The U.S. Air Force has revealed more of its long-term plan for acquiring a new model of the venerable F-15 Eagle fighter.

The flying branch is proposing to buy eight F-15EXs in 2020 and 18 each in the years 2021 to 2024, for a total of 80 new Eagles over the next five years, according to Air Force budget documents.

In the years after 2024, the Air Force has proposed to purchase another 64 F-15EXs for a total fleet of 144 of the upgraded, twin-engine fighters. The new planes in total would cost $7.8 billion.

The likely result, by the mid-2020s, would be a mixed fleet of old and new Eagles. As of early 2019 the Air Force operated 217 F-15E Strike Eagles for air-to-ground missions and 240 F-15Cs and Ds for air-to-air missions and training.

Nine squadrons fly the F-15C and D -- three in the active force and six in the Air National Guard. Another nine units, all but one of which are in the active force, operate the F-15E.

The F-15Es mostly date from the 1990s. The F-15Cs and Ds are 1970s- and 1980s-vintage aircraft. Each has racked up more than 8,300 flight hours, on average, the Air Force stated. That's roughly double the service-life that builder McDonnell Douglas, later Boeing, designed the plane for.

The F-15Es have tougher airframes than the F-15C and Ds have. The flying branch is upgrading the Strike Eagles with new radars as part of a plan to keep the fighter-bombers in service for decades to come.

Many of the F-15Cs, however, suffer from structural fatigue that limits their top speed. "Many F-15C/Ds are beyond their service life and have serious structures risks, wire chafing issues and obsolete parts," the Air Force stated. "Readiness goals are unachievable due to continuous structural inspections, time-consuming repairs and ongoing modernization efforts."

The F-15C and Ds "won't make it to 2030," Air Force major general David Krumm, the service's director of strategic plans and requirements, told Air Force magazine reporters John Tirpak and Brian Everstine.

The Air Force plans to swap out old Eagles for new ones in squadrons that already fly F-15s, thus minimizing disruption to operations. Replacing the oldest and more tired F-15Cs and Ds for combat missions could allow the healthier old Eagles to continue flying in the training role.

Converting from the F-15C/D to the F-15EX would require 18 months, Krumm said, compared to up to three years for a squadron to convert to new F-35 stealth fighters.

The F-15EX features better sensors, jammers and avionics than the F-15C and D have, but the airframes -- and thus training and support requirements -- are similar across the different models. "F-15EX logistics, maintenance and training will heavily leverage existing the F-15 infrastructure," the Air Force stated.

But upgrading an F-15C unit to the F-15EX could require some changes to the squadron's training and staffing. An F-15C is a single-seat, single-role plane. "The last gunslinger," in the parlance of one high-profile magazine feature.

The F-15EX, by contrast, is a two-seat plane. "With two seats, it will be multirole-capable and operable by one or two aircrew," according to the Air Force. Assigning a new, two-crew air-to-ground mission to the existing F-15C squadrons could require the Air Force to add aircrew to the units and alter their training and deployment profiles.

The Air Force also is considering arming the new F-15EXs with a hypersonic missile, potentially making the Eagle force the branch's first units to perform a new kind of specialist strike mission. Hypersonic missiles likely would be too big to fit inside the F-35's weapons bay.

There's precedent for transforming an old fighter design into a new, hypersonic striker. The Russian air force in 2018 organized a small contingent of modified MiG-31 interceptors and armed them with fast, long-range air-to-surface rockets for a niche strike role.

Critics in the media pointed out that the Air Force didn't actually want the new F-15s. Instead, the U.S. Defense Department's Cost Assessment and Program Evaluation office, one of the military's many analytical organizations, convinced the office of acting defense secretary Patrick Shanahan to add eight F-15EXs to the department's 2020 budget proposal.

Other Eagle skeptics include retired Air Force generals who prefer for the flying branch to buy only radar-evading F-35s. The blocky F-15 lacks stealth features.

But Krumm pointed out that an F-15EX should cost around half as much to maintain as an F-35 costs. The lower sustainment cost and the ease of adding F-15EXs to the force both could be powerful arguments in favor of the acquisition -- to say nothing of the new Eagle's potential hypersonic-strike capability.

Congress must approve the Air Force's budget proposal. The F-15EX could be the subject of lively debate as lawmakers begin weighing spending options.

David Axe serves as Defense Editor of the National Interest. He is the author of the graphic novels  War Fix, War Is Boring and Machete Squad.