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Just How Stealthy Is The U.S. Air Force’s New F-15?

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Pictured – An F-15EX Eagle II assigned to the 40th Flight Test Squadron, 96th Test Wing, out of Eglin Air Force Base, conducts aerial refueling operations above Northern California on May 14, 2021. U.S. Air Force photo by Ethan Wagner.

Nearly a decade before the U.S. Air Force, in a surprise move, tapped Boeing to build potentially hundreds of new F-15EX Eagle IIs—an upgraded version of the five-decade-old classic warplane—there was another new F-15 in the offing.

The Silent Eagle was a stealthy F-15. At least, it was as stealthy as Boeing’s engineers could make the big, blocky, twin-engine jet with its radar-reflecting right angles and round engine nozzles.

It’s possible—likely, even—that after a decade some of the Silent Eagle’s advancements have made their way onto the F-15EX. The new Eagle II is not a stealth fighter. But it very likely is a stealthier fighter. And that could matter a lot in wartime.

In early 2008, Boeing realized it had a problem. Lockheed Martin’s single-engine F-35 stealth fighter was scooping up more and more international orders in addition to the Air Force’s own huge requirement—1,700, nominally—for the type.

Boeing had not developed a new fighter since its X-32 lost out to the X-35—Lockheed’s demonstration for what would become the F-35—in the Air Force’s Joint Strike Fighter competition.

Desperate to compete, the Chicago plane-maker dusted off the F-15’s 1960s-vintage blueprints. Engineers looked for every opportunity to improve the fighter’s stealth. They chose three main facets of the design.

Underwing stores are a major source of radar reflectivity, so Boeing added internal weapons bays to the fuselage-hugging conformal fuel tanks that many F-15 models are compatible with.

A fighter also tends to give away its position when it emits radar energy. To boost the F-15’s emissions stealth, Boeing added a passive, non-emitting infrared sensor than a Silent Eagle’s pilot could use instead of radar.

Perhaps most importantly, engineers applied radar-absorbing materials to the front of the Silent Eagle. The RAM helped to give the new F-15 a frontal radar-cross-section roughly similar to the frontal RCS of an F-35, Boeing initially claimed.

The company eventually walked back that claim. Still, it was apparent the Silent Eagle possessed some stealth qualities in certain situations and from certain angles.

But actual fighter-operators never got a chance to test that proposition. Boeing never sold the Silent Eagle in its full configuration. Yes, it continued to win sizable orders for new F-15s—most notably from Saudi Arabia and Qatar—but all the buyers opted for more modestly-enhanced models.

The Saudi F-15SA and Qatari F-15QA both have infrared sensors. But no one bought the internal weapons bays. And it’s not clear the SAs and QAs have RAM—the Pentagon and U.S. State Department are still pretty picky about which Middle East countries they will export stealth technology to.

The U.S. Air Force’s 2019 decision to buy new F-15EXs surprised many observers, as the flying branch since 2001 had bought only F-22 and F-35 stealth fighters.

Then-defense secretary Jim Mattis ordered the Air Force to buy the Eagle II in order quickly to replace 1980s-vintage F-15Cs in squadrons whose main role is defending the continental United States from intruders. Aging F-15Cs also fly from USAF bases in the United Kingdom and Japan.

Today the Air Force has a requirement for at least 144 F-15EXs to replace around 250 F-15C/Ds—it’s already paid for 20. The service also has floated the idea of buying addition Eagle IIs, at a cost of around $88 million per plane, eventually to replace the 220 F-15E fighter-bombers it acquired starting in the early 1990s.

As the Air Force mulls a bigger future fleet of Eagle IIs, it’s also adding missions to the type. The F-15EX unlike other fighters is big enough to carry the Air Force’s big new hypersonic land-attack missile and the “out-size” air-to-air missile it’s developing.

The Eagle II also packs a powerful radar-jammer called the Eagle Passive-Active Warning Survivability System, or EPAWSS, which could allow the F-15EX to blind enemy radars and clear a path through air-defenses for other aircraft.

The F-15EX like the F-15SA and F-15QA has an infrared sensor. So the Eagle II is stealthier than an F-15C or F-15E is. But the EX like the SA and QA lacks internal bays. So it’s not as stealthy as a Silent Eagle would have been.

But what about radar-absorbing material? When you’re flying into battle against Russian or Chinese forces, every degree of protection matters.

It wouldn’t be hard for Boeing to add to the Eagle II the same RAM that it had planned for the Silent Eagle. In terms of observability, the F-15EX might represent a compromise between an Eagle and a Silent Eagle.

The Air Force declines to comment on this aspect of the F-15EX’s design. Boeing likewise is mum. When Combat Aircraft reporter Rob Coppinger asked Boeing about RAM on the Eagle II, the company told him that information is “likely to be classified.”

This article was written by David Axe from Forbes and was legally licensed through the Industry Dive publisher network. Please direct all licensing questions to legal@industrydive.com.

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