Yeah, the trijet design seems failed in general. Unless you can design it to tolerate any wing+tail dual engine failure -- in which case, why have the tail engine at all?
It wasn’t failed. It was designed for a very specific reason and served that purpose well.
Once the reason went away, better designs took over.
They were designed to allow smaller jets to fly over the ocean further than a two engine jet was allowed (at the time). Airlines didn’t want to waste all the fuel and expense of a huge 4 engine jet, but 2 wouldn’t do. Thus: the trijet.
The rules eventually changed and two engine jets were determined to be safe enough for the routes the trijets were flying.
Using two engines that were rated safe enough used less fuel, so that’s what airlines preferred.
It was never designed to be used anywhere else as a general design. Two engines did that better.
You've framed this as disagreeing with me, but I don't think you are. I agree the design made sense in the 1960s, when we didn't know any better and requirements were different.
No, you really can't. Even if it were the same size a dramatically more powerful engine would need a larger "tail" to maintain control in case of an engine out scenario. But a 50% more powerful engine is also likely to be much bigger meaning that major components like the landing gear (and everything around them). A 50% more powerful engine is also likely to be much heavier necessitating its support structures (a.k.a. the wing or tail) be redesigned.
The 737 MAX suffered a number of bad design decisions to accommodate its newer, more powerful engines. Its engines topped out at about 8% more powerful than the 737 NG engines.
Look at thrust on the 737 Max vs thrust on the original 737.
There's a lot of other changes, of course, but more powerful wing engines let you build a bigger plane in the same kind of shape. Changes in flight rules are also significant; if twin jets can't serve all your routes, you most likely want trijets to cover the routes that can't be served by twins and don't demand a quad ... with current flight rules and current engines, twin engine covers pretty much everything.
Essentially every new design is a twinjet, so it's clearly possible to make appropriate decisions in that design space. And both Boeing and Airbus have given up on quadjets.
The MD-11 isn't a new design. It's a stretched version of a first generation widebody whose design dates back to the mid-1960s. Before the MD-11 was developed, McDonnell-Douglas toyed with the idea of a dual engine variant before settling on a three engine version of the DC-10. Trijets in general came about because the engines of the day were too unreliable and too small to work in twin engine configuration at that scale.
The plane which ended up being the final nail in the MD-11's coffin, the 777, didn't start development until the 90s. Of its three initial engine choices, two were derivatives of engines that were around when the trijets came to be. The initial version of that Rolls Royce engine was so late (and so unreliable) that it essentially killed the Lockheed trijet. The third option, the GE90, was the largest turbofan engine at its introduction until it was succeeded in 2020 by the GE9X.
Scaling these earlier engines up to fit an MD-11 sized twin was never an option.
When I replied to this thread[0], with this comment[1], both the comment I was replying to and my comment were talking about trijets in the abstract, not MD-11s in particular, and the current year, or perhaps as early as the 1990s, but definitely not as early as the 1960s.
Several comments, including yours, seem to have misconstrued that to mean I think the MD-11 in particular could be retrofitted into a twinjet. That's, uh, wildly mistaken, and not something I've ever claimed. I just think trijets in general are a design dead-end. Again, that doesn't mean it didn't make sense in 1960s when the DC-10 was being designed.
>both Boeing and Airbus have given up on quadjets.
It is possible “to make appropriate decisions” up to a certain size. They didn’t stop making new quadjets because the design doesn’t work as well as a twin engine, but because airlines don’t need/want aircraft that large. You wouldn’t build a successor to the A380 as a twin engine.
Airlines currently don't want them (which is not even 100% accurate since airlines pulled A380s out of storage, and continue to push back plans to retire them). You started this by saying "You know you can just make the wing engines 50% more powerful, right?". You weren't talking about commercial decisions, you were talking about engineering decisions and capabilities. So, no you can't just make twin engines bigger in all situations. If airlines want large capacity aircraft again, they will be quad jets, not super powerful twin engines.
