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  • XB-70
    Rank 4 Registered User
    • May 2018
    • 350

    The Stealth Thread

    I told AJ that I would start a dedicated thread for this. This can be used for pimping your fave stealth designs or trash talking supposedly fake stealth fighters. But I'll start it off with a general discussion of the science and mythology behind stealth.

    First the basics. Please briefly review: https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=2710&context=qnde

    The paper is a discussion of the results of testing the propagation of electromagnetic waves (X-band) through various materials. (Test setup shown on Fig. 1) Several on this thread insinuate that a propagating radar wavefront will 'bounce off' a surface. This isn't true. Quantum mechanics comes into play - the electromagnetic wavefront will always enter into the material. How far it travels is dependent on many factors - but multi centimeter penetrations are sometimes possible (see results for Kevlar and glass composites). A half millimeter steel or aluminum plate would have completely blocked it but this wasn't shown. In order to understand the technology it is imperative to understand that the wavefront enters into the material and interacts with it.


    This is a stealth thread so I won't go into the non-conductive materials discussion. With a stealth aircraft, the surface is conductive. (https://www.defensenews.com/smr/global-strike/2019/04/29/what-it-takes-to-keep-the-b-2-bomber-flying/) A metal plate is also conductive, and yet an airframe constructed of metal is said to be unstealthy. Although this may seem a contradiction, it really isn't. The reason has to do with Eddy Currents and Skin Depth. Skin depth is "a measure of how far electrical conduction takes place in a conductor" (https://www.microwaves101.com/encyclopedias/skin-depth). From Gauss's Law, the electric field *inside* the conductor is always zero. This means that when a conductor is exposed to a rapidly changing electromagnetic field (such as the wavefront from a radar transmitter) it will be confined to the exterior of the surface (but it nevertheless does enter into it). A single skin depth will attenuate the bulk of the field so that only around 37% is left. Five skin depths will attenuate practically all of it. Thus, a metal surface five skin depths thick is a solid object from an electromagnetic perspective. It is free electrons within the conductor which is mostly responsible for this effect.

    The Laws of Conservation of Energy and Momentum are pertinent, and I will get back to them. (Photons do not have mass) The electromagnetic energy entering the surface of a conductor causes a force on free electrons within it. This causes their movement within the conductor (electrical current). These current paths (Eddy Currents) will be doughnut shaped all across the exposed surface. The image here (a heat map showing the current density) is the effect with the difference that the doughnut shaped current paths will be all across the surface for a transmitter far away. (https://www.nde-ed.org/EducationResources/CommunityCollege/EddyCurrents/Physics/depthcurrentdensity.htm) One thing that you can see from the heat map is that the region of highest current density is just below the surface. The reason for this is that as the electromagnetic field enters the material it loses strength. The energy and momentum is taken by the free electrons as they become "excited" and start their current paths. Energy absorption.

    However, the excited electrons will then lose that gained energy by one of two ways. The first is by coulomb force interactions with other charged particles within the conductor. If the travelling electrons displace the conductor's nuclei through interacting with their bound electron shells, then the energy is transferred to thermal energy (this is your I^R losses). This is the basic principle behind stealth. On the other hand, the excited electrons can simply give up their energy (particles will always tend to their rest state). Since energy and momentum must be conserved, they give up their energy and cease their circular motions by re-emitting the photons in the reverse direction. This is wave reflection, is not desired. And, as that heat map shows, since great bulk of the current is just below the surface, if the electrons there give up their energy through photon emission then the photons are almost guaranteed to escape the material and travel back to the receiver.

    So, back to the defensenews article on the B-2, you have to bury your surface eddy currents. "...225 different types of materials...we'll do paint, we'll do different types of tape..." If the outermost layer has a very low impedance (like it would if you built your aircraft out of aluminum or so) then there is very little energy transferred to thermal energy through I^R losses and almost all of it will be re-emitted as the electrons return to rest. On the other hand, if the outermost layers have a higher resistance (less conductivity) than the surfaces below them then the profile of the surface eddy currents will be forced deeper within the skin structure. And, if the current density is deeper (buried) then it is less likely to escape, because many of the re-emitted photons will be recaptured by electrons and then converted into thermal energy. This can be done with multiple materials. However, it can also be aided by controlling material thickness. The resistance of a wire is proportional to its length and inversely proportional to its cross sectional area. (https://phet.colorado.edu/en/simulation/resistance-in-a-wire) The eddy currents don't travel in a wire, but they do have a travel length and cross sectional area. The same behavioral characteristic applies. Conveniently, there are tools available to experiment with this. However, thickness values are rarely available.

    That said, the layers of the ITO coating of the Su-57 is said to be between 70 to 90nm. (http://www.defense-aerospace.com/articles-view/release/3/199009/russia’s-su_57-fighter-gets-advanced-stealth-coating.html) A stealth fighter has to control the reflected energy. A designer could utilize surface normal almost exclusively (the F-117 was built like this). But that still leaves a lot of reflection, which could be a problem - especially if every normal vector isn't pointing upward as it was on the Nighthawk. (Only the tail provided downward pointing surface normal on the Nighthawk).

    I'll provide two web based skin depth calculators.
    https://www.pasternack.com/t-calculator-skin-depth.aspx
    https://chemandy.com/calculators/skin-effect-calculator.htm

    Neither of these provide a selection for Indium Tin Oxide (but it is variable as it depends on the ratio of the elements used). However, gold has a higher conductivity and will bound it. Using gold and selecting a frequency of 10GHz, a skin depth value of 753nm is provided. The layers that the Russians use in their canopy coat are kept much less than this, which means that there will be both very little wave reflection and very low wave absorption on the outermost layer - guaranteed because of material thickness. This is what you want. The majority of the wavefront's energy is transmitted into the next layer which should have a higher conductivity (by a different composition and it may also be thicker). As such, it will absorb more energy and also reflect more energy than the previous layer did (but some of that would be absorbed by the higher layers. And so on.

    When it comes to transparent surfaces - like the canopy or IRST domes - the total layer is going to have to be thin. (The materials are transparent, but not that much). So, some of the energy will pass through and interact with whatever is inside the canopy or IRST dome. It will reflect off of various surfaces. Only the part that initially reflects off of the surface in the threat direction and the part that passes through twice and propagates in the threat direction will add to RCS. So, rather than something that one can eyeball, this is in fact a very complex process. The goal is to find a minimum value.

    So, with all of this given, I'll finish off with some truths and 'truths'.

    1) You can eyeball stealth.
    This is a myth. As the above discussion detailed, many things you cannot see come into play.

    2) Spherical objects cannot be used in a stealth design.
    This is a myth. The generalized RCS for a spherical object (RCS = pi*r^2) does not hold true in an absolute sense. The generalization relies on the assumption of full wave reflection. In other words, it will only be true for an object which is - from an electromagnetic perspective - a solid. For conductive material, that means 5 skin depths. For non solid objects the relation is quite complex, as the total energy balance is: the energy absorbed in the conductor, the energy reflected off the conductor surface in the threat direction, the energy reflected off the conductor surface in non-threat directions, the energy absorbed in internal components, the energy reflected off of internals in the threat direction, and the energy reflected off of internals in non-threat directions. Of these, only the two terms in the threat direction contribute to RCS. Since these can be minimized through design, the actual RCS contribution of IRST domes and such can be quite small.

    (As an addition to this. Try the skin depth calculator with a frequency of 1GHz. For gold again, the skin depth is now 2.38 micrometers. This means that the wavefront passes through easier - both directions - and so more will be reflected off the internals of the canopy or IRST. For lack of a better term, I'll call this the canopy effect.)

    3) Current stealth technology tends to break down at lower frequencies.
    This is true. The canopy effect is one reason for this. The wavefront passes through the absorber/coating materials more easily and then non-stealthy shapes such as the instrumentation and the pilot's helmet start contributing to RCS. There is also the more widely known case of Mie scattering. The Su-57 - with its many thin, transparent surfaces - will have a very pronounced canopy effect. This is why they had to build Okhotnik. The Su-57 isn't as suited for deep penetration tasks as the F-35 or J-20 is because it is particularly vulnerable to detection at low frequencies.

    4) You can tell which design has greater stealth by comparing surface normals.
    In general, there is a lot of truth here. The underlying assumption is that the build is similar. I could be wrong, but I believe this will normally produce an accurate result.

    5) A degraded or scratched surface greatly degrades stealth.
    True. Material depth is one of your shaping parameters - one of your most important. You degrade both materials and shaping this way.

    6) The saying that stealth is about "shape, shape, shape, and materials" is accurate.
    True. RCS is a product of the objects size, its projected cross section, and its reflectivity.
    Size is a function of shape, the projected cross section is a function of shape, reflectivity is a function of shape (material depth) and materials.

    7) Russia's much touted ROFAR is a counter to stealth.
    This is theoretically true - but the extent remains to be seen. Use the calculator again but with frequency set at 110GHz. The skin depth for gold is now only 227 nanometers. This means that all of your layers are now thicker relative to skin depth - which means that they are more reflective. Everything. Your RAM paint, canopy coat, all of it. Possibly too reflective. (And this shows that more RAM is not better. If the technicians apply the coats too thick it actually degrades stealth.
    Now go back to airpoweraustralia's stealth analyses and you will see a large reason why they were so wrong
    )

    8) ROFAR could let one see a pilot's face.
    False. It would be the lower frequencies which would penetrate through the canopy. They don't have the resolution to produce more than a blur. The Russians do have a habit of exaggeration.

    I may build upon this later. It depends on any following discussions. Amazing how much of this is public knowledge (if you know what to look for) isn't it? It's the design and build techniques that makes this aerospace grade that is classified...simple physics isn't.

  • XB-70
    Rank 4 Registered User
    • May 2018
    • 350

    #2
    Oh, one addition that just popped in my head.

    9) The Sukhoi patent is an accurate assessment of the Su-57's stealth properties.
    Myth. The patent is an aerodynamic patent - it specifically says so. Thus, it does not in any way describe material depth. So it doesn't even accurately cover shaping and anyone who doesn't know that is ignorant of even basic RF engineering. Because this isn't some super secret stuff. If you want to build radios, microchips, touchscreens, and most every electronic thing produced today you have to understand and work with skin depth. Anyone touting it doesn't understand the subject.

    Comment

    • panzerfeist1
      Rank 6 Registered User
      • Feb 2018
      • 399

      #3
      7) Russia's much touted ROFAR is a counter to stealth.
      This is theoretically true - but the extent remains to be seen. Use the calculator again but with frequency set at 110GHz. The skin depth for gold is now only 227 nanometers. This means that all of your layers are now thicker relative to skin depth - which means that they are more reflective. Everything. Your RAM paint, canopy coat, all of it. Possibly too reflective. (And this shows that more RAM is not better. If the technicians apply the coats too thick it actually degrades stealth.



      Now go back to airpoweraustralia's stealth analyses and you will see a large reason why they were so wrong
      )

      According to the Chinese they say the noise level drops 100 times lower and looking at the latest RTI source that I have provided earlier the head chief says the size reduction of obtaining FICs would be 12 times smaller. So I wonder if the 100 times lower noise is just ROFAR or having the same amount FICs as an aircraft has MMICs if that is so than I wonder if they can put 12 times as much FICS into their aircraft to lower the noise even further
      8) ROFAR could let one see a pilot's face.
      False. It would be the lower frequencies which would penetrate through the canopy. They don't have the resolution to produce more than a blur. The Russians do have a habit of exaggeration.

      I found sources that have said 1hz and 1ghz and KRET's head official saying UHF frequencies. There are radars that use lower frequencies this has been shown even by Chinese research papers that ultra-high resolution is achievable at long ranges.




      https://spie.org/news/zhang-photonic...solution?SSO=1

      More experiments done by them.

      Zhang_Fig3




      They confirm ultra-high SAR is achievable at long ranges but I am sure everyone is curious to see RTI's radar prototype for their aircraft designs. Went through one of their articles they have tested radar balloon prototypes to track low altitude targets faraway and seem to also be working on expanding the frequency to the terahertz range. They use Photodetectors multiply frequency and modulators to de-chirp their frequency. This is their summary, "
      In summary, we have designed a novel photonics-based radar applying photonic generation and de-chirp processing of broadband radar signals. The radar has great potential in real-time target detection and imaging with an ultra-high resolution."

      As you said lower frequencies are needed to penetrate the canopy but in their summary they can multiply broadband radar signals and than de-chirp them without having to worry about a signal to noise loss doing it.

      More of their experiments


      https://www.osapublishing.org/Direct...eq=0&mobile=no

      the words are all screwed up for me to post their quote in that source. but on number 4 discussion and conclusion they have tested a 4ghz bandwidth on a regular radar and have demonstrated a ka-band radar with a bandwidth of 12ghz having better range resolution than just the phased array radar.

      more of their high frequency radar tests using lower bandwidths achieving high resolution

      W-band inverse synthetic aperture radar (ISAR) imaging systems are very useful for automatic target recognition and classification due to their high spatial resolution, high penetration and small antenna size. Broadband linear frequency modulated wave (LFMW) is usually applied to this system for its de-chirping characteristic. However, nearly all of the LFMW generated in electronic W-band ISAR system are based on multipliers and mixers, suffering seriously from electromagnetic interference (EMI) and timing jitter. And photonic assisted LFMW generator reported before is always limited by bandwidth or time aperture. In this paper, for the first time, we propose and experimentally demonstrate a high-resolution W-band ISAR imaging system utilizing a novel logic-operation-based photonic digital-to analog converter (LOPDAC). The equivalent sampling rate of the LOPDAC is twice as large as the rate of the digital driving signal. Thus, a broadband LFMW with a large time aperture can be generated by the LOPDAC. This LFMW is up-converted to W band with an optical frequency comb. After photonic-assisted de-chirping processing and data processing to the echo, a high-resolution two-dimension image can be obtained. Experimentally, W-band radar with a time-bandwidth product (TBWP) as large as 79200 (bandwidth 8 GHz; temporal duration 9.9 us) is established and investigated. Results show that the two-dimension (range and cross-range) imaging resolution is ~1.9 cm ~1.6 cm with a sampling rate of 100 MSa/s in the receiver.

      US

      https://www.sciencedirect.com/topics/engineering/x-band

      The feasibility of a photonics beamformer at L- and X-bands has been demonstrated [65]. The use of one beamformer capable of operating at widely separated bands of frequency is a major achievement.

      Photonics-based beamformers can have an instantaneous bandwidth extending from 3 to 6 GHz and are 75% lighter and smaller than their electronics counterparts, and the incurred loss of about 14 dB is tolerable [66]. The weight and volume advantages of photonics-based beamformers make them attractive for airborne/satellite-borne applications.

      A 5-bit photonic switchable delay line for a wideband array has been reported for an airborne application [67]. The array occupies an area of 0.9 2.8 m2 and consists of 96 elements. It is designated to operate in the 8501400 MHz range and follows the contour of a jumbo jet fuselage. The 96 radiating elements were divided into 24 columns with three combined into one subarray which was controlled by a 5-bit time shifter. The photonic time shifter provides the coarse delay steps ranging from 0.25 to 7.75 ns while a 6-bit electronic delay line in the T/R modules provides refined differential delays ranging from 0.01 to 0.5 ns. The array has a 50% instantaneous bandwidth and the L-band module has a 38 dB loss, but efforts are being made to reduce this."

      This was back in 2000, and if the Chinese and Russians see feasibility in photonics I am sure the US has made the efforts to already reduce the noise loss. if you read further in this source even the US also referenced them as FICs photonic integrated circuits and speak of the same benefits of it as well.

      https://uanews.arizona.edu/press-rel...tonics-project

      Frank Jaworski, program manager for emerging technology at Raytheon Vision Systems, added: "Raytheon regards the integration of photonic integrated circuits with focal plane arrays as a critical path for the development of future Department of Defense imaging systems vital to the nation's security. We look forward to the University of Arizona's leadership of the consortium and utilizing their expertise in developing this key technology."

      Neil Supola, chief of the infrared focal plane array branch at the Army's Night Vision and Electronic Sensors Directorate and government program manager for AIM Photonics, said: "This program is a great opportunity for the Department of Defense to leverage advances in integrated photonics manufacturing being realized by the Manufacturing USA program together with its state, industrial and academic partners. The scope of industrial participation on this project highlights the relevance photonic integration has within the Department of Defense community, and this project's inherent potential to make a large impact."
      Last edited by panzerfeist1; 15th July 2019, 05:42.
      I thought the fall of western civilization was a tragedy, now I realize it's a comedy.

      Comment

      • LMFS
        Rank 4 Registered User
        • Feb 2018
        • 520

        #4
        The Su-57 - with its many thin, transparent surfaces - will have a very pronounced canopy effect. This is why they had to build Okhotnik. The Su-57 isn't as suited for deep penetration tasks as the F-35 or J-20 is because it is particularly vulnerable to detection at low frequencies.
        Great post. I wonder what is the difference in your opinion between the surfaces on the Su-57 and those on the F-35 / J-20. The F-35 at least has also several EO apertures, wouldn't they be also rendered transparent by lower frequencies?

        Comment

        • XB-70
          Rank 4 Registered User
          • May 2018
          • 350

          #5
          The F-35 at least has also several EO apertures, wouldn't they be also rendered transparent by lower frequencies?



          Yes. But most of its apertures - DAS - are smaller. And the EOTS is underneath a facetted canopy - which will allow for a more controlled directivity from the part of the wavefront that is reflected (until Mie scattering happens). The F-35 also probably has the cleanest cockpit interior. That said, the USAF freely admits the RCS of their designs experiences a step change for the worse at lower frequencies. And, if they didn't, any EE with a background in RF would know they are lying.

          With the Su-57, if you will remember, the early prototypes didn't have the DIRCMs. I think once the Okhotnik-B drone started coming along they abandoned the deep penetration role for the Su-57 as it was redundant.

          Some ideas floated around over recent years for the sixth generation fighter concept were smart skins and a pilotless platform. If you didn't have the canopy then that eliminates a large part of the canopy effect (you can sink the surface currents deeper with composite structures), and if you could shrink your apertures sufficiently and spread them out across the surface that would take care of most of the rest. No current 6th gen concept does this. The technology for it does not yet appear to exist.

          Comment

          • mig-31bm
            Rank 5 Registered User
            • Oct 2013
            • 2140

            #6
            Originally posted by XB-70 View Post
            3) Current stealth technology tends to break down at lower frequencies.
            This is true. The canopy effect is one reason for this. The wavefront passes through the absorber/coating materials more easily and then non-stealthy shapes such as the instrumentation and the pilot's helmet start contributing to RCS. There is also the more widely known case of Mie scattering. The Su-57 - with its many thin, transparent surfaces - will have a very pronounced canopy effect. This is why they had to build Okhotnik. The Su-57 isn't as suited for deep penetration tasks as the F-35 or J-20 is because it is particularly vulnerable to detection at low frequencies.
            I want to add that low RCS at VHF is a requirement of F-35
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            Comment

            • LMFS
              Rank 4 Registered User
              • Feb 2018
              • 520

              #7
              Originally posted by XB-70 View Post


              Yes. But most of its apertures - DAS - are smaller. And the EOTS is underneath a facetted canopy - which will allow for a more controlled directivity from the part of the wavefront that is reflected (until Mie scattering happens). The F-35 also probably has the cleanest cockpit interior. That said, the USAF freely admits the RCS of their designs experiences a step change for the worse at lower frequencies. And, if they didn't, any EE with a background in RF would know they are lying.
              Ok I see. The only additional apertures I know are the ones for the DIRCM turrets, probably that is what you are referring to. The MAWS apertures are similarly small. I guess we would need to know what happens to the wave front once it goes through those surfaces (what are the properties of the materials behind) and what kinds of other RCS reduction methods can be used (maybe destructive interference can be used around certain, specially relevant frequencies).

              But I get the basics, that many of the most RCS relevant design elements are simply not visible.

              Some ideas floated around over recent years for the sixth generation fighter concept were smart skins and a pilotless platform. If you didn't have the canopy then that eliminates a large part of the canopy effect (you can sink the surface currents deeper with composite structures), and if you could shrink your apertures sufficiently and spread them out across the surface that would take care of most of the rest. No current 6th gen concept does this. The technology for it does not yet appear to exist.[/SIZE][/FONT][/COLOR][/LEFT]
              The DAS technology could be used when mature enough to have the pilot shown a virtual image of the exterior, I think there was a British concept like this going around some time ago. But by then maybe the AI on the plane is enough to get rid of the pilot altogether

              Thanks for the discussion in any case. I still need to research quite a bit to properly understand the topic but this helps a lot.

              Comment

              • XB-70
                Rank 4 Registered User
                • May 2018
                • 350

                #8
                Thanks for the old news article, Mig-31. You saved me a lot of time. I'll use it a bit later.

                Comment

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