[nluken]

> Concorde burned 52% of its fuel just taxiing and taking off

and later in the article:

> Remember, Concorde burned 52% of its fuel just taxiing down the runway.

Setting aside that these are completely different claims, the author does not cite this claim at all and it fails my personal gut check. Where is this information coming from?

[kens]

The claim in the article, "Concorde burned 52% of its fuel just taxiing down the runway", is completely wrong and kind of ruins my confidence in the article. A Concorde used less than 1% of its fuel taxiing down the runway, not 52%.

Source: Air France Flight 4590 Accident Report states that the plane had 95 t of fuel on board when the aircraft started out and used 800 kilos of fuel during taxiing (page 17) and 200 kilos after taxiing before takeoff (page 159). https://www.faa.gov/sites/faa.gov/files/2022-11/Concorde_Acc...

(Since there's a bunch of discussion about how to reduce taxiing consumption, I'll point out that one tonne of aviation fuel is about $700, so there's not much money to be saved by creating battery-powered tugs or whatnot.)

As far as takeoff, "at the start of cruise 20% of the total fuel burnoff will have been consumed while only 9% of the total distance will have been covered." From "Operation Experience on Concorde", a paper by the Design Director. While 20% is a lot, it is much less than 52%. https://www.icas.org/icas_archive/ICAS1976/Page%20563.pdf

[consp]

9% of the distance but 100% of the altitude. That statement completely ignores the hardest part of the flight (with respect to building potential energy) of getting at altitude.

[labcomputer]

> (Since there's a bunch of discussion about how to reduce taxiing consumption, I'll point out that one tonne of aviation fuel is about $700, so there's not much money to be saved by creating battery-powered tugs or whatnot.)

Probably the biggest win in aviation emissions would be converting all the ground support vehicles to electric. They’re currently classified as off-road vehicles, so don’t have to adhere to the same emission standards and normal cars and trucks. Additionally, they already spend a lot of time parked at the gate, which makes charging convenient and means that workers are never “waiting” for the vehicle to charge.

[masklinn]

Yes, it sounds like the repetition of a mangled version of the SR71 stories. Burning 45 tonnes of fuel on the runway would be completely insane.

Checking various links on taxiing burn yields about 2 tonnes which is a lot more realistic and reasonable (a previous HN comment indicates the 767 burns about a tonne taxiing: https://news.ycombinator.com/item?id=24283386 concorde burning twice that sounds fair)

The OP might have gotten confused reading articles like https://simpleflying.com/concorde-fuel-consumption/ stating concorde burned half its tank from the gate to cruise (mach 2 at FL600)

[Reason077]

> ”the 767 burns about a tonne taxiing”

This seems incredibly inefficient. Is there a future for hybrid aircraft, which would feature both traditional turbofans and large batteries for energy storage?

Batteries would eliminate the need for an APU and power the aircraft during taxi, allowing the engines to be started just before actual takeoff, and shut down immediately after landing.

Either the batteries could power wheel motors directly during taxi, or the aircraft could mix turbofans with e-fans (which could also allow energy recovery during descent and help power the aircraft during cruise, reducing fuel consumption further).

[tetromino_]

> This seems incredibly inefficient.

Very inefficient but good for safety: if an engine is failing, you hopefully might discover that while taxiing rather than when you are in the death zone 25 meters up in the air.

[Reason077]

Not an expert, but intuition suggests this probably isn’t true.

If an engine is going to fail spontaneously it’s almost certainly going to happen at high thrust, not while at idle or very low thrust values during taxi.

[philipwhiuk]

Your intuition is wrong.

Engines experience issues when changing speeds (especially start-up) not when at steady thrust output.

[Reason077]

Fair enough. But delaying engine start-up until the aircraft has nearly finished taxiing wouldn’t have any effect on the way the engine operates during start-up. It just means it would spend less time at idle or near-idle speeds during taxi.

[masklinn]

Electric taxiing (on APU) has been in development for over a decade, but it's mostly intended for single aisles (the shorter the flight the more the taxi overhead), and the relatively low fuel prices has led to these projects mostly dying off: L3 shuttered their effort in 2013, Honeywell and Safran's EGTS joint venture was dissolved in 2016, and wheeltug... apparently still lives (with no support from either boeing or airbus), though it was initially supposed to enter service in 2018.

[Reason077]

Fuel saving would be only one of the benefits.

Airlines would also significantly reduce engine operating hours, reducing engine wear and thus maintenance costs. I’ve been on flights out of Heathrow that seem to spend almost as much time taxiing as they do in the air (due to weather or ATC delays or whatever), so for short-haul operations this seems really significant.

Local air quality is also a concern for airports: the air in the neighbourhoods around Heathrow often stinks of jet exhaust, sometimes you can smell it from miles away. Presumably, much of those emissions come from taxiing aircraft.

[labcomputer]

The limiting factor for most turbine engines isn’t really operating hours, but “cycles”, which is to say starts and stops. From a maintenance perspective it’s not terribly important whether you start the engine at the gate or the runway.

Also, as far as maintenance goes, engine hours are weighted by operating power. So, an hour at idle doesn’t count as much as an hour at cruise power. One of the reasons airlines started using not-full power on takeoff when conditions allow it is because of “power by the hour” maintenance contracts, which incentivize that.

[Reason077]

> ”engine hours are weighted by operating power”

Interesting - I didn’t know this!

[kamarg]

> This seems incredibly inefficient. Is there a future for hybrid aircraft, which would feature both traditional turbofans and large batteries for energy storage?

I would assume the extra weight would make it not really worth the added cost and complexity.

[elmomle]

Honestly it sounds like the "right" way to do it would be electric ground vehicles pulling the planes into position, as with tugboats in water. Plane never need carry batteries into the sky and saves a literal ton of fuel.

[masklinn]

IIRC towing to and from the runway has two major issues:

- standard towing tractors are really slow when towing, nowhere near taxiing speed, so you need a fleet of heavier duty "fast tow", possibly dedicated (depending on price)

- more traffic around the runway, which creates more airport complexity

Taxibot does exist tho, and is certified, and used in a few airports. Though I think it's only hybrid not electric.

[TylerE]

Bigger issue is that the engines need to be idled for a while anyway to get up to proper temps, etc. you don’t want to start the engines and jam them into full takeoff thrust 5 seconds later.

[fragmede]

Because jet engines are notoriously simple devices to begin with.

[toast0]

Airport tugs might be a better fit to improve ground operations efficiency?

[tim333]

I think they've looked at that kind of thing but not found if practical so far. One innovation has been airbus jets taxiing with just one engine which cuts fuel use a lot as it mostly goes to just spinning the engines.

[jelder]

eTAXI is just one such solution

https://www.safran-group.com/videos/e-taxi-safran-unveils-it...

[prof-dr-ir]

The article is just generally sloppy.

> .. my recent trip from Abu Dhabi to LA. 24 hours door-to-door. We have the technology to reduce that to under 10.

The direct flight (by Emirates) takes 16h15 mins, so that leaves 7h45 mins not in flight. If we want to bring that down to 10 hours just by making the flight supersonic then that would require a flight time of 2h15, corresponding to a (ridiculous) speed well over Mach 4.

[notahacker]

In fairness, Astro Mechanica and Hermeus claim to have a pathway to Mach 5. Not saying I expect to see it, particularly not for regular people flights to the Middle East, but believing in it is kind of the premise of the article.

(I must admit I was more curious about Astro Mechanica's engine tech before they also threw in the intention to operate Uber for business jets...)

[Reason077]

Not ridiculous if you’re flying above the atmosphere. SpaceX has proposed point-to-point rocket-powered hypersonic flights that connect New York to Paris in around 30 minutes.

Obviously the real problem with this idea is environmental: emissions would be substantial and nobody wants an extremely noisy rocket port near their city.

[LargoLasskhyfv]

How do you imagine that? First thing coming to mind is the loudness of rocket starts and powered landings. Even for airports that would be too loud. At least with current regulations. So you'd probably waste time getting to some dedicated facility, far out in the midst of nowhere to care about, and getting out of a similar hole on the other side of the trip. And again regulations regarding the closure of airspaces and seas for starts and landings, as it's currently done. Which seems rather incompatible with the current system of commercial flight ops, as it's currently done. Other relevant regulations coming to mind are evacution procedures/general survivability provisions for conventional commercial flights, which are mandatory by law.

However I turn that idea, no matter from which point I'm looking at it, I'm not seeing it going anywhere.

[jrjeksjd8d]

Whenever I hear people talk about rocket flights I think of the Stephen King short story "The Jaunt". Humans develop near-instant transportation but you have to be unconscious while travelling. A kid avoids being sedated and is driven insane by whatever interdimensional stuff he sees in transit.

Likewise for every fit 20-something being launched at Mach 5 you'd have 10 octogenarians dying of cardiovascular complications.

[general1465]

And furthermore you would be able to start only in good weather window for takeoff and landing and Gs on Gemini flights (which were doing the same thing) weren't comfortable either.

[ghaff]

Musk has proposed lots of things.

[pixelesque]

Roughly that figure (45%) was used to get to Mach 2.0 at 60,000 feet, about 45 minutes after takeoff from LHR (normally over the Bristol channel) to JFK.

Takeoff and climb / accel to Mach 1.7 was done with re-heat (afterburners), which did use a lot of fuel. After that, normal power (no re-heat) was used to get to Mach 2.0 and cruising (supercruise).

[vablings]

https://news.ycombinator.com/item?id=24283386

They did burn a crazy amount of fuel on getting up to supersonic speeds though.

[wat10000]

It used about half of its fuel for taxiing, takeoff, climb, and acceleration to cruising speed. Maybe that's where the number came from originally and it got mangled in translation.

[saalweachter]

When I looked into this in another context (not supersonic jets), while "a lot" of fuel was used just getting the jet up to speed going down the runway, "most" of the fuel was going from 1 foot off the ground to N0,000 feet.

(I was curious if there was any opportunity for some sort of system to power take-off from the ground, be it catapults like on air craft carriers or just power-transmission for electric planes, and the numbers I found were that while a surprising amount of fuel was used by the time the plane lifted off, it was more like 5% than 50%.)

[dwroberts]

American coverage of the Concorde has to try and make out that it was technically bad, otherwise they would have to face up to the fact that their country squashed the possibility of supersonic travel, through political bullying and protectionism of their own aircraft industry

[BobaFloutist]

And also through supersonic travel being annoying as hell and super expensive.

[fragmede]

Though, given the investment into the Concorde led to Airbus and all of their planes, disrupting Boeings dominance of that industry, I think they might have gotten the last laugh.