Al-21F is basically a modern copy of J-79 engine based on the samples captured by Soviets from the crashed Phantoms during the Vietnam war. It is an excellent choice to replace J-79s, as well as supporting the grounded fleet of Fencers. It is superior to J-79 almost in all aspects.
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I agree, it is superior in every way. As I mentioned before (in a response to PeeD), Iran can easily fasten these into F4s instead of J79. Iran also has had long record of use and maintenance of AL21F (Su24).
This is a simple project, does not cost a lot, and it can make a major difference in range as the 17,000lb non-afterburner thrust is quite sufficient to avoid using afterburner and wasting precious fuel. Which is almost the same as J79 WITH afterburner.
My logic behind this comes from years of analysis. Iran has a great workhorse fleet of F4Es and a few F4Ds, it makes sense AT THIS POINT (not a few years from now), to keep these operational and even enhance their performance temporarily until Iran can get a decent replacement for the next 20+ years.
I am sure both myself, PeeD, and many others here would love to see Iran have 240 SU30s and SU35s, but we live in a real world. Russians (3 times, not the 2 times most people know about and talk about) promised to sell Mig31 to Iran (once even got paid in advance, large sum as well), and every time it refused to do so after pressure from U,S. and some major threats or offers to buy.
Iran for Russia and China are simply one thing: Leverage over U.S. behavior towards them.
So Iran has to build its own leverage. With Russia and China in mind, they need to get their existing jet to a certain degree of competency so she does not look so desperate (if such a thing is possible). I am sure right about now Peed is probably thinking that the major problems with F4 don't go away with a slightly better performance and better fuel engine. The landing gear is a major problem, it has low landing maximum weight, and there are other issue with airframe, radar signature, and I can think of another 23 things wrong with it.
But for certain functions, still F4 is a great aircraft. If it has extended range (AL21F would help, and two bolt on fuel tank on top by the fuselage would help), then it can carry 3 anti-ship missiles which force an adversary to MAKE PLANS AROUND that kind of threat/scenario.
The more of your adversary's resources to bog down to cover potential scenarios, the more you can find a hole in its defense/offense to act against it. Iran just needs one or two shots in an all out war, then the whole game changes.
Iran did not get that shot during Operation Praying Mantis.
But in the next conflict OR TO AVOID ONE (which is what it has so far, remembering that George @$$wipe W. Bush did want to attack Iran in December 2006 and only the Joint Chief of Staff's threat of resignation forced him to change his mind).
Iran also needs training new pilots and new tactics like the one they have with "sandwich". Great air combat tact to use multiple planes for a specific advantage (hence F14 + Mig 29 + a F5e). However this is not that ideal and secure data linking is still an issue IF USED AGAINST U.S.
If one could have his cake and eat it too, I would love for Iran to have a AL41F license for production and installation/maintenance. No doubt Russia does not want that unless the price was just so sweet one could not ignore. But Iran cannot pay that price.
Iran more than anything needs STANDARDIZATION in engines.
The LTC4B-8D (a great engine for Iran's hot and high) is another perfect example. Iran can use this engine for heavy attack helicopters, for medium transport helicopters, even 2 of these for something like a EH101 or CH148 for the Navy.
This engine is a derivative of the same engine in CH53D.
Standardization is key for Iran to keep costs down, keep maintenance under control. Even if these engines are not fuel efficient, so what? Iran's biggest problem is not fuel access.