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Comment by mhandley | Hacker News Reader

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.

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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.

http://www.airbusgroup.com/int/en/news-media/media~item=be67...

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:

http://eandt.theiet.org/magazine/2014/10/rise-of-electric-ai...

just remove the fan from the turbine altogether

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.

[1] https://en.wikipedia.org/wiki/Turbofan [2] https://en.wikipedia.org/wiki/Turbojet

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.

https://en.wikipedia.org/wiki/Bypass_ratio

Tom Clancy wrote a book many years ago in which he discusses this in laymans terms really well: http://www.amazon.com/Fighter-Wing-Clancys-Military-Referenc...

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.

Is this why on the Antonov-70 and the military Airbus, they have those strange curved multi-blade props?

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.

Unless you're experimenting with madness: https://en.wikipedia.org/wiki/Republic_XF-84H

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.

https://en.wikipedia.org/wiki/Auxiliary_power_unit

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?

It's not that bad. Use 300 volts and it's only 50amp. That kind of amperage is routinely used by oven/stove/ranges.

And 300 volts is not considered high voltage.

If they upped the voltage to 1,500v (which is the limit before you need much more complicated stuff) it's only 10amp - a simple thin wire would do it.

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).

Yes, I misread kW for W :( :(

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.

1: http://www.aerospaceweb.org/question/propulsion/q0195.shtml

2: http://www.pprune.org/tech-log/423988-concorde-question.html

High-Tc superconducting cables?

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

And noise reduction is actually a really important benefit of this engine as some airports have restrictions on noise.

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.

> 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.

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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