The Lockheed Tristar
and DC-10 share a similar configuration, however the biggest difference is the mounting of the tail engine. The Tristar's tail-mounted engine, as with the 727, is located in the rear fuselage and fed through an S-duct
The McDonnell Douglas DC-10
does not use the S-duct design, instead its tail engine is mounted above the fuselage in a "straight" layout.
One issue with trijets is positioning the central engine. On most trijets this is placed at the tail along the centerline, producing some technical difficulties.
The most common configuration is having the central engine located in the rear fuselage and supplied with air by an S-shaped duct – this is used on the Hawker Siddeley Trident, Boeing 727, Tupolev Tu-154, Lockheed Tristar and, more recently, the Dassault Falcon 7X. The S-duct design has the advantage of reduced drag, additionally since the engine outlet is located lower down, the aircraft will be easier to handle in the event of an engine failure. However, S-ducts are complicated and costly designs. Furthermore, the central engine bay would require structural changes in order to accommodate future larger-diameter engines in the case of the Boeing 727 and Lockheed TriStar. The 727's central bay was only wide enough to fit a low-bypass turbofan and not the newer high-bypass turbofans which had greater output as well as being quiet enough to meet new noise regulations of the 1980s, but Boeing decided that a redesign was too expensive and ended its production instead of pursuing further development. The Lockheed Tristar's S-duct cross section was too small to fit an existing double spool engine as it was designed only to accommodate the new triple spool RB211 engine, and delays in RB211 development in turn pushed back the TriStar's entry into service which affected sales. The DC-10 and related MD-11 use an alternative "straight" layout, which allows for easier engine installation, modification, and access. However, this has inferior aerodynamic properties when compared to the S-duct designs. Also, as the engine is located much higher up than the wing-mounted engines, an engine failure will produce a greater pitching moment, making it more difficult to control.
Placement of the remaining two engines varies. Most smaller aircraft, like the Hawker Siddeley Trident, the Boeing 727 and the Tupolev Tu-154 have two side-mount engine pylons in a T-tail configuration. The larger widebody Lockheed L-1011 TriStar and DC-10/MD-11 mount an engine underneath each wing. Preliminary studies were done on the TriStar to reuse the fuselage and wing for a twinjet design though these never materialized due to Lockheed's lack of funds, while in the late 1990s Boeing which had taken over McDonnell Douglas had considered removing the tail engine from the MD-11 to make it a twinjet but ending up instead cancelled MD-11 production altogether.