Radar Absorbing Materials and Structures (RAM / RAS), stealth (LO) coatings

[Hanse]

I thought about posting this in the J-20 thread but maybe those posts could be duped/moved here or something.

This picture of the F-22 has always made me do a double-take. The paint doesn't seem to be the same as that on the current aircraft. And while it's obviously not making the aircraft transparent it doesn't seem to be your average paint. Thoughts?

 

[kcran567]

Awaiting re-painting, or testing to measure how stealthy it still is with degraded coatings?

 

[Hanse]

Awaiting re-painting, or testing to measure how stealthy it still is with degraded coatings?

Forget the gray panels and edges. I'm talking about all of that in between.

 

[sublight_]

Forget the gray panels and edges. I'm talking about all of that in between.

It's going to be hard to do any analysis with that photo. It has some weird over exposure problems that were fixed after the fact. Look at the background along the top 15% of the photo. When you get 3/4 of the way over to the left side, it suddenly transitions from over exposed to correctly exposed. If you can get the unedited original it might give you a better idea of what you are looking at.

 

[Hanse]

Forget the gray panels and edges. I'm talking about all of that in between.

It's going to be hard to do any analysis with that photo. It has some weird over exposure problems that were fixed after the fact. Look at the background along the top 15% of the photo. When you get 3/4 of the way over to the left side, it suddenly transitions from over exposed to correctly exposed. If you can get the unedited original it might give you a better idea of what you are looking at.

Wish I could but that was the only copy I've ever seen of that shot (and it was from 1999). The port vertical stab almost looks translucent.

 

[J.A.W.]

Translucent, S?

Are you suggesting some form of 'cuttlefish' (if not 'Philadelphia') type of effect is in use?

 

[Hanse]

Translucent, S?

Are you suggesting some form of 'cuttlefish' (if not 'Philadelphia') type of effect is in use?

No, I said it almost looked translucent, not that it was.

 

[J.A.W.]

I get that S, but I am asking, is it an 'active*' effect & not just paint, or even a reactive (tonal) light response?


* akin to the Rorschach face mask in the 'Watchmen' flick.

 

[quellish]

I thought about posting this in the J-20 thread but maybe those posts could be duped/moved here or something.

This picture of the F-22 has always made me do a double-take. The paint doesn't seem to be the same as that on the current aircraft. And while it's obviously not making the aircraft transparent it doesn't seem to be your average paint. Thoughts?

1. Photo has been heavily altered.
2. Aircraft is in primer that was used earlier in the program (1997-1998ish).
3. Aircraft is not wearing any stealth coatings.

IF you ever watch the movie "Armageddon", there is a scene at EDW where the drill crew is introduced to NASA/USAF's "best astronauts". Those guys are (mostly) the F-22 test team at the time.

 

[Vanessa1402]

How thick is the Radar absorbing coating of fighter?. Is it 1 mm? 2 mm? or 4 mm?

 

[BAROBA]

I would say as thin as possible... RAM is build out of different materials.The metal outerskin could be anything from 1 layer of atoms to a few 100 micrometers. Then you got a much thicker and much lighter isolated-layer that forms a buffer between the outer-skin and the frame. This layer can be up to 5 cm (or more) thick on some spots, but it is unknown if it is so on all spots of a stealthfighter and if all stealth-fighters has such thick layers.

 

[Vanessa1402]

I asked this because even on the same aircraft, the thickness of the radar absorbing material seem extremely inconsistent
In some photos the RAM layer seem very thick, like 3-4 mm thick




In some other photos, it look like the RAM is not even 0.1 mm thick

 

[TomcatViP]

@Vanessa1402 : It depends on how much energy need to be absorbed. Hence the relative geometry of the plane regarding the said panel is driving the RAM thickness.
It's also important that there isn't any discontinuity in conductivity. Hence you can't simply coat with a panel with a thicker layer simply saying that the value will make for all panels in an area.
Also a single panel might also have varying RAM thickness on its own surface in function of the two point above (aircraft geometry and surrounding panel properties).
Most RAM are ideally applied via a robotic process. However, manual modifications may have to be done in squadron and an iterative process and stringent procedures have to be followed.
Notice that there is a delta and an Aircraft might have varying overall rcs depending of maintenance history. Everything is accounted for the value staying in a coherent and manageable range.

6th Gen representations are often seen maintained throughout robotic pit stop for this reason. Will that be true in the future?

 

[Ronny]

I don't know about thickness but current RAM allegedly can reduce reflection by 6-10 dB

 

[quellish]

I don't know about thickness but current RAM allegedly can reduce reflection by 6-10 dB

There are a lot of different variables at work - frequency, etc. - but ~-20db is more typical for modern aircraft. And that is approaching the theoretical and practical limits of RAM.

The depth of the coating stack on a given aircraft will vary. In some areas it will be deeper than others. Maybe the angle to a ground threat radar means that it needs to be deeper in one place. Or maybe in an area where energy striking another surface collects / focuses. Or the whole stack is deeper / more complex because of the properties of the underlying structure (i.e. the structure is plastic and needs several layers of other stuff to reflect and absorb, etc.).

On one modern, large aircraft the layer of magnetic RAM in the coating stack varies from 0.5mm to 2.25mm

 

[Ronny]

I don't know about thickness but current RAM allegedly can reduce reflection by 6-10 dB

There are a lot of different variables at work - frequency, etc. - but ~-20db is more typical for modern aircraft. And that is approaching the theoretical and practical limits of RAM.

The depth of the coating stack on a given aircraft will vary. In some areas it will be deeper than others. Maybe the angle to a ground threat radar means that it needs to be deeper in one place. Or maybe in an area where energy striking another surface collects / focuses. Or the whole stack is deeper / more complex because of the properties of the underlying structure (i.e. the structure is plastic and needs several layers of other stuff to reflect and absorb, etc.).

On one modern, large aircraft the layer of magnetic RAM in the coating stack varies from 0.5mm to 2.25mm

Would you mind me asking where you get that from? and by large aircraft, do you mean like B-2 or F-18 E/F?
Some sources I can find show that the absorbing value is about 5 dB for 2 mm thick layer and 10-30 dB for 3 mm thick layer. 1mm layer seem kinda useless. Though maybe that the characteristic of that specific RAM

 

[Ronny]

Fun fact, the inlet of F-35 and F-22 use some sort of RAM with white color, unlike the outer skin

 

[flateric]

I asked this because even on the same aircraft, the thickness of the radar absorbing material seem extremely inconsistent

it's extremely inconsistent even on small parts

 

[Vanessa1402]

I asked this because even on the same aircraft, the thickness of the radar absorbing material seem extremely inconsistent

it's extremely inconsistent even on small parts

View attachment 679763

What is that image?. Is it the skin of the F-35 or something else?. Does the number represent the thickness in mm or nanometer?
How do you know that is the RAM thickness and not the full skin thickness?

 

[Vanessa1402]

@Vanessa1402 : It depends on how much energy need to be absorbed. Hence the relative geometry of the plane regarding the said panel is driving the RAM thickness.

imho, the energy to be absorbed will be the same as it is on the same fighters, both the wing and fusalage are exposed parts.

 

[flateric]

What is that image?. Is it the skin of the F-35 or something else?. Does the number represent the thickness in mm or nanometer?
How do you know that is the RAM thickness and not the full skin thickness?

this is F-35 DSI bulb area; of course, RAM thickness numbers are deliberately distorted

 

[Ronny]

I asked this because even on the same aircraft, the thickness of the radar absorbing material seem extremely inconsistent

it's extremely inconsistent even on small parts

View attachment 679763

Can you tell us where that screenshot come from?

 

[Vanessa1402]

Are those RAM panel?

 

[quellish]

If you mean the green panels, no those are coated in primer. They have an edge treatment that is conductive or impedance matching.

 

[Vanessa1402]

If you mean the green panels, no those are coated in primer. They have an edge treatment that is conductive or impedance matching.

No, I mean the gray panel, like in this photo

 

[quellish]

If you mean the green panels, no those are coated in primer. They have an edge treatment that is conductive or impedance matching.

No, I mean the gray panel, like in this photo
View attachment 679877

On the F-35 those are carbon skin panels. They may or may not have a coating of RAM applied.

 

[Ronny]

On the F-35 those are carbon skin panels. They may or may not have a coating of RAM applied.

To be fair though, isn't carbon fiber itself also absorb radar wave?

 

[TomcatViP]

I asked this because even on the same aircraft, the thickness of the radar absorbing material seem extremely inconsistent

it's extremely inconsistent even on small parts

View attachment 679763

Can you tell us where that screenshot come from?

I would double down on that one Very curious to see where those comes from.

Let'snot forget that it's a Navy plane (a C model?). There might be some differences with how the A is built to reflect the increase wear due to carrier operations.

 

[quellish]

On the F-35 those are carbon skin panels. They may or may not have a coating of RAM applied.

To be fair though, isn't carbon fiber itself also absorb radar wave?

It can reflect or absorb depending on how it is constructed. On the F-35 the skin panels reflect

 

[Vanessa1402]

If you mean the green panels, no those are coated in primer. They have an edge treatment that is conductive or impedance matching.

No, I mean the gray panel, like in this photo
View attachment 679877

On the F-35 those are carbon skin panels. They may or may not have a coating of RAM applied.

Afaik, the carbon fiber skin (Graphite exposy or/and BMI) are the one with the green primer

 

[Vanessa1402]

I asked this because even on the same aircraft, the thickness of the radar absorbing material seem extremely inconsistent

it's extremely inconsistent even on small parts

View attachment 679763

Can you tell us where that screenshot come from?

I would double down on that one Very curious to see where those comes from

Count me in, I also want to know

 

[quellish]

Metallic structures are coated in primer as well, not just carbon.

On that illustration the RAM is not shown, it is below the top coat.

 

[Hanse]

If you mean the green panels, no those are coated in primer. They have an edge treatment that is conductive or impedance matching.

No, I mean the gray panel, like in this photo
View attachment 679877

On the F-35 those are carbon skin panels. They may or may not have a coating of RAM applied.

Afaik, the carbon fiber skin (Graphite exposy or/and BMI) are the one with the green primer
View attachment 679981
View attachment 679980

Where are these pictures from?

 

[Ronny]

On the F-35 those are carbon skin panels. They may or may not have a coating of RAM applied.

To be fair though, isn't carbon fiber itself also absorb radar wave?

It can reflect or absorb depending on how it is constructed. On the F-35 the skin panels reflect

Isn't that a bit counter productive for stealth aircraft?.

 

[quellish]

On the F-35 those are carbon skin panels. They may or may not have a coating of RAM applied.

To be fair though, isn't carbon fiber itself also absorb radar wave?

It can reflect or absorb depending on how it is constructed. On the F-35 the skin panels reflect

Isn't that a bit counter productive for stealth aircraft?.

No.
You can always reflect more energy than you can absorb. That was the breakthrough in stealth in the 70s, designing to reflect energy away from the emitter. Shape is the cake, materials aren’t even the icing. Shape is all about reflection

 

[Ronny]

It can reflect or absorb depending on how it is constructed. On the F-35 the skin panels reflect

Isn't that a bit counter productive for stealth aircraft?.

No.
You can always reflect more energy than you can absorb. That was the breakthrough in stealth in the 70s, designing to reflect energy away from the emitter. Shape is the cake, materials aren’t even the icing. Shape is all about reflection

I don't think you understand my question. For reflection, you can always coat the lowest layer with thin layer of metal. But if the layers on top are RAM, then they can help absorb the radar wave before they are reflected. Thicker the RAM, the lower the effective frequency and also better absorbing capability

 

[quellish]

It can reflect or absorb depending on how it is constructed. On the F-35 the skin panels reflect

Isn't that a bit counter productive for stealth aircraft?.

No.
You can always reflect more energy than you can absorb. That was the breakthrough in stealth in the 70s, designing to reflect energy away from the emitter. Shape is the cake, materials aren’t even the icing. Shape is all about reflection

I don't think you understand my question. For reflection, you can always coat the lowest layer with thin layer of metal. But if the layers on top are RAM, then they can help absorb the radar wave before they are reflected. Thicker the RAM, the lower the effective frequency and also better absorbing capability

The thin layer of metal is a maintenance nightmare. The reflective skin panels are an attempt at a solution.

Thicker ram doesn’t necessarily mean it absorbs more of the target frequency and it is heavy and difficult to maintain.

 

[Ronny]

The thin layer of metal is a maintenance nightmare. The reflective skin panels are an attempt at a solution.

Why would the layer of metal be a maintenance nightmare if it is underneath the RAM layer and completely shielded from the environment? at least that how it seem on F-22, the conductive coating is below the primer and top coat

Thicker ram doesn’t necessarily mean it absorbs more of the target frequency and it is heavy and difficult to maintain.

with the same permittivity and permeability, it would seem logical that thicker layer of RAM will absorb more radiation or at very least have lower effective frequency. I vaguely recall reading somewhere that RAM on F-35 is slightly different as it is imbedded inside the composite and called the fibermat

 

[quellish]

Why would the layer of metal be a maintenance nightmare if it is underneath the RAM layer and completely shielded from the environment? at least that how it seem on F-22, the conductive coating is below the primer and top coat

- Easy to damage, difficult to repair/remove
- Any other maintenance requires repairs to the conductive coating and verification
- Corrosion and voids
- Degrades over time
- Expense
- Too many more reasons to list

The conductive coating is responsible for the majority of the maint. hours on the F-22.

with the same permittivity and permeability, it would seem logical that thicker layer of RAM will absorb more radiation or at very least have lower effective frequency. I vaguely recall reading somewhere that RAM on F-35 is slightly different as it is imbedded inside the composite and called the fibermat

No, RAM does not necessarily behave that way. Thicker is rarely "better". There is an ideal depth for a given material. Modern aircraft often have several stacked layers of RAM that are meant to work together. For example, a low frequency coating that allows higher frequencies through with some attenuation, and below that a high frequency material. The inlets of the F-22 have about a dozen layers of coating.

"Fiber mat" is an RF reflective carbon composite, as mentioned previously. It is intended to replace the need for a conductive coating on the OML. After a layer of primer RAM is applied to that material.

 

[BAROBA]

As I understand it is that Stealth is 80-90% shaping of the plane.You should be able to make a plane out of metal in a specific shape, and it would reflect less then a plane that is not shaped.
RAM coating is to catch those rays that come from another angle then that the plane is designed for. All beams in nature get at least a little diffused by the environment and will come in from angles that will reflect in such a way that they will end up straight in the enemy's radar. You don't want that. What you can do is diffuse the incoming angle so that the outgoing angle is nowhere near the original angle. So that is why the RAM is filled with little metal pieces that act like little mirrors. It still wont catch every ray, but you get rid of a lot of random noise that could tell you are there.
I think RAM is also used to stop electrical currents from riding on the plane and being emitted from any point that sticks out of the fuselage, being a break in the surface and/or a wingtip or even a screw. ( Don't know much about electricity and related stuff, maybe one of the experts can enlighten us.) But read everything Quellish writes, he knows his stuff