J-20... The New Generation Fighter II

My calculation for J-20's edge-diffraction radar return is 0.000016

From a frontal perspective, the obvious weakness in the J-20's stealth design in comparison to the F-22 is the presence of the J-20's canards. I have made an attempt to quantify the effect on the radar return from those canards to determine whether it is significant or not. Here is my post from another forum.



I made a mistake in calculating the edge-diffraction radar return. It should be lower. I was looking at Gambit's diagram for reflection and I realized that the J-20's RAM coating would absorb the incoming radar signal twice, not just once.

Therefore, the correct calculation is as follows:

1% x 1% x 40% x 40% = 0.01 x 0.01 x 0.4 x 0.4 = 0.000016 of the radar energy is reflected back due to edge diffraction. This is extremely small and it should be ignored.

Basically, I'm saying that curve-shaped canards, which are made from composite material and RAM-coated, should have no effect on the J-20's overall RCS.

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Let's do some math to get a feel for the insignificance of edge diffraction.

A. Pretend that you're shining a flashlight (e.g. an electromagnetic source in the visible-light range) at the edge of a piece of paper. How much light is hitting that paper's edge? Let's just say 1% of the total light being emitted by the flashlight. (Initial scatter)

B. A significant portion of the radar waves that impact upon the RAM-coated canard of the J-20 will be absorbed. Let's say 60% is absorbed and 40% is reflected. (Initial absorption)

C. Of the 1% of light that scatters due to edge diffraction, how much of it could theoretically bounce back toward the light source? Given the continuous curved surface of the J-20's body and shaped nose, let's say 1% makes it back. (Secondary scatter)

D. A significant portion of the radar waves that impact upon the RAM-coated body of the J-20 will be absorbed. Let's say 60% is absorbed and 40% is reflected. (Secondary absorption)

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"In electromagnetic wave propagation, the knife-edge effect or edge diffraction is a redirection by diffraction of a portion of the incident radiation that strikes a well-defined obstacle such as a mountain range or the edge of a building.

The knife-edge effect is explained by Huygens-Fresnel principle, which states that a well-defined obstruction to an electromagnetic wave acts as a secondary source, and creates a new wavefront. This new wavefront propagates into the geometric shadow area of the obstacle."



"2. A second way of stopping radar reflections is by coating the plane with material that soaks up radar energy. These typically consist of carbon or a magnetic ferrite-based substance. The result is that the B-2 is reported to have the same RCS as a child's tricycle! Radar-absorbing paints tend to consist of liquid chemical bases mixed with carbon or iron oxide powders. Still other coatings come in the form of sheets impregnated with iron oxides. These sheets, which are used on the Lockheed F-117 and Northrop B-2, can be cut or shaped to fit the outside surface panels of the host aircraft."

Refining my estimate for J-20 canards' initial scatter to 0.000032%

The J-20 has two canards. I've never been to an air show, but I'm a good guesser. I'll estimate that each canard is 1m (or 3 feet) long. I'll further estimate that each canard is 0.5m (or 1.5 feet) high. The total canard area facing an enemy radar is 1m x 0.5m x 2 [canards] = 1 m^2.

I don't have the faintest idea of the illumination cone for a directed military radar. However, to be of any use, I'll estimate that the illumination cone is 1km in radius. If the illumination cone is significantly smaller than 1km, I don't see how you can find an enemy fighter within a reasonable amount of time.

Area of a circle = pi * r^2 = 3.14 * (1km*1km) = 3.14 * (1,000m*1,000m) = 3.14 x 10^6 m^2

Initial scatter ratio = (1 m^2) / (3.14 x 10^6 m^2) = 3.185 x 10^-7 = 0.0000003185. My revised initial scatter percentage is now 0.000032%. My initial guess of 1% initial scatter was way too high.

Martian said:

From a frontal perspective, the obvious weakness in the J-20's stealth design in comparison to the F-22 is the presence of the J-20's canards. I have made an attempt to quantify the effect on the radar return from those canards to determine whether it is significant or not. Here is my post from another forum.

I made a mistake in calculating the edge-diffraction radar return. It should be lower. I was looking at Gambit's diagram for reflection and I realized that the J-20's RAM coating would absorb the incoming radar signal twice, not just once.

Therefore, the correct calculation is as follows:

1% x 1% x 40% x 40% = 0.01 x 0.01 x 0.4 x 0.4 = 0.000016 of the radar energy is reflected back due to edge diffraction. This is extremely small and it should be ignored.

Basically, I'm saying that curve-shaped canards, which are made from composite material and RAM-coated, should have no effect on the J-20's overall RCS.

----------

Let's do some math to get a feel for the insignificance of edge diffraction.

A. Pretend that you're shining a flashlight (e.g. an electromagnetic source in the visible-light range) at the edge of a piece of paper. How much light is hitting that paper's edge? Let's just say 1% of the total light being emitted by the flashlight. (Initial scatter)

B. A significant portion of the radar waves that impact upon the RAM-coated canard of the J-20 will be absorbed. Let's say 60% is absorbed and 40% is reflected. (Initial absorption)

C. Of the 1% of light that scatters due to edge diffraction, how much of it could theoretically bounce back toward the light source? Given the continuous curved surface of the J-20's body and shaped nose, let's say 1% makes it back. (Secondary scatter)

D. A significant portion of the radar waves that impact upon the RAM-coated body of the J-20 will be absorbed. Let's say 60% is absorbed and 40% is reflected. (Secondary absorption)

----------

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"In electromagnetic wave propagation, the knife-edge effect or edge diffraction is a redirection by diffraction of a portion of the incident radiation that strikes a well-defined obstacle such as a mountain range or the edge of a building.

The knife-edge effect is explained by Huygens-Fresnel principle, which states that a well-defined obstruction to an electromagnetic wave acts as a secondary source, and creates a new wavefront. This new wavefront propagates into the geometric shadow area of the obstacle."

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"2. A second way of stopping radar reflections is by coating the plane with material that soaks up radar energy. These typically consist of carbon or a magnetic ferrite-based substance. The result is that the B-2 is reported to have the same RCS as a child's tricycle! Radar-absorbing paints tend to consist of liquid chemical bases mixed with carbon or iron oxide powders. Still other coatings come in the form of sheets impregnated with iron oxides. These sheets, which are used on the Lockheed F-117 and Northrop B-2, can be cut or shaped to fit the outside surface panels of the host aircraft."

Click to expand...

Refining my estimate for RAM coating effectiveness up to "99.999%" reduction





"McGraw-Hill Science & Technology Encyclopedia:
Radar-absorbing materials

Materials that are designed to reduce the reflection of electromagnetic radiation from a conducting surface in the frequency range from approximately 100 MHz to 100 GHz. The level of reduction that is achieved varies from a few decibels to greater than 50 dB, reducing the reflected energy by as much as 99.999%.

Two methods have been widely adopted in order to produce such absorbers. The first is to avoid a discrete change of impedance at the material surface by gradually varying the impedance. The removal of the discontinuity at the surface allows the microwave energy to be transmitted into the absorbing medium without reflection. This transition from the impedance of free space to that of the bulk material is commonly achieved by a geometric profile. The carbon-loaded foam pyramids used as the lining of anechoic chambers are typical of this type of absorber. To produce such absorbers, it is necessary in practice to taper the material over distances which are large compared with the wavelength of the frequencies to be absorbed. Therefore, practical absorbers of this type giving greater than 20 dB absorption vary in thickness from about 0.8 in. (2 cm) at 10 GHz and above to 6 ft (2 m) at 100 MHz and above. The absorber performance improves with increasing thickness until the point is reached where all of the energy that enters the material is absorbed and only the front-face reflection is left. While this type of absorber is capable of producing a very high degree of absorption over a broad bandwidth, it is at the same time a relatively thick material. See also Anechoic chamber.

The second method of absorber design produces much thinner absorbing layers which are capable of producing good absorption (≥25 dB) with restricted bandwidths. These materials achieve the absorption by a combination of attenuation within the material and destructive interference at the interface. The electromagnetic properties and the thickness of the layer are such that the initial reflected wave and the sum of the emergent rays resulting from the multiple reflections within the material are equal in magnitude and opposite in phase. The thickness of the layer is close to a quarter-wavelength at the frequency of operation, giving a 180° phase difference between the interface reflection and the emergent waves. See also Interference of waves.

Microwave-absorbing materials are widely used both within the electronics industry and for defense purposes. Their uses can be classified into three major areas: (1) for test purposes so that accurate measurements can be made on microwave equipment unaffected by spurious reflected signals; (2) to improve the performance of any practical microwave system by removing unwanted reflections which can occur if there is any conducting material in the radiation path; and (3) to camouflage a military target by reducing the reflected radar signal. See also Electronic warfare; Microwave; Microwave measurements; Reflection of electromagnetic radiation."

China stealth fighter 'copied parts from downed US jet'

The technology behind China's J-20 stealth fighter may have come from a US plane shot down during the Kosovo war, Balkan military sources say.

Adm Davor Domazet-Loso, Croatia's military chief of staff at the time, claims Chinese agents took parts of a downed F-117 stealth jet in 1999.

The F-117 had been shot down by a Serbian anti-aircraft missile during a Nato bombing raid.

China's J-20 stealth fighter had its first test flight earlier this month.

Currently, the United States is the only nation with a fully operational stealth plane.

'Reverse-engineer'
Nighthawks were the world's first stealth fighters - planes that are almost invisible to radar.

When the F-117 was shot down in 1999 during Nato bombing, it was the first time one of the jets had been hit.

Military officials and experts say they believe that some of the parts found themselves in Chinese hands, which allowed China to replicate them to develop similar technologies.


The Pentagon has downplayed concerns over China's stealth fighter
"At the time, our intelligence reports told of Chinese agents criss-crossing the region where the F-117 disintegrated, buying up parts of the plane from local farmers," Adm Domazet-Loso told the Associated Press news agency.

"We believe the Chinese used those materials to gain an insight into secret stealth technologies... and to reverse-engineer them," he said.

A senior Serbian military official confirmed that some of the pieces were removed by souvenir collectors, and that some ended up "in the hands of foreign military attaches".

Alexander Neill, head of the Asia Security Programme at the Royal United Services Institute, said the proposition was highly possible, as Serbia and China had a very close relationship during this time and routinely shared intelligence.

"At that time the Chinese had a close relationship with Serbian military intelligence and in that regard - just as an intelligence-sharing relationship - a windfall such as a F-117 would be gold dust, and any modernising military worth its salt would examine anything of that nature extremely closely," he told the BBC.

He said it would be very difficult to know for sure, but that China had during that time a "rapacious appetite" for technology which would help them with their modernisation programme.

"Given the history of Chinese espionage, it is clear that China has had a very covert history of acquisition," he said.

'No interest'
Parts of the F-117 wreckage, including its left wing, cockpit canopy, ejection seat, pilot's helmet and radio, are on show at Belgrade's aviation museum.

Zoran Milicevic, deputy director of the museum, said: "I don't know what happened to the rest of the plane. A lot of delegations visited us in the past, including the Chinese, Russians and Americans. But no-one showed any interest in taking any part of the jet."

China's Chengdu J-20 stealth jet is expected to be operational some time between 2017 and 2019.

Some analysts have claimed this sign of military strength will worry the US government, but the Pentagon has played down concerns over the fighter.

"Developing a stealth capability with a prototype and then integrating that into a combat environment is going to take some time," US director of naval intelligence Vice-Admiral David Dorsett said earlier this month.

China's official military budget quadrupled between 1999 and 2009 as the country's economy grew. In 2010 it stood at $78bn (£50bn).

But the US has by far the largest defence budget in the world at just over $700bn.

I'm surprised it took so long for this accusation to appear. A lot of news sites are using titles like, "China's Stealth Fighter Uses Stolen US Technology." big and bold at the top of the article, and then adding, "Claims some official." buried in the middle of a paragraph. BBC was the only site I've seen so far that at least put the accusation in quotes in the title, lol.

They just won't resist any chance to be ignorant...oops I mean "delivering the truths"

I do think that the Chinese may have gotten hold of some samples for studying in the early years of the project's development, but I quite believe the stealth technology used on the J-20 today is made in China, developed indigenously in China, and can only be found in China. What I mean is that while stealth tech was prioneered by the US, I'd believe the Chinese developed its own stealth technology that has its own design, characteristics, that would make it unique on its own. In other words, its own industry. Therefore it's fallible and ignorant to say it copied off something else. We can't say Linux copied Windows just because they have similar interface, or that Andriod is a copy of Iphone because both are smart phones. Each of them are their own brands, with their own technology and approaches. Monkeys really do run the tabloids in mass newspaper outlets all over the world.

Yeah, this news is one that we cannot deny.

That being said, I'd want to pull some of its conclusions in dishonour.
Chinese agents were present, picking up pieces? Sure. Lots of people were. The article points it out. If I said Norwegian agents were present picking up interesting information about RAM, would that be interesting news? No.

Until China launched their own stealth jet, this was surely uninteresting news. Something most bureaus overlooked (the news was made in the late 90s!). It barely made the news! Now that China's made their jet, they suddenly collect every news bit they can related to it and say: Hey! look at this, we knew all along, and it's old.

It's about conforming news to your 早知道 (had-I-known-earlier)-viewpoint. I'd hate to say this, but this is exactly the same kind of reporting we see after each terror attack (I'm referring to the Scandinavian ones here, to not step on anyone's toes), where every paper is emphasizing how every intelligence bureau had all knowledge prior.

OK, so my point is, this is purely conforming prior events to current conclusions (Aha, the Japanese bombed Pearl Harbour, so every action they took from the 1930s-on was leading up to a showdown with the US). And in this case I feel it's been proven that this talk about the F-117 is too outdated to give any complete awareness about stealth that would be required making this aircraft.

Maggern said:

Yeah, this news is one that we cannot deny.

That being said, I'd want to pull some of its conclusions in dishonour.
Chinese agents were present, picking up pieces? Sure. Lots of people were. The article points it out. If I said Norwegian agents were present picking up interesting information about RAM, would that be interesting news? No.

Until China launched their own stealth jet, this was surely uninteresting news. Something most bureaus overlooked (the news was made in the late 90s!). It barely made the news! Now that China's made their jet, they suddenly collect every news bit they can related to it and say: Hey! look at this, we knew all along, and it's old.

It's about conforming news to your 早知道 (had-I-known-earlier)-viewpoint. I'd hate to say this, but this is exactly the same kind of reporting we see after each terror attack (I'm referring to the Scandinavian ones here, to not step on anyone's toes), where every paper is emphasizing how every intelligence bureau had all knowledge prior.

OK, so my point is, this is purely conforming prior events to current conclusions (Aha, the Japanese bombed Pearl Harbour, so every action they took from the 1930s-on was leading up to a showdown with the US). And in this case I feel it's been proven that this talk about the F-117 is too outdated to give any complete awareness about stealth that would be required making this aircraft.

Click to expand...

In other words, you're referring to hindsight bias

Of course there are those that believed that China obtained some of the wreckage and for that the embassy was bombed to destroy it even more. But none of them believed China, especially then, was capable of learning that much from it because of the same reason why so many were surprised with the emergence of the J-20. Those who believe the embassy was bombed to prevent China from learning from it aren't official from the government. The real reason they want to believe that was the case is because they don't want to believe that the US could possibly make a mistake like that. I bet a couple years from now the skeptics of the J-20 will tell everyone that they always believed China was fully capable of producing a stealth fighter.

China is simple doing what other countries,including US to obtain any info or intelligent regarding specific technology.