barit1 & Bearfoil
Well now that the "EGT" has been pretty well wrung out, I have been thinking about something in the Trent 900 engine. The most unique feature of this engine is the counterrotation, e.g., The HPC/HPT spool rotating in the opposite direction from the IPC/IPT spool. This is the first high bypass fan engine to contain this feature. The second and third will be the Trent 1000 and the GEnx engines. The difference between the Trent engines and the GEnx is this; In the GEnx, the fan, fan booster and LPT rotate in the opposite direction from the core engine, e.g., compressor and HP turbine spool. For all three engines, this is relatively new technology to save weight and improve efficiencies.
Referring to Rolls Royce's presentation sent to Qantas on 11/18/2010, Rolls says "the Trent 900's HP/IP support structure would be subject to high severity if the engines were operated to 540 psi at P30 on more than 75 flight cycles." In the Trent 900 there are 4 support structures which are commonly referred to as "frames". Of the 4, one supports the the rear of the IP compressor and the forward of the HP compressor and one supports the rear of the HP compressor and the HPT rotor and the IPT rotor. Contained in or supported by the frames are the bearings.
Frames are probably the second most difficult components to design next to HPT air-cooled blades. However, there is a 60 year history accumulated in frame design and design analysis to assist the designer. But there isn't a long history on designing frames were bearings contained in the frame or supported by the frame are rotating in the opposite direction at different speeds. Are there some unique design issues in play here?
I think the most critical of these two frames is the one supporting the rear HPC spool, the HPT rotor and IPT rotor. The biggest concern on this frame would be the stress and thermal gradients experienced during an engine cycle resulting in generation of LCF. Additionally, vibrations or slight unbalance could lead to HCF in certain areas if not accounted for properly. Contained within these frames are the oil feed tubes and return tubes which supply oil to the bearing and from the sumps.
I think Rolls Royce is telling Qantas (and maybe their other customers) a problem has been detected and we need to limit compressor outlet pressure, therefore, fuel flow and the generated maximum thrust to prevent failure of the support structure (frame) until a final modification can be inserted into your engines.
I am not dismissing the spline wear as a non-issue here, it could be contributing in some way.
Turbine D