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 L-1011 TriStar, and, more recently, the Dassault Falcon 7X. The S-duct has low drag, and since the third engine is mounted closer to the centerline, the aircraft will normally be easy to handle in the event of an engine failure. However, S-duct designs are extremely complex and costly. Furthermore, the central engine bay would require structural changes in the event of a major re-engining. For example, the 727's central bay was only wide enough to fit a low-bypass turbofan and not the newer high-bypass turbofans which were quieter and more powerful. Boeing decided that a redesign was too expensive and ended its production instead of pursuing further development. The Lockheed Tristar's tail section was too short to fit an existing two-spool engine as it was designed only to accommodate the new three-spool Rolls-Royce RB211 engine, and delays in the RB211's development, in turn, pushed back the TriStar's entry into service which affected sales.