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The Boeing 747 can carry a fifth engine on the side. As the airframe looks quite symmetric, I think that it would not be big work to hang a sixth engine on the side as well.

From here, we seem to be quite near to the six-engine aircraft - a few extra pipes and wires are probably all we need to get these engines turning.

Was a six-engine 747 present at some time of its development history? I think this could be, assuming:

  • less powerful engines than eventually were available
  • maybe it could take off with less runway
  • some special uses with very heavy payload.

There are no six-engine variants mentioned, built or proposed, on Wikipedia.

The expected answer would mention some sources relevant to the design decisions through the history of this aircraft.

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    $\begingroup$ For what it's worth, Boeing did make at least one 6-engine aircraft model, though not a 747. Incidentally, looking around in the categories, there are some truly bizarre looking planes, including some with up to 14 engines... $\endgroup$
    – Darrel Hoffman
    Aug 28 '19 at 17:33
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    $\begingroup$ As a systems guy, "a few extra pipes and wires are probably all we need to get these engines turning" makes me want to sit you down and figure it out. Not saying you're necessarily wrong, but most likely you are. $\endgroup$
    – Mad Physicist
    Aug 28 '19 at 20:04
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    $\begingroup$ why stop at six.... $\endgroup$
    – Anilv
    Aug 29 '19 at 5:51
  • $\begingroup$ The real question to ask it why don't we have a 747 with 4 ge90s yet.. $\endgroup$
    – DatsunZ1
    Aug 29 '19 at 19:35
  • $\begingroup$ This would be like asking "My car has a cup holder, surely it would be easy to turn the car into a bulldozer." Heh ! $\endgroup$
    – Fattie
    Aug 29 '19 at 19:39
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No, it wasn't considered during the development. (Bowman)

The 747 came from Boeing's studies for the USAF CX-Heavy Logistics System program, which was won by the Lockheed C-5 Galaxy. See: What was Boeing's competitor to the C-5?

That project called for four engines, and an engine was designed for that purpose. See: How was the high-bypass concept invented?

So the powerful engine was available.

Regarding requiring less runway, it would have been a bad product. Airplanes are sized according to both takeoff and cruise:

enter image description here
Y-axis is takeoff thrust/max takeoff weight and X-axis is max takeoff mass/wing area (Preliminary Sizing - HAW Hamburg)

Shortening the takeoff by adding more engines, or overly powerful engines (a lot more than cruise requires), would lead to poorer cruise economy due to the increased drag (if two additional engines or bigger engines are used) and higher fuel rate per thrust unit – gas turbines get better fuel rate per thrust unit if they're running near their design limit, that won't be the case if there's a lot more power than needed in cruise.

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  • $\begingroup$ @ymb1 The thing is, if the power was added in such a way that cruise wasn't affected that much, it just moves the takeoff constraint down and to the right, both of which make the aircraft better/cheaper $\endgroup$
    – costrom
    Aug 28 '19 at 17:08
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    $\begingroup$ @costrom: If I'm reading your comment right, if the mass is fixed, down to the right is lower thrust. And it moves into the hatched zone for cruise. And excess cruise thrust is less efficient as explained in the last paragraph. $\endgroup$
    – ymb1
    Aug 28 '19 at 17:18
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    $\begingroup$ Pedantic nitpick: the denominator of the y-axis is max takeoff weight, not mass, due to the multiplication by g, which makes the scale nicely dimensionless $\endgroup$
    – llama
    Aug 28 '19 at 18:04
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    $\begingroup$ While the plot of wing loading vs TWR is clear, it may be best to explain how the two are traded off (and how any excess is just wasted fuel), for any casual visitors. $\endgroup$
    – Therac
    Aug 28 '19 at 20:53
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    $\begingroup$ @Therac: I think I have for the part that is relevant, wing loading won't be. Also the linked PDF is undergrad level and easy to follow through for the extra curious. $\endgroup$
    – ymb1
    Aug 28 '19 at 20:57
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The "fifth engine mount" option on the 747 is not designed to handle a running engine. It was an option used only by Qantas as a means of ferrying spare engines to remote locations, where flying a plane for a long distance to a maintenance facility on three engines was not possible. Only four of the Qantas fleet of 747s (totalling more than 60 aircraft) had this fifth engine option.

The mount for the fifth engine was not designed to transmit any thrust the engine would have delivered if it was running, and it fact the engine is partially disassembled by removing the fan to reduce drag (which reduced the loads on the 5th engine mount, as well as drag on the plane as a whole).

Some pictures and videos here: https://www.flightradar24.com/blog/how-qantas-ferried-an-engine-on-the-wing-of-a-747/

To convert this into "a 5 or 6 engined plane" would require a lot more than just "a few pipes and wires". The wing structure would have to be redesigned to handle an extra 50,000 pounds (or more) thrust from each extra engine, plus the extra weight of a proper pylon and nacelle structure. Considering the aerodynamic wing flutter problems with the initial 747 design, sticking another two engines on the wing would most likely have required a complete redesign of the wing.

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  • $\begingroup$ Pratt & Whitney Canada (PWC) use 2 747SP for engine testing that have an engine mount on the top fuselage and another cradle under wing for different configuration, here's an old article: 747sp.com/pratt-whitney-pw1200g-first-flight . Unlike Quantas, these are mount are custom fabricated and fully functionnal for testing purposes. (see l3t.com/mas/latest-news/2019/06/20/…) $\endgroup$
    – Gabriel Mongeon
    Aug 29 '19 at 12:39
  • $\begingroup$ In the early 80s, I flew on a British Airways 747 that was taking an engine this way from London to New Delhi. I was wondering if it was normal for the inboard flap to jiggle, even though retracted, when the captain explained what was going on. I suppose BA could have bought or leased this aircraft from Qantas. $\endgroup$
    – sdenham
    Aug 29 '19 at 18:14
  • $\begingroup$ "On the image above you can see the specially designed stub wing, which is placed just aft of the co-pilot's seat." – "just aft"? it’s at the other end of the upper deck! $\endgroup$
    – jbg
    Aug 30 '19 at 11:13
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[This answer refers to the original version of the question before it was edited.]

Literally speaking, yes. At least you have thought about it.

Seriously, the target is not to fit as many engines as possible, but how to fulfill the performance and safety requirements with as few engines as possible.

More engines means more power and redundancy, but fewer engines means less cost, complexity, weight, higher fuel efficiency, and a lower probability of a single engine failure.

The big four engine aircraft are currently losing market share, and we can observe a transition to two engine aircraft. There are many studies and articles on this topic, for instance "Size matters for aircraft fuel efficiency. Just not in the way that you think" by Dan Rutherford on the ICCT blog.

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    $\begingroup$ Sound funny but logically correct. This was not the idea, however. The question has been edited now. $\endgroup$
    – h22
    Aug 28 '19 at 8:17
  • $\begingroup$ @h22, forgive me, but it is not possible to substantiate with sources that Boeing did not think or consider putting 6 engines on the 747. I'm pretty sure the architects have explored this option at the time. Which kind of source could possibly tell the opposite? $\endgroup$
    – bogl
    Aug 29 '19 at 7:01
  • $\begingroup$ There are books that explore the design history of various aircraft. The soon-to-be-published 'American Secret Projects 3 - US airlifters since 1962 likely contains a section on 747 concepts. At least one of its competitors (from Douglas) had 6 engines. $\endgroup$
    – Hobbes
    Aug 29 '19 at 8:51
  • $\begingroup$ Why does reducing the number of engines reduce the probability of a single engine failure? $\endgroup$
    – Lightness Races in Orbit
    Aug 29 '19 at 17:42
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    $\begingroup$ It might be interesting to add an edit note to this answer as it doesn't reflect the question any more and feels slightly awkward $\endgroup$
    – everyone
    Aug 30 '19 at 1:08

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