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Since more and more airlines are dropping the 747 for the 777 or equivalent due to fuel efficiency, a theoretical question came to mind.

What do you think is preventing Boeing from creating a 747 twinjet (two engines instead of 4).

I know there are probably design/engineering issues associated with such a conversion, but I reckon that if it was done it may increase sales to go up against the A380? Not sure the math as well, but perhaps the engine from the 777 could be used or configured for higher thrust?

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    $\begingroup$ What do you think is preventing Boeing from creating a 747 twinjet because it makes no sense and there is no demand? $\endgroup$
    – Simon
    Commented Aug 28, 2015 at 21:26
  • $\begingroup$ Do people ever ask "Why don't we put a BMW engine inside a Kia"? $\endgroup$
    – curious_cat
    Commented Aug 29, 2015 at 13:35
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    $\begingroup$ @curious_cat you would be surprised what people actually ask ( and do ) $\endgroup$
    – Federico
    Commented Aug 29, 2015 at 14:19
  • $\begingroup$ The A380 hasn't made any sales for well over a year. The market for such an aircraft is quite small. $\endgroup$
    – Ben
    Commented Aug 30, 2015 at 1:01
  • $\begingroup$ @Federico: Fascinating! $\endgroup$
    – curious_cat
    Commented Aug 30, 2015 at 3:29

2 Answers 2

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Currently, the 777 has engines that have a max thrust of 115,000 lbf, for a total of 230,000 lbf of thrust. The 747-8 has engines with a max thrust of 66,500 lbf, each, for a total of 266000. And just to throw in for comparison, the A380, currently has engines each producing 72,000 lbf of thrust, and a total of 288,000 lbf.

Right now the 777 has some of the highest thrust producing engines in commercial service. This does not take into account the extra thrust that is required for engine out issues. With a 2 engine design you have to have enough extra thrust for loss of engine during take-off at max take off weight. Whereas with a 4 engine design, you only have to have enough thrust to account for loss of 1 engine. So for a 747 twin engine aircraft, as a rough estimate you'd need to have each engine produce at least 150,000 lbf of thrust if not more.

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  • $\begingroup$ So in theory, if they revised the 777 engine to achieve 150,000, it could still be an option to keep the queen alive? $\endgroup$
    – chickenricekid
    Commented Aug 28, 2015 at 22:15
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    $\begingroup$ If they could get an engine designed and built to have the necessary performance, it could be built. Also, realize the 777 engine is about the same diameter as the 737s fuselage, so scaling up to the needed thrust for a 747 would be larger, and would entail a lot more design changes and rescaling of the wing and gear of the 747s design. Is it worth it? From what I can see, no, it just couldn't be redesigned in an effective manner for an appropriate cost. $\endgroup$
    – slookabill
    Commented Aug 28, 2015 at 22:59
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    $\begingroup$ Don't forget that the B747 was designed almost 50 years ago. If you tried to make that sort of design change (and assuming one of the engine manufactures was feeling crazy enough to try designing something with 150,000lb thrust for it) , it would be like your grandfather's army knife that is still the same "knife," despite having new wings, new landing gear, a new fuselage, a new cockpit, and a new tail. If it was a remotely sensible commercial programme, you might as well also change the only thing left from the original - i.e. the name. $\endgroup$
    – alephzero
    Commented Aug 29, 2015 at 0:19
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    $\begingroup$ "Right now the 777 has some of the highest thrust producing engines in commercial service." Not just some of. The GE90-115b (the 777-300ER engine) holds the world record for thrust produced by a commercial jet engine (and I'm not aware of anything in the works that is likely to exceed it.) $\endgroup$
    – reirab
    Commented Dec 28, 2015 at 16:39
  • $\begingroup$ @alephzero: Granted, it wouldn't be the first time Boeing's done something like that - look at the 737 MAX, which has basically nothing in common with the 737-100 except the basic configuration. $\endgroup$
    – Vikki
    Commented Oct 17, 2018 at 21:30
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For a wing design it's better to have more engines because the loads evenly spread along the wing. This allows a lighter wing design.

In addition you must provide a minimum yaw control ability at one engine in-operating during take-off. This means you have to enlarge the vertical stabilizer. This enlarges the drag during normal operation.

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  • $\begingroup$ But isn't it also true that the engine load torgue further out on the wing increases loads? I might have to have less strong engine MOUNTS, but the wing is till taking on the same approximate load (20K lbs for GE90 on 777 vs 12K lbs x 2 for GEnx on 747) accounting for thrust, you still have to engineer the wing root and inner wing for 110K lbs of thrust. $\endgroup$
    – Chris K
    Commented Jun 22, 2017 at 22:18
  • $\begingroup$ Yes. But take the impact of the worst case into account for this calculation. If one structure fails... this leads to different safety margins. So it isn't that easy to compare this only by taking mass and thrust related loads. Additionally the engines you compare are from different age: 95 to 04. So I would say the thrust to mass ratio is not compareable. $\endgroup$
    – Alex44
    Commented Jun 22, 2017 at 23:04

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