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Comment by bertil | Hacker News Reader

“game-changing” and “10% efficiency” don’t seem to match that well…

I suspect that the ability to handle pure Sustainable aviation fuel (SAF) is the critical improvement here, but the article doesn’t provide the context that:

- current engines can only run with 5% SAF

- airlines are trying to get as much SAF as they can but struggle to find more at an affordable price

- how much SAF we could make based on current food trends and what is blocking us from reaching that level.

The announcement phrased as it is makes sense for professionals, which is why I understand RR PR department has phrased it that way, but the BBC shouldn’t expect its readers to know all that.

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10% efficiency gain in such a mature technology as the jet engine is huge.

Indeed, but "game-changing" would be something like a 50% or 90% efficiency increase. The game has not changed with a 10% change.

I don't think you're considering the scale of the airline industry.

Winglets cut fuel consumption by 4-6% and literally changed the game.

I agree that 5% efficiency justifies widespread adoption, but that didn’t require much more than a few hours of maintenance to set wingtips up.

I’m not sure that 10% would justify switching to a new engine, a new airplane type, and investing in re-qualifying all your pilots. I remember hearing hesitation around RISE and braced wings. Those would “only” save 20% fuel but required too many changes to be adopted before planes needed to be replaced.

On the other hand, working with a different fuel but with a similar power package, that could presumably be adopted before 2030, if the savings are indeed almost an order of magnitude of carbon footprint. This is why I think the article should have said a lot more about the supply chain and price of SAF.

> I agree that 5% efficiency justifies widespread adoption, but that didn’t require much more than a few hours of maintenance to set wingtips up.

Retrofitting winglets on an airliner costs a million+ upfront and grounds the plane for a week.

It’s not to the level of buying a new plane, but this is not a few hours work on the tarmac between two flights, they’re not airfoils off of wish you glue onto your rust bucket.

>didn’t require much more than a few hours of maintenance to set wingtips up.

Try 14+ days for commercial aircraft.

10% at scale is substantial.

It’s huge but not game changing.

For some perspective, Boeing started the whole 737MAX ordeal over ~5% fuel economy.

10% is close to the point where airlines would retire their planes earlier in order to reduce their fuel cost, as planes spend their value on fuel every 5 odd years.

Don't they replace engines independently of planes, anyway? There's no suggestion they need to retire airframes to retrofit the new engines.

Edit: this is answered down thread, seems it's more complicated than that.

10% from engines helps in multiple ways.

Suppose you want to reduce carbon emissions by 50% but SAE cost 40% more. Replacing half your fuel with SAE = 0.5 * 140% + 0.5 * 100% = fuel costs go up by 20% and ticket prices go up by half that or 10%.

If instead you reduce total fuel need by 10%. 10% engines + 40% SAE + 50% normal = 50% carbon savings. But you’re paying 0.1 * 0% + 0.4 * 140% + 0.5 * 100% = 6% more on fuel and ticket prices only go up 3%.

On top of that you don’t need to carry as much fuel which further increases efficiency or let’s you increase cargo capacity.

I thought that aircraft could use much more than 5%, but that the maintenance regime was different - e.g. [1]. Current engines assume a proportion of impurities and are tuned to work with that, but could be modified to run with 100% SAF if it was available.

Just to add - 10% increase on the state of the art is nothing small!

[1] - https://www.gov.uk/government/news/royal-air-force-completes...

What is sustainable aviation fuel? Is it meaningful more “sustainable”?

The article explains briefly: fuel that is not from fossil sources. Essentially, organic waste converted to petrol-like hydrocarbons. Used frying oil from fast food is a large, consistent source. Ethanol from corn is also common in the US, but mostly for petrol meant for car. Not sure if there’s corn-based SAF but that would feel counter-productive (corn-based ethanol takes a lot of petrol to make).

There seems to be issue with the relative portion of octanes, so it cannot be used in large portion in both car and aviation: I remember reading something about lead in petrol and “knocking”, ie the air-petrol mix exploding too early. I’m assuming that this engine handles that problem better, somehow. Given that lead is still in aviation gas, that would be beneficial in many ways.

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