J-20 5th Gen Fighter Thread IV (Closed to posting)

[Skywatcher]

Stealth is mostly about the exterior shaping. To quote Lockheed Martin engineer Denys Overholser, stealth is dependent on

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. The RAM coating which you are making a big deal out of is not a major factor in stealth. The coating is merely icing on the cake to further reduce the last few percents of EM energy that propagates omni-directionally and cannot be handled shaping.

That's the point!

The last few percentages of EM energy matter a lot with a target as small as the F-22A (remember, 90% of the high performance is relatively easy. The remaining 10% is where programs go to die). It won't matter with a F-15 or Su-27, but the F-22A's RCS is several magnitudes is smaller.

Wave Cancellation


Given that some of the radar waves will penetrate the skin of the aircraft and possibly bounce back towards the radar emitter, how do you minimize the impact from these waves? Well, you try to have them bounce from an internal structure (like a second skin inside the first skin) that is one-fourth of a radar wavelength inside in the airplane. In other words, you set up a second skin, which is one-fourth of a radar wavelength under the surface of the airplane. When radar bounces off this internal skin and leaves the airplane, it meets up with the radar reflection of the real surface. But the path of the radar wave that reflected internally is half a wavelength longer than the path of the wave that bounced off the surface (one quarter of a wavelength in, one quarter of a wavelength out), which means they are now approximately out of phase, and largely cancel each other out. That’s the theory, anyways.

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Um, you were saying?

 

[kwaigonegin]

radar can indeed be used to analyze the shape of an object. That's what radar imaging satellites do, that's what modern fighter radars with Synthetic aperature mode do (albeit usually for large targets and topography), that's how longbow radar on apache identifies targets, that's how maritime patrol radars indentify their targets (ISAR is basically a similar thing, only over a longer period of looking at the target, without necesarily moving the radar in relationship to the target)

Of course, downside is that current technology isn't advanced enough to be militarily usable for what is being suggested here. Resolution of such synthetic images is still not good enough for targets with so few radar return points to be observed from a militarily useful distance. What good is it if you can look at f22 with such radar and conclude "yes, it is indeed a f-22" from only 5 or 10 km away? Those figures are just illustrative, but actual ranges are sure to be well under 100 km. One needs a big radar that has a large aperture antenna, lots of power behind it and modern processing to be able to do it with older gen aircraft at reasonable distances. Even that being said, identification of airborne targets via radar is even today a very hard thing to pull off at large distances. One needs moving parts, like helicopter rotor blades, or even roating turbine blades for aid. Even so, there's been an interesting promo pamphlet for russian buk 2me system. It says it can detect a stationary, hovering helicopter at 30 meter altitude at 15 km away. And that's with a fairly modern radar array, against a target whose RCS is oscillating a lot more than airplane's turbine blades.

that is true.. SAR can potentially be use for target identification but against moving targets especially one at high speed the level of technology is not even close yet. For the most part most planes 'look' fairly similar so SAR or not, it would be extremely difficult to identify based strictly on ''shape'.

 

[hardware]

Radar doesn't work that way.

possible,according to some aviation writer,believe APG-77 able to tell the different between Mig-21 and Mig-23.just by scanning there shape and compare it with on board data bank.
and back in the 90's,aviation week teported that lockheed developed new solfware that allow ground surveillance radar to scan for target shape,it look like SAR in reverse.

 

[vincent]

Just an idea. What if the radars in an area encode their locations and time in their emissions and set up passive stations in the same area. If a signal is received by the passive receivers, can they calculate the target's location using the location of the emission station and the emission time?

 

[kwaigonegin]

Just an idea. What if the radars in an area encode their locations and time in their emissions and set up passive stations in the same area. If a signal is received by the passive receivers, can they calculate the target's location using the location of the emission station and the emission time?

theoretically yes but that plays only a small part of the detection process. It's also quite a likely something similar might be used to track and detect the F-117 shot down over Serbia in 1999 by a couple of SA 3s. They modify the radar to longer wavelengths , mayhe change the dpler pulse rate and uses triangulation via multiple arrays to detect the F-117. Longer wavelength has the disadvantage of not being able to properly identify the aircraft however they can still detect and range the object. Since all the aircrafts flying that day are NATO aircrafts they don't really need to identify as much as track and detect. Part of it is due to ingenuity but like most things in war luck also played a role.

If the F-117 had come in at a different flight path, altitude and the weather has been different things may not have happened like it did. From what I know the stealth birds pretty much had the same ingress/egress routes every night so that played a role as well in the downing of the nighthawk.

 

[Engineer]

That's the point!

The last few percentages of EM energy matter a lot with a target as small as the F-22A (remember, 90% of the high performance is relatively easy. The remaining 10% is where programs go to die). It won't matter with a F-15 or Su-27, but the F-22A's RCS is several magnitudes is smaller.z



Um, you were saying?

"Last few percentages" and "a lot" do not go together. Materials only reduce a few percentages because material is least significant to RCS begin with. If you have actually read your own source thoroughly, you would have realized that. Let us go through a few paragraphs that you have obviously missed.

Denys Overholser, a Lockheed mathematician and electrical engineer who had the brilliant idea of using a highly swept, wedge-like, faceted design for stealth (which eventually became the F-117), once famously said that there are four elements that are important in reducing the radar reflection of an airplane: “shape, shape, shape, and materials”. So you can guess which one of these really matters.

Summary: shaping is the most important. In the context of our discussion, it means just by knowing the exterior shape of the F-22, one can already gain a fairly accurate estimate on RCS.

Like I said, there are three ways of going about reducing RCS: absorbing radar (skin material), reflecting radar waves that cancel themselves out (skin thickness, “depth” of internal components), and reflecting radar away from its origin (aircraft shape). Of these, the last is by far the most important. So let’s start with what is the least important, and today the least commonly used:

Radar-Absorbent Materials (RAM)

Summary: material, which you are making a big deal out of, has least effects on RCS.

Reflection

This is the most important challenge of all, the one that really allows stealth aircraft to be stealthy and that causes them to be shaped so strangely.

Summary: shaping is the most important and is what contribute most to RCS reduction.

As to your quote from the article, it doesn't support your notion that an actual F-22 has to be reversed-engineering and built for RCS estimation. There are multiple reasons for this.

First, thickness is only one parameter. Reverse-engineering requires knowledge of all parameters. One simply does not need to know all the parameters as a prerequisite to find out one parameter.

Second, use of thickness only works against a narrow band of radar waves. It so happens that the thickness is also calculated based on the frequency that an opponent's radar uses. So, the opponent only needs to look at their own radars, then use the same calculations to figure out the skin thickness.

Given China has designed and built a stealth fighter, the country is already knowledgeable in stealth technologies. There is no need for China to reverse-engineer an F-22. All the talk about reverse-engineering is merely a requirement that you invented and doesn't exist in reality.

 

[Skywatcher]

"Last few percentages" and "a lot" do not go together. Materials only reduce a few percentages because material is least significant to RCS begin with. If you have actually read your own source thoroughly, you would have realized that. Let us go through a few paragraphs that you have obviously missed.

*cuts out the irrelevant strawmen*

Let me put this in simple terms that even you can understand.

So with shaping, lets' say that the RCS has been reduced by 90-95% by shaping compared to the F-15, which is about 15m2.

Now for that remaining 5-0.01%, that's where the RAM comes in, and also various other RCS tricks. Those are very complex things.

Second, use of thickness only works against a narrow band of radar waves. It so happens that the thickness is also calculated based on the frequency that an opponent's radar uses. So, the opponent only needs to look at their own radars, then use the same calculations to figure out the skin thickness.

If that's true, the AESA radars should have made stealth obsolete long ago.

Given China has designed and built a stealth fighter, the country is already knowledgeable in stealth technologies. There is no need for China to reverse-engineer an F-22. All the talk about reverse-engineering is merely a requirement that you invented and doesn't exist in reality.

So China uses the same stealth technology as the US? Pardon me, but that about as plausible as the USN and PLAN sharing the same nuclear propulsion technology. Just because you use technology based on similar technology doesn't mean you can predict how the other guys' technology will behave.

PS: A bit of basic education on terminology for you. Reverse engineering "is the process of discovering the technological principles of a device, object, or system through analysis of its structure, function, and operation" according to Eilam, Eldad & Chikofsky, Elliot J. (2007). Reversing: secrets of reverse engineering. John Wiley & Sons. p. 3.

 

[Blitzo]

*cuts out the irrelevant strawmen*

Let me put this in simple terms that even you can understand.

So with shaping, lets' say that the RCS has been reduced by 90-95% by shaping compared to the F-15, which is about 15m2.

Now for that remaining 5-0.01%, that's where the RAM comes in, and also various other RCS tricks. Those are very complex things.

And it is that final 5-0.01% that will be significant.

I think what engineer saying isn't incorrect, it is true that shaping determines the vast majority of an aircraft's RCS reduction, and China may be able to completely replicate the shaping of an F-22, but the difference between an F-22 with shaping and Chinese applied RAM and the difference between an F-22 with US RAM may be only a few square centimeters, however with today's radar technology that few square centimeters may be highly significant.

Continuing the F-15 analogy:
An F-22 without RAM may have a 95% reduced RCS compared to F-15
An F-22 stand in with Chinese RAM may have a 97.5% reduced RCS compared to F-15.
An F-22 built by lockheed may have a 99.5% reduced RCS compared to F-15 (due to say, better RAM, and some "minute" differences in structure and serration that add up)
So while the difference is "only" 2% between a test dummy and the real thing, it may not be operationally negligible due to the power of modern radar. Unless we know what that difference means in terms of say, detection and tracking ranges by modern PLA IADS, AEWC and fighter FCRs, we cannot definitively say whether the differential in test RCS and real RCS is practically significant.

In short: a 1950s radar may not be able to tell the difference between a "chinese" F-22 stand in and a lockheed F-22 (i.e.: it won't be able to detect either), but a modern 21st century AESA may be able to detect one at longer ranges than the other.

 

[latenlazy]

And it is that final 5-0.01% that will be significant.

I think what engineer saying isn't incorrect, it is true that shaping determines the vast majority of an aircraft's RCS reduction, and China may be able to completely replicate the shaping of an F-22, but the difference between an F-22 with shaping and Chinese applied RAM and the difference between an F-22 with US RAM may be only a few square centimeters, however with today's radar technology that few square centimeters may be highly significant.

Continuing the F-15 analogy:
An F-22 without RAM may have a 95% reduced RCS compared to F-15
An F-22 stand in with Chinese RAM may have a 97.5% reduced RCS compared to F-15.
An F-22 built by lockheed may have a 99.5% reduced RCS compared to F-15 (due to say, better RAM, and some "minute" differences in structure and serration that add up)
So while the difference is "only" 2% between a test dummy and the real thing, it may not be operationally negligible due to the power of modern radar. Unless we know what that difference means in terms of say, detection and tracking ranges by modern PLA IADS, AEWC and fighter FCRs, we cannot definitively say whether the differential in test RCS and real RCS is practically significant.

In short: a 1950s radar may not be able to tell the difference between a "chinese" F-22 stand in and a lockheed F-22 (i.e.: it won't be able to detect either), but a modern 21st century AESA may be able to detect one at longer ranges than the other.

I go back to what I said earlier about how knowing the science means understanding the best and worst case scenarios in a simulation.

 

[Engineer]

*cuts out the irrelevant strawmen*

Let me put this in simple terms that even you can understand.

So with shaping, lets' say that the RCS has been reduced by 90-95% by shaping compared to the F-15, which is about 15m2.

Now for that remaining 5-0.01%, that's where the RAM comes in, and also various other RCS tricks. Those are very complex things.

Let me make it simple for you in one statement: materials' contribution to RCS is least significant.

That has been made clear multiple times in a

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, which you now embarrassingly try to ignore by claiming it to be irrelevant.

The implication of this information is that your argument doesn't work, since your argument is based on an assumption that materials play a significant role. This information is thus very relevant.

If that's true, the AESA radars should have made stealth obsolete long ago.

Non sequitur. That particular method of stealth is obsoleted as a result of radar employing other frequencies. It has nothing to do with invention of AESA radar. This has also been mentioned in your own source.

So China uses the same stealth technology as the US? Pardon me, but that about as plausible as the USN and PLAN sharing the same nuclear propulsion technology.

Strawman argument.

Just because you use technology based on similar technology doesn't mean you can predict how the other guys' technology will behave.

Wrong. You can absolutely predict how the other guy's technology will behave, since laws of physics is the same to everybody.

PS: A bit of basic education on terminology for you. Reverse engineering "is the process of discovering the technological principles of a device, object, or system through analysis of its structure, function, and operation" according to Eilam, Eldad & Chikofsky, Elliot J. (2007). Reversing: secrets of reverse engineering. John Wiley & Sons. p. 3.

Quoting something which I already know isn't going to make your arguments more correct. You have yet to provide proofs that estimating F-22's RCS requires reverse-engineering a physical aircraft.