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.

▲ loeg 4 hours ago | parent | next [-]

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.

	▲	harpiaharpyja an hour ago \| parent [-]
		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.

▲ inferiorhuman 4 hours ago | parent | prev [-]

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.

  It was never designed to be used anywhere else as a
  general design. Two engines did that better.

Not quite. Dassault still makes a three engined bizjet and in theory the Chinese fly a three engined stealth jet.

	▲	MBCook an hour ago \| parent [-]
		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!