Rakesh wrote: ↑19 Dec 2023 22:18
Saar, I am not
young like you! 
Can we please stop reducing the font sizes? Thank You.
... moi and young, oh boy!!! ...
Rakesh wrote: ↑19 Dec 2023 22:18A Safran equivalent of the F414 would mean that the industrial offsets - from a MRFA contract - would be invested into a new generation turbofan that will primarily be for the AMCA and a variant of this turbofan will likely find its way into the Tejas Mk2. But I am completely speculating here.
Maitya-ji can perhaps provide the possible technical hurdles, if such a plan is indeed being looked at.
I think you are alluding to the long-dead (for what, maybe 1.5+ decades now????)
M88-4 variant (of 95-105kN) program ...
That program was aiming at keeping the HPT/HPC of M88-2 as is ... while tinkering with the LPT/Fan combo to achieve the upthrusting (via increased Mass Flow route).
Pls refer to my many posts on this aspect (of upthrusting via the "crude" mass-flow route) spanning atleast a decade now(and you are still calling me "young"
), littered all over the Kaveri thread.
So, the "requirement" there (in M88-4 program) was to have,
1) A larger fan
2)
A new low-pressure turbine
3) Afterburning of
both gas flows
(plus a variable-area convergent-divergent nozzle)
No idea what Saffran finally did with that program - but I think they simply didn't fund it.
So with our funds (as is always the case - we are experts at funding furrin R&D programs, there are never fund shortfalls etc in such cases
e.g. the ICE upgrade), maybe doable - yes!!
But minm a decade of R&D/Dev, Testing and certification work required, nevertheless.
Remember, we are talking about a
20-25% growth of dry thrust levels, from 51Kn (of F404 or M88-2) to 61-62Kn (of F414/M88-4 levels).
So, technologically, at a very very high level, the aim is to:
Lower unit-weight of LPT blades -> Lesser Centrifugal force to deal with (note rpm would be at ~10K levels) by the disc -> allowing various other type of turbine disc design (e.g. BLISKs and even BLINGs - refer to my posts in Kaveri thread) -> more work extracted by LPT complex -> higher rpm at the shaft -> higher Fan rpm -> better Fan/LPC PR and more bypass mass-flow (plus higher exhaust vel - of the bypass mass) -> more/better dry thrust rating.
Now in the F404 -> 414 "upgrade" by GE, they used the "traditional"
path of improving the Gas Generator itself - so, improving HPT+HPC (primary goal) along with smaller improvements to the LPT+Fan/LPC systems.
So they went with SC Ni-Superalloys for the LPT blades as well (which in itself is a challenge, for the "larger" blades of the LPT etc - will not delve into it today, maybe some other day), even though the ambient temp requirements were much lower than those of the HPT cases.
They mitigated the tech risks by essentially keeping the
turbine material techs similar for both HPT and LPT - while dealing with equally (if not more) demanding tech challenges on the following three aspects:
1)
Keeping the cumulative LPT weight lower via Blisk design - SC LPT blade weights would be higher (and thus the resultant centrifugal forces) requiring heavier disks to support them, making the overall LPT quite heavy. Using advanced "joining techniques" (e.g LFW etc) a Blisk design was possible, keeping the weight increments in check.
2) The larger LPT blades are again notoriously difficult to SC investment cast - there's a concept
called the LAB effect.
Not sure if I'd posted on it here, but it's part a writeup I'd started during the ToAsT deal surfaced (one of many such unfinished writeups), which as usual, is safely lying in HDD, untouched for many months now.
Anyway, what it essentially means is that, during practical casting of SC blades and vanes, it is impossible to maintain perfect [001] crystal orientation – so some levels of low angle boundaries (LAB, are formed between parallel single crystals), occurs during the SC solidification process (and gets further exacerbated by formation of recrystallized grains forming during solution heat treatment phases).
This results in unacceptably high rejection-rates of blades etc, and
the costs goes up exponentially - and can only be mitigated by developing the associated technologies wrt much
tougher control of SC casting parameters, which in turn requires 3-4 decades worth of dedicated R&D with unlimited funding etc.
Which, of course, GE (and P&W etc) have had access to.
3) These LPT blades can be solid, and also wouldn't require any TBC coating etc - as the ambient temp requirements are way lower than what the SC casted ones are designed for. Some env-protection coating would eb required though.
Solid blades are much easier to cast, comparatively speaking, as the whole 3-4 tech towers wrt mould fabrication that has intricate internal cooling passages inbuilt into it etc are not required.
(Yes, yes, you are right - I agree - the hand-me-down SC Casting Tech of the AL-31FP program, is as cutting edge as it gets and we should have copied it to Kaveri program etc).
Coming back to the topic - I think have digressed quite a lot, as usual.
M88 program (including M88-2 used in Rafales) on the other hand kept the LPT blade weights lower by using a completely different material technology. Trying the
γ-TiAl alloy route, which is equally risky, as at that time (early 90s etc), it was a pioneering field, with no proven product fielded in Aerospace field.
These γ-TiAl alloys, are almost half the weight of Ni based superalloys* - but at the same time, dealing with γ-TiAl at a product engineering (as opposed to sample-size-based R&D efforts)
level is notoriously difficult. Many advanced countries (US, Fr, UK, Ger, Japan et all) have spent enormous amounts of funds and R&D effort over multiple decades with very limited (and painfully slow) iterative tech dev.
One such example can be, to be able to engineer turbine blades,
TiAl-based alloys should exhibit a reasonable ductility at room temperature (more than 1%) - and along with being able to
maintain strength at the service temperature (700–800deg C for LPT applications, whereas 1100-1150 deg C for HPT ones).
Plus typically, a LPT balde geometry would be of a slim foil and thick root, twisted from hub to tip - machining such geometries in an inherently low ductility material is a huge challenge (unless you take the casting route). Moreover, the thermal conditions vary pretty wildly across the entire blade geometry from tip (highest temp) to hub (lowest temp).
Mechanical properties in γ-TiAl alloys are strongly governed by the microstructure, which in turn is affected by the chosen processing route (for example casting).
Casting, in general, results in coarser microstructure - this results in lack of strength of the part itself. For Ni Superalloy based blades, the extensive heat treatment (and strengthening) steps post casting etc, tries to alleviate this issue, but it's not that successful with Ti-Al casted parts.
This was mitigated by various
Powder Metallurgy (PM) techniques wrt refining TiAl microstructures, as follows:
1) PM
Metal Injection Moulding (MIM) for producing turbine blades in Ger
2)
Spark Plasma Sintering (SPS) rapid processing technique in Fr
3)
Laser Metal Deposition (LMD) technique (developed by ONERA, Fr)
Both 2 and 3 above helped Snecma achieve
fine equiaxed lamellar microstructure of γ-TiAl alloy based
near-net shape LPT blades, bumping up the
operating temp levels of these LPT blades to around 800-850deg C levels (exactly what a standard LPT blade would be looking for, for a military TF application).
I'll refrain from going into more details of these (as there are further complications, tech hurdles etc), as this post has now become a too long anyway.
So, coming back to the topic ...
M88-4 program was aiming at the more "aggressive" approach of improving the LPT/Fan only, and thus increasing the mass-flow levels (from M88-2 levels) of the bypass (primary) - and maybe there was a secondary aspect of small increments to the core-mass-flow as well (I don’t know for sure though).
But with Kaveri design already
allowing higher mass-flow (thru core), better still would have been to try and
up-thrust the K9 itself via this lighter LPT (and improvements to the Fan/LPC) route.
That is, beg/borrow from Safran et all, the lighter LPT blade tech, via the γ-TiAl alloy route (already developed atleast 2+ decades back in the M88-2 program), and
increase the dry-thrust levels majorly from the increment in bypass mass-flow (and it's velocity as well).
But industry wide "standards/thumb-rule" is ~10-12% increments via only the LPT/Fan-LPC route - beyond that some amount of work/improvement is required in the core Gas generator (i.e. the HPT+HPC) itself. So any such attempts would require a decade long design/dev/testing/certification life-cycle anyway.
Also why on earth anybody consider any such tech-transfer proposal with any seriousness - when the baseline TF architecture is itself not flight-tested and certified. A decade went by doing nothing, as the MoD Baboons cut whatever trickle-feed funding was available to the program.
Oh well ...
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*Note:
From Technology pov, and just to re-iterate only (nothing is there below, that I've not pointed out, scattered across my various posts in the Kaveri thread):
TiAl alloys have
1) half the density of Ni based superalloys : (∼4g/cm3 vs ~8g/cm3)
2) high specific modulus, mechanical strength and a very good oxidation resistance, all essential pre-requisites high-temperature applications
3) GE implemented Ti-Al (specifically Ti–48Al–2Cr–2Nb, figures representing % composition) based LPT blades for their GEnX series in 2007 (actual impl was on 2011, IIRC)
4) But the GE solution was based on Investing casting tech - so had the basic drawback of coarse-grained structures (results lack of strength) remained
5) CFM partially overcame this weakness in 2016 (in their LEAP engine program) via centrifugal casting tech
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I seriously need to re-cosnider my approach towards posting here - what was originally a 2-3line post replying to Rakeshji's jibe (at me being young etc), turned out to be such a long rant.
Take it FWIW pls ...
