Why is the J79 30% heavier than the R-25?

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For engines of similar size and performance that developed during the same time period, The GE J79 is listed as having a weight of about 3,800 pounds while the Turmansky R25 is listed at 2,600 pounds.

Are they so dissimilar of design that the J79 ends up that much heavier or is equipment not included in the R25 weight that are added into the J79 number?

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For engines of similar size and performance that developed during the same time period, The GE J79 is listed as having a weight of about 3,800 pounds while the Turmansky R25 is listed at 2,600 pounds

Same period? There's 10+ years between them - that's a lot considering how fast the state of the art was evolving at the time.

Are they so dissimilar of design that the J79 ends up that much heavier or is equipment not included in the R25 weight that are added into the J79 number?

Both, I would guess. The J79 certainly has a *lot* more compressor & turbine stages, with associated heavy, forged rotor discs (17+3 stages, as opposed to 8+2 on the R-25) and apparently the blading in the compressor is stainless steel throughout, rather than titanium in the first few stages on the R-25. So I'd expect the R-25 to be a fair bit lighter in an equal comparison, but I'm not sure this fully accounts for a 30% difference.

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As Trident notes, 30% isn't that surprising given that the J79 ( -17 ) has about 32% higher compression ratio than the R-25-300 and a vastly better time-between-overhaul ( 2000+ versus 400 hours ). More spinning stuff, longer lasting, better made. The Soviet engine was really designed for a short, high-power usage given that the expected combat life of its host would be measured in single-digit hours.

An interesting investigation might be between the J79 and the R35, both about the same weight and compression ratio but very different in thrust. Again the R35 'suffers' from a very low TBO in exchange for higher performance. The candle that burns twice as bright...

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J-79 engine was captured in Vietnam. Lulka studied and made the engine AL-21F

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Profile picture for user MadRat

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Soviet engines have less than stellar harmonics and suffer from a degree of shaft twisting when pushed hard. They wear out disproportionately in shorter time, because the engineers pushed a false narrative popular with leadership. Keep it simple, create it to be producible, and (most importantly) make it work.

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Suppose an entity/country decided to buy up all the surplus Mig 21's and rebuild them for sale on the open market. In their quest for sales, they have determined that they would have a far better product if they were to re-engine the Mig 21 with a Western engine....sort of like the Israeli's did with the Mirage/Kfir.

What Western engine would you use? Would it be the J79? I understand that you may wish to use a Russian sourced engine but this is not your decision. It must be a Western engine because they believe they will get most of their sales from the West...Would any Western engine, in your opinion, make it a better or worse, aircraf?

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From published numbers all of the following fit with space to spare, are lighter, give more thrust & have better sfc than the R-13.
EJ200
F414
RM12
M88
F404

I expect that they're also much more reliable.

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Since those engines actually have more life than the airframes, you only need one engine for every four or five airframes. :)

Honestly, if you could make it stealth then a new build might be worthwhile as a pointblank interceptor. But it would be hard to justify anything costing more than $35 million each. At that point you essentially are pushing for a trainer that goes Mach 2. Not too many modern jets that size get anywhere near that performance, so you're asking for something completely new build. I think that's why many of us were hoping the Northrop Grumman N400 was going to revive something akin to an F-20A.

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Since those engines actually have more life than the airframes, you only need one engine for every four or five airframes. :)

Honestly, if you could make it stealth then a new build might be worthwhile as a pointblank interceptor. But it would be hard to justify anything costing more than $35 million each. At that point you essentially are pushing for a trainer that goes Mach 2. Not too many modern jets that size get anywhere near that performance, so you're asking for something completely new build.

= JF-17 / FC-1
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Since those engines actually have more life than the airframes, you only need one engine for every four or five airframes. :)

Since you are generally talking about Western engines in comparison to the Russian ones. A simple oogle search and wiki gives us these approx figures...

Limiting it currently to American fighters and PW F-100 powerplants.

Total F-16 (all blocks) produced till now - 4,573 nos
Total F-15 (all blocks) produced - 1,198 nos

Of that

62% (approx 2,835) of all F-16 uses PW-F100 series engines = 2,835 engines
99% (approx 1,186) of all F-15 uses PW-F100 series engines = 2,372 engines

Total = approx 5,207 x PW-F100 engines for 4,021 x F-16/15.

But PW says that they have produced over 7,200 x F100 engines for the teen fighters till now.

And your theory says there is hardly any need to change a western engine for the entire life of an aircraft. If that is the case, for what purpose did the PW make those extra 2,000 x F100 engines for???

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From published numbers all of the following fit with space to spare, are lighter, give more thrust & have better sfc than the R-13.
EJ200
F414
RM12
M88
F404

I expect that they're also much more reliable.


Can you also put the unit cost of each of those engines to give a clearer picture as to what kind of upgrade in financial terms we are looking at?
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No I can't, because I don't know the prices. AFAIK prices aren't fixed for such things, but are decided contract by contract.

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intuitively, the 404 swap will be the most economical.

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There are more Mig 21 airframes laying about than almost any other make of jet fighter....How many thousands of those things did they make? And they were good aircraft! It seems that if they were able to be reclaimed instead of letting them rot in the weeds and put into service with reliable, serviceable engines a great many more airforces would be able to be equipped with good quality aircraft that will last and not bankrupt their nation.

Just a pipe dream...but a refreshed 404 equipped Mig 21 would be cool....sorta of a model T fighter jet...just out gas in it and fly.

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There are more Mig 21 airframes laying about than almost any other make of jet fighter....How many thousands of those things did they make? And they were good aircraft! It seems that if they were able to be reclaimed instead of letting them rot in the weeds and put into service with reliable, serviceable engines a great many more airforces would be able to be equipped with good quality aircraft that will last and not bankrupt their nation.

Just a pipe dream...but a refreshed 404 equipped Mig 21 would be cool....sorta of a model T fighter jet...just out gas in it and fly.


Whatever you have described here, has already happened.. The line at AVIC was churning out new-built F-7s until mid 2013.. There was no need to put anything as expensive as an F404 in there, WP-13F did the job solidly at a fraction of the price. And since the original design was greatly limited by the size of the cone, they needed a version with lateral intakes which could incorporate a relatively decent radar, a decent turbofan and a decent glass cockpit.. and that is the JF-17.. (now discounting the JL-9 which might be even more a direct successor to the type, albeit with very limited potential, IMHO).
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Can you also put the unit cost of each of those engines to give a clearer picture as to what kind of upgrade in financial terms we are looking at?
Ballpark figures:

EJ200 - GBP £3.4 million in FY2009
F414 - US $4.8 million in FY2000
M88 - US $5.5 million in FY1995
F404 - US $2.84 million in FY1995, $4.16 million in FY2007
RD-93 - US $2.67 million in FY2005

http://www.rolls-royce.com/media/press-releases/yr-2009/310709-typhoon-tranche-3.aspx
https://www.flightglobal.com/news/articles/snecma-boss-refuses-to-bow-to-demands-for-m88-price-28080/
http://www.defense-aerospace.com/articles-view/release/3/5680/ge-wins-f404-engine-contract-(june-21).html
http://www.defense-aerospace.com/articles-view/release/3/3059/us-navy-orders-super-hornet-engines-(sept.-22).html
http://www.geaviation.com/press/military/military_19951101.html
http://www.geaviation.com/press/military/military_20070207.html
http://www.globalsecurity.org/military/world/china/ws13.htm

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intuitively, the 404 swap will be the most economical.

Yeah, I expect so. And the cheaper the better, because as MadRat implies, the airframes probably won't have much life left, except for F-7s.
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I'm thinking that they could significantly reduce overall size from several perspectives using a much lower volume engine. The original MiG-21 was around 13,000 lb st thrust wet and could push through Mach 2. Smaller engines can reach that thrust at far lower volume, weight, length, and width. And modern electronics would make them only about 400% more capable at 65% of the original's weight.

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Yeah, I expect so. And the cheaper the better, because as MadRat implies, the airframes probably won't have much life left, except for F-7s.

That would be interesting semantically: how would you then call an engine swap? A molt?