Comment by loeg
20 hours ago
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?
20 hours ago
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.
A design that was once useful but no longer has a use is not the same thing as a failed design. Which is what the disagreement seems to be about.
In the case of the quad jets, Boeing tried the 747-SP and had minimal marketing success.
In the case of the trijets the MD-11 lived on as a freighter because it had a much higher capacity than anything else smaller than a 747.
Not quite. Dassault still makes a three engined bizjet and in theory the Chinese fly a three engined stealth jet.
I didn’t know there was a three engine business jet, my knowledge is mostly passenger airliners and even then just from an amateur perspective.
Other than being able to identify a couple of famous ones I don’t know a ton about military airplanes either.
Thanks!
> in which case, why have the tail engine at all?
"you know what this motorized piece of anything needs, less power"
-nobody, ever
You know you can just make the wing engines 50% more powerful, right?
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.
> just make the wing engines 50% more powerful
You realize this is not quite how aerospace engineering works, right?
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