Most turbofans have a low- and high-pressure spool, whereas the R-R engines have three: a high, intermediate and low spool. R-R believes that this is more advantageous because each spool can be better optimised aerodynamically and, therefore, works more efficiently. The fan and booster run together on the LP spool in the two-shaft engine. As the booster therefore runs at the slower speed of the fan, it limits the booster pressure rise.
In the three-shaft engine, the fan runs on a separate shaft and is connected to the LP turbine and nothing else, therefore running at the optimum speed. The HP compressor, in the three-spool engine, is relieved by the IP compressor, which runs faster than the two-shaft engine's booster.
As the second-stage turbine on the IP shaft runs at a lower speed, the mechanical stresses are lowered to the point where un-cooled blades can be used. As the three-shaft engine therefore requires fewer stages of compression, turbine and variable guide vanes, it is shorter than the equivalent two-shaft power plant. The UK engine manufacturer believes the more stocky rigid engine is therefore, less prone to shaft whipping which impacts on engine life.
Source: Flight International
