Key Point: That Silent Eagle would have to remain substantially lower than the much stealthier F-35A’s projected $85 million cost, or there would little point to procuring them.
A recent article concerning Boeing’s pitch to the U.S. Air Force of a cheap, modestly upgraded F-15X fighter elicited lamentations from some commenters that the Air Force was not enticed by a more ambitious proposal for the F-15SE Silent Eagle—an F-15 with a reduced radar cross section.
Reduced radar cross-sections (RCS) are a common feature in the cutting-edge o 4.5 generation fighters. Before reduced-RCS engineering was widely understood, manufacturers designed fighter like the F-15 or F-16 that had an RCS of around 3 to 5 m2 or greater.
By comparison, the U.S. military’s newest fourth-generation fighter, the the FA-18E/F Super Hornet, might as well be called the Silent Hornet with an RCS ranging between .1 and 1 m2. The French Rafale has a 1m2 RCS, and the Swedish Gripen and Eurofighter Typhoon around half that at .5m2. Relatively small, RCS-optimized fighters also include the F-16C (1.2 m2) and Chinese J-10 (1.5 m2) Even Russia boasts that its longtime counterpart to the F-15, the Flanker, has been improved with an RCS between 1 to 3 m2 in the most advanced Su-35S model.
However, RCS is not uniform across all aspects of a plane, so these designs are doubtlessly more visible from certain angles. Furthermore, RCS is often calculated from frontal radar visibility of a ‘clean’ airplane not carrying weapons. Of course, fighters will be carrying weapons, so the observability for fourth-generation fighters will usually be inferior in practice to these optimal figures.
In 2009 Boeing unveiled a Silent Eagle demonstrator with stealth characteristics stemming from its use of a conformal weapons bay to store radar-conspicuous weapons internally, application of expensive Radar-Absorbent Materials (RAM), and limited airframe modifications including outwardly canted vertical stabilizers. (A conformal weapons bay has also been proposed for the Super Hornet, but the Navy has so far demurred.) There were also update that have since migrated to the proposed F-15X including an Infrared Search and Track system, an improved APG-82 radar, modernized fly-by-wire controls, and an upgraded defensive suite.
In the early 2010s, Boeing marketed the Silent Eagle to F-15 operators including Israel, Japan, Saudi Arabia and South Korea—but none of them jumped at the opportunity. There’s a reason why air forces haven’t been too enticed by a quasi-stealth fighter, and that’s because there’s a steep difference in performance between quasi-stealthy and the real thing.
This is because for every order of magnitude (factor of ten) RCS decreases by, detection range decreases only by 43.7 percent. Thus a 1 m2 RCS will not be detected at one-tenth the distance that a 10 m2 RCS will, but something more like three-fifths the distance. That’s why true stealth jets like the F-22 and F-35s have had their cross sections reduced to around the size of a golf-ball and a marble: .001 to .0001 m2.
An F-15SE is thought to have a cross-section comparable to the Super Hornet with possibly a .1 m2 cross-section from the front and 1 or higher from other angles. This is a useful edge—but not a decisive one.
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First, note manufacturer claimed detection ranges for average jet fighters for the following radars—then consider the benefit of reducing RCS by an order of magnitude or two. (Manufacturer claims are noted; the rest are derived from the above formula, and thus not authoritative but seem to conform with claims.)
Irbis-E PESA radar on the Su-35S 3 m2 230 miles (Manufacturer)
.3 m2 131 miles
.03 m2 75 miles
.01 m2: 56 miles (Manufacturer)
96L6E “Cheeseboard” Radar on S-300 and S-400 surface-to-air missile (Russia’s top SAM system)
4 m2 242 miles (Manufacturer)
.4 m2 138 miles (Manufacturer claims 142)
.04 m2 78 miles*
* Manufacturer claims ‘anti-stealth’ range of 94 miles.
KLJ-7A Radar on Chinese JF-17 fighter 5 m2
105 miles (Manufacturer)
.5 m2 60 miles
.05 m2 34 miles
Keep in mind that the F-35 has an RCS another order of magnitude smaller at around .001 m2, reducing detection range to just a few dozen miles.
To be clear, these are ballpark figures—in reality, radar detection is complicated by many additional variables such as background clutter, and an aircraft is rarely immediately detected at the maximum possible distance. Furthermore, range figures often use the radar’s narrowly focused tracking mode, rather than the broader ‘Volume’ area-search mode. Different bandwidth radars on other platforms may also prove more effective than the X-Band radars on a fighter.
Now consider the effective range of notable Beyond Visual Range radar-guided anti-aircraft missiles, excluding very-long range types such as the 40N6 or R-37 that are not designed to hit a fighter-type target.
48N6E2/E3 (Mach 5.9 missile used in S-300 and S-400 SAM system) 120-160 mile
R-77-1 (Standard Mach 4 BVR missile for Russian fighters) 68 miles
K-77M (Next-gen BVR missile on Russian fighters)120 miles
PL-12 (Standard Mach 4 BVR missile on Chinese fighters) 43-62 miles
In effect, the reduced detection range is roughly around the maximum range of the deadlier missiles, and well above the range of less cutting-edge weapons like the PL-12 and R-77-1. Realistically, though, a fighter would be unlikely to launch at the farthest extent of its range envelope, as that would give an adversary a large window of opportunity to evade.
Therefore, we can see that reduced RCS gives a fourth-generation fighter a situational awareness edge against another non-stealth opponent. However, that advantage would rarely limit the range at which the reduced-RCS fighter could be fired upon. That advantage could also be lessened if adversary fighters are directed by ground-based sites or AWACS aircraft with longer detection ranges.
Designers of clean sheet 4.5-generation fighters certainly should aim for lower RCS. However, Boeing is seeking to sell the F-15X on the premise of its affordability and low operating costs. However, a Silent Eagle-style F-15 would entail a modified airframe and require expensive Radar Absorbent Materials, diminishing the cost savings.
That Silent Eagle would have to remain substantially lower than the much stealthier F-35A’s projected $85 million cost, or there would little point to procuring them. The fact that a figure of $100 million per Silent Eagle was widely floated around in the past does not seem encouraging on that front.
Of course, a Silent Eagle would still possess certain speed and range advantages relative to an F-35, but compared to the proposed F-15X it would actually sacrifice range (the conformal weapons bays being in the place of fuel tanks) and payload (up to 22 externally mounted missiles on the F-15X, versus four to six internal missiles on the F-15SE.) And for routine air defense and ground support, low-observable characteristics would be of little value.
Low-observable fourth-generation jets do have a tangible advantage—but far from a decisive one when compared to stealth jets. Spending extra on stealthier F-15s would likely come at too high a cost for too little benefit when it comes to replacing the F-15C.
Sébastien Roblin holds a master’s degree in conflict resolution from Georgetown University and served as a university instructor for the Peace Corps in China. He has also worked in education, editing, and refugee resettlement in France and the United States. He currently writes on security and military history for War Is Boring.
This article first appeared in August 2018. It is being republished due to reader interest.