radken
This is off the main line direction in this thread as to the whys and wherefores this engine seems to be suffering from some "built-in" shortcoming(s), but I was wondering if one of you could address a few questions as to the failure mode of the LPT disk itself. As a non-engineer, I've been musing about the "normal" operating speed of these turbines vis-a-vis the rotational speed necessary for self-destruction. (I'm retired... and I can afford to play golf and muse)
My questions start with a very basic one, which is, in the quest for efficiency and low mass, are these units designed to spin in a near supersonic regime, or do they experience tip velocity excursions approaching SS, say at the highest power settings, like N1 does?
Its moot to the cause of this engines demise, but, regardless of whether the LPT may or may not ever operate near the "shock" regime, at some point after the output shaft separation, and prior to disk impingement with the stators, could the free wheeling disk (blades) have been almost instantly accelerated past Mach 1 prior to the disk burst point? If so, could the blades have failed first from "swallowed shock," their sequential departure thus initiating intolerable out of balance, and, therefor, premature disk rupture?
If management of near supersonic blade tip velocity is, in fact, a parameter in this engines software, its actions in this regime and any harmonic issues impinging on the bearing box would have been completely addressed... No surprises there, right?
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Happy to see that you are at least prone to asking questions before forming ill advised opinions
You questions about speed have two parts imbedded in them. One part has to do with aerodynamics which of course has something to do with shock waves. The secoind part is a structural question which has a great deal to do with stress and strain.
Talking about speed of sound at the largest diameter (of a rotor) in the fan is easier to comprehend since it operates at a more familiar ambient temperature and pressure. On the other hand deep inside the engine the pressures and temperatures are quite extreme so the speed of sound would be far different. Simplistically the engine revolves around a defined cycle and as such the designer goes for the most effeciency per pound of thrust per pound of fuel. Thus one designs the compressor to operate at the highest pressure that can be obtained without tip stall or any other perturbation you might want to call it, including shock waves etc. The turbine driving the compressor had about the same tip diameter so its surface speed at its tips are about the same.
The bottom line after this is to ensure that centrifugal stresses are kept within an acceptable envelop with substantial margins against combinations of overspeeds and vibratory modes that might combine and produce rapid fatigue cracking. This is basic regulatory stuff and codified in the design regulations.
The problem comes in when the operation of the engine is outside of the certifcated conditions. Either from mis-operation of the engine (including maintainence) or mis-manufacturerd parts.
Vibratory modes are predictable and calibrated via experience including the hundreds of hours in development testing before the engine is certified. So obviously no mis-designed or undertested severe failure conditions are expected in service before any recommended maintainence cycles. assuming of course no mis-operation or mis-manufacturerd parts.
I've tried to keep this brief and cover some nuggets of your questions.
Just one editorial comment; don't confuse low mass with low cost or low safety margins. Margins of safety are regulated and it's stiffness and dampening that make it possible to design for light weight and still meet safety margins, else all the plumbing visible around the outside of engines wouldn't be called tubing but would be made out of thick steel pipe