Throughout its long gestation, KF-X has faced repeated objections: that it is unaffordable, or at least unjustifiable; that the country lacks the skills to develop it, or at least has too few engineers, especially if it pursues civil airplane and military helicopter programs at the same time; that the U.S., as a technology supplier, would seek to block KF-X sales; and, perhaps above all, that the South Korean fighter cannot offer much that is not already on the market at a lower price.
But backers, particularly ADD, present KF-X as the keystone in South Korea's future military aviation development. It would not just be a home-produced fighter; it would become the host aircraft of South Korean combat aircraft systems, such as sensors and weapons, promoting wider advances in the defense industry. South Korea would be in complete control of its configuration, not needing foreign permission to integrate its systems, as it has for the T-50 supersonic trainer and its combat variants.
In evolving the design and program, ADD has sought to address doubts about South Korea's technological capacity and the aircraft's technological adequacy. In 2009, the developers acknowledged that South Korea could not build a fully stealthy aircraft, equivalent to the F-35. They relaxed the radar cross-section to the level of such aircraft as the Boeing F/A-18E/F Super Hornet and Eurofighter Typhoon.
That radar cross-section is likely to be in the range of 0.1-1 sq. meters (1.07-10.07 sq. ft.), compared with 1-10 sq. meters for old F-4 Phantom and F-5 Tiger fighters in South Korean service, says a former air force officer who has been involved in planning for KF-X and other programs. KF-X's intended cross-section “is low enough,” he says, pointing to an official but unpublished study showing gaps in the coverage of North Korean surveillance radars when dealing with such a target. Those gaps exist even without electronic countermeasures degrading the performance of the radars, the study shows.
While the designers relaxed the stealth specification, they notably did not much change the aircraft's external shape and configuration, the foundation of low radar cross-section. The KF-X kept classic stealth features such as parallel edges and surfaces, forward fuselage chines and curved engine inlet ducts. That has allowed ADD this year to propose reintroduction of high-grade stealth in later KF-X versions. Block 2 would have more stealth coatings, radar-absorbing structural materials, tighter control of gaps, “integrated” (presumably flush) aerials, and a weapons bay. It would be as stealthy as an F-117, ADD estimates. Further, unstated improvements would advance Block 3 to the level of the F-35.
These improvements would be added with new systems. So KF-X would, in the end, significantly outperform current fighters—just not immediately.
Since settling on moderate stealth, ADD has been studying two main variables in its design: the number of engines, and the location of horizontal stabilizers. It has settled the first issue—the aircraft will have two engines—but the second issue will depend on the origins of the experienced foreign partner. The KF-X will have conventional aft stabilizers, following concept Design C103, if a U.S. company helps develop it; and forward stabilizers, for Design C203, if a European partner is chosen.
Size appears to have been set by the choice of two engines, the preference of the air force. “We do not have a rubber engine,” says an engineer familiar with the project, meaning that the designers must choose one off the shelf, not have one designed for their specification. So they see available afterburning thrust as 17,700 lb. (from the General Electric F404), 20,200 lb. (from Eurojet EJ200) or 22,000 lb. (from GE F414). The Snecma M88 is not mentioned as a candidate.
The thrust ratings straddle what is available to the Typhoon, and so it is not surprising that the airframes, for both C103 and C203, have been sized for an empty aircraft mass very close to that of the European fighter. Reflecting the great volume typical of stealth designs (partly because snaking inlet ducts demand a bulky fuselage), C103 and C203 each have more internal fuel than the Typhoon.