Stand at the window of any airport terminal today and play a game: try to tell the airliners apart by shape alone. You can’t, really. Boeing or Airbus, regional jet or long-haul giant, they are all the same basic object — a smooth metal tube with two big engines slung under a pair of swept-back wings and a tail at the back. It is one of the most successful designs in engineering history, and one of the most monotonous.

It was not always this way. For decades, airliners came in a glorious variety of shapes, and the world’s drawing boards held far stranger ones that never made it off the page. This is a tour of the airliner’s lost variety — the shapes we built, the shapes we only imagined, and why, after seventy years of the tube, the weird ones may finally be coming back.

When airliners had personality

The jet age began with real diversity. In the 1950s the French Sud Aviation Caravelle hung its engines at the very back of the fuselage, leaving the wings clean and the cabin quiet — an idea copied by the Boeing 727 (with its distinctive third engine fed through an S-duct and a tall T-tail), the DC-9, the Vickers VC10 and the Soviet Il-62. Look up at a 727 and you see three tails’ worth of hardware clustered at the rear, nothing under the wings at all.

Then there was the vertical dimension. The Boeing 377 Stratocruiser carried a downstairs cocktail lounge; the 747 gave us the most famous hump in aviation; the Airbus A380 went to a full double-deck, two complete passenger floors from nose to tail. Others were stranger yet: the German VFW-614 mounted its jet engines on pylons above the wing. Go back further and the first true airliners were not land planes at all but flying boats — the vast Dornier Do X and the Boeing 314 Clipper landed on water. And on the Junkers G38 of the 1930s, a lucky few passengers actually sat dentro the wing, looking out through windows in its leading edge.

The shapes we only imagined

For every airliner that flew, a dozen wild ones stayed on paper. The most gloriously mad was the Caproni Ca.60 of 1921 — a nine-winged, eight-engined flying houseboat meant to carry a hundred passengers across the Atlantic. It was actually built; it broke apart on its second flight and was never rebuilt. Decades later, America’s answer to Concorde, the swing-wing Boeing 2707 supersonic transport, was cancelled in 1971 before a single one was completed. Engineers in the 1950s seriously sketched nuclear-powered airliners that would fly for weeks without refuelling, and NASA and manufacturers have drawn passenger flying wings — “spanloaders” with the cabin spread across the wing itself — again and again. Almost none of them was ever certified to carry a fare-paying passenger.

Why they all converged

So why did the airliner settle on one shape? Because the boring answer is brutally efficient. A cylinder is the perfect pressure vessel — the cheapest, strongest, lightest way to hold cabin pressure at 40,000 feet. Wings and tail bolt on to that tube and can be scaled up or down for any size of aircraft. Hanging the engines under the wing makes them easy to reach for maintenance and helps relieve the bending loads on the wing. And, crucially, every airport gate, jet bridge, baggage system and certification rule on Earth is built around a tube. A radically better shape has to beat not just the tube’s aerodynamics but that entire mountain of infrastructure.

The next act: beyond tube-and-wing

For the first time in a long while, that may be about to change. The most promising challenger is the blended wing body, which merges fuselage and wing into a single smooth lifting shape so the whole aircraft helps carry the load. Boeing and NASA flew a subscale demonstrator, the X-48, and studies point to fuel savings of anywhere from 20 to 50 percent; a startup, JetZero, is now working toward a full-size version. Others hedge their bets: MIT’s “double-bubble” D8 keeps a fuselage but makes it wide enough to add lift and tucks the engines at the tail, while the NASA and Boeing X-66 hangs an extremely long, thin wing from the fuselage on a supporting strut.

None of these is guaranteed to reach an airport gate; the tube-and-wing has buried plenty of promising rivals before. But for seven decades the airliner has been a metal cylinder with wings — unglamorous, unmistakable, and quietly perfect. If the blended wing finally breaks through, the next generation of travellers may look out of a terminal window and, for the first time in their lives, not be quite sure what they are looking at. Which would be wonderful.

Sources: NASA (“Beyond Tube-and-Wing,” Bruce I. Larrimer; X-48 programme); Hackaday; Leeham News; MIT; Wikipedia.