some design trade-offs. "there may be a performance penalty, marked by slower cruise speeds for the same thrust setting, possibly slower max cruise speeds, slower green dot (best L/D speed), slightly shallower climbs, and possibly degraded single-engine performance. The single engine performance difference is expected to be the most prominent, with a possibly larger yaw, and a definitely reduced climb gradient, consequently lowering obstacle clearances."
My impression based on just reading that article is that, rather than a technological breakthrough enabling the geared approach, Pratt and Whitney have explored a rather neglected area of the design space, to arrive at a more efficient, lower noise engine with slightly worse performance in some other respects. And had to conquer numerous 'ordinary' engineering challenges to make it work.
Thanks, that article was indeed much better to understand what's going on. Still, as mentioned, high-speed gearing of the kind does offer some design challenges, but apparently its all secret sauce.
So basically this seems to be a extra-high-bypass turbofan achieved by gearing down the fan. But delayed hugely because giant megacorps move very very slowly.
Because of safety. For a brand new concept engine, you don't want to have any doubts at all. That means lots of long term testing. Since it will be powering jetliners, it has to be as close to flawless as possible. And even then, they have to convince the airline companies to use it.
30 years. I do wonder if that is true. Did they actually developed it under a time period of 30 years.
or do they mean it more in a way that it is a continuation of previous models.
It's more of a 'The concept is 30 years old' thing. Some of the reasons driving the commercial development only became factors in the market more recently; things like fuel efficiency and airport noise restrictions. Also, one of the less discussed advantages (as a customer) for the geared turbo fan is the decrease in expected maintenance/repair costs on the life of the engine. That's big business for engine manufacturers and there will be less of it with this design.
It's a bit easier to be blasé about testing when your worst-case-scenario is that somebody misses a ride, vs when you kill ~150 people. The rapid cycle of innovation is a tradeoff that SV is able to make (and finds amazingly beneficial) because we're working with soft tools and have backups of our data. I'm certain that were human life so cheap, Pratt and Whitney would have a much shorter dev cycle, too.
Curiously, I read that comment as saying: Silicon Valley is too preoccupied with short term profits to be able to concentrate for a long time and come up with such 'tangible' and actual benefits in the domain of transportation. This is not the next 'snapchat/tumblr/facebook' ... That's how I interpreted his comment at least.
It's also a very different mindset. I moved from a long career designing software for Medical Devices to desktop S/W and the hardest part for me is getting used to a much more relaxed approach to development process, documentation and testing.
It's weird when I realize I can just push something out the door and not have to wait for the reams of paperwork to be complete.
Shorter project sprints came about to try to fix the problem of project failure. Failure to create a loop that feeds discoveries in to a project's knowledge, and to communicate knowledge between teams is exactly what makes projects come to catastrophic ends, and that's what the shorter cycle time attempts to correct.
The fact the product benefits from faster innovation is a function of more efficiently sharing knowledge and data, not spending less time on individual components.
Designing and testing an engine can be done in an agile style. There's nothing intrinsically "software only" about the methodology.
NB: I ran a startup that made software to manage requirements in big projects. I've spent lots of time thinking about different ways to manage things. Including how not to, but that was after we failed.
Yes it is intrinsically software-only. When a software test fails you fix the bug and run it again. When a jet engine test fails you just blew up a $10M prototype.
A software test is equivalent to a testing small part in an engine design. The cost of failure is low. The cost of a full production test in an engine design is clear, but it's the equivalent of a product handover and acceptance testing in software. The cost of failing that test could well run to millions of dollars.
The only difference for a SaaS startup is that they're their own customer for product development (in the sense that they build and test in-house rather than someone else paying them to build a product). The cost of failure is still there, it's not measured.
> I'm certain that were human life so cheap, Pratt and Whitney would have a much shorter dev cycle, too.
I read a book recently about the history of Supermarine, and what jumped out was the attrition rate of their test pilots. Ditto for the post-war British jet industry.
There is a trade-off point between safety and fast development, but I'm glad we've moved to the side of safety.
Yeah I don't think this is an argument the aerospace industry can win. To whatever degree SV lacks patience, the airplane industry has the opposite problem. Tolerance of needless complexity leads to expensive systems that aren't necessarily any better for it. There's a real sense that design has absolutely no place and every problem can be engineered around, even if it takes two more layers of parts to achieve it.
Not only that, they have a bad habit of adopting disproven solutions simply because they 'are mature'.
That is I believe Musk's premise with his rocketry division. It couldn't possibly be this complicated, can it? I hope he turns out to be right. But if he is, gonna be a lot of smug engineers having to take early retirement.
> That is I believe Musk's premise with his rocketry division.
One hull loss per nineteen attempts is not an acceptable failure rate for "the airplane industry", is it? It's difficult for me to put into words the level of respect and admiration I have for the risk-averse, plodding, and methodical aerospace culture that has created modern aviation. (Seriously, have you ever considered the absolute miracle of engineering that is a modern jetliner?) To lump that profession under the same rubric of engineering as the one where a bunch of overcaffeinated "disruptors" write software to get people to click on ads seems... unjust, to put it mildly.
You can't compare a giant pipe bomb with passengers on one end to an airplane either, but I'm the one who opened that door without a suitable qualifier, so that's on me.
What I meant to convey is that I expect someone to do to commercial aviation what Musk is trying to do to rocketry, and what I expect that to look like is for more effort to be placed into reducing single points of failure by designing complementary systems instead of just putting three of everything on the vehicle and then adding two more layers of stuff to manage it. And also for them to act like progress in software development processes actually applies to them now instead of in thirty years. We got no end of grief for refusing to do pure waterfall, and they were conspicuously silent when we're were able to fix fundamental up front design flaws in 3-8 weeks, and without doing a reenactment of Macbeth starring William Shatner, set in a rehab clinic.
Then again, I expect something to happen to improve the caliber of software developers out there as well, so we'll see how that goes. Often when I expound on the qualities of my profession I am intentionally and consciously imagining that the 30% of my peers that are responsible for 70% of our problems either don't exist, or are off doing something that's not important.
Or it could be that the FAA and other airline regulatory bodies around the world (especially in UK and Europe) keep innovations from happening, and so development times can be long.
If we could distribute a new gear design which fixes flaws in some over-the-air patch it'd be a lot easier to iterate and improve.
Selling thousands of engines too soon and realizing they all suffered from a critical engineering issue could bankrupt your company. There's a lot on the line!
Bankrupt your company, and probably do some serious damage to the airlines or plane manufacturers that bought them from you, and also kill a lot of people. But, you know, move fast!
You can have an idea, but getting it into production, especially if the entire design hinges around it, is an entirely different story.
From the article, the new engine is to wide to fit a number of existing planes.
> what may be most remarkable about the engines is that they took almost 30 years to develop. That’s about 15 times as long as the gestation period of an elephant
Who the hell is running their analogy department?!?
"Thirty years! That's a big number! We need some kind of comparison that will instantly give the reader perspective. Of course, in this, as in so many other things, the answer lies in pachydermian pregnancy patterns."
As long as we are ripping on their numbers… "75% less noise" almost certainly means 75% less power in the noise which means about 6 decibels quieter. That's noticeable, but not huge.
Smart phones haven't even existed long enough for anyone to spend 30 years making an app for them... And the laws of physics have much better backwards compatibility than each new fly-by-night smartphone OS
