SEOUL — South Korea’s indigenous fighter jet development program has entered the phase of prototype development following critical design review, or CDR, , according to developers.

The KF-X program for a 4.5-generation fighter, worth $7.4 billion, seeks to develop an advanced twin-engine fighter jet on par with the latest F-16 variant of Lockheed Martin by 2026, with the rollout of the first prototype happening in 2021. Korea Aerospace Industries, or KAI, is responsible for the systems development and integration.

During the CDR session at the end of September, members of the Defense Acquisition Program Administration, or DAPA, examined nearly 400 kinds of technical data to see if the technologies meet the capability requirements before giving the green light to prototype development.

“The KF-X program now enters the prototype development phase as its CDR was approved,” said Jung Kwang-seon, chief of DAPA’s KF-X development team. “We will strive to develop and deploy the KF-X aircraft with advanced capabilities meeting the combat requirements.”

The jet’s full-sized mock-up was unveiled for the first time at the Seoul Aerospace and Defense Exhibition, or ADEX, which is taking place from Oct. 15 to 20.

The model has six under-wing hard points: two for external fuel tanks, two laser-guided bombs, and two other IRIS-T short-range air-to-air missiles. Four MBDA Meteor beyond-visual-range air-to-air missiles are nestled under the fuselage, while a mock-up of the Lockheed Martin Sniper targeting pod is mounted on the right cheek station.

KAI spokesman Kim Ji-hyung told Defense News that the KF-X is still open to U.S. missile systems. Originally, the DAPA hoped the KF-X would be equipped with U.S. armament, such as Raytheon-built AIM-120C advanced medium-range air-to-air missile, and AIM-9 Sidewinder missiles, but the U.S. government has yet to approve the export license of the missiles.

“It’s easy to integrate U.S. missiles into the aircraft, and we’re open to the possibility,” Kim said. “It’s just a matter of U.S. export controls of weapons systems.”

Fitted with a homegrown active electronically scanned array, or AESA, radar, the jet has a max take-off weight of 25,600 kg and a max payload of 7,700 kg, according to KAI. The jet can fly as fast as Mach 1.8 and has a cruising distance of 2,900 km.

The KF-X Block I will not have an internal weapons carriage, which is planned for subsequent production blocks. The initial version will also lack air-to-ground striking capability since the homegrown long-range air-to-ground missile is to be developed by the mid-2020s. The Korean version of the Taurus air-to-ground missile is being developed by LIG Nex1, the country’s precision guided weapons maker.

“Though it’s called a 4.5-generation aircraft, the KF-X bears similarities to the fifth-generation F-35A,” KAI said in press material. “It’s operating cost is half of the U.S. stealth jet and features high-tech maneuvering capability next to the F-35A.”

Despite development progress, there are signs of challenges in the jet fighter program, including a potential funding loophole. That’s because Indonesia, the only international partner of the KF-X, has been backtracking from its original commitment to investing 20 percent of the development costs. KAI is obliged to pay for 20 percent, and the government is to fund the remainder.

Under a 2016 deal, Indonesia is obliged to pay around $1.3 billion to acquire up to 48 jets called IF-X in Indonesia and get the transfer of fighter jet technologies.

But the South Asian nation has paid only $190 million, some 13 percent of its financial commitment, citing domestic budgetary constraints. As of July, Indonesia has funding shortfall of $250 million, according to DAPA officials.

Jakarta, instead of cash, has offered to make payment in kind, including the provision of CN235 transport aircraft produced by Indonesian Aerospace, also known as PTDI, under a license.

Indonesia also reportedly asked to renegotiate the terms of deals on the KF-X/IF-X, with a focus on getting more technology transfer from South Korea.

“It’s a thorny issue,” a DAPA source said, asking not to be named. “The two governments have been in consultations over the funding issue but have yet to narrow a gap.”

Much attention has been on the development of the indigenous AESA radar, which experts see as the toughest challenge of the KF-X program.

In May, the DAPA announced the CDR of the AESA radar, developed by Hanwha Systems, was completed for the first production prototype to be disclosed in the second half of 2020.

Hanwha Systems, formerly known as Samsung Thales, has completed the AESA hardware with the help of Israel’s Elta Systems.

In April, airborne tests of the hardware systems were carried out with technical assistance from Italy’s Leonardo, according to Hanwha officials. The flight test bed was carried out onboard a 737-500 airplane, and the radar is to be further tested in South Korea.

“The radar is scheduled to be tested aboard an actual KF-X prototype aircraft in 2023 with the goal of completing all aspects of development by 2026,” said Jang Bo-seop, a marketing manager of Hanwha Systems.

He said the KF-X AESA has more than 1,000 transmit-receive antenna modules to perform close to the Northrop Grumman APG-83 Scalable Agile Beam Radar that equips the F-16V.

“Developing AESA software and integrating it into the hardware is a tough task,” said a retired Air Force officer who serves as a member of DAPA’s KF-X advisory group. “There are risks down the road in spite of progress in the early development phase.”