Avarachanji, reg the conservative engine technology roadmap for Kaveri, selected by the GTRE folks, well, let me put it this way.Avarachan wrote:Maitya, thank you for these very informative posts.
I have a question for you. Given that the Kaveri is to be used in IUSAV (the unmanned strike platform), do you think that the conservative design choice was the right one? After all, because GTRE went with the conservative choice, at least India will soon have an engine it can use for other purposes, apart from the Tejas.
There's simply no other choice - 2nd (1a-2b) and 3rd (1b-2a) quadrant choices couldn't have been taken, as in the late 80s (when this decision was being taken) the status of,
1) Prevalent Materials R&D and, more importantly, indigenously available manufacturing and engineering base to translate these designs to manufactured parts/products etc.
AND
2) Low experience on the mechanical and CFD (from non-flying testbeds like GTX-37U and UB etc.) aspects of an aero-engine design
To give a short example of 1)
How many times here in BR we have heard that if only we could have mastered Blisk-manufacturing technology, most (if not all) issues of Kaveri would be sorted. What never gets discussed or thought thru is what it really means in terms of constraints that are being tried to overcome.
I'll not go into too much detail (will reserve that for Material write-up, if it ever gets finished - most likely it won't, just like my engine-design related write-ups - all lying around at 50-60% completion level ), let me try to bring out a small dichotomy (in context of blisk manufacturing usage).
Let's look at a HPT stage of a Turbine - now the blades will be required to withstand 1600-1700deg C temp and tip-rotor speed of about 1.5M. But what about the disk - the temp there seldom will reach beyond 800-900 deg C and speed maybe 0.9M.
Big difference, isn't it? But that's not all.
Look at the picture of an military turbojet/fan HPT, for example that of a F-110 as shown below:
Now if you compare the mass of the disk and that of the blades, it's obvious that the mass of disk is many order-of-magnitude more than that of the blades.
So between the disk and the blades of the same HPT stage, you have the following:
- Factors ----------------- Disk ------------- Blade
-----------------------------------------------------
Operating Temp -------- 1700degC -------- 850degC
Speed -------------------- 1.5M ------------ 0.9M
Mass -------------------- easily 10-12 times of Blades (cumulative)
Cycle Fatigue ----------- Low ------------ High
So the mechanical pressure due to good-old centrifugal force on a disk is multiple times more than that of the blades - but the operating temp regime is also very different.
So for the disk you would ideally be looking for some material with very good tensile ductility, high tensile yield and ultimate strength (and LCF too - more on this on some other day) - while the temp operating environment gives you a lot of leeway (compared to blades), so much so you can get away with even equiaxed-casted materials.
But for the blades the requirements are high Thermal Mechanical Fatigue (TMF) resistance (aka higher melting points), Creep-rupture strength and HCF - while lot of leeway on mechanical strength aspects like tensile yield/strength and ductility.
So for the blades you are constrained to have casted materials that have directionally oriented grains parallel to airfoil axis (aka DS or SC) - but that's not all, due to high-temp operating env, it needs to have a good ability to accept TBC as well (plus higher oxidation resistance properties - again more on this on a later day). But the operating word is "casted" alloys.
And there-in lies the problem.
As casted alloys generally have lower tensile properties, worser ductility and lesser homogeneity making them unsuitable for a disk application - to have those kinds of mechanical properties you need wrought alloys.
So, coming back to the topic of blisk manufacturing etc - you are basically asking/looking for an "integral" manufacturing process where the disk part is made of wrought superalloy, while the blade part being from casted superalloy.
This should give us an idea about what challenging the material/manufacturing R&D and engineering aspect could be.
I'll later bring out a suitable example from 2 as well.
But coming back to your original point about design choices - this above example, would be sufficient to demonstrate that how many options the GTRE folks would have had then, given our experience on integral casting etc, to reject the possibility of blisked turbine stage development route. Instead, it would have been less riskier to have accepted the then prevalent bolted-blades-on-disk philosophy and live with the resultant compromise on the TeT itself. And that's what the GTRE folks did.
Pls note the above example is not some "virtual" one - something quite similar actually happened with Kaveri where-in the in-house developed HPT disk had to be rejected (when the TeT was increased, as the blades were found to be able to accept a few tens of degrees C more TeT) and import (from USA) the HPT disks (while the HPT blades remained indigenous DS ones). But that story for another day.