Experimental, Modern Day

F-15SE “Silent Eagle” – the Stealth F-15

With all the talk of the F-15EX, we thought we’d take a look at an older upgrade to the F-15, the F-15SE Silent Eagle.

Despite being a 50-year old platform that has provided stellar service to its users, the F-15 once again finds itself being discussed in aviation circles. This is because of the newest iteration of the aircraft, the F-15EX Eagle II.

However, Boeing have attempted to offer major upgrades to the F-15 for decades, with one of these being the unsuccessful F-15SE Silent Eagle, which gave the F-15 platform fifth-generation features, and a significantly reduced radar cross section all the way back in 2009.

So, what did this aircraft do, and why wasn’t it chosen for production?


The F-15

The F-15 was developed in the 1960s and ’70s as an air superiority fighter for the US Air Force, as it lacked a truly dedicated aircraft in for this role.

Part way through its development (when it was known as the F-X) the US discovered the existence of the Soviet MiG-25. This sent the US into a bit of a frenzy, and prompted McDonnell Douglas (its designer) into increasing the capabilities of the F-X.

The F-15 would evolve through a number of different models, like the F-15A and the F-15C, but in the late 1980s the first new major model arrived; the F-15E Strike Eagle.

An early F-15A in 1976.
An early F-15A in 1976.

As a strike fighter, the Strike Eagle could perform highly accurate attacks against ground targets while still maintaining an air superiority capability.

Like the standard F-15, the Strike Eagle was produced in a number of variants, with most being types customised to certain customers.

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There have been many upgrades to the F-15, but since the Strike Eagle in the 1980s the platform has not had a major variant introduced. All this changed in 2018 though, when the US Air Force and Boeing began work on a new variant based on the F-15QA.

F-15E Strike Eagles in flight.
F-15E Strike Eagles. Note the conforming fuel tanks next to the air inlets.

It would receive significant upgrades to its tracking systems, radar, electronic warfare capabilities, and could carry up to 22 air-to-air missiles.

Named the F-15EX, this aircraft would allow the F-15 to integrate into modern and future systems. It is a two-seater, but can be flown by a single crewmember, and it will be able to operate with unmanned wingman systems.

The F-15EX will essentially serve as a “missile truck”, where more advanced units, such as the F-35, will travel ahead and select targets. The F-15EX will then serve as a missile dispenser against these targets.

F-15EX test aircraft in flight.
The F-15EX. Boeing has hinted that this version may have radar absorbent paint in some areas.

However, there is actually a missing variant of the F-15 that never came to be; the Silent Eagle.

The Silent Eagle

The Silent Eagle was a new variant of the F-15 proposed by Boeing in the late 2000s. This aircraft, named the F-15SE ‘Silent Eagle’, was to bring 5th generation fighter technology and stealth characteristics to the F-15 airframe.

This latter point was one of the main focuses of the project; to reduce the radar cross section of the rather bulky F-15.

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It was hoped that the Silent Eagle could refresh the F-15 airframe enough to serve as a less capable, but much cheaper alternative to Lockheed Martin’s new F-35 Lightning II for the export market.

F-15SE Silent Eagle mock-up.
F-15SE Silent Eagle mock-up. Image courtesy of Boeing.

The F-15 platform was already in wide-scale use around the world, so it would be easy for those users to upgrade to the F-15SE as their logistics, training and supply chains wouldn’t change much.

Boeing was never hoping to beat the F-35’s stealth, but with the F-15 having a radar cross section bigger than a B1-B Lancer, they could definitely make it better.

The biggest change was transforming the conformal fuel tanks that ran down the sides of the aircraft into weapons bays. These internal bays would greatly simplify the surface of the F-15 and produce less radar returns.

Internal weapons bays are key characteristics on true stealth aircraft, and can be found on ones like the F-22 Raptor, F-35 Lightning II and the Chinese Chengdu J-20.

Of course, this did reduce the aircraft’s fuel load and range, but it could be boosted with external tanks when stealth was not as important.

The two large vertical stabilisers were modified too, being angled outward at 15-degrees. This alone reduced the radar cross section, but it also generated a small amount of additional lift and slightly increased the range of the aircraft.

F-15SE missile launch.
F-15SE launching a missile out of its internal weapons bay. Image courtesy of Boeing.

In addition to these features, the F-15SE Silent Eagle was also given a coat of radar-absorbent paint. Instead of reflecting radar waves away from the enemy radar receiver, it simply absorbs a small portion of them.

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All in these changes are estimated to have given the Silent Eagle a radar cross section one-fifth of the standard F-15.

Of course, the aircraft received electronic upgrades too. One change was moving the infrared search and track sensor (which can detect targets without using its radar) into the fuselage from a wing pod.

F-15SE Silent Eagle model in wind tunnel.
A wind tunnel model of the SIlent Eagle. Note the bulges either side of the inlets, these are the conformal weapons bays. Image courtesy of Boeing.

It was planned to be equipped with Raytheon’s APG-82 AESA radar, which they say can find and track multiple targets simultaneously and link with other units to create a detailed view of the battlespace. This radar is only now being installed on the F-15EX a decade later.

A multitude of weapons could have been carried on board, including laser guided bombs, as the Silent Eagle retained the Strike Eagle’s ground-attack capabilities. It would have also been capable of carrying AGM-88 high-speed anti-radiation missiles (HARM), which are able to hunt enemy radars by following their radar emissions.

Combining this with its partial stealth characteristics, the Silent Eagle could have flown SEAD missions (Suppression of Enemy Air Defenses).

F-15SE pilot in cockpit, wearing a  Joint Helmet Mounted Cueing System.
The F-15SE was to come with a Joint Helmet Mounted Cueing System. This helmet tracks the pilot’s head movement, and overlays information in the visor. It can be slaved to the aircraft’s weapons. Image courtesy of Boeing.

Power was to come from two Pratt & Whitney F100 turbofans that produced 29,000 lbs of thrust each. Depending on the source, the Silent Eagle was either as fast, or slightly slower than the standard F-15.

So with all these upgrades, why was the F-15SE Silent Eagle abandoned?

One word: F-35.

Project Failure

Boeing build a demonstrator F-15 Silent Eagle and first flew it in July of 2010. A missile-launch was successfully achieved in November 2010. At the time, it was likely the most advanced non 5th-generation fighter in the world.

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But no matter what Boeing did, under the layers of radar absorbent paint was still an F-15; a large, loud and powerful aircraft that simply couldn’t compete with purpose-built stealth aircraft.

Boeing showed the aircraft to various nations who were looking to purchase new fighter and strike fighters, but it came with a rather costly price tag of $100 million per aircraft.

Silent Eagle mock-up.
The Silent Eagle could have still performed as an air superiority fighter, while retaining its strike capabilities. Image courtesy of Boeing.

South Korea very nearly purchased the Silent Eagle during the F-X fighter program, which sought procure them advanced new aircraft to replace older models like the F-4 Phantom.

The Silent Eagle faced off against the F-35 and Eurofighter Typhoon and nearly won, as it was cheaper and easier for South Korea to transition into, but eventually they decided to go with the F-35 instead.

This was partly motivated by a deal between South Korea and Lockheed for South Korea to use advanced technologies used in the F-35 in their new KF-X domestic fighter (now KF-21).

Israel showed interest in the Silent Eagle, as did Japan and Saudi Arabia, but all of them eventually went with the F-35 instead.

F-35 Lightning II from the side.
The F-15SE Silent Eagle simply couldn’t compete with the more advanced F-35.

With this disappointing defeat at the hands of Lockheed, Boeing cancelled the program in 2015.

A few years later, Boeing would unveil their new F-15EX, which is already being procured by the US Air Force.

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So when you see or hear about this new iteration, remember that the Silent Eagle walked so the F-15EX could run.