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Recently I found this article this article about the concept of directly sit on a pile of intermodal containers and lock them in place. Whether it is plausible to open such a huge opening ventral of such monstority, I found the idea of using intermodals as air freights attractive, as it eliminates the hurdle of taking the items two and fro between two incompatable systems.

Courtesy of Boeing

Also similar but more radically engineered are the spanloader concepts, where the intermodal containers are just tucked into the wide, rectangular fuselages, or even into chords of giant flying wings themselves. Please refer to this thread (in which many of links require Wayback Machine to access and takes time to load alrge pdfs) to see their applications including even fitting ICBMs inside.

Courtesy of NASA

There are even far smaller concepts of ground effect freighter which is just one container-sized.

Then I am curious: why the current jumbo freighters, instead of unsightily truncated ULD, do not directly load intermodal containers onto their cargo decks?

P.S. Size and shape of fuselage should not be the problem, as such odd shapes were meant for thinner cabins like this answer, while not a problem for new-gen wide bodies like this, not to mention jets modified to carry oversized cargo.

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  • $\begingroup$ Look up relative fuel efficiency of freight aircraft vs. trains and tractor-trailer trucks. Then compare that hoverfreighter's consumption. $\endgroup$
    – Zeiss Ikon
    Dec 13, 2021 at 19:19
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    $\begingroup$ You're talking about steel freight containers that easily weigh 2 1/2 tons empty? $\endgroup$
    – TomMcW
    Dec 13, 2021 at 19:28

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There are probably multiple reasons, but one big one is weight. A standard shipping container is made of solid metal and weighs several ton(ne)s empty. An air cargo pallet on the other hand weighs only a few hundred pounds.

Given how critical weight is to aviation costs, transporting around tons of 'dead weight' in the form of steel shipping containers doesn't seem very attractive.

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  • $\begingroup$ I am comparing intermodal with Unit Loading Device (ULC), which are basically aluminum cargo containers. A bit of Googling and I found there are several retailing companies selling aluminum containers in addition to Corten. $\endgroup$ Dec 13, 2021 at 20:00
  • $\begingroup$ @CrystallizedRefresher I'm not sure I understand what you're comparing. Your third link shows that even an aluminum container weighs around 2000kg empty. $\endgroup$
    – Pondlife
    Dec 13, 2021 at 20:13
  • $\begingroup$ The ULC in average have effiency twice by weight and thrice by space compared to aluminum shipping container! I don't get the reason behind such advantage. Is square-cube rule working at this scale? $\endgroup$ Dec 13, 2021 at 20:36
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    $\begingroup$ @crystallizedrefresher it is probably down to the fact that shipping containers are designed for hard usage - they are expected to be banged around, sprayed with seawater, cope with having other containers stacked on top for weeks, etc. A container for aircraft holds doesn't have to put up with this so can be a lot less robust - and thus lighter. $\endgroup$
    – Andrew
    Dec 13, 2021 at 21:09
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The size and the shape of the fuselage are a problem.

Almost all large freighters currently in operation are cargo versions of passenger aircraft. This means they share the circular or elliptical cross-section of the original passenger aircraft. This saves a lot of money when retired airliners are reused for freight.

Even when they don't come from an airliner, e.g. the An-225 Mriya, an elliptical fuselage is very practical both structurally and aerodynamically. It can also be pressurized, which is relevant for some types of cargo.

Container ships don't have the aerodynamics issue, nor the pressurization one. The submerged part is streamlined, the top doesn't have to be. Also, they're far larger, so there's no reason to shape the containers as anything other than a box.

It's completely technically feasible to produce an air freighter shaped to efficiently fit intermodal containers. The containers themselves can be readily made lighter by using aluminum walls and ultra-high-strength steel beams.

A study into CFRP containers has shown that a 40' can weigh less than 1 ton. There are some issues with ultra-lightweight containers, e.g., a steel container doubles as a security safe. They can be overcome, and the time savings could be worth it.

But an aircraft designed for this role would be unpressurized, useful for intermodal freight and nothing else. There just isn't enough demand to support the development and certification of such a dedicated cargo-only aircraft.

So, currently, the chosen solution is to use ULD for air freight, which maximize the use of aircraft internal volume, and then spend extra time transloading cargo from and into trucks.

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    $\begingroup$ Modern container ships aren’t particularly streamlined to be honest - fairly cuboid shapes to help pack as many containers in as possible $\endgroup$
    – Tim
    Dec 14, 2021 at 11:12

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