Can Korea turn the KF-21 into its first homegrown stealth fight?

The KF-21 fighter jet during its test flight on Oct. 24, 2024 [NEWS1]

 
Korea is stepping up efforts to integrate ground attack and stealth capabilities into upcoming domestically developed fighter jets as the country seeks to bolster its own security and arms exports.
 
The Defense Acquisition Program Administration (DAPA) said Aug. 8 that it will accelerate its KF-21 upgrade plan, aiming to complete the development of the fighter’s air-to-ground attack capabilities by the first half of 2027 from the original schedule of late 2028.
 
But that’s not all. The KF-21EX, the Block III variant of the fighter, is poised to upgrade the KF-21 into a fully stealth fighter by internalizing its weapons bay, enhancing stealth functions by reducing its radar cross-section, and is set to be developed by the early 2030s.
 

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Importance of KF-21 

The KF-21 Boramae fighter represents a significant milestone for the Korean Air Force and the entire aviation industry. The country has relied on U.S.-made fighters, with the T-50 Golden Eagle training jet and the FA-50 Fighting Eagle light fighter being the lone domestically developed exceptions.
 
The KF-21 could also be a tempting option for countries that cannot purchase fifth-generation fighters from the United States for geopolitical reasons, such as the United Arab Emirates, Saudi Arabia, the Philippines and Peru, according to NH Investment & Securities researcher Rhee Jae-kwang. 
 
The UAE signed a letter of intent regarding the KF-21 in April, with the Gulf state's deputy minister of defense sitting in the back seat of a KF-21 during his visit to Korea. 

 

The KF-21 fighter jet outside a Korea Aerospace Industries hangar in 2023 [JOINT PRESS CORPS]

 
"Saudi Arabia and the United Arab Emirates, in particular, are interested in the KF-21, and they may even turn out to be the fighter's co-development partner as it is upgraded to a full fifth-generation fighter," the analyst said. 
 
The Korean-made fighter is not merely a reconstructed machine with hand-me-down parts from the United States; instead, Korea aims to achieve a 65 percent localization rate. Around 600 Korean defense firms and parts manufacturers are involved in production, from Hanwha Systems' Active Electronically Scanned Array (AESA) radar to LIG Nex1's Infra-Red Search and Track (IRST) electronic warfare suite.
  
Notably, Hanwha Systems announced on Aug. 8 that it had begun production of the KF-21's AESA radar system, which it developed with the Agency for Defense Development. A crucial component of 4.5-generation fighters, the advanced AESA radar system acts as the aircraft's eyes, detecting and tracking multiple targets faster than conventional mechanical radar. The United States had declined to transfer its own AESA technology for security reasons.
 
The Korea Research Institute of Standards and Science (Kriss) recently developed software to design the frequency selective surfaces in radomes, the enclosures that house radar arrays. They play a crucial role in a fighter's stealth ability as they must absorb enemy frequencies to remain undetected while transmitting friendly signals for communications.
 
Kriss said the software development represents a valuable step towards developing a Korean-made radome for stealth fighters, but would require further progress to be ready for production.
 
However, at the very core of the KF-21 are two U.S.-designed General Electric F414 engines, the same powerplants used by the F/A-18E Super Hornet. Hanwha Aerospace signed a 556.2 billion won ($400 million) deal with DAPA in July 2024 to produce 40 of the F414 engines under license.
 
President Lee Jae Myung pledged to locally develop the engine for the successor of the KF-21 during the election and soon designated the task a National High-tech Strategic Technology, with the government to support development. 

The KF-16, left, and the KF-21, right, during a test flight on Feb. 19, 2025 [KOREAN AIR FORCE]

 
KF-21’s future stealthy upgrade
 
Work to make the KF-21 a true stealth fighter is also progressing, according to industry experts. The KF-21 is not classified as a fully stealth fighter because its weapons bay is outside the aircraft, rather than inside, making it more vulnerable to radar. However, the machine was purposely designed as a stealth jet — by minimizing its radar cross-section (RCS) — from the ground up and already encompasses a space for an internal weapons bay.
  
This means that instead of having to design the exterior of the KF-21 from the ground up to be fully stealth-ready, the fighter can receive relatively minor design changes.
 
“It’s inefficient for our air force to have a fleet that only consists of stealth fighters,” Professor Chang Jo-won at the Korea Aerospace University, who specializes in aeronautical science and was involved in the early process of the KF-21's development, told the Korea JoongAng Daily. “Having the weapons bay inside the plane significantly limits the total number and weight of arms carried by each fighter.”
 
The professor added that the KF-21 was designed as a semi-stealth fighter, not due to the Korean firms’ lack of technology, but because it would be more effective to develop a semi-stealth fighter first, which could then be improved upon in the future.
 
The KF-21 wasn’t developed to replace Korea’s F-35 Freedom Knight fighter — classified as a “high-end” aircraft — but rather to replace the aging third-generation F-4 Phantom and KF-5 fighters the Air Force had used since the 1960s.
 
“The KF-21 was never intended to compete with the latest F-16F or the F-35, so to see the potential of the KF-21, which may compete and even excel when compared to generally used fighters, is an achievement in its own right,” Kim Ki-won, Dakeyung University Professor of military studies, said.
 

The KF-21, right, and the KF-16t, left during a test flight on Feb. 19, 2025 [KOREAN AIR FORCE]

 

BY CHO YONG-JUN [cho.yongjun1@joongang.co.kr]