17
$\begingroup$

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.

$\endgroup$
8
  • 4
    $\begingroup$ 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... $\endgroup$ Aug 28, 2019 at 17:33
  • 26
    $\begingroup$ 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. $\endgroup$ Aug 28, 2019 at 20:04
  • 4
    $\begingroup$ why stop at six.... $\endgroup$
    – Anilv
    Aug 29, 2019 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, 2019 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, 2019 at 19:39

4 Answers 4

39
$\begingroup$

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.

$\endgroup$
3
  • $\begingroup$ 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/…) $\endgroup$ Aug 29, 2019 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, 2019 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, 2019 at 11:13
31
$\begingroup$

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.

$\endgroup$
7
  • $\begingroup$ @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, 2019 at 17:08
  • 1
    $\begingroup$ @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. $\endgroup$
    – user14897
    Aug 28, 2019 at 17:18
  • 1
    $\begingroup$ 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, 2019 at 18:04
  • 1
    $\begingroup$ 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, 2019 at 20:53
  • 1
    $\begingroup$ @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. $\endgroup$
    – user14897
    Aug 28, 2019 at 20:57
11
$\begingroup$

[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.

$\endgroup$
10
  • 6
    $\begingroup$ Sound funny but logically correct. This was not the idea, however. The question has been edited now. $\endgroup$
    – h22
    Aug 28, 2019 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, 2019 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, 2019 at 8:51
  • $\begingroup$ Why does reducing the number of engines reduce the probability of a single engine failure? $\endgroup$ Aug 29, 2019 at 17:42
  • 1
    $\begingroup$ 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, 2019 at 1:08
3
$\begingroup$

The answer is actually Yes, Boeing explored both 5- and 6-engine variants during the design phase of the 747-100 series. So, your first rationale was the correct one.

I read about it in an older book on the 747, written when the plane was only a few years in service. I can't remember the author or title at the moment, I'm afraid, but it had extensive quotes from Joe Sutter and other engineers, and discussed the early teething troubles in detail.

In the early 747 development, one of the biggest problems was the JT9D engine, one of the most powerful engines conceived at the time. The initial prototype version, the JT9D-1, had insufficient thrust even when it was working, which wasn't often. Pratt & Whitney weren't sure how much more power they could eke out of it in time for EIS, and the wear rates were abysmal.

So, uncertain that the plane would have sufficient thrust at the required MTOW for the intended passenger, baggage, and fuel load, Boeing and P&W considered two potential alternatives.

  • One option was a 5th engine (another JT9D-1) buried in the rear fuselage and fed through an S-duct intake. This was a desperate crutch, since it would increase fuel burn, increase maintenance, and decrease dispatch reliability, but it would guarantee enough power. See note below.
  • The second option was stranger. They considered installing two JT3D engines alongside the rear fuselage, with a T-tail design. The JT3D was the first-generation turbofan used on the 707, DC-8, and C-141, so just imagine two 707 turbofans mounted like the engines of a DC-9, and you've got the idea. I'm not sure they would have been used in cruise flight.

In the end, the "improved" (but still hideously unreliable and finicky) JT9D-3 engine provided enough power to (just) meet the requirements, and the 5- and 6-engine options were shelved. From there, the -3/-3A variants were replaced by the more powerful and actually reliable JT9D-7 series, and the rest was history. For growth projects after that point, they focused on increasing power -- first with the JT9D-23 planned for the Phase B program, then on with PW4000, etc.

There were several other non-standard engine configurations pondered, too.

Trijets

Note: The center-engine placement for the 5-engine concept came back twice in the 747's history, both in the context of 3-engine 747s. It came up first early in the 747's history, even before EIS. Boeing proposed three-engined variants to compete with the DC-10 and L-1011, as mid-range alternatives, which they dubbed the 747-300 (sometimes, 747-300B) the so-called 747-300B. Different inlet and outlet configurations were considered (the 747-305B and -306B were two of them, plus another with a boundary-layer annular inlet). This was abandoned due to lack of interest. A few years later, with the 747 actually in service, Boeing tried again with the 747-3, the idea being that airlines might now like the fleet commonality it allowed. The 747-3 centered around a proposal for for United Airlines. It would have used an almost straight-through center engine built atop a redesigned aft fuselage. There's no trijet that really resembles it. Most of the trijet proposals were for shorter-body 747s, but unlike the SP, these were designed for shorter-range, domestic and transcontinental routes. The 747-3, for example, would have been less than 200 feet long (and lost 20 feet of wingspan), carried 301 passengers, and only flown 2,600 nautical miles.

In appearance, the trijets varied. Some proposals had T-tails, others had cruciform-style tails like the Sud Caravelle, and some had fairly conventional tails (like the 747-3). One "747-300" proposal even had a stretched upper deck for 44 people, even though it had a shortened body just 174 ft. long and spanning 160 ft. (about 57 ft. shorter and 25 ft. narrower than the 747-100) [FI, 1968-03-28].

I've even seen claims some 747SP development ideas included a trijet with an asymmetrical configuration: twinned engines on one pylon on one wing, a single nacelle on the other. I have no idea how that would have worked safely on takeoff, or even if it's true; that's an internet-forum claim.

Twinjets

Early in the 747 development, there were musings from Boeing about a "big twin" version. This was actually called the 747-200 in some initial articles in Flight International and other publications. There's not a ton of public info to be had, but the references talk about 1,200 - 1,800 n. mi. ranges, 200-240 passengers, but still using JT9D engines. This was conceived as a competitor more to the original Airbus A-300 concept, before the actual A300 [FI, 1968-05-02].

Derated quads

Finally, in the unusual engines department, Boeing considered several reduced-power 747s for US domestic operators. One, a 747-100B with a stretched upper deck (SUD), was used in Japan, but Boeing considered a version powered by four JT9D-7R4E engines (the 767 configuration).

Then, still in 1982, they considered a down-rated 747SP for transcontinental routes. This would have had four engines, too, but they would have been PW2037 or RB211-535E4 models, the same engines as the 757.

Everything else

So, that summarizes pretty much all (known to me, anyway) 747 projects that weren't quadjets, plus the weaker quads. For everything else, Boeing felt that four engines would be powerful enough, if the engine manufacturers developed increased-thrust variants.

The heaviest 747 proposals would have had (or do have) far more power.

  • 1970s stretch studies: JT9D-23, 60,000 lb. thrust range
  • 747 Super, 747-500, 747X: GE90 derated variant (GE90-68?), GE uprated CF6-80E1, PW ADP, PW4168, Rolls-Royce Trent 700, 70,000-75,000 lb. thrust
  • 747-500X and 747-600X: Rolls-Royce Trent 900 or Engine Alliance GP7176, 76,000-80,000 lb. thrust
  • 747X and 747X Stretch: Rolls-Royce Trent 600 or Engine Alliance GP7168 / GP7172, 68,000-72,000 lb. thrust
  • 747-8 Intercontinental: GEnx 66,500 lb. thrust

So, when you think about the change from a 43,500 lb. thrust JT9D-3A, the 747-8I is carrying six engines worth of thrust!

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.