The F-15SE Silent Eagle could have an RCS less than one-fifth of the original F-15 Eagle but it was never built. Here’s why.

Reducing the radar cross-section (RCS) of the Eagle was the number one factor in the F-15SE Silent Eagle’s tweaked design.

The F-15EX Eagle II, the most advanced version to date of the iconic F-15 fighter, will fall in on the mission of the F-15C/D, but since in the 2030s the F-15E comes to the end of its service life the EX could shift to more of the Strike Eagle model’s ground-attack mission. However, for now the US Air Force (USAF) has officially said it intends to fly the aircraft with a single pilot although the EX will feature two cockpit positions.

In July 2020, the USAF awarded Boeing a contract to build the first jets. Future plans call for as many as 144 aircraft.

As told by Bertie Simmonds in his book F-15 Eagle, the F-15EX Eagle II has won a production contract, but Boeing has tried before to land a lucrative deal with a number of advanced F-15 derivatives. There were plans to produce a newer/updated version of the F-15 fighter airframe adopting some 5th-generation capabilities and one was called ‘Silent Eagle’. In early 2009, Boeing put on display its F-15SE demonstrator which amalgamated certain elements of a 5th-generation fighter into the proven Eagle airframe. This was to be a multirole Eagle, but one with all the abilities of the fighter versions then in service but with some big improvements.

Reducing the radar cross-section (RCS) of the Eagle was the number one factor in the Silent Eagle’s tweaked design. To this end, Boeing found that a canting out those huge twin vertical tails by 15° helped reduce the F-15’s RCS. Other technology, such as the use of radar-absorbing materials (RAM) as used in 5th-generation fighters and four conformal weapons bays (CWB) which would be in the same location as the former FAST conformal fuel tanks, would reduce the big fighter’s radar signature still further. For `non-stealthy’ missions, weapons could still be hung from the various wing and fuselage hard-points.

Under the skin other advances included a full fly-by-wire system, the Raytheon APG-82 AESA radar as well as an up-to-the-minute electronic warfare suite packaged in the main airframe. The aircraft would also field advanced targeting pods, including an infrared search and track system. Overall, it was considered that such new-build Silent Eagles — possibly taking advantage of the latest variants of both the Pratt & Whitney F100 and General Electric F110 would have improved performance, most notably in longer range thanks to the aircraft being lighter and more fuel efficient.

The end result was an F-15 that — compared to the original machine could (and let’s stress the ‘could’ here) have an RCS one-fifth, or less than one-fifth of the original F-15 Eagle. This is of course dependent on the aspect of the airframe to the radar that is looking at it: impressive, but not in the same league of a 5th-generation fighter with stealth ‘built in’ to the aircraft’s blueprints. It’s also worth noting here that thanks to political pressures, the aim was be stealthy’ only against air-to-air threats — not to more advanced ground radars, should these ‘Silent Eagles’ be sold abroad.

Despite possible customers at home and abroad (Israel, Japan and Saudi Arabia especially could have benefitted from a ‘stealthy’ Eagle on charge), no orders were forthcoming. This was despite the fact that the conformal weapons bay was tested with a live firing from an F-15E demonstrator in 2010 which also trialled various RAM coatings and paint finishes. The flyaway cost of the SE was thought to have approached $100 million dollars per aircraft, including spares and support which compares to $150 million for an F-22. Some aspect of the Silent Eagle programme found their way into export F-15E variants, notably some of the RCS reduction material and some of the electronic and warfare equipment which found its way onto South Korea’s F-15 Strike Eagle fleet. South Korea was also partnered with Boeing on the manufacture of the CWBs should the Silent Eagle find orders.

Other exciting tech developed on the Silent Eagle was a next generation helmet display/targeting system. The Joint Helmet Mounted Cueing System was integrated on the Silent Eagle trials aircraft (an F-15E devoid of the canted-out twin-tails). This allowed the F-15SE’s weapons to be aimed where the pilot was looking. A refinement of what had been developed previously by Vision Systems International, this latest version was ergonomically superior, lighter and more reliable than previous systems.

Eventually, despite initial interest from Israel, Saudi Arabia, Japan and South Korea, no firm order materialised although the F-15SE was initially the winner of South Korea’s 2013 F-X III fighter competition, but this was overturned and now ROK has placed orders for around 60 Lockheed F-35 Lightning Ils. Sadly, those big canted tails were only it seen in Boeing concept art and wind-tunnel models.

F-15 Eagle is published by Mortons Books and is available to order here.

Photo credit: Boeing and U.S. Air Force