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A Little Gear That Could Reshape the Jet Engine

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Re: A Little Gear That Could Reshape the Jet Engine

#41
post #30

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)

Re: A Little Gear That Could Reshape the Jet Engine

#42
post #30

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.

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

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

Re: A Little Gear That Could Reshape the Jet Engine

#43

Earlier quoted context omitted.

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

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

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.

Re: A Little Gear That Could Reshape the Jet Engine

#44

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…

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.

Re: A Little Gear That Could Reshape the Jet Engine

#45
post #19

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.

Re: A Little Gear That Could Reshape the Jet Engine

#46
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.”

Re: A Little Gear That Could Reshape the Jet Engine

#47

Earlier quoted context omitted.

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

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

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.

Re: A Little Gear That Could Reshape the Jet Engine

#48

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

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.

Re: A Little Gear That Could Reshape the Jet Engine

#49
post #34
post #7

> 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 pregnanc…

Thirty years is: ~ 0.36 x orbital period of Uranus ~ 1.02 x orbital period of Saturn ~ 5.7 x half-life of colbalt-60 source: http://www.wolframalpha.com/input/?i=30+years

Psssht, it's not even half a Jovian year. What's the big deal.

Re: A Little Gear That Could Reshape the Jet Engine

#50
post #18

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…

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

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.

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