No one watching Patrick Henry’s polished displays in the F-15 at Paris in 1975 could have guessed that the aircraft would still be in production 44 years later, albeit under the auspices of Boeing and not McDonnell Douglas. That Boeing would be about to build a new variant of the mighty Eagle for the U.S. Air Force would have strained credibility even further, 18 years after the last USAF F-15s were ordered. And yet that is exactly what is happening, with the USAF’s Fiscal Year 2020 budget request including $1.1 billion to buy the first eight of 144 planned F-15EX aircraft. Under this plan, the Air Force is scheduled to receive two F-15EXs in 2022, six more in 2023, and a total of 80 aircraft over the next five years.

Nor would anyone have been able to predict that others would be carrying on the patient work of McDonnell Douglas flight operations director Joseph Dobronski, who demonstrated the aircraft with potential customers in the back seat. These efforts have culminated in 108 recent Eagle sales to Saudi Arabia and Qatar, and these sales have in turn contributed to some $5 billion of investment in the development of ever more advanced models of the F-15.

Some years ago, the USAF staked its future on an "all stealth" combat aircraft force based on the Northrop B-2, and Lockheed’s F-117, F-22, and F-35. Cost constraints and the rise of stealthy stand-off weapons led to a more realistic bomber mix, but the air force remained wedded to the idea that it would procure only low observable (stealthy) fifth-generation fighter aircraft, phasing out fourth-generation fighters like the F-15 and F-16 as quickly as it could replace them.

Cost caps brought production of the F-22 to a premature halt, however, while delays to the Joint Strike Fighter have forced the USAF to operate fourth-generation fighters for far longer than it had planned.

The USAF soon came to realize the value of operating fourth- and fifth-generation aircraft together to provide a more lethal and cost-effective force than would be possible by using fifth-generation platforms alone. With the right datalinks and tactics, the distinct advantages of each generation could be leveraged, while at the same time avoiding or mitigating against the inherent design weaknesses of either generation.

While fifth-generation platforms like the F-35 can use stealth and advanced sensors to operate within contested airspace, aircraft like the F-15 offer higher performance and large external weapons payloads.

In contested regions, teaming F-15Cs with fifth-generation aircraft can make the most out of both platforms. A combined force incorporating both generations is more lethal and cost-effective and will create complex problems for adversaries attempting to defend against it.

Under the U.S. Air Force’s original plans, 749 F-22s would have completely replaced the F-15C, but the F-22 buy was prematurely terminated in 2009, leaving a significant shortfall in air superiority fighters (a role for which the F-35 is poorly suited). The USAF, therefore, continues to operate 234 F-15C/D aircraft primarily from six CONUS-based Air National Guard installations and two active-duty bases at Lakenheath in the UK and Kadena in Japan. These are rapidly running out of life, however, and the F-15EX promises to plug the gap quickly and easily, boosting the USAF’s shrinking force structure, and reinforcing its air defense capabilities without disrupting the larger F-35 procurement initiative

The F-15X can use approximately 70 percent of the existing Eagle spares inventory and is compatible with the F-15C/D’s existing ground support equipment and infrastructure. Conversion for pilots and groundcrew would be quick and easy, allowing an active duty F-15C unit to convert from the F-15C/D to the F-15EX in 12 months, compared to 18 months for the F-35.

The F-15EX would also allow today’s F-15C/D units to embrace a multi-role mission rather than the pure air-to-air role that they have traditionally been assigned. The F-15EX is a derivative of the latest export standard Advanced Eagle. Successive export variants of the F-15 have progressively introduced new capabilities to the basic two-seat F-15E Strike Eagle airframe, culminating in the so-called Advanced Eagle.

The first step on the road to the Advanced Eagle was the development of the F-15K Slam Eagle for Korea, which introduced a number of features not found on the USAF F-15E, including an AAS-42 Infra-red search and track system, a refined Tactical Electronics Warfare Suite and the AN/APG-63(V)1 radar. This incorporates digital processing, making it upgradable to APG-63(V)3 standards via an Active Electronically Scanned Array (AESA) antenna upgrade. The F-15K also introduced new weapons, including the AGM-84K SLAM-ER, the AGM-84H Harpoon Block II, and the KEPD 350.

The F-15SG for Singapore was fitted with an AN/APG-63 (V)3 AESA radar and a BAE Systems Digital Electronic Warfare System (DEWS). The F-15SA for Saudi Arabia was the first variant to actually use the Advanced Eagle/F-15 Advanced name. The F-15SA introduced a fully digital cockpit and a digital fly-by-wire flight control system. The latter allowed the reactivation of two outboard underwing hardpoints, which had been deactivated early in the F-15 program due to stability issues.

Qatar has ordered the further improved F-15QA Advanced Eagle, with large area displays and other improvements, and the F-15EX is closely based on this configuration. The USAF approached Boeing to acquire a similar F-15 variant, then dubbed F-15X, in early 2017. This proposed USAF F-15X (known as the F-15CX in single-seat form or as the F-15EX in two-seat form) combined the advanced features of the F-15SA and F-15QA with an AN/APG-82 AESA radar—as used by upgraded F-15E Strike Eagles. The new aircraft will offer a 20,000-hour service life and dramatically reduced hourly operating costs compared to the legacy F-15 or to the F-35A.

In 1975, Aviation Week cited the F-15 as an example of a new generation of burgeoning U.S. technology, dismissing European competitors as “sadly representing the final refinement of an earlier generation.” Today it is the F-15 that represents the ultimate enhancement of the previous generation of fighters, and yet it is successfully winning orders in the face of competition from aircraft like the Dassault Rafale and Eurofighter, which are, in many respects, representative of an entirely new generation.

Even more remarkably, the new F-15EX stands ready to write a new chapter in the Eagle’s illustrious history.