This is obviously not detailed information, but it seems to me that even a very quick and dirty, back of the envelope analysis yields some interesting and informative inferences. I did a quick cross-comparison of the KF-21's numbers (above) with those for a range of modern Western fighters, including the F-35, the F/A-18 Super Hornet, the Rafale, the Eurofighter Typhoon, and the F-22.
Since I'm not privy to
sekret sources, I will rely mostly on Wikipedia, sadly, although in this case it should generally be OK, I think, since I'm only comparing very simple data.
The data points I've compared are: Max Thrust (with Afterburner); Max Take-off Weight (since this number is readily available for all of these fighters for the purposes of this very casual comparison; comparing combat loads would be a far more difficult and arcane exercise); Thrust to Weight Ratio (Max Thrust/MTOW); Max Speed; Max Range.
(If anyone feels that any of the Wiki-based numbers I've highlighted below are grossly incorrect, please feel free to provide a source.)
Lockheed Martin F-35 Lightning II - Wikipedia
Boeing F/A-18E/F Super Hornet - Wikipedia
Dassault Rafale - Wikipedia
Eurofighter Typhoon - Wikipedia
Lockheed Martin F-22 Raptor - Wikipedia
Max Thrust: F-35: 43,000 lbf; Super Hornet: 44,000; Rafale: 34,000; Typhoon: 40,000; F-22: 70,000; KF-21: 44,000
MTOW: F-35: 70,000 lbs.; Super Hornet: 66,000; Rafale: 54,013; Typhoon: 51,809; F-22: 83,500; KF-21: 56,320
Thrust to Weight (Max Thrust/MTOW): F-35: 0.61; Super Hornet: 0.66; Rafale: 0.62; Typhoon: 0.77; F-22: 0.84; KF-21: 0.78
Max Speed: F-35:1,960 kph; Super Hornet: 1,915; Rafale: 1,912; Typhoon: 2,125; F-22: 2,414; KF-21: 2,200
Max Range: F-35: 2,800 km; Super Hornet: 2,346; Rafale: ~3,000 (?); Typhoon: 2,900; F-22: 3,000; KF-21: 2,900
Some observations that immediately leaped out on viewing the numbers:
-Based on Thrust to (MTOW) Weight, the six fighters can be put into two groups: 1) F-35/Super Hornet/Rafale, which cluster around 0.6-ish; and 2) the Typhoon/F-22/KF-21, which cluster around 0.8-ish. IMO, this is not a coincidence. The former group comprises fighters that are generally understood to emphasize their strike (or multi) roles. The Typhoon and the F-22, on the other hand, are air superiority fighters first, although they can also perform strike roles (especially if the air forces in question chose to put in the effort to upgrade them). By the numbers, the KF-21 falls squarely in the second category.
-Similarly, there is a similar demarcation in Max Speed. The F-35/Super Hornet/Rafale group clusters around 1,900-1,960 kph, while the Typhoon/F-22/KF-21 group clusters well north of the first, at 2,125-2,414 kph.
-The Max Ranges are all quite similar, except for the Super Hornet, which looks a bit short-legged. This is possibly due to the fact that its weight is relatively high compared to engine power; the KF-21 uses essentially the same engines (2 x F414), while its MTOW is 10,000 lbs. lighter.
-The development menu for the future weapons loadout as listed above is yet another data point that hints very strongly at the the expectation of a Low Observability fighter in many of the required operational roles. Everything that we know about the KF-21's design thus far indicates that the eventual end-goal is an LO fighter. Things like the painstaking stealth shaping, which comes with performance trade-offs, and the precise space needed for an Internal Weapons Bay being built into the airframe, both of which exist from the the get-go, makes no sense otherwise.
-Now comes a bit more speculative inference: let's look toward what the KF-21 might look like nearer the end of its development phases, in later Blocks. Let's assume that the KF-21 transitions to an upgraded EPE version of the GE F414 engine, which is generally expected to provide about a 20% boost to performance.
Balanced against this would be modifications for a wide range of strike capabilities and weapons, as well as stealth modifications such as a fully-developed Internal Weapons Bay, and fully conformal sensors/electronics, and possibly more fuel capacity, all of which would increase the weight of the aircraft. If the MTOW increases by, say, 10%, to around 62,000 lbs., which would approach that of the Super Hornet (66,000 lbs.), the Thrust to Weight would be still 0.85, or equivalent to the current F-22 (0.84).
Even in a more extreme scenario, in which the KF-21 goes full bomb truck, the Thrust to Weight would still be extremely high. If we assume a 20% increase in MTOW, which at almost 68,000 lbs. would bring the KF-21 near the F-35 (at 70,000 lbs.), the Thrust to Weight would hover at around 0.78. This is what the Air-to-Air 56,000 MTOW lbs. Block I version of the KF-21 is at now (according to the numbers provided above). I'm not sure that grossing up the KF-21's takeoff weight to that extreme is a very good or practical idea, since it doesn't seem to be initially structurally designed for it (it has the graceful lines and proportions of the F-22, just smaller), but if it were possible, it would be a real monster. Imagine a Low Observability fighter with the carrying capacity of the F-35 but the power and kinematics of the F-22.
IMO, the signs of the Koreans' long term ambitions are all there to see, and it will be interesting to follow over the next decade or so to see if they pull it all off. On the evidence of their modern economic, industrial and technological history, however, if there's one thing that they have proven again and again, it's that they are very bold and good at reaching for stretch goals.
(Someone once told me an interesting story about the development of their electronics industry, which started from very humble beginnings, focusing on things like assembling PCB boards in the 70s. At the time, industry experts said that South Korea would never be able to develop what were then considered very high-end, high-tech items, such as microwave ovens. Samsung Electronics, which at the time was a relatively modest company that was unknown outside of Korea, decided that they were going to crack this market segment. They put together an R&D team that painfully struggled to develop a working prototype. The team basically lived at the R&D center, laboring round the clock. After a long period of trial and error, they emerged looking like death warmed over, but with a microwave oven that worked. Some years later, the Koreans were among the global leaders in the white goods market.)