We had an engineer who had worked at Douglas when the DC-10 was being designed. He said that the reason the DC-10 didn't have an S-duct for the middle engine was that they couldn't afford to design / develop one. I don't know whether the shortage was entirely money or was also time. Thay may well have not had enough computing power. You'll note that the MD-11 got a pinched-waist housing for the middle engine to reduce drag. The DC-10 should have had that. It almost looks as though quick and dirty won the day with the '10.