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We started to build the C-5 and wanted to build the biggest thing we could

https://en.wikipedia.org/wiki/Lockheed_C-5_Galaxy

The An-225 is 50% larger and has a shorter life.

Did they mean the C-5 was actually the largest physically possible aircraft or just that anything larger would have weakening service life?

Question reposted, originally posted from an account that had to be deleted.

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    $\begingroup$ Interesting side note: Boeing and Lockheed both bid for the contract to build the C-5 for the US military. Lockheed won. Trying to deliver (almost / did ) bankrupt the company; they merged with Douglas a short time later. Boeing decided not to waste all the engineering studies etc. they had done making their presentation, and used that knowledge to build the 747. The 747 did NOT bankrupt the Boeing company. :)! $\endgroup$
    – Greg
    Commented Jun 20, 2021 at 23:54
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    $\begingroup$ Not really. As you suggested, AN-225 is physically bigger than C-5. However at the first flight it is the biggest plane at the time. $\endgroup$
    – vasin1987
    Commented Jun 22, 2021 at 5:25
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    $\begingroup$ Note that "the biggest aircraft we can build" does not mean "the biggest aircraft physically possible." Physics doesn't break down at a certain size. It just gets harder and more expensive, and the advantages don't keep pace. $\endgroup$
    – TypeIA
    Commented Jun 22, 2021 at 5:58
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    $\begingroup$ @TypeIA : unless you get to sizes which can collapse into a singularity, then physics do break down :) But that's far, far beyond the scope of aircraft engineering. $\endgroup$
    – vsz
    Commented Jun 22, 2021 at 16:16
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    $\begingroup$ "biggest thing we could" $\endgroup$
    – Criticizing Israel not allowed
    Commented Jun 22, 2021 at 23:55

2 Answers 2

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The largest airplane possible is a function of time. At any point in time, the then current technology will have a practical maximum size due to square-cube effects mentioned in this answer:

However, technology advances. The C-5 was developed in the 1960s, with a MTOW of 417 ton. Compared to the 142 ton of the then current 707-320, this was a groundbreaking engineering effort at the time. And no, mounting more engines also runs into problems at the then current technology, like the Spruce Goose experienced, which did not fly out of ground effect.

https://en.wikipedia.org/wiki/Hughes_H-4_Hercules#/media/File:H-4_Hercules_2.jpg

Did they mean the c 5 was actually the largest physically possible aircraft or just that anything larger would have weakening service life?

Lifespan is indeed one of the design parameters for size, however the C-5M is a C-5 with upgraded engines and avionics according tho the wiki article, so some of the lifespan of the original planes was already consumed. The An-225 with its MTOW of 640 tons was developed in the 1980s.

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    $\begingroup$ Your claim that the Hughes H-4 Spruce Goose “could not fly out of ground effect” seems like uninformed speculation. aviation.stackexchange.com/questions/12500/… $\endgroup$
    – Mike Sowsun
    Commented Jun 22, 2021 at 12:41
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    $\begingroup$ Perhaps "did not fly out of ground effect" would be less ... problematic? $\endgroup$
    – CGCampbell
    Commented Jun 22, 2021 at 13:27
  • $\begingroup$ I would reword the first sentence to "The largest airplane possible is a function of ingenuity and available resources." $\endgroup$
    – Ryan
    Commented Jun 22, 2021 at 13:41
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    $\begingroup$ @koyovis No, it is not speculation to understand that the Spruce Goose on Nov. 2nd 1947 was at a weight considerably less than it’s max design weight and could have easily flown much higher than ground effect. It had a designed service ceiling of 20,900’. The only warranted speculation would be whether or not it could operate efficiently at it’s max design weight of 400,000lbs. $\endgroup$
    – Mike Sowsun
    Commented Jun 22, 2021 at 15:02
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    $\begingroup$ Hughes didn't fly it out of ground effect because his only objective was to show that it flies and then put it away, the requirement for it having kind of evaporated. $\endgroup$
    – John K
    Commented Jun 22, 2021 at 17:27
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There are other considerations at work here which need to be taken into account.

First of all, neither the An-225 nor the C5 represent the "largest possible plane". If you install enough engines, you could fly a plane larger than either of them, at which point the question becomes where can you land it (maximum runway loading in lbs/sq.ft and wingspan clearance) and where can you shelter it for maintenance. There are few airports in the world that can support the double-decker Airbus because it is too heavy for their runways and their terminal buildings can't load passengers from four skyways into one plane at the same time.

Planes of unusual size tend to be built in support of a specific business plan or use model i.e., transporting objects by air which cannot be transported by ground. Examples include ballistic missiles and the like. But the "divide-by-zero" in this context is that anything you want to put on the plane had to get to the airport by ground in the first place, unless it was built next door to an airport and delivered for use next door to one. These are rare applications for which it is hard to justify constructing a plane from scratch in service of that application which might only be used a hundred times (for emplacing surface-to-surface nukes) from program start to finish.

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    $\begingroup$ It may be worth mentioning that the An-225s sole reason to exist was the transport of Buran - just as you said - from its place of construction to the launch site. Similarly, the Mil W-12 (largest helicopter ever) was shrunken to the Mi-26 series version when the size of the Soviet 8K67 intercontinental missiles, for the transportation of which it was intended, could be reduced by the introduction of the new R-29 type. $\endgroup$
    – Peter Kämpf
    Commented Jun 20, 2021 at 6:08
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    $\begingroup$ I beg to differ on the sole purpose of the 225. It may have been the official reason, but why go through all the trouble with the cargo hold if it was only to transport the Buran on its back? $\endgroup$
    – Jpe61
    Commented Jun 20, 2021 at 21:47
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    $\begingroup$ @Jpe61 To also be able to carry the booster engines. $\endgroup$
    – Davidw
    Commented Jun 21, 2021 at 0:28
  • $\begingroup$ @Davidw no, the An-225 was designed for ferrying ICBMs and other large loads to launch sites, including rocket stages for space launchers. It wasn't just designed for the Buran, though it was adapted for that during its design phase. $\endgroup$
    – jwenting
    Commented Jun 24, 2021 at 19:49

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