TriStar S duct.
I saw an explanation about the design of the S duct from Lockheed. When originally concieved the L10 was going to be the proverbial 'bus' and so quick on and off loading was required. To make use of the centre doors with airstairs or bridges would have brought the wing engines very close to equipment using those centre doors.
The solution was to move the engine farther out on the wing which incidentally also improved wing bending relief. The downside was the need for a bigger rudder to keep the Vmcg/Vmca speeds under control.
Lockheed considered a DC-10 type arrangement but settled on the S duct to give a full length rudder. As has been mentioned before this also minimised the pitching effect of a high mounted #2 engine.
At the time of the L10 line closure they were also considering using the space above the S duct as an extra fuel tank to give the balance effect now often achieved with a tailplane tank.
