Does anyone know why the McDonnel Douglas Corporation was the only trijet airliner manufacturer in histroy to use the straight-through (or tube) rear engine design on both the DC-10 and MD-11? Every other trijet airliner ever built (Trident, B727, Tristar, Tu-154, Yak-42) are all S-duct designs. Even the smaller regional jets like the Yak-40 and Falcon are all S-ducts.Thing is that as far as I can gather the S-duct design is generally more complex and therefore more costly than the tube design so one would expect more manufacturers to have gone with the tube?I suspect that the popularity and sheer numbers of the Boeing 727 (1832 built) had a lot of influence not only on Lockeed but also on Tupovlev?Konrad

I'm a different variety of engineer, but my guess is the MD11 design optimizes the intake airflow (operating efficiency)... call the intake a tube if you like but it is really not a tube but a transition between ram air and the fan, which is required in any modern design. Any plumbing beyond the transition introduces some flow restriction (less efficiency). The downside to the design is an increased complexity of the vertical structure by having the engine above the tail rather than in it (more weight, less efficiency). I assume they ran the numbers based on the economic data of that time and picked the configuration for reasons I can only wonder about. Today's engines have much greater thrust available and much improved specific fuel consumptions; hence, the popularity of twins.

Having flown both the DC-10 & KC-10 I was told in both ground schools it was because Douglas did not want to pay Boeing for use of its patented s-duct and so designed the straight thru you see.Gary

Gary - Patent costs are a potential showstopper for Dougals I had not considered but are you sure that Boeing actually own the S-duct patent, seeing as the Trident was designed in the late 50's (by de Havilland) and flew almost a year before the B727?An interesting quote from Wikipedia with regards to this:"In fact, de Havilland had invited a group of engineers from Boeing to see the DH121 {Trident 1A} design and development program in its early stages, partly because Boeing was looking to develop a similar medium-range version of its 707 design."The above would have happened in 1960 sometime. Would be interesting to compare the dates of this visit and Boeing's supposed filing of the S-duct patent back in the US...Dan - thanks for the clarification. Don't forget jet engine reliability in your list - one of the main reasons behind the easing of the 60 minute rule and introduction of ETOPS. Still, nothing like three engines for high 'n heavy operations, one of the key factors behind the MD-11's existence today!Konrad