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Comment by saagarjha | Hacker News Reader

> A pair of fuel-powered generators inside the central fuselage produce the electricity that powers four ducted fan propulsors mounted on top of the aircraft's flying-wing fuselage.

Is this efficient?

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Yes. The turbine engine wants to run really fast for more efficiency. And the fan wants to be big and run really slow for more efficiency. This is a mismatch. If you connect them with an axle (like in a turbofan), you have to make a lot of compromises, lowering efficiency.

You can put a gearbox in there but it's heavy, maintenance intensive and expensive. Efficiency of the gearbox is pretty good, but not 100%. Then you have a geared turbofan or a turboprop. Geared turbofans are rare for a reason.

With a generator and electric motors, you can have a different amount of fans and turbines, and they don't need to be even close to each other. You don't need the expensive and maintenance prone gearbox. You can optimize the propulsion system more -> more efficiency.

Turbines don't scale down very well - they lose efficiency the smaller they get. This means, electric propulsion is promising for small aircraft since you can have one or two turbines but can have more propulsors.

The big problem with gas turbine gearboxes is that they have to operate at such high RPM that only the best metalworking tech works.

Same with driveshafts and any kinds of transmissions. Any off-axis power diversion from turbines is extremely hard. Imagine a driveshaft spinning at 1/10 of the turbine RPMs, it will still require extreme stiffness or it will disintegrate from vibrations.

Very high RPM generators are not that efficient too because of eddy current losses. It's not so bad in big industrial turbines spinning at 5-7k RPM or so, but losses mount at 15k+ RPM.

This is why there is such a high interest in making superconducting generators for direct drive by turbines. They can be very efficient, and very small — two usually mutually exclusive qualities for an electrical machine.

And even with the best metalworking, materials, and lubricants, high speed gearboxes don't last long, and require frequent inspections for fatigue, and overhauls.

P&W 1000G series have 32000hp gearbox rated in excess of the core cycle life and hours, but still apparently needs mandatory defectoscopy.

I was also skeptical, given that each power conversion step will necessarily be less than 100% efficient. I was surprised to learn that high wattage electrical motors can commonly achieve better than 95% efficiency. I assume the generator can be similarly efficient, which just leaves the combustion engine itself.

Pretty much. The thermodynamics are the same if your turbine drives a propeller or a generator, and the losses due to friction and resistance are small. Diesel-electric locomotives have used the principle for 100 years.

More dense energy-wise than batteries can get. Its required for this concept to be a reality.

That would be true if they weren't using fuel to power it, which they are. Part of the article shows the alternator they were possibly going to use.

You and the GP are in agreement.

The GP posted a sentence fragment, the expansion of which is presumably "[These generators and fuel are] more dense enegry-wise than batteries can get".

I think the better question is: Is this efficient enough?

If this design brings the noise levels they want, perhaps they are only worried about small distances to be covered?

Jet-a or diesel or petrol is incredibly more energy dense in Wh/kg compared to li ion batteries, the very best of which are about 255Wh/kg. Even after losing 40% to waste heat.

it can be. If you can operate a thermodynamic engine at a single 'operational point' meaning fuel/air flow in and energy out then you can heavily optimize its efficiency in terms of thermal efficiency and noise produced at that specific point...or at least a few points.

You can also muffle it if your optimization point is not just thermal efficiency but overall system efficiency.

It's really difficult to muffle a turbjoet or turbofan...the exhaust flow is the energy source.

I think it's meant to have a low heat signature, so efficiency was probably part of the consideration (less heat to shed).

Reminds me of E.E. "Doc" Smith's diesel-powered stealth spaceship in First Lensmen (1950)

https://gutenberg.ca/ebooks/smithee-firstlensman/smithee-fir...

If it has a little battery it could for a short duration have an even lower heat signature by turning off the turbine.

I think the benefit here is to be able to build and fly an electric drone without going knee deep into battery issues.

Doesn't matter, the fuel gives a much longer loiter time over targets.

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