The F-35 still has reasonable rear hemisphere stealth, but it cannot come close to the F-22 in that regard.
The F-22 Raptor defines air dominance. The 5th Generation F-22’s unique combination of stealth, speed, agility, and situational awareness, combined with lethal long-range air-to-air and air-to-ground weaponry, makes it the best air dominance fighter in the world.
In short, the F-22 was designed to be a no compromise air to air killer that would be able to defeat anything in air to air combat. The cost was painful even for the US Air Force (USAF) as a result of this attitude. Other designs that are supposed to be much cheaper don’t have the same F-22’s all-aspect stealth let alone the same stealth from the rear.
“The F-22 exhaust nozzles are very expensive and are the only exhausts to provide rearward stealth over a reasonable radar frequency range in a fighter plane,” Mark Knight, a pilot and an aviation expert, says on Quora. “To provide thrust vectoring using a circular nozzle would produce slightly more thrust from the same engine, but a circular nozzle would compromise the stealth of the F-22 from many rearward angles. Yes, the F-35 has a circular nozzle, and yes, it does compromise the stealth of the F-35 from some rear hemisphere angles. The philosophy with the F-35 design was to concentrate on forward hemisphere stealth and not to spend so much on rearward stealth. The F-35 still has reasonable rear hemisphere stealth, but it cannot come close to the F-22 in that regard.”
The radar in fact will see the F-35 nozzle as a curved continuous surface which will generate specular reflections. “In a very narrow band of frequencies it is stealthy from the rear, but nowhere near as wide a range of frequencies as the F-22,” Knight explains.
“This is one of the most subtle problems that has to be solved in the stealth design world,” he points out. “The fact that radar systems don’t necessarily see the shape of the actual object, but their own version of the shape dependent upon the relationship between surface feature sizes, surface curvature and wavelength. As wavelength significantly exceeds surface feature size the feature effectively becomes rounded over to the radar and it will reflect as if the object was a different shape. That’s why the subtle curvature of the F-22 nozzle is better, because it is effective until much lower frequencies than the F-35 nozzle.”
Knight continues. “The Su-57 was going to have F-22 style nozzles developed for it but once the engineers got started and discovered how expensive it was going to be the Russian government decided that the improved rear aspect stealth was not worth the extra cost. So, the Su-57 was developed with frontal stealth as a priority. The Chinese did not even consider such designs for their stealth fighters as front hemisphere stealth was all that they considered important.”
However, the rectangular exit port of the F-22 nozzle design is not the most important part of it from the point of view of its stealth. “The stealth comes from the pointed projections and the complex shape of the exterior parts of the nozzle. I have highlighted some of these features on the picture included [Below].”
Knight concludes: “These stealth considerations forced the cross section of the nozzle into the rectangular shape as a side effect. The exterior parts that provide the stealth are not rectangular but have a series of subtle curved and saw-toothed shapes that avoid reflecting radar energy back the way it came from a wide range of angles. This cannot work so well with a circular nozzle.”
Photo credit: Mark Knight via Quora, Capt. Dale Greer / U.S. Air Force