@moon-light – You can go to any industrial facility in the world and find that control systems, process monitoring systems, data storage, power conversion, etc. is most often stored inside metallic cabinets. You are able to do this because metals – even the most conductive ones – are not perfect reflectors. They do absorb a small percentage of incident RF energy. If they didn’t, then the RF emissions from microprocessors and other equipment inside industrial cabinets would keep increasing, building up upon itself, until all the circuitry fried itself just as surely as if submitted to an EMP. Since they do absorb, however, the RF emissions inside such devices stabilize to tolerable levels even though the control circuitry continue to emit RF energy as long as it is in operation.

Metal does not absorb RF energy fast. This is why direct path off of a metal surface is such a problem for RCS. Two and three bounce paths (such as with corner reflectors) are as well. But after 10 or 20 reflections there is almost no signal energy left. And in a metal cabinet there is nothing for an RF wavefront to do except make contact with a surface. And so each RF pulse from every switch or clock just attenuates away.

Stealthing up an engine is done by giving it the “cabinet treatment”. If you make the blades of your turbine out of a highly reflective material (metal) then you will need to use a radar blocker for all aspect VLO. The blocker is made reflective itself with just enough absorption to nullify specular reflections – but it is shaped to steer the majority of the reflected signal through a torturous path to the turbine. Now the turbine blades will strongly reflect the energy back, but that’s OK. Because the blocker is not shaped to pass the RF energy through the other way. So the RF energy is trapped between the blocker and the turbine and attenuates away over multiple reflections.

If you made the turbine blades out of a transparent material (with enough absorption to nullify any specular reflection from slight impedance mismatches…you can never get perfection so you need to build some margin), then you pass the RF energy deep into the engine so that it can again get the “cabinet treatment”. Only this time the energy will attenuate away over multiple reflections between the engine’s rotor and the interior casing. You wouldn’t need or want the whole engine to be transparent. Any components within that path which would reflect back outward would need to be treated though.

“How does distance play any role here when the blocker block the direct view?”

Specular emissions reflect off a surface in an arc vice a straight line path. A portion of that arc will escape the blocker. The longer the duct is – from the engine front to the inlet or the engine rear to the outlet – the great the chance for that escaped energy to make contact with your ducting material and further attenuate. Sorry that I can’t show a picture. I’ve tried before and don’t seem to have the rights. I don’t know whether users have to be promoted a few levels before they can do that. The higher ranked ones do it often.

@QuantumFX and latenlazy – If they really do have the manpower and the technical and material resources to pursue all of this at once then more power to them. You are right that, in that case, they should. Obviously, there will be a cloak of secrecy on military projects and so I don’t have any idea how things are progressing. However, I believe their civilian CJ-1000 program shows a cautionary tale. Their design is close, but not quite, a LEAP engine and the program has had multiple delays till now it looks like it may enter service nearly 15 years after LEAP. Now this might simply be the case that one can only progress at so rapid a rate. The Chinese only fully mastered 1980s designs around a decade ago. But this could also be a sign of shortages…which may be affecting other programs too.

Normally technological advances for gas turbines aren’t primarily pushed through the the specific development of a new engine design, but the independent and ongoing development of the basic component technologies that make up gas turbines. New engine designs draw from and are assembled through the work done on these more basic components.

True, but in a successful program they are heavily interlinked. The actual turbofan group needs to receive timely relevant information from the component and the materials groups, and the component and materials groups need timely reports on turbine testing (relevant info only). And so there should be multiple lateral interfaces.

Care to elaborate about specifically which part of his results? Obviously absolute RCS numbers are going to be incorrect, but imo his test gave great insight into the relatively high RCS (specular) aspects of the targets based on shaping.

@ActionJackson – all of them. He used a ray tracing method on the visible surface assuming that it was always reflective, which was pretty far off the mark. And he made assumptions on RAM thickness and effectiveness that don’t always hold true. Take a look at the picture of the backside of the F-22 here: https://www.wired.com/2010/12/lockheed-cross-breeding-raptors-joint-strike-fighters/

Do you see those triangular sections outboard of the engine nozzles (and also at the base of the tail)? Those are called re-entrant surfaces. The basic idea is that the outer skin is mostly transparent allowing RF energy to reach these sections where the highly acute angle and reflectivity of these surfaces directs the energy up into its vertex, providing for a very large amount of absorption with only a thin layer of RAM. And it makes the aircraft appear (to radar) something more like this: https://en.wikipedia.org/wiki/Anechoic_chamber

There is a picture description of its operation here: https://www.comsol.com/blogs/modeling-rf-anechoic-chamber-using-periodic-structures/

Other aircraft have these features too. The Su-57 has similar structures partly visible beneath the vertical tail (aligned more horizontally than vertically) and you can see they used a similar technique with the venerable SR-71. https://sploid.gizmodo.com/fascinating-photos-reveal-how-they-built-the-sr-71-blac-1683754944

But he didn’t factor in such shaping features in his studies because he would have had to have seen them being built. Then there are pattern absorbers. The basic idea is shown in Figure 7 here: http://www.rfcafe.com/references/electrical/ew-radar-handbook/radar-cross-section.htm

If you look at what happens on the left side of the plot you will see that as the wavelength (lambda) becomes much larger than your feature size (r) then the radar cross section drops dramatically. Because of this phenomena, people developed the pattern absorber. The idea is you imprint microscopic patterns into your surface and it partially masks an object – making it display a significantly smaller RCS than it would have otherwise. But Dr. Kopp couldn’t take those into account because he would have to examine their structures under a microscope to see if the aircraft he studies had them.

Then there are impedance matching and conductivity gradients which allows the F-35 (and possibly soon the Su-57) to show this (40 sec mark) and it not be a problem for RCS: https://www.youtube.com/watch?v=0uL0LaeTPqA

Dr. Kopp was concerned about the F-35’s rear shaping. He generously considered the engine a perfect absorber to make his analysis easier and showed that a reflective nozzle such as the F-35’s would produce vulnerabilities all by itself. But you throw some gradients in there and you can bring that reflection way down…and then RF waves penetrate further into the engine to simply dissipate away within the labyrinthine structure. The same with the blades of the low pressure turbine like in that blurry (sorry) video.

There are plenty of other tricks too. Some of them work well together and some of them don’t. In fairness to Dr. Kopp, he stated that his work was only a preliminary analysis. But he was digging a deep hole for himself because he never could have gathered sufficient information to perform a detailed one. You cannot determine whether an aircraft is stealth by looking at the finished product! But anyone can fall victim to confirmation bias…and Carlo Kopp did.

“…the generation gap much more important than present advertised one.”

No it won’t be. Moore’s Law is at work here. The performance of ICs (systems on a chip and such) continues to improve and their costs continues to fall. Fourth and 5th gen aircraft operating at that time will get the same sort of networking upgrades and will be operating as a part of system of systems too.

@panzerfeist1 – The F-22’s RCS spec in X-band is -40dBsm. The F-35’s is -30dBsm. Nothing has changed and these values remain true. However, RCS can vary greatly with frequency, and the F-35 was designed from the ground up with improved absorber material that, in addition to being more weather tolerant, also remained useful across a much larger frequency band. And so it is not a contradiction to hear quotes saying both “the F-22 is stealthier” and “the F-35 is stealthier”. It depends entirely on the frequency in question. Infrared is a factor too and there the F-35 likely wins by leaps and bounds (having two engines and exhausts right beside one another is not optimal for heat transfer and leads to poor hotspot and plume reduction).

“There have been good discussions of showing which aircraft had more stealthy features.”

CAN’T be done by the eyeball brigade. The best visual example of a stealth bomber is the very first one ever attempted. I’m not talking about the F-117 or the WWII Horton bomber. I’m talking about Germany’s WWI Linke-Hofmann R.1. It is instructive because it has a mostly clear cellophane fabric covering which forms its aerodynamic surface. It wasn’t perfectly transparent, but at a distance you would have trouble seeing it. And so the visual surface was smaller and composed of the wings and wooden supports. And if you were to place it on a radar test stand you would find that cellophane and dry wood are generally poor reflectors and the great bulk of your signal return is from the engines mounted within the fuselage as well as their driveshafts and metallic fasteners and cables. Three surfaces in this case and what the bomber so clearly illustrates is that they do not have to be the same. In a mature stealth design you decouple your surfaces as needed to achieve optimal results. The Linke-Hofmann R.1 is the first example where this was attempted.

Eyeballing aircraft isn’t going to get you any kind of accuracy at all. Dr. Kopp did that in great detail with ray tracing and got erroneous results. Neither the F-35, J-20, or the Su-57 appeared to possess all-aspect stealth. If he would have analyzed the B-2 or the F-22 he would have found the same thing. And it was because his assumptions were wrong. Some of those visual surfaces are actually transparent at the frequency of interest and allow for better attenuation deep within the structure. As an example (with visual light) you can take a flat sheet of glass and pass light through. You take a piece of vantablack with the same shape and it absorbs it almost perfectly. If you use a polished mirror then you reflect it back. The shape of your object was exactly the same each time but the shaping of the field was completely altered in each case, due to using different materials. The scientific basis of why this is is Quantum Mechanics. The electrons orbiting molecules and atoms are fixed in discreet and well defined allowed values. The designers in Lockheed know the wavelengths used and thus the energy of the photons at those wavelengths. So their work is a materials challenge to find options where there is no allowed electron-photon interactions that scatter back to the receiver. Passing it through is fine. Passing it through and absorbing it later into thermal energy is too, as is deflecting it along another path. Of course, it has to be able to fly too. The materials science involved is not easy.

Their materials databases, test results, computer simulations, etc. are all classified but the general approach – a hundred year old aircraft and a hundred year old theory – is historical knowledge and cannot be. And that alone shows that it is foolish to try to assess an aircraft’s cross section at bands within the radio frequency spectrum using the visual spectrum!

…just look at the state of Russian Engine tech.

It might be premature to dismiss them. The Russians are currently in flight trials with tech which will allow them to attain all aspect VLO. The Chinese and everyone else likely won’t have that for ten years or more.

Also, i do not understand why all so obsessed with izd.30. AL-41F1 is quite capable engine.

It mostly has to do with the capability for all aspect VLO. You can see the blades of the low pressure turbine when looking at an engine from the rear. That isn’t good for your RCS if they are made of metal (see pic in link).

https://www.nextbigfuture.com/2017/09/china-catching-up-on-fighter-and-commercial-jet-engines.html

They should be composite in the product 30. The Russians have been tight lipped about exactly what composites are in the product 30, but you can learn a lot by following unclassified programs. They outright state that the PD-35 will have ceramic matrix composites…and it has a remarkably short development cycle planned for something using new materials. This suggests they aren’t that new.

https://www.flightglobal.com/news/articles/contract-docs-reveal-plans-for-russias-new-widebody-445047/

There are other reasons but they are more minor. The new engine really does make or break the program.