KAI KF-21 Boramae


The KAI KF-21 Boramae is a South Korean twin-engine fighter aircraft. The initial goal of the program was developing an indigenous multirole fighter for the Republic of Korea Air Force. The airframe of the Blocks I and II adopts semi-stealth technology with partially embedded external weapons, with full stealth—including internal bays—and MUM-T capabilities planned for the Block III or KF-21EX variant.
As South Korea's second domestic fighter jet development program following the FA-50 series, the KF-21 makes the nation the fourth in the world—joining the United States, Russia, and China—to produce an aircraft featuring stealth avionic architecture.
The Block I program is led by the South Korean government, which holds 60% of the shares. An additional 20% is held by the manufacturer Korea Aerospace Industries, with Indonesia holding the final 20% stake. Later, in August 2024, Indonesia's stake was reduced to 7.5% due to Indonesian government request. The Block II program, which focuses primarily on the serial integration of air-to-ground and air-to-ship armaments, will be executed solely by South Korea through early 2027. The Block III, or KF-21EX, program will be carried out with newly formed international partners.
In April 2021, the first prototype was completed and unveiled during a rollout ceremony at the headquarters of KAI at Sacheon Airport. It was named the Boramae. The first test flight was on 19 July 2022. The serial production started in July 2024. 40 Block I units are scheduled for delivery beginning in March 2026; the ROKAF expects to complete additional 80 Block II units deployment by 2032. It will also be available for export. The ROKAF will initially begin replacing its F-4D/E Phantom II and F-5E/F Tiger II jets with KF-21s, which will eventually also replace the aging KF-16s and F-15Ks in service.

Background

The KF-X project, intended to produce an advanced multirole jet fighter to replace South Korea's aging F-4D/E Phantom II and F-5E/F Tiger II aircraft, was first announced in March 2001 by South Korean President Kim Dae-jung at a graduation ceremony of the Korea Air Force Academy. In 2002, research and development requirements were determined by the Joint Chiefs of Staff. The project was considered extremely ambitious; the Korea Institute for Defense Analyses, a state-owned defense ministry think tank, expressed serious doubt about the country's ability to complete such a complex undertaking. The ROK Air Force expressed concerns that a delay or failure of the KF-X program could compromise its future security readiness. The program evaluation and feasibility phase was fraught with delays and postponements while its economic cost and potential risks were debated, but the project received renewed interest after a feasibility study in 2008 and attacks by North Korea in 2010. The feasibility study concluded that, despite the project's risks and the significantly higher per-unit cost compared to foreign alternatives, developing the domestic defense industry was considered a matter of national importance and expected to generate positive ripple effects across high-tech sectors. The study still required a foreign partner to mitigate financial risks.
To fulfill the foreign partnership requirement, South Korea first offered Turkey a 20% stake in the KF-X project; however, Turkey declined, seeking greater control over the initiative. The South Korean government expected to commit 60% of the development cost, with the remaining 40% to be covered by domestic and foreign partners. As an alternative, Indonesia agreed to fund the 20% share through Indonesian Aerospace and purchase 50 of the approximately 150–200 planned aircraft. On 15 July 2010, South Korea and Indonesia formalized a partnership, launching the KF-X development program with a target to begin aircraft deliveries in 2026.

Design and development

The initial goal for the program was to develop a single-seat twin-engine
multirole fighter with stealth capabilities exceeding the Dassault Rafale, Eurofighter Typhoon, F-16 Fighting Falcon, but less than those of the Lockheed Martin F-35 Lightning II. The Weapon Systems Concept Development and Application Research Center of Konkuk University advised that the KF-X should be superior to the F-16 Fighting Falcon, with 50% greater combat range, 34% longer airframe lifespan, better avionics, active electronically scanned array radar, more-effective electronic warfare, and data link capabilities. Their recommendations specified approximately of thrust from two engines, supersonic interception and cruising capabilities, and multi-role capabilities. The Block I configuration requirements were later finalized by the ROKAF and DAPA to a 4.5 generation fighter with semi-stealth capabilities.

Configuration development and selection

At the start of configuration development, South Korea had secured about 65% of the technologies required for the KF-X program, prompting efforts to both develop the remaining capabilities domestically and seek international partnerships to fill critical gaps. The Agency for Defense Development proposed two primary KF-X concepts: C103, resembling the F-35, and C203, modeled after European fighters with forward canards. These configurations were outlined for potential partnership with the United States or European partners.
On the other hand, Korean Aerospace Industries and the Defense Acquisition Program Administration proposed the C501, a third design that prioritized cost reduction through a smaller, single-engine configuration. While more affordable, the C501 offered lower performance compared to the F-16 and was considered unsuitable for operations over Indonesia's large airspace.
Besides, ROKAF favored a twin-engine design for its superior combat performance, enhanced safety, and larger airframe capable of accommodating future upgrades. These enhancements could eventually allow the aircraft to be reclassified as a fifth-generation fighter, whereas the C501 remained closer to a fourth-generation standard. By December 2015, the development team had begun designing the KF-X, with initial research and design based on the C103 configuration. After receiving the basic drawings of the C103, C104, and C105 from ADD, the team built an experimental model of the C105 and conducted wind tunnel tests, which were then repeated on the newly designed C107 featuring a larger airframe and higher maximum takeoff weight. Around the same time, an audit in 2015 estimated that 87% of the technologies required for the KF-X program had already been secured.
For the remaining technologies, Lockheed Martin agreed with South Korea on transfer of 25 technologies to KAI under an offset trade agreement linked to a contract to acquire 40 F-35As. However, the U.S. later disapproved the transfer of four core technologies - AESA radar, infrared search and track, electro-optical targeting pod, and radio frequency jammer technology—requiring South Korea to develop these capabilities domestically. For the rest of 21 technologies, the U.S. government approved the transfer to South Korea in December 2015, including avionics operational flight program software, head-mounted displays, and aerial refueling control units. Despite its previous partnership on the T-50 design, Lockheed Martin’s role in the KF-21 Boramae program was limited. While the company provided guidance to KAI's engineers during the KF-21's development and testing, Lockheed does not possess any design rights to the fighter jet.
For the initial armaments, the U.S. government delayed integration approval for the Raytheon AIM-120 AMRAAM and AIM-9X Sidewinder, South Korea moved decisively to secure alternatives, quickly integrating the European MBDA Meteor and Diehl IRIS-T to maintain the KF-21's development timeline. These European systems were selected to diversify the platform's strike options and arm initial production blocks while South Korea fast-tracks the development and integration of its own indigenous air-to-air missiles; notably, the U.S. later granted approval for its weapons systems only after the European integration had been finalized.
Following approximately two years of wind tunnel experiments and continued domestic technology development, the C109, jointly proposed by ADD and KAI, was selected in 2018 as the prototype design for the KF-X. The preliminary design was finalized in June 2018, and a critical design review in September 2019, examining 390 technical data sets, confirmed that the KF-X met ROKAF's requirements.
KAI
C501		ADD
C103		ADD
C104		ADD / KAI
C105		ADD / KAI
C107		ADD / KAI
C109
Empty weight9,300 kg
10,900 kg
11,090 kg
11,100 kg
11,600 kg
11,800 kg
Max takeoff weight20,900 kg
24,000 kg
24,100 kg
24,500 kg
25,800 kg
25,400 kg
Internal fuel3,600 kg
5,400 kg
5,400 kg
5,400 kg
>5,400 kg
5,400 kg
Wingspan9.8 m
10.7 m
10.7 m
11.0 m
11.2 m
11.2 m
Length15.2 m
15.7 m
15.7 m
16.0 m
16.8 m
16.9 m
Wing area37.1 sq m
42.7 sq m
42.7 sq m
42.7 sq m
46.5 sq m
Engine1 × P&W F100
or GE F110		2 × EJ200
or GE F414		2 × GE F4142 × GE F4142 × GE F4142 × GE F414
Maximum speedMach 1.89Mach 1.97Mach 1.97
Hardpoints91010101010
Weapons bay-Space securedSpace securedSpace securedSpace securedSpace secured