Was a six-engine 747 ever seriously considered by Boeing?

Question

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.

Answer 1

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.

Answer 2

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:

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.

Answer 3

^{[This answer refers to the original version of the question before it was edited.]}

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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.