
Are Passenger Jet Engines Hitting Their Technical Limits? - pseudolus
https://www.bloomberg.com/opinion/articles/2019-11-12/why-passenger-jet-engines-are-reaching-their-technical-limits
======
Merrill
>The first patent for using a gas turbine to power an aircraft was filed in
1921 by Maxime Guillaume.

[https://en.wikipedia.org/wiki/Jet_engine](https://en.wikipedia.org/wiki/Jet_engine)

After close to a century of development, one would think it would be near its
technical limits.

Fuel efficiency trends for new commercial jet aircraft: 1960 to 2014

[https://theicct.org/publications/fuel-efficiency-trends-
new-...](https://theicct.org/publications/fuel-efficiency-trends-new-
commercial-jet-aircraft-1960-2014)

[https://theicct.org/sites/default/files/publications/ICCT_Ai...](https://theicct.org/sites/default/files/publications/ICCT_Aircraft-
FE-Trends_20150902.pdf)

Current airliners appear to burn half the fuel per passenger mile as did early
'70s airliners. This reflects both propulsion and aerodynamic improvements.

~~~
freehunter
> After close to a century of development, one would think it would be near
> its technical limits.

Internal combustion engines in cars have been around longer and we’re still
pushing record amounts of power and record amounts of efficiency.

~~~
Merrill
This is partly due to the application of computer controls and partly due to
the application of things like direct fuel injection, turbocharging, etc. that
were previously available only on racing or aircraft engines.

For example, the Cosworth DFV normally aspirated Formula 1 engine with 182
cubic inches was making over 520 horsepower (2.9 / cubic inch) by the end of
its career in 1983.

~~~
fhood
Combustion engine development _was_ oddly stunted though. We have known the
basic formula for power for a very long time (forced induction + high
compression + high octane fuel). Superchargers are ancient, turbos aren't new
either, although I guess fuel injection had to wait for computers to be
reliable and cost effective. Even then I don't fully understand why it took 30
years for automakers to figure out how to produce high power engines that met
emissions standards.

~~~
abakker
It didn't take long to know how to do it. It took a long time to make cost
effective parts that were reliable in doing it for the service life of a car.
the bearings in a modern vw turbocharger are incredible. Imagine getting 100K
Miles out of a turbocharger which can regularly spool to 200K RPMs. those
bearings need to be made to incredible tolerances, which were not previously
cost effective.

~~~
Merrill
People generally overestimate the difficulty of designing a product and
underestimate the difficulty of designing the equipment, processes, and
systems needed to mass produce the product economically.

~~~
nostrademons
Is that true? There are a number of interesting corollaries if so, notably
that China should get a lot more respect for extremely cheap mass-production
of products, the U.S's position as "designer of products that are built
elsewhere" is quite precarious, that software and other IP (where mass
production is basically free) are basically the perfect businesses, and that
services that can be provided without mass-producing a product will grow in
importance.

I could buy all 5 of those statements, but it's interesting that the combined
effect of them would be to push people out of making things and into
manipulating brains (which itself is a statement I could believe).

~~~
snagglegaggle
It is true, and _some_ Chinese production lines are truly feats of
engineering. However, it's a little tricky to determine whether this is really
Chinese values and engineering or the values and engineering they have
imported from other countries. There's a lot of production lines in China that
produce nonfunctional goods.

In the microelectronic and mechanical areas the strength of the Chinese market
seems to be very, very lax IP law. While most of the interesting parts don't
make it over to the western market, they have very featureful, accessible, and
novel components that are made by cobbling together (potentially stolen or at
least unlicensed) IP.

Historically US manufacturing used to be the best cost/performance optimized
and this is what helped win WWII.

------
rossng
A friend in the industry explained to me recently that part of the problem is
requirements creep while the engines are being designed. I am definitely _not_
a mechanical engineer - so take what I say with a grain of salt - but I did
think there were some interesting parallels to software development.

New engines are normally designed alongside a particular aircraft. The
aircraft manufacturer optimistically estimates how much thrust will be
required. Over years of development, the predicted weight of the aircraft
increases and the aerodynamics get slightly worse. To compensate, the engine
manufacturer needs to find a way to squeeze more thrust out of the engine -
but without changing its size or overall architecture.

This can be resolved by adding extra complexity in the form of extra
compressors or stages. Alternatively, you can try to run the engine at higher
temperatures and pressures. While it's still possible to certify the engine
and run it safely, you will probably experience many more maintenance issues
than if the engine was running inside its original design parameters.

~~~
aunty_helen
When an aircraft is released engine development doesn't stop. These things are
always undergoing revisions, performance upgrades, ADs, and even just
performance upgrades / downgrades / operating procedure changes based on data
from in field running.

I doubt an engine would undergo the addition of a stage as that's pretty major
but there's no reason a mfc couldn't charge you a mil for better compressor
blades and give you a thrust rating upgrade for your patronage.

Edit: I'll keep talking as more info is coming to my head. Jet engines have
fadec computers that get to decide what your maximum power level is. This
maximum is programmed into the engine as a thrust rating. Let's say you engine
has 8 possible power levels to configure, when you test the engine when new or
after maintenance, you will look up a chart based on parameters that your test
cell has logged, ie thrust vs EGT (exhaust temp) is a big one, so too is
vibration at the different bands. This chart will tell you what to program the
ECU with (forgot the actual acronym, its analogous to an ECU on a car). So to
your point, when you get more performance out of an engine, you can set these
levels higher, so you wouldn't expect more degradation from you engine for a
higher power level as the manual will tell you what power level the engine can
be set to which will keep everything within its maintenance limits. (Sorry if
that's not super clear)

Edit 2: Funny implementation detail of setting a thrust rating, it's literally
a jumper based system similar to old motherboards, except you manually wire a
blanking plug to bridge the connections. A blanking plug is an electrical
connector that has a cap on it with a little bit of space to run wires from
one pin back to another.

Also the correct acronym is EEC.

------
inferiorhuman
The Bloomberg article doesn't seem to touch on what specifically is going on
with the Trent 1000 (but my understanding was that protective coatings weren't
holding up to air pollution). In any case, General Electric is having problems
specifically with the air-cooled blades that Bloomberg's pinning Rolls'
troubles on. Keeping the blades solid at those temperatures is almost
wizardry. Fascinating stuff.

A Thai Airways lost an engine (GE90) on one of their 777s recently due to
problems with the air cooling. As a result eight GE90 engines are now subject
to an additional inspection. Here's a video touching on the GE problem by a
tech who works on GE turbines for a living:

[https://www.youtube.com/watch?v=0GxvEQZil2U](https://www.youtube.com/watch?v=0GxvEQZil2U)

~~~
kylehotchkiss
Pollution causing jet engine issues is scary. The countries and engineers
building the engines don't encounter extreme pollution and the places
experiencing the worst pollution aren't major hubs for aircraft development.
It's an "out of sight/out of mind" type of issue during a record period of air
travel growth all around the world

I hope ICAO can begin studying the affects of pollution on aircraft more
heavily because compressing air with an AQI over 1,000 (like India has
experienced recently) could have very unknown consequences elsewhere in the
engine after the blades, (unless it just pushes all the pollution particulates
back into the cabin, which is another issue that needs to be solved)

~~~
xadhominemx
Airborne particulates are absolutely not “out of sight/out of mind” for
engineers designing commercial jet engines. That’s an absurd statement. It’s
one of the top considerations for those engineers.

------
baq
note that there isn't that much efficiency left on the table: the best turbine
is about 62% thermal-efficient[1] (note - not an aircraft engine). compare
that to the theoretical carnot cycle at approximately jet operating
temperatures - it's about 80%. these things are approaching dark magic
territory at ~75% theoretical thermodynamic maximum.

[1]
[https://www.ge.com/power/about/insights/articles/2016/04/pow...](https://www.ge.com/power/about/insights/articles/2016/04/power-
plant-efficiency-record)

~~~
credit_guy
By my calculation the carnot efficiency for the jet operating temperature is
about 85.5%: 1-T_cold/T_hot, where T_cold ~ 230K (it's quite cold at high
altitudes) and T_hot ~ 1588K (or 2400 F, which is the hottest temperature in
the Leap engine produced by GE, which powers the 737-Max [1]). This is only
possible though because of GE's usage of the ceramic matrix composites (CMC)
which allows for 150-200K hotter temperatures than classical alloys.

Now the current CMC method employed by GE uses silicon carbide, which has a
melting temperature of about 3000K. If one day they could switch to hafnium
carbide, with a melting temperature of more than 4100K, the carnot efficiency
could reach more than 89%.

[1] [https://www.ornl.gov/news/ceramic-matrix-composites-take-
fli...](https://www.ornl.gov/news/ceramic-matrix-composites-take-flight-leap-
jet-engine)

~~~
baq
didn't know they get so hot. i assumed 1170K (900C) exhaust temperature and
-50C cold reservoir. this is even darker magic than i suspected :)

------
rshnotsecure
They were hitting their limits by the mid 70’s.

The mid 70’s was golden era of supersonic research. Supersonic engines are
just regular jet engines with a Turbofan. Add in wings swept further back in a
Delta pattern like the Concorde and you have...supersonic aircraft that
comfortably can do Mach 1.5-Mach 2.0.

Today, I think there is just 1 functioning wind tunnel in the US that can do
supersonic research, so sad. If you live in the Bay Area you pass it, just
look at Moffet Field on your way north of San Jose.

The Japanese and Australians, due to their remote location in part, have done
most of the significant research of any last few years.

Why did SuperSonics not go anywhere with essentially every major civilian
project being mothballed?

1\. Materials Science: we simply don’t have reliable data for how long XYZ
Metal lasts at extreme temperatures of supersonic flight. If you read Michael
Crichton’s Airframe from 1996 (great novel), they mention that aircraft are
designed with a lifespan of a human. 80 years. It’s insane how long these
things fly. The heat is so extreme, the Concorde would fully extend 1 foot (it
was made in part of titanium) during flight. Many crashes have occurred, such
as Alaska Airlines Flight 261 in 1999 from Mexico to California, because the
maintenance time frames of these parts is still not fully understood. And
that’s for subsonics!

2\. Noise: No way around it. The decision to build aircraft close to cities
was essentially finalized in preparation for World War II. You can dampen
significantly, but it is painfully loud. Landing outside the city with fast
train back is best solution.

3\. Fuel costs and burn. Supersonic research got underway right as Earth Day
started. Lots of supersonic tech relies on afterburners, terrible mileage. It
was bad timing.

4\. 2003 Concorde crash where a tire was hit on the runway, and the entire
aircraft exploded. This should have not happened for a tire collision.
SuperSonics are fragile because of all the design tradeoffs one has to make
(currently).

~~~
cameldrv
Previous commenter had it right. Supersonic aircraft have terrible lift/drag.
This means that they will always burn much more fuel. Since the extra speed is
really only useful for long flights, a supersonic aircraft has to carry a
higher fraction of its weight in fuel, further reducing efficiency.

This makes supersonic airliners infeasible for the really long routes that
they would truly help with. The Concorde only shaved 3.5 hours off the NY-
London time, and cost about 8x as much to operate.

The extremely high costs then mean that you have to sell what are essentially
coach seats at greater than business class prices. There aren't many people
willing to pay business class prices, so you can't run the planes very often,
and you can only run them between very large, rich cities.

Since the whole point of supersonic is speed, you can't effectively sell
connecting flights, because if you fly, say, Boston-London through NYC, the
direct flight from Boston subsonic will arrive at nearly the same time as the
supersonic flight with a connection.

This leads to a very small passenger market. It also means that supersonic
planes will operate on an infrequent schedule. Many people in a position to
pay for a supersonic flight will prefer a subsonic flight that meets their
schedule over a supersonic flight that doesn't.

With the advent of lie-flat beds, in-flight wifi and power, and higher cabin
pressurization, being on an airplane an extra 3.5 hours isn't even so bad
anymore, so the practical advantage isn't nearly as great.

All of these factors make supersonic a very small niche market. This means
that few planes will be produced. High performance aircraft have high
development costs, and those need to be amortized over the production run.
Concorde was a complete financial disaster because they only produced 14 of
them, for all of the reasons above. The only solution is to somehow make the
airframe far more efficient.

~~~
RcouF1uZ4gsC
> With the advent of lie-flat beds, in-flight wifi and power, and higher cabin
> pressurization, being on an airplane an extra 3.5 hours isn't even so bad
> anymore, so the practical advantage isn't nearly as great.

I think this is huge. From what I read, the cabin of the Concorde was very
cramped. I would prefer business class on a 787 or A380 vs being cramped and
arriving faster.

~~~
cameldrv
The size of the seats was a little bit worse than what airlines sell today as
premium economy.

It's interesting to see what Boom is doing here relative to Concorde. They
have a similar sized cabin, but they propose to configure it with fewer but
larger seats.

They will have more efficient engines and a lighter airframe, but the lower
efficiency at higher speeds means they can't match subsonic range, so they'll
have to make a fuel stop on transpacific flights, which hurts comfort and will
cost at least an hour per flight. Economically, transpacific routes may make
more sense even with the fuel stop. Concorde probably could have done this
too, but Air France and British Airways didn't serve these routes. All of the
very long routes they might have used it on crossed Europe and Asia where they
couldn't go supersonic.

------
_ph_
I can't read the linked article, but in general, any technology constantly
tries to push the limits. Jet engines are a very mature technology, so there
are less improvements by just improving the general design. The next step is
optimizing each part, e.g. by selecting new materials to make the parts from.
This usually means a large performance leap - new materials can mean a way
better performance - but also a time where there are struggles to make a new
material working reliably. With time and experience, reliability grows and
progress slows down till the next large step, which might reduce the
reliability somewhat.

On top of those things, I heard stories that GE for example laid off a large
amount of their experienced engineers. This certainly increases the risks with
new designs.

------
jokoon
Yeah I guess that's a good way to start discussing investments in railroad and
nuclear energy.

I don't think a more efficient jet engine will make air traffic environment-
friendly anytime soon.

Sorry to hijack the conversation.

~~~
Shivetya
neither solve the issues air travel does. the amount of rail needed to funnel
the same amount of traffic one of the larger airports generate would be
phenomenal let alone where could you even put it down? then of course comes
the small problem of how many miles cross ocean which puts rail right out of
the equation.

Seriously, look at an airport. get the number of flights and where they come
from and their frequency, now imagine trying to build a sufficient number of
rail lines to do the same.

air travel saves time that no rail line could except in short hop scenarios
where extra time to process through an airport might tip a balance. let alone
you can have nearly an number of planes in flight between two points

~~~
cnorthwood
London Victoria rail station averages ~207k passengers per day. London
Heathrow airport average ~211k. Doesn't feel that unfeasible.

~~~
briandear
Does London Victoria have trains going to Canada, Hong Kong, Moscow? I am
pretty sure most Heathrow flights aren’t to Leeds.

~~~
mschuster91
The point is: take airplanes where it makes sense (long distance > 600km), and
a well-meshed train network for everything else.

------
baybal2
No, turbine inlet temperatures are physically possible to get higher.

Mitsubishi has been developing a turbine for 1700C inlet temperature, and it
seems they are getting close.

Second, it is possible to reduce compression losses in 3 spool designs.

Fan designs are getting better, and more efficient. Gearboxes for mid-sized
engines are planned, and it's possible we'll see them in GE90 class engines
eventually.

Lastly, it's entirely possible to slap a recuperator, intercooler, and
reheater on the engine if things will get too desperate.

~~~
dfgsgsgedag
"Mitsubishi has been developing a turbine for 1700C inlet temperature, and it
seems they are getting close."

And how long does the blade stay together? What happens to the blades if a
foreign object is ingested? These blades are being made to run at T that make
them very ductile, the centrifugal forces literally bend the blades.

Mitsubishi isn't the only one trying to increase the T.

"Second, it is possible to reduce compression losses in 3 spool designs." And
increase the losses due to mechanical coupling and decrease reliability.

"Lastly, it's entirely possible to slap a recuperator, intercooler, and
reheater on the engine if things will get too desperate. " At what weight and
aerodynamic cost?

Its not my intention to contradict you, but to highlight that it's not obvious
that any of these things will ever commercially.

~~~
baybal2
> And how long does the blade stay together?

Perspiration cooling, the blade itself will always stay around 1200C

> And increase the losses due to mechanical coupling and decrease reliability.

Three spool engines are already there, and flying

------
maith1
The biggest issue for innovation in aircraft, is due to the sheer capital it
requires to develop both jet engines and the aircraft, which results in
companies developing these separately. For there to be more innovation in the
space there needs to be closer alignment between aircraft
manufacturers(Airbus, Boeing) and jet engine manufacturers (Rolls Royce, GE,
P&W). There's talks of blended wing bodies etc, which would improve
aerodynamic efficiency but I can't see anything like this happening given the
way the industry is set up.

------
golergka
So, this have been bugging me for a while - what when we actually come to
limits of the new technological and scientific advances that got us through
the 20th century? I doubt that anyone here doesn't see a comparison to Moore's
law. Supersonic passenger flight isn't happening, Musk's spaceship (or at
least, it's use for passenger travel) is a huge gamble...

What if, among all the possible visions of the future, 2119 will have roughly
the same tech as 2019? I don't doubt that there will be cosmetic differences,
I'm talking more about about the fundamental stuff: energy efficiency, FLOPs,
bandwidths, etc, being in the same order of magnitude. What if 2219 and 2319
still says the same, and the hockey stick of growth turns into another plateu?

TBH, this future frigthens me even more than nuclear war or climate change.
For both these old horrors, we at least know the theoretical solutions; but if
we simply run out of new things to discover that make a difference for us as a
species, what then?

~~~
mahart
Biotech is still in its infancy and mastering DNA would open up a new frontier
where humans can edit the genetic code to what makes a human. Assuming it
isn't universally banned.

The only truly depressing future to me is if interstellar travel really is as
hard as it seems to us now. There kind of needs to be a physics/propulsion
discovery on the order of magnitude of electricity or the journey can only be
feasibly made by the immortal AI robot race that replaces us.

~~~
Robotbeat
Interstellar travel IS hard, but the current laws of physics allow it to be
done. Crewed interstellar travel can be done on timescales of ~50 years to the
nearest star system using ~10Terawatt of solar power in space to power
efficient beamed macron propulsion with the benefit being that most of the
investment is in the macron beamer and power supply which stays in the solar
system and can be reused like once every few months, sending a stream of ~100
ton spacecraft on magsails to Alpha Centauri (using the magsails to brake
against the interstellar medium near the destination). Two way trips are not
feasible in a human lifespan as it currently is, but one-way settlement of
nearest stars is, with a stream of supplies and settlers arriving every few
months.

To really make this feasible, improvements in hibernation tech could help.
Induced torpor over a week or two already works and could be used repeatedly
so that psychological time in transit is only a few years (and the reduction
in metabolism from torpor may also extend calendar life of the crew, but that
is not yet proven). Even modest life extension (i.e. increasing healthspan
from ~mid-60s to mid-80s or longer) would significantly increase the viability
of human settlement of Alpha Centauri, and we're just getting to grips with
the biochemical tools needed to fight aging, so who knows over a few centuries
what will be possible. I think increase in human healthspan and lifespan is
more likely than a dramatic practical but fundamental physics breakthrough
cutting down the time.

------
hilbert42
This is like the 737 Max problem all over again, the regulators need to step
in and say that there's not enough margin in these designs to be reliable and
safe. This would force the Airbuses and Boeings of this world to go back to
four engines in their new designs, which they should have never abandoned in
the first place.

As I've said for many years, cutting air travel prices to the bones is a very
dangerous affair—as it results in substandard/shithouse quality engineering
being installed in aircraft (cost-cutting results in designs that have
insufficient safety/reliability margins).

Travelers need to get used to paying about 30% to 40% more per trip—and the
regulators should be spruiking this message loud and clear from upon high
until the plebs get the message!

(It's all pretty basic really—it's a no-free-lunch argument.)

~~~
laurencerowe
It seems far from clear to me that increasing prices for air travel in pursuit
of further increased safety would help overall when it is already incredibly
safe. Higher prices may encourage air passengers to switch to less safe forms
of transport (bus, car) leaving more people killed in transportation accidents
overall.

------
einpoklum
> Unlike carmakers, the airlines lack viable technological alternatives.
> Biofuels have potential but fully electric large commercial aircraft are
> probably decades away.

Huh? Sure they have alternatives: Fuel cells, or non-polluting combustible
pairs of compounds, e.g. Hydrogen + Oxygen (or even Hydrogen + Air?) . Also,
I'm no physicist, but I'm pretty sure it's more efficient to convert to
kinetic energy (almost) directly rather than through electricity - meaning
that electric engines don't make much sense.

Correct me if I'm wrong!

Also, even with current technology - passenger aircraft engine turbines are
quite far from their maximum potential efficiency; see @baq's comment.

~~~
dfgsgsgedag
"I'm pretty sure it's more efficient to convert to kinetic energy (almost)
directly rather than through electricity - meaning that electric engines don't
make much sense."

An electric motor is >90% efficient. Sure you're added an extra stage
converting to electricity, but so what?

Electric drives are so efficient, _diesel_ ICE locomotives have had electric
drives (in lieu of transmissions) since the 1950s. That is, they have massive
diesel engines that generate electricity instead of moving the train directly.

------
rwmurrayVT
Only tangentially related, but when I was at VT there was a lot of interesting
research going on to move engine testing to the ground. There's
engine/aircraft compatibility issues around blended wing body aircraft. You
can read more about it at the link below.

[https://www.researchgate.net/publication/289985065_An_overvi...](https://www.researchgate.net/publication/289985065_An_overview_of_recent_results_using_the_StreamVane_method_for_generating_tailored_swirl_distortion_in_jet_engine_research)

------
discordance
Anyone on HN got a link that shows the improvement of commercial jet engines
over the last 50 years or so?

~~~
baybal2
I can show one which shows just how much of a runway is left:
[https://www.yumpu.com/en/document/read/49793471/active-
cooli...](https://www.yumpu.com/en/document/read/49793471/active-cooling-air-
cooling-newac)

There is a lot of space to improve in

------
gorgoiler
If we let companies race to the bottom leaving only a duopoly, then one of
those companies mortally wounds themselves with flawed profit-margin-enhancing
tech (787 MCAS, VW diesel, Galaxy battery fires, Deskstar HDDs) are we not at
risk of entering a technological dark age?

~~~
rstuart4133
Nah. The magic is not the company. It's the people in it.

When Nokia's retail division went up it a ball of flames, the engineers didn't
just disappear and become janitors or something. They formed HMD, who is the
one of the very few mobile phone makers seeing ear on year growth in
shipments.

Raw human talent is the major driver of innovation. Companies are just one way
they organise themselves. Open source is another. Different organisations suit
different circumstances, but you always need the raw human talent.

------
nickhalfasleep
I wonder how much fuel and maintenance could be saved on airliners if airports
had giant electro-magnetic catapults like naval carriers to accelerate and
launch aircraft without consuming as much fuel at high power.

~~~
hodgesrm
Very little. By one estimate jets burn about 10% of fuel to reach _cruising
altitude_. [1] Unless the catapult got you up to altitude it would not help
much. Catapults solve a different problem, namely taking off from a short
runway.

[1] [https://www.quora.com/How-much-fuel-is-burned-during-take-
of...](https://www.quora.com/How-much-fuel-is-burned-during-take-off-until-an-
aircraft-reaches-normal-cruising-speed-Is-this-the-most-fuel-consuming-part-
of-the-flight)

~~~
TheOtherDave
On the plus side, a catapult that could get the airplane up to cruising
altitude in one piece would make for a heck of a ride.

------
TheOtherDave
I wonder if this means Boeing/Airbus/etc will have to move towards flying
wings or other designs which might risk some degree of passenger comfort to
achieve efficiency goals?

------
neonate
[http://archive.is/OoWW6](http://archive.is/OoWW6)

