A quarter of a century after the requirement emerged, South Korea is within months of finalizing its F-X fighter program. A decision on F-X Phase 3 will be made by June, and three international companies are vying for the contract.

The Pentagon officially notified Congress of the potential sale of 60 Boeing F-15 Silent Eagles or Lockheed Martin F-35s as South Korea's minister of foreign affairs, Yun Byung-Se, was meeting earlier this month with U.S. Defense Secretary Chuck Hagel at the Pentagon. These formal offerings coincidentally come as North Korea grows increasingly bellicose against its southern neighbor and the U.S.

Boeing is proposing a hybrid sale where the F-15 Silent Eagle would be excluded from the foreign military sale of the subsystems, and thus not disclosed in the price. The company says the cost of the subsystems—the active, electronically scanned array radar; infrared search-and-track targeting system; secure radios; advanced displays and mission-planning systems as well as an airborne GPS receiver is $2.408 billion.

The price of the actual aircraft was not disclosed. However, Brad Jones, who managed the concept at its unveiling in 2009, said at the time that the cost of a new Silent Eagle would be roughly $100 million.

Lockheed Martin's offering of F-35As is priced at $10.8 billion, including the aircraft, each with a Pratt & Whitney F135 engine and nine spares, and the 3F software package, says Eric Schnaible, a company spokesman. The 3F is slated for use by the U.S. Air Force, and includes more weapons and offensive capabilities than the rudimentary 3I, the first release slated for foreign customers.

The third available type is the Eurofighter Typhoon, and operational issues are not the only factors in Seoul's decision. Korean Aerospace Industries (KAI) would likely build, or at least assemble, 48 Typhoons if South Korea bought that aircraft, according to industry officials. KAI would also be assured of at least as large a role for the Silent Eagle. The Korean manufacturer is already building major assemblies of the F-15K, which won the earlier F-X Phase 1 and 2 competitions, held last decade to partly fill a requirement that first emerged in 1988.

South Korea could make the center wing box and tail surfaces of the F-35 as a second-source supplier for the global program, says Lockheed Martin. The company is negotiating with Japan on how much of the stealth fighter will be built in that country, which chose it in late 2011. Japan will also have an assembly line but South Korea is not seeking one.

In the South Korean competition, as elsewhere, rivals have offered a range of options for local production, officials say. It is always cheaper to fully import fighters, taking them from factories that are well experienced in building them, but if the customer wants to pay for a lot of local production, it can do so. One question will be how much work there will be in making detailed parts and building assemblies, as distinct from simply mating parts sent from the U.S. or Europe.

The newer F-35 and Typhoon offer higher manufacturing technology than the F-15 can. The F-35, in particular, would bring the technology for building to the tight tolerances required for a stealthy aircraft.

Eurofighter cites its freedom from U.S. regulations as a strength for technology transfer. The Typhoon could serve as the basis for a proposed indigenous fighter, the KF-X, that is sought in the future, or Eurofighter could supply technology for an all-new aircraft. Lockheed Martin and Boeing are also offering to support development of the KF-X. They are handicapped by the U.S. government's restrictions, but Eurofighter's governments will surely not give it a completely free hand.

The Silent Eagle, based on the F-15E, is stealthier, thanks to limited internal weapons carriage and tail fins counted outward by 15 deg. When stealth is unneeded, after enemy air defenses have been suppressed, it can be adapted to carry larger loads. Boeing signed an agreement in late 2010 with KAI to design and build the conformal weapons bays for the aircraft.

An F-35 selection, by contrast, would bring with it the cachet of South Korea's participation in the largest fighter procurement in history. The per-unit price of the F-35 would likely be higher. But, interoperability with allied forces would be simplified by operating identical platforms. Also, the F-35 would offer a higher-end stealth solution for South Korea to complement its purchase of F-15Ks already on order.

The drawbacks for an F-35 sale, however, could include the following: limited payload versus the F-15 Silent Eagle's capacity and the uncertainty associated with maintenance and sustainment cost. Also, the Pentagon is hopeful that development will wrap up by October 2017, but this could be subject to change if a major problem crops up in flight testing. So, there could be risk associated in the delivery.

Risk, however, is not minimal for the Silent Eagle, which has yet to conduct a full suite of testing.