Originally Posted by Turbine D
I don't agree that large fan three spool engines are more fuel efficient than two spool designs in the same thrust category. In fact, generally, it is just the opposite. The advantage of a three spool design is a shorter and generally lower weight engine. When coupled with a shorter nacelle, less drag is produced. With these two factors combined (shorter engine/shorter nacelle), parity may be reached with two spool engines. While there may be some advantages of better matching of various compressor section in a three spool design, that is only one part of the total package to gain high efficiency (low fuel burn).
Its all related. The very reason that they can theoretically be shorter and lighter is because they can remove compressor and turbine stages, and the only reason that they can remove stages while increasing efficiency is because all the spools are spinning at much closer to optimal speeds.
Originally Posted by Turbine D
If three spool engines were indeed more fuel efficient, wouldn't we all be flying on three spool engines?
Well come on... ramjets are indeed faster than turbofans, so why aren't we all flying around at Mach 5 [/sarcasm].
I'm no expert, but I would think it has a lot to do with the fact that the 3-spool is an inherently more complicated setup which can induce higher maintenance costs, and so far 3 spools have only made sense for applications where there are relatively many flight hours per engine cycle. However, I have read that Rolls Royce is readying a lower output/ short haul optimized three spool in time to compete for use on the 737 replacement around 2020.