
A Little Gear That Could Reshape the Jet Engine - nether
http://www.bloomberg.com/news/articles/2015-10-15/pratt-s-purepower-gtf-jet-engine-innovation-took-almost-30-years
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Animats
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.

~~~
iammaxus
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](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.

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fancyketchup
> 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.

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mhandley
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.

~~~
blobert
This idea would unfortunately negate one of the top benefits of a high-bypass
turbofan. Noise reduction.

~~~
mhandley
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.

~~~
WalterBright
> Yes, but why is a high bypass turbofan 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.

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PeterWhittaker
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.”_

~~~
TeMPOraL
I suppose the world is big enough for both. Sadly, media follows the kids more
often than the adults.

~~~
gilgoomesh
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).

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brwnll
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.

~~~
WalterBright
Re-engining an airframe is a big deal. A lot of engineering has to go into it.

~~~
dexterdog
Why can't they just have a standard mount like an LCD panel?

~~~
Gibbon1
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.

~~~
nether
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.

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dognotdog
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?

~~~
rm445
This article has much more detail:

[http://theflyingengineer.com/flightdeck/pw1100g-gtf/](http://theflyingengineer.com/flightdeck/pw1100g-gtf/)

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.

~~~
dognotdog
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.

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readams
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.

~~~
ckozlowski
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.

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callesgg
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.

~~~
mschip
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.

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joncp
> "At times, the extent of the operation had to be protected from bean-
> counting Pratt executives"

Yep, that's how UTC operates. It's amazing they make any money at all.

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JSeymourATL
“The Silicon Valley guys seem to have the attention span of 3-year-olds.”

~~~
GauntletWizard
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.

~~~
onion2k
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.

~~~
ams6110
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.

~~~
onion2k
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.

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knodi123
> 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_."

~~~
avar
I thought the next example was even more inane. It goes on to say: "and
unimaginably longer than it takes to pop out a smartphone app".

~~~
craftkiller
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

