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Comment by willyt | Hacker News Reader

Supposedly Concorde used more fuel taxiing from the stand to line up on the runway than a 737 flying London-Paris, but the fuel consumption PPM was comparable with a 747 once the plane was at super cruise. The problem is an building an engine that is efficient throughout the entire flight envelope. Engine efficiency improvements have come from gradually increasing the bypass ratio while solving some tricky problems with the tips of the blades approaching supersonic speeds, but, as I understand it, you can't use high bypass engines at supersonic speeds, so it will be interesting to see what technological advancements Rolls Royce have made over the original Olympus engines that powered the Concorde.

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> Supposedly Concorde used more fuel taxiing from the stand to line up on the runway than a 737 flying London-Paris

This sounded ludicrous to me, so I tried to check it. Still not entirely convinced.

The closest source I could find was a Vice article [1]:

> A Concorde’s taxi to the end of a runway used as much fuel as a 737’s flight from London to Amsterdam.

The article links to a now-defunct 2012 story on "scotsman.com", which I found on the Wayback Machine [2]. This story interviews Jim O'Sullivan, the former general manager and chief engineer of British Airways' supersonic fleet, but does not explicitly attribute this taxi fuel consumption claim to him or anyone else.

A simulation-based fuel calculator I found says that the 737-800 would use 94,000 lb of fuel flying LHR-AMS [3]. The Concorde's fuel capacity was 211,000 lb [4].

Could any aviation nerds weigh in on how plausible it is to claim that the Concorde would somehow burn through HALF of its entire fuel load just by taxiing? Maybe "taxiing" here includes takeoff?

[1] https://www.vice.com/en_us/article/ezz8q7/concorde

[2] https://web.archive.org/web/20170626124207/http://www.scotsm...

[3] http://fuelplanner.com/index.php

[4] https://en.wikipedia.org/wiki/Concorde#Specifications

You are correct in your common sense test, the article is not correct.

Check out the Concorde wiki[1] It states ~4.4K lbs for taxi which passes the common sense test. Taxi time is generaly planned as 15 minutes, so ~10K lbs/hr which is more than what a Boeing 767 burns at cruise - quite a lot! For comparison Concorde super sonic cruise fuel burn was 40K-55K lbs/hr.

For another reference Boeing 767s typically plan to burn 2K lbs of fuel for start-up/taxi/takeoff run.

[1] https://en.wikipedia.org/wiki/Concorde

Thank you for doing the research to find the numbers for this.

I think it's worth adding, as is reflected by your B767 ground movement figure that all jet engines are very inefficient at low power outputs, not just the Concorde's/supersonic aircraft. This is as a large amount of energy is needed to create sufficient compression to allow them to function.

Finding ways to minimize the amount of time spent in these states has been the industry's approach to date.

Thanks, I read it in a newspaper article once and I should probably have checked but I was lazy and I knew someone else would do it for me! So I just wrote 'supposedly' and waited...

I just remembered that this is a great site for Concorde facts http://www.concordesst.com.

Also this OmegaTau podcast episode is pretty fantastic, it's so detailed you could almost jump in a Concorde and fly off after you've listed to it. http://traffic.libsyn.com/omegataupodcast/omegatau-166-flyin...

Didn't it need to use the afterburners for takeoff? It if were just for taxiing, a trivial solution would just to drag it to takeoff position.

The afterburners were only used for taxiing and going supersonic. The Olympus engines actually generated so much thrust at idle that only the outboard pair were used for taxiing, and still apparently it was tricky to control.

I have seen various proposals for having some form of external taxi for aircraft, but nothing close to being adopted which I have put down to jet fuel still being cheap and the capital & operational costs being quite high for a lot of tractors with qualified drivers.

If fuel were that costly and if a plane expended a significant enough amount of it taxiing that the its range is affected, then a taxi that takes the aircraft to its takeoff position would be a no-brainer.

A jet-turbine can use a bypass to exchange some of the speed of its exhaust for a greater mass of exhaust moving at a slower speed. This is desirable as matching speed of the exhaust to the airspeed increases propulsive efficiency. A modern jetliner flies relatively slowly to its (un-bypassed) jet exhaust speed, therefore it will slow it down a lot and move a large mass of air - this is a high-bypass turbofan. A jet flying at supersonic speeds obviously has less need to slow down the exhaust, therefore such a high bypass ratio would actually be counterproductive.

Despite having zero bypass the Olympus engines in the Concorde were the most efficient heat engine of their day, achieving 43% efficiency [0]. Their propulsive efficiency was in the 50%s (compared to the 60%s for a B747-100) [1]. Despite this going at supersonic speeds does require an awful lot of energy over subsonic speeds so even with its high efficiencies it burnt a lot more fuel than an equivalent subsonic flight.

Even if Boom could produce the most efficient engines seen, it would still require a lot more energy for the total flight, and therefore would have worse total fuel consumption.

[0] https://en.wikipedia.org/wiki/Rolls-Royce/Snecma_Olympus_593

[1] https://www.nap.edu/read/23490/chapter/6#37

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