With the first version of the Trent XWB engine for the A350 poised for the start of test flights, Rolls-Royce is already stepping up studies of what comes next in its big engine line-up.
One key question is whether the recent birth of the new Rolls-Royce mid size engine joint venture with Pratt & Whitney will increase the chances of geared technology spreading to the next generation of higher thrust turbofans beyond the Trent XWB? For now it seems unlikely, mainly because Rolls believes its existing three-shaft architecture provides multiple jumping-off technology options for improving both thermal and propulsive efficiency.
However geared technology in longer term Rolls engines will not be limited to the mid-thrust range. Both General Electric and Rolls have acknowledged that the appeal of the GTF concept increases in proportion to thrust. Even before the newly forged relationship with Pratt was announced in October, Rolls openly said ultra-high bypass ratio geared fans may be required to meet the challenging targets of future emissions goals such as those outlined in the European Union’s Flightplan 2050 vision for future air transport.
Overall Rolls shows no signs of diverging from the guidelines of its existing research strategy which, early last decade, grouped technology acquisition within three broad, rolling time bands – up to around five years, around ten years, and up to 20 years and beyond. This it calls its Vision 5, Vision 10 and Vision 20 programs, with Vision 5 originally covering engines such as the Trent 900 for the A380, and later the Trent 1000 for the 787.
Study concepts include a bladed spinner (Rolls-Royce)
Rolls basis its future near term big engine development squarely on the Vision 10-derived Trent XWB architecture and is starting to reveal more details about what the technology options are. Speaking at this year’s International Society For Air Breathing Engines meeting in Sweden, Rolls Future Programs senior project engineer John Whurr, says “the core architecture of the Trent XWB, in particular the two-stage intermediate turbine, facilitates development of the core thermodynamic cycle.”
It does this, he adds, by enabling increased compressor pressure ratios, and overall pressure ratio, without compromising the efficiency of the high and intermediate pressure turbines. Improvements in the core could also come from introducing a ‘cooled’ cooling air system to enable higher compressor temperatures without compromising component lives. Higher turbine inlet temperatures, and hence better thermodynamic efficiency, could be gained by introducing ceramic matrix composites in the turbine.
Other concepts include a 'mini-mixer'. (Rolls-Royce)
Driven by the need to be greener, Rolls sees an increasing trend towards lower specific thrust and improved propulsive efficiency. The low pressure system will play a big role in this with the likely adoption of lightweight composite fan blades and containment casings, lighter LP turbines using advanced materials such as titanium aluminide, as well as slimline nacelle concepts that will mitigate the drag and weight of the larger diameter engines of the future.
Further advancements under study include increased use of fuel as a heat sink, greater use of CMCs and possibly the introduction of variable area exhaust nozzles similar to those in development for the PW1000G.
It is the raft of technologies under consideration for the longer term ‘Vision 20’ period that begin to stir the imagination. They include intelligent control systems with advanced prognostics/diagnostics and connectivity, as well as a fuel-cooled intercooler. For the first time in more than a decade, the engine could also look significantly different with radical changes to the fan. Rolls is studying a bladed spinner, for example, which integrates the root of the fan blades with the spinner in an arrangement which “allows us to squeeze more flow through a fan of given size,” says Whurr.
Internally, the Vision 20 engine could feature stator-less contra-rotating high and intermediate pressure turbines and a casing for the intermediate pressure compressor formed from a bladed drum made from metal matrix composites. It could also be configured with all-ceramic, self-diagnostic bearings and embedded centerline electric motors/generators.
To further boost efficiency, Rolls is also studying a novel concept called a mini-mixer, which would be wrapped around the aft casing. The forced mixer would be “one of the best ways to improve transfer efficiency” between the core stream and bypass stream says Whurr. Taken as a whole, the studies show there is no shortage of ideas when it comes to squeezing yet more efficiency out of conventional turbofans.