The Airbus A330neo program has come a long way since our 29th of December article “A330neo prospect gains traction.” With the Farnborough Air Show days away, we understand there are now Airbus internal job postings for engineers to join the program. The speculation then reduces to “when” the program will be announced, not “if.” Another would be what improvements are foreseen for the Boeing 787-derived engines that may power the neo.
Rolls Royce reportedly gains exclusivity
Reuters recently reported that Rolls Royce might get an exclusive engine deal for the A330neo. There are many reasons Airbus might give Rolls Royce or General Electric exclusivity on an engine for the A330neo, especially if Airbus sees the likely sales of the updated aircraft to stay below 500 units. The reasons can range from how much of the $2B estimated program cost the engine manufacturer would pay to what efficiency improvements they would undertake on top of what is already on the way for their 787 engines. There is every reason to believe the GEnx-1B can match the fuel consumption performance of a further developed Trent T1000-TEN. We understand Rolls Royce will leverage developments from the A350 TXWB engines but GE can just as easily leverage developments from the LEAP program.
The picture shows the Rolls Royce Trent 1000 carbon fibre fan demonstrator engine from the companies ALPS (Advanced Low Pressure System) program. Is this also the looks of the Rolls Royce A330neo engine?
If the reason for Airbus giving Rolls Royce exclusivity would be technical, we think it might rather be that Rolls Royce have finally realized a long standing potential in the Trent’s engines weight domain. The more efficient 787-derived A330neo engines are a full tonne heavier per engine then the engines they replace. As an A330-300 classic and its competitor, the 787-9, have similar empty weights, the heavier engines will set a A330neo back with a further 2.5-3 tonnes, knock-on effects included. This extra weight will be felt on the 2,000-3,000nm missions these aircraft often fly where the engines improved fuel consumption will be less noticeable. Any weight that can be saved in an A330neo engine will therefore be of utmost importance to Airbus.
The lighter Tri-shaft
Contrary to what one assumes, a triple-shaft turbofan is principally lighter than a two-shaft architecture, everything else being equal. This comes from the fact that a two-shaft engine couples the low pressure compressor, the Booster, to the rotational speed of the fan. The fan is spun so that the outer third of the fan blades have supersonic speeds, the middle part transonic and the inner part subsonic. As the booster comes directly after the subsonic inner part of the fan and shares its rotational speed, it also works with subsonic blade speeds. The result is very low pressure gains per stage, about +10%, compared with +30-40% per stage for an optimal spinning compressor like the ones in GEnx high pressure compressor. This results in more stages for a given pressure ratio and to not lose even more efficiency, the booster diameter is kept as large as possible, thereby rendering all discs and frames heavier then the comparable tri-shaft compressor part. In the equal scale cut-through of the GEnx-1B and Trent 1000, it can be seen that the GEnx’s low pressure compressor diameter is essentially double that of the Trent’s low pressure compressor, the IPC.
This then repeats on the turbine side. The turbine section of the GEnx is longer and has a larger diameter than the T1000.
General Electric have mitigated any weight disadvantage their chosen architecture would have by going to a CFRP fan already in 1995 with the GE90. Rolls Royce tried a carbon fiber fan for the RB211 (Hyfil fan) but changed to the heavier Ti fan due to Hyfil toughness problems. The end result is that today’s 787 engines both weighs around 6.1 tonnes but Rolls Royce should be able to lower that to 5.9 tonnes should it decide to use the CFRP fan and fan case from its ALPS program.
Design and production system
Rolls Royce have always maintained that the hollow Ti fans it produces can be made thinner and therefore more aerodynamically efficient than CFRP fans and that the weight disadvantage is minimal. After a decade of research RR now claims it has a CFRP fan blade design and production system that can compete with Ti blades. The end result of this development is the ALPS demonstrator engines that now run on the Derby test beds and which will fly on its 747 test aircraft this fall. RR also perfected a 3D woven carbon fiber production system which uses resin infusion, which was done together with GKN in the CTAL joint venture where Rolls Royce has now bought out GKN.
With both design and production being ready for prime time, we see no reason to wait until 2020 for the Advance engine to use this technology. An A330neo could use it to equal advantage in the 2017-2018 time frame.
By Leeham Co EU