South Korea Plans Phased-Development, Typhoon-Size Fighter

In all of the West, only one all-new fighter, the Lockheed Martin F-35, is in full-scale development. If it outlasts its predecessors, as new products usually do, it could find itself to be the last man standing.

The F-35 will have Russian and Chinese competition, but only the U.S. fighter is likely to be engineered to standards that facilitate integration of Western weapons and sensors. For many countries, there will be no real alternative.

Or maybe there will be: a Western fighter from the East.

After at least a decade of studies, the design of South Korea's proposed KF-X fighter is becoming clearer. If it goes ahead, and if it is not heavily revised, it will be a two-engine fighter of the size of the Eurofighter Typhoon, perhaps following the Typhoon and other European fighters in mounting its horizontal stabilizers forward (see specifications table, page 48). It will be designed for Western, especially U.S., weapons and sensors, although later South Korean equipment will be fitted.

First mooted within the government in 1999, and announced as a national objective in 2002, KF-X has been under study for 14 years, repeatedly failing to gain authorization for full-scale development, which it still awaits. Early in the program, the targeted in-service date was 2015; now, it cannot fly before 2021, and therefore cannot be operational until middle 2020s, if it survives powerful opposition (see page 49).

In the air force's planning, KF-X will be a medium fighter, at first serving alongside and then replacing the KF-16, the locally built version of the F-16. Fighters above the KF-S rated as “high” grade—mainly meaning a greater payload range—would be the Boeing F-15K and whichever aircraft is chosen for the current F-X Phase 3 competition (AW&ST April 15, p. 52). Below it will be the Korean Aerospace Industries (KAI) FA-50.

The driving force for a fighter to fill this role has been the defense ministry's Agency for Defense Development (ADD). Program Director Lee Daeyearl answers to ADD chief H.Y. Paik. The ADD is handling the preliminary design and will oversee full-scale development and integration, if KF-X is approved, say government officials. So KAI, the country's combat aircraft specialist, would be only a supplier and, presumably, the detail designer of the airframe. It would not be surprising if the aerospace division of Korean Airlines made some parts. A foreign company would supply the engines. Electronics would come from several manufacturers.

At the top level, a foreign partner will be needed, probably the winner of F-X Phase 3, which means Boeing, Lockheed Martin or the Eurofighter partners. Most South Korean advocates of the program play down the intended role of outsiders. The KF-X will be led by South Koreans, they emphasize. Indonesia, which has contributed engineers and 20% of the funding since 2011 and proposes to order 50, is a junior partner, which is why the aircraft is sometimes called KF-X/IF-X. Attempts to enlist Turkey failed, partly because the South Koreans insisted on leadership; other partners are possible, but none have appeared so far.

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.

Also relying on airframe volume, the designers are contriving to work a weapons bay into the Block 2 version. In the Block 1 variant, four Raytheon AIM-120 air-to-air missiles are mounted in recesses under the fuselage, a favorite approach introduced by the Phantom's designers about 60 years ago (see cover photo). Those missiles must move inside the bay for the Block 2 KF-X; there is not enough space to have both a bay and external under-fuselage weapons. Provision for the weapon bay will be in the Block 1 aircraft, ADD says, which must mean that internal equipment will be packaged in some way to easily make space available.

Six more hard points for weapons and other stores are on the wing, the outer two available only for Raytheon AIM-9X short-range air-to-air missiles. ADD's drawings show the others with air-to-ground weapons: GBU-39, GBU-53, CBU-105, GBU-31, GBU-38 and GBU-24 guided bombs and AGM-65 missiles. External fuel tanks are an option for the inner pair of hard points, and models show cruise missiles mounted in those positions, too. There are no wingtip hard points, presumably to restrict radar reflections, but models show sensor pods on the lower corners of the fuselage.

A gun is mounted internally above the left inlet duct.

The prime sensor for KF-X will be a radar with an active, electronically scanned array. A foreign radar will be installed first, while a later version of the fighter will be outfitted with a set based on work that South Korean electronics company LIG Nex1 is undertaking. The designers are also specifying an electro-optical targeting system, an infrared search and track sensor, data link, GPS-INS navigation, “advanced threat warning and countermeasures” and internal electronic countermeasures.

The cockpit will incorporate a helmet-mounted display, a head-up display and multifunction head-down displays. In integrating electronics and weapons, the program intends to follow Western design standards, hence the advantage that KF-X, if built, will hold over Chinese and Russian rivals. Introducing new U.S., European or indeed South Korean equipment should be easier.

The wing of the C103 (tail-aft) version has full-span flaperons and leading-edge flaps for variable camber. The planform is a diamond shape, with 40 deg. leading-edge sweep and 10-deg. forward sweep for the trailing edge.

When the C103 and C203 designs were discussed in February, there was a hint that the two-seat option had been rejected, along with such variants as a single-engine aircraft. But ADD's latest description of the aircraft does show a version with a second seat that replaces the forward fuselage fuel tank. Although the U.S. has decided that simulation obviates the need for a trainer version of the F-35, a second seat is becoming popular again in other fighters for a reason that was familiar decades ago but then fell out of fashion: two people can handle the work of a combat mission more easily than one.


Source: By Bradley Perrett - Aviation Week & Space Technology News – 1 April 2013

Photo: The South Korea Air Force KF-X Fighter Aircraft (Photo by aviationweek.com)


(1.04.2013)