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I saw this image on a NASA page talking about hybrids and I was wondering if the back-engine placemente is actually viable and if it is why hasn't been done yet? enter image description here

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    $\begingroup$ "why hasn't been done yet?" one word: tailstrikes $\endgroup$ – Federico Jun 8 '17 at 5:13
  • $\begingroup$ Doesn't any single-engine jet effectively have a back engine - or at least back exhaust - placement, given obvious weight & balance limitations? Not to mention a number of pusher prop planes. $\endgroup$ – jamesqf Jun 8 '17 at 6:09
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    $\begingroup$ This configuration (STARC-ABL) is also aimed to reduce drag and energy consumption by absorbing the turbulent fuselage boundary layer in the continuation of Boeing SUGAR program. $\endgroup$ – mins Jun 8 '17 at 8:31
  • $\begingroup$ Until somebody loads the plane improperly and tips the tail. $\endgroup$ – kevin Jun 8 '17 at 11:50
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As you look closely to the picture, there are also engines located under the main wings. Thus this is a trijet, a concept that has been used in the past by for example the MD-11, Lockheed tristart and Boeing 727. The main reason for choosing a trijet configuration was the ETOPS requiring twin engine aircraft to always be within a certain range of an airport. This was in a time when engines where less reliable. With three engines the aircraft did not have to bother with ETOPS regulations allowing it to fly routes impossible to fly for twin engine aircraft. But as engines became more reliable and the ETOPS got lengthened the trijets became redundant.

A big downside of the trijet is the structural complexity of an engine in the back, the fuselage needs to be reinforced to withstand the extra forces. From a maintenance perspective it is difficult to access the middle engine because it is high of the ground requiring a stair for visual inspection. Then there is the economical aspect, if you can do it with less engines it will always be more fuel economical. The next iteration of 777 can carry as many passengers as the 747 with half the engines.

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    $\begingroup$ I don't think this is right ... what's unheard of on this design is not the rear choice for the engine, but more the fact that the rear engine is centered on the fuselage while a tri jet such as the MD11 have it offset (up) $\endgroup$ – Antzi Jun 8 '17 at 16:28
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    $\begingroup$ You completely missed the reason the aft engine is here. It's not for escaping ETOPS, but to suck air flowing along the fuselage instead of the free air stream in the hope this will reduce aircraft total drag. Conventional trijets exist, this is not one of them. $\endgroup$ – mins Jun 8 '17 at 17:46
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    $\begingroup$ The question mentions only "the placement" of the back engine, not the shape of the intake. $\endgroup$ – Koyovis Jun 9 '17 at 1:37
  • $\begingroup$ @mins, I'm aware that the reason NASA put the engine here is for the boundary layer ingestion, but the question asked here was more general about placement of the engine in general indicating that the asker is not aware of the trijet concept. $\endgroup$ – Brilsmurfffje Jun 9 '17 at 9:16

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