KAI KF-21 Boromae is a 4.5+ gen multirole twin-engine fighter with AESA radar, stealth features, Mach 1.8 speed, and 25,400 lb thrust engines.
The KAI KF-21 Boromae is a twin-engine, multirole fighter aircraft developed by Korea Aerospace Industries (KAI) in collaboration with Indonesia. Designed as an advanced 4.5+ generation platform, it incorporates stealth shaping, Active Electronically Scanned Array (AESA) radar, Infrared Search and Track (IRST), and electronic warfare systems. The aircraft is powered by two General Electric F414-GE-400K turbofan engines, each producing 22,000 lbf (98 kN) of thrust with afterburner. It reaches a top speed of Mach 1.8 and an operational ceiling of 50,000 ft (15,240 m). The KF-21 supports air-to-air, air-to-ground, and maritime strike missions, carrying 7.7 tonnes of armament on ten hardpoints. With a combat radius of approximately 500 nautical miles (926 km), it bridges the gap between the KF-16 and F-35 in the Republic of Korea Air Force. It first flew on July 19, 2022, and is planned for operational deployment by 2026–2028.
History of the development of the KAI KF-21 Boromae (KF-X)
The development of the KF-21 Boromae, originally known as KF-X, began as a response to the aging fleet of KF-16s and F-4/F-5 fighters in South Korea. Discussions on an indigenous fighter began as early as 2001, driven by the need to reduce foreign dependency and maintain regional parity against neighbors such as China, Japan, and North Korea.
The formal KF-X program was launched in 2010 under the direction of the Republic of Korea’s Defense Acquisition Program Administration (DAPA). The Korea Aerospace Industries (KAI) was selected as the prime contractor. In 2014, Indonesia joined the program with a 20% stake, securing 48 aircraft under the name IF-X. South Korea retained a 60% stake, while the remaining investment came from local defense firms.
The goal was to develop a 4.5+ generation multirole fighter, incorporating partial stealth, advanced sensors, and flexible weapons integration. Unlike full fifth-generation fighters, the KF-X would be more affordable, maintainable, and ready for export. The program targeted interoperability with NATO and U.S. assets, including the use of GE F414 engines and AESA radar developed by Hanwha Systems with Israeli assistance.
On August 2, 2011, a dedicated KF-X Research and Development Center was opened in Daejeon. Between 2011 and 2015, KAI worked on conceptual studies involving two variants: KF-X-101 (single-engine) and KF-X-102 (twin-engine delta-wing with canards). The latter was selected due to better performance and future scalability.
Full-scale development was approved in January 2016. The rollout ceremony took place on April 9, 2021, and the first flight occurred on July 19, 2022, in Sacheon, South Korea. The name “Boromae”, meaning “Young Hawk” in Korean, reflects the air force’s vision for future air dominance.
The program was challenged by technology restrictions. The U.S. refused to export key subsystems such as electro-optical targeting pods, infrared search and track systems, and radar jammers, forcing South Korea to develop these independently. Despite setbacks, the program advanced with six prototypes constructed by 2022, including two twin-seat variants.
The first batch, Block I, focuses on conventional weapons and low observability. Block II, expected after 2026, aims to introduce internal bays and expanded stealth capabilities.
The KF-21 does not have a NATO reporting name yet, as it remains in pre-operational status. However, it is already being positioned as a competitor to export options like the Dassault Rafale, Eurofighter Typhoon, and Saab Gripen E.
South Korea intends to produce at least 120 KF-21 units by 2032, replacing F-4s and F-5s. Indonesia’s participation has faced financial delays, but the partnership continues, with localized assembly expected for Indonesian units.
Design of the KAI KF-21 Boromae (KF-X)
The KF-21 features a twin-engine, mid-wing monoplane design, optimized for reduced radar cross-section (RCS). While it lacks internal weapon bays in its initial configuration, it incorporates angled air intakes, serrated edges, and composite structures to minimize signature. The aircraft measures 55.4 ft (16.9 m) in length, with a wingspan of 36 ft (11 m) and height of 15.7 ft (4.8 m).
The airframe is constructed from 50% carbon fiber reinforced polymer composites, reducing radar reflectivity and airframe weight. The wing area is approximately 460 ft² (42.7 m²), optimized for lift and maneuverability.
The cockpit uses a glass avionics suite, featuring a wide-area display (WAD), hands-on throttle and stick (HOTAS) controls, helmet-mounted display systems (HMDS), and digital fly-by-wire controls. Pilot interface is designed for high situational awareness in multirole scenarios.
The AESA radar developed by Hanwha Systems, with Thales and Elta support, offers simultaneous tracking of multiple air and ground targets. The aircraft integrates IRST, EW self-protection systems, and data link systems for network-centric warfare.
Power comes from two GE F414-GE-400K turbofans, each rated at 22,000 lbf (98 kN) with afterburner. These engines provide redundancy, increase thrust-to-weight ratio, and allow supercruise potential in future blocks. Fuel capacity is over 7,700 kg (17,000 lb), distributed in internal tanks and external drop tanks.
Weapons are carried on ten hardpoints, including six underwing, two under-fuselage, and two wingtips. Total external payload capacity is 7,700 kg (17,000 lb). Compatible weapons include Meteor BVRAAM, AIM-120 AMRAAM, JDAM, GBU-12, CBU-105, Taurus KEPD 350, and anti-ship missiles.
The aircraft incorporates modular architecture, allowing easier maintenance and future upgrades. However, the current absence of internal bays limits stealth compared to full 5th generation fighters. The design balances performance, cost, and export adaptability, though some compromises exist in stealth fidelity.
Landing gear is a tricycle retractable configuration, designed for rough runway operation and high sortie rates. The aircraft uses quad redundant flight control systems with electrically actuated surfaces, ensuring stability and survivability.
Performance of the KAI KF-21 Boromae (KF-X)
The KF-21 Boromae is powered by two General Electric F414-GE-400K turbofan engines, each delivering 22,000 lbf (98 kN) with afterburner and 13,000 lbf (58 kN) in dry thrust. These engines are a derivative of those used in the F/A-18E/F Super Hornet, offering high reliability and maintainability.
The maximum speed is estimated at Mach 1.83 (2,255 km/h or 1,401 mph). While it does not feature internal bays in its Block I configuration, which increases drag and limits stealth, the aircraft retains competitive acceleration and climb rates due to its high thrust-to-weight ratio, especially when lightly loaded.
The service ceiling is rated at 50,000 ft (15,240 m), allowing it to engage targets at high altitude. The combat radius is approximately 500 nautical miles (926 km), while the ferry range with external tanks extends up to 1,800 nautical miles (3,330 km). Internal fuel capacity is estimated at 7,700 kg (17,000 lb), supplemented by up to three external fuel tanks for extended range.
Takeoff distance is approximately 1,200 m (3,937 ft) and landing distance around 1,000 m (3,280 ft), depending on payload and altitude. This provides sufficient flexibility for operations from short runways and dispersed bases.
The aircraft’s agility is supported by a fly-by-wire system and digital flight control laws, enabling high angle-of-attack performance and stable maneuvering. It is designed for a sustained load factor of +9g, with good transonic and supersonic handling characteristics. Roll rate and pitch responsiveness are optimized for dogfight scenarios as well as precision strike missions.
When compared to peer aircraft:
- Eurofighter Typhoon: KF-21 has lower supercruise capability and lacks canards but offers better radar signature management and multirole integration.
- Saab Gripen E: KF-21 has a higher thrust rating and payload capacity, but Gripen offers advanced electronic warfare and cost efficiency.
- Dassault Rafale: Similar payload and multirole capability, though Rafale has more operational maturity and proven combat record.
- F-35A Lightning II: KF-21 lacks full stealth and internal bay configuration but is less expensive, easier to maintain, and faster in certain loadouts.
The KF-21’s performance profile, though not fully comparable to true fifth-generation fighters, is sufficient to dominate 4th generation adversaries and support multirole missions with precision. Future Block II upgrades, including internal bays and improved EW, will narrow the gap with fifth-generation standards.
Variants of the KAI KF-21 Boromae (KF-X)
The KF-21 program is structured around two main blocks and multiple mission-configured variants.
- KF-21 Block I: This is the baseline production variant. It includes low-observable shaping, external weapons configuration, and multirole capability. It will support air-to-air and air-to-ground roles, equipped with AESA radar, IRST, electronic warfare systems, and standard NATO weaponry. It lacks internal weapon bays but serves as a transition platform for full stealth integration.
- KF-21 Block II: Planned for production post-2026. This variant introduces internal weapon bays, enhanced stealth features, and potentially supercruise capability. It will support domestically developed targeting pods, EW suites, and expanded avionics, aiming to bring the aircraft closer to 5th generation standards.
- KF-21N (proposed): A navalized carrier-capable version is under conceptual review. It would feature strengthened landing gear, arrestor hooks, and short takeoff capability for light aircraft carriers. This version targets South Korea’s future CVX-class carriers.
- KF-21B/IF-X: The two-seat variant used for advanced training, mission command, or electronic attack roles. While it sacrifices internal fuel volume for the rear seat, it allows expanded mission profiles, including tactical command and control functions.
The modular airframe allows flexible configuration. Future variants may include unmanned loyal wingman control nodes or electronic warfare-specialized versions. As of now, only six prototypes (including two two-seaters) have been built, with production variants expected from 2026 onward.
Military missions of the KAI KF-21 Boromae (KF-X)
The KF-21 Boromae is designed to fulfill a wide spectrum of military missions, enabling it to perform both air superiority and strike operations under diverse combat conditions. Its mission architecture is modular, supporting roles such as interception, close air support, suppression of enemy air defenses (SEAD), precision ground strike, and anti-ship missions.
The aircraft is configured with ten external hardpoints, capable of carrying a total payload of up to 7,700 kg (17,000 lb). These stations support a range of weapons:
- Air-to-air missiles: AIM-120 AMRAAM, AIM-9X Sidewinder, MBDA Meteor (with active radar homing and beyond-visual-range engagement exceeding 100 km).
- Air-to-ground munitions: JDAMs, GBU-12 Paveway II, CBU-105 sensor-fused weapons, and Korean-made KGGB glide bombs.
- Cruise missiles: Taurus KEPD 350, with a range of 500 km (310 mi), enabling deep-strike capability against high-value targets.
- Anti-ship missiles: SLAM-ER and Korean-developed SSM-700K Haeseong, ensuring maritime strike capabilities.
The KF-21 integrates electronic countermeasure pods, targeting pods, reconnaissance sensors, and IRST for multi-domain situational awareness. The AESA radar allows simultaneous tracking of more than 20 airborne targets and enables interleaved air-ground operations without performance compromise.
In peacetime, the KF-21 will serve in quick reaction alert (QRA) roles, border patrols, and training missions. During conflicts, its primary roles shift to offensive counter-air (OCA), defensive counter-air (DCA), deep interdiction, and support of naval task forces. With its strike radius, precision munitions, and networking capabilities, it is also suited for SEAD/DEAD missions, especially in hostile air defense environments.
Compared to competitors:
- The Eurofighter Typhoon has superior supercruise capability and agility but lacks stealth shaping.
- The Dassault Rafale offers excellent multirole integration but at higher operational cost.
- The F-35A has internal weapon bays and stealth but is less agile in visual-range combat and has higher lifecycle costs.
- The Gripen E provides electronic warfare strength but has reduced range and payload.
The KF-21 Block II, with internal bays and enhanced sensors, will reduce radar signature and expand first-strike viability. The modular software architecture also allows integration of new weapons and subsystems without complete redesigns, improving sustainability in long-term deployment.
Export potential is a strategic goal of the program. Indonesia, despite funding delays, remains a co-developer. Malaysia, the Philippines, and Colombia have shown interest, but no firm deals have been signed as of 2025. South Korea aims to present the KF-21 as a cost-effective alternative to F-35A for allied air forces unable to afford fifth-generation platforms.
The KF-21 remains in active pre-series evaluation with six flying prototypes. Production is scheduled to begin in late 2026, with operational squadrons for the Republic of Korea Air Force (ROKAF) expected by 2028. The aircraft will replace KF-16, F-4E Phantom II, and F-5E Tiger II units. The platform is planned to remain in frontline service well beyond 2045, with ongoing upgrades and variant expansions.
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