747 Trijet

In the late 1960s and early 1970s Boeing studied a shortened, three-engined 747 to compete with the Lockheed L-1011 TriStar and the McDonnell Douglas DC-10.

On paper it was the better aircraft. The 747 trijet would have carried more payload, flown further, and seated more passengers than either rival — it was, after all, built on a much larger aeroplane’s wing and systems.

The layout

The centre engine would have been mounted in the tail with an S-duct intake, similar to the L-1011’s, rather than straight through the fin as on the DC-10.

Why it died

The wing killed it. Engineering studies showed that removing two engines and shortening the fuselage would require a total redesign of the 747 wing.

That is not obvious until you think about what the outboard engines do. On a four-engined aircraft they act as substantial masses distributed along the span, and their weight counteracts the upward bending load on the wing in flight — the wing is designed around their presence. Take them away and the load distribution changes fundamentally.

There was a second constraint that mattered just as much commercially: maintaining the same handling characteristics as the 747 was a priority, so that pilots would not need extensive retraining. A wholly redesigned wing made that far harder to guarantee.

What Boeing built instead

The company abandoned the trijet and pursued a shortened four-engined 747 instead — which became the 747SP. Same wing, same engines, same handling, less fuselage. Forty-five were built.

There is a footnote worth knowing: the designation “747-300” was originally proposed for this never-built trijet. Boeing later reused it for the stretched-upper-deck aircraft launched in 1980, which is why the name appears attached to two entirely different aeroplanes in older sources.

The market it was aimed at, twenty years later

The trijet was meant for a segment below the full-size 747. That segment did eventually get filled — by the Boeing 777, a long-range twinjet that entered service in the 1990s and was smaller than the 747-400.

The 777 did what the trijet was supposed to do, with two engines instead of three, and went on to take over most of the routes the 747 had held. In that sense the trijet’s market was real; Boeing was simply twenty years and one engine generation too early.

For the other 747s that stayed on paper, see the 747-500X, -600X and -700X and the 747-400XQLR.