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The rules of stealth

BAROBA

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Hello all,

I was working a time ago on a design book for planes, I always loved drawing planes.
And I like the lines on stealth aircraft.
So I was wondering if anybody knows all the rules for designing a stealth aircraft?
This forum is a great resource for finding out about a lot of the different rules, but some posts have altered my perspective on how solid those rules are.

Also the rules seem to have changed throughout the years, did different companies have different rules?
It seems only logical that after time companies discovered all the rules and applied them to their designs.

How would the future of stealth look like, with the emergence of new technologies that could radically change the looks of an aircraft?

Any comments ideas and insights are welcome :)

Cheers,

Rob
 

Kryptid

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True "low observable" aircraft typically apply several techniques simultaneously in order to produce very low radar cross-sections (RCS). Here are some of these techniques:

Radar-absorbing Materials: Stealth aircraft typically have special coatings on their exterior which are designed to absorb radio emissions and convert them into heat or infrared radiation. This reduces the amount of radio waves that return to the radar of the enemy aircraft.

Reduced Metal Content: Metal is a very good reflector of radio waves, so reducing the amount of metal used in an aircraft helps to reduce its RCS. New aircraft tend to have increased amounts of composite materials in their design. These not only reduce RCS, but they are also lighter and stronger than many metals.

Internal Weapons Bay: Missiles, bombs and fuel tanks that are carried externally serve as good reflectors of radio waves. Carrying them internally hides them from radar. The F-22 can carry 8 missiles internally (6 AMRAAMS and 2 Sidewinders). Doing this also reduces drag because the missiles are not out there in the wind.

Hidden Engine Face: The blades that make up the fan and compressor section of a jet engine are very good at reflecting radio waves. For this reason, you would want to keep the engine face out of view of a prying radar. This can be achieved in one of two ways:

Curved Inlet Ducts: This is how the F-22 hides its engines from radar. If you look at pictures of the F-22, you may notice that you are unable to see the engine face, no matter what angle you view it from. The air can flow down the curved inlet channels to the engines, but radio waves must bounce around inside of the inlet channels and lose energy.

Radar Blockers: The solution utilized by the X-32 was to implement a radar-blocking device to hide the engine face from radar. The exact nature and design of this device is classified. Radar blockers are generally believed to cause extra drag when compared to curved inlet ducts. However, other design considerations may make its usage unavoidable.

Shaping: The vast majority of RCS reduction in a stealth aircraft can be attributed to its shape:

Faceting: An early way to decrease RCS was to design the aircraft out of sharply-angled facets that would reflect radio waves well away from a radar source. The more sharply swept each facet is, the less radio waves that are reflected back to the radar that sent them. Take a look at the F-117. It has a wing sweep of 72.5 degrees. This is exceptionally high for a subsonic aircraft. The shaping of an F-117 reduces its signature down to the size of a bird or a ball-bearing (depending on which source you hear it from). Unfortunately, faceting is not good for aerodynamics.

Planform Alignment: A newer way to decrease RCS has been implemented on the B-2, F-22 and F-35. The technique of planform alignment involves minimizing the number of different angles on the aircraft in order to reflect radio waves in particular directions. Take the F-22 for example. The wingsweep is 42 degrees. The sweep of the stabilators, inlet lip, and vertical stabilizers is also 42 degrees. By doing this, the radio waves that hit the F-22 on these surfaces are all reflected by the same amount away from the radar. This is as opposed to a standard aircraft planform which reflects radio waves at many different angles at once.

Blending: Aircraft that implement planform alignment also implement blending. This involves smoothly blending different parts of the aircraft together so as to create smooth, continuous curves that radio waves can flow around. Take a look at the F-22. The fuselage, canopy, wings, and engine nacelles are all blended smoothly into each other. This makes the aircraft far more aerodynamic than a faceted aircraft would be.

Tailless-ness: Removal of the vertical stabilizers can dramatically reduce RCS from the side hemispheres. The B-2 is designed this way. The X-36 is also tailless. 6th generation fighter planes may be tailless as well (Take a look at the F/A-XX design proposed by Boeing).
 

Firefly 2

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Plasma stealth.

I first read about in an Air International feature of the MIG 1.44. It's big in the latest few pages of the PAK FA thread.

http://www.secretprojects.co.uk/forum/index.php/topic,5644.0.html

This will not help you, I'm sorry, I just wanted to post this for the sake of correctness.
 

SOC

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Cockpit canopy glass must also be treated, usually with gold, to make it non-transparent for radar. Otherwise, the cockpit and all of the internal structures can be a good source of reflected energy.

Emission control is also important for designing a truly LO aircraft. This is where LPI radars, passive detection systems, and the like come in, as they greatly reduce the chance of an enemy locating your aircraft using ESM systems. If you forget about this part, then you've basically wasted your effort.

Super Hornet also uses radar blockers.

F-117 used an external radar blocker in the intake in the form of a screen designed with openings small enough to prevent fire control radar wavelengths from entering the intake duct.

Antenna location and design is also important. It won't do to have a clean profile and then clutter it up with old-style blade antennae for IFF and other systems. Look at the Raptor, specially designed antennae are mounted flush with the skin of the aircraft.
 

BAROBA

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A remark and a question :
The f-22 does seems to have a full metal skin, if you look at this picture.
( http://www.aerospaceweb.org/aircraft/fighter/f22/f22_09.jpg )
In the natgeo- special 'Hitlers stealth fighter', the Northrop guys said that the skin has to be as conductive as metal in order for stealth to work. Is this a picture of a F-22 without RAM-materials applied?

How much detail can you leave on a plane without it getting un-stealthy? Something in the order of centimeters? because the F-35c liftfan-doors on its back show pretty large spaces between the fuselage and the doors.

thanks in advance,

Rob
 

quellish

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BAROBA said:
A remark and a question :
The f-22 does seems to have a full metal skin, if you look at this picture.
( http://www.aerospaceweb.org/aircraft/fighter/f22/f22_09.jpg )

What makes you think that is metal?
 

dragon72

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BAROBA said:
A remark and a question :
The f-22 does seems to have a full metal skin, if you look at this picture.

That's just it's normal paint with a trick of the light. If you look on the right vertical stabilizer you can see the different paint tones.
Here's a more natural tone.
 

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BAROBA

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Metal has a very board specular reflection.( the wide smear of light on the surface) Paint is has a much smaller specular reflection(it would show only a small dot or thin lines), pretty much like rubber or glossy plastic in the example picture that I link to. http://www.evvisual.com/maxscript/images/ContactSheet-3.jpg
Specular reflection is something you see when the light-source is across you on the other side of the surface in question. So the picture that dragon72 posted doesn't show any specular reflection, because it is impossible to see it from that angle.
I work as a 3d artist and I am pretty good at recreating realistic metal (effects) and this F-22 looks as if it is made out of metal. If it ain't metal ( which I also presume) what else is it? The common explanation of RAM-coating is rubber with metal flakes in it. But rubber doesn't have that broad specular reflection which you can see on the photo.
 

overscan (PaulMM)

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Getting back to the Rules of Stealth - here is an RCS plot of a very early B-2 model. You can see that the effect of aligning the edges of the platform gives 4 spikes where the radar cross section is concentrated. Outside those narrow angles, the radar cross section is much smaller.

 

sferrin

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BAROBA said:
Metal has a very board specular reflection.( the wide smear of light on the surface) Paint is has a much smaller specular reflection(it would show only a small dot or thin lines), pretty much like rubber or glossy plastic in the example picture that I link to. http://www.evvisual.com/maxscript/images/ContactSheet-3.jpg
Specular reflection is something you see when the light-source is across you on the other side of the surface in question. So the picture that dragon72 posted doesn't show any specular reflection, because it is impossible to see it from that angle.
I work as a 3d artist and I am pretty good at recreating realistic metal (effects) and this F-22 looks as if it is made out of metal. If it ain't metal ( which I also presume) what else is it? The common explanation of RAM-coating is rubber with metal flakes in it. But rubber doesn't have that broad specular reflection which you can see on the photo.

Never heard of the F-22 being coated with metal impregnated rubber. Call it "funky paint" and leave it at that. And there are at least two types of paint that have appeared on the F-22 that I've seen. One looks like plain old paint, the other looks "metallic". There are also shots of the V-22 with this "metallic" looking paint on it. (That contact sheet looks like someone's been to the V-ray Materials site. ;) )
 

Sundog

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I've heard the new paint on the F-22, which is also now being used on V-22's, referred to as metallic, but I think that's a reference to it's 'reflective capabilities'. The main reason to have that is, at least visually, a pilot usually spots another aircraft at a distance as a block dot, because it blocks out the diffuse light, which is why you can see it at a distance. However, if the aircraft reflected just enough light at a distance, you could theoretically make it blend in at long distance much better than with standard camo schemes where you get the black dot effect just as with any aircraft.

The military hasn't given a reason for going to this new paint scheme, to the best of my knowledge, but I just sort of think of it as a "Passive Yehudi Light System," IMHO.
 

sferrin

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Exactly. There are a lot of pictures out there that demonstrate that. The F-22 (the paint on it anyway) seems to do a pretty good job of picking up the surrounding color. There are a couple of the 2nd F-22A over Edwards where parts of the plane almost look transparent.
 

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The best summary I ever heard was "make a very bad antenna, and then make it fly".

The steps to doing this are:

1 - Decide what threats you want to defeat by stealth
2 - Shape to scatter as much radar energy as possible away from the transmitter
3 - Where energy might be backscattered in the direction of the radar, focus it into spikes that will create only transient echoes as the position relationship of the radar and target changes
4 - Clean up remaining scattering with RAM/RAS, to the greatest practical extent
5 - Minimize apertures and suppress what you must have by location and edge treatment
6 - Use non-emitting systems where it is possible
7 - Use LPI techniques when it is not
8 - Match visual, IR, acoustic, contrail and smoke emissions to RF detection range
 

Woody

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It might not be strictly on topic but why is the detecting antenna always positioned at the same point as the illuminating antenna? It would seem to me that the easiest way to negate most radar stealth strategies would be to put the two some distance appart then all those efforts not to return radio waves back to the transmitter would be for nothing. Just a thought.

Cheers, Woody

PS: I'd put the illuminator in an expendable drone with an laser data link as it would obviously become a target :).
 

AeroFranz

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You are not alone in thinking that ;)

this is the principle bi-static radar works on.
 

donnage99

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Woody said:
It might not be strictly on topic but why is the detecting antenna always positioned at the same point as the illuminating antenna? It would seem to me that the easiest way to negate most radar stealth strategies would be to put the two some distance appart then all those efforts not to return radio waves back to the transmitter would be for nothing. Just a thought.

Cheers, Woody

PS: I'd put the illuminator in an expendable drone with an laser data link as it would obviously become a target :).
I'm pretty ignorant in radars, but here's my 2 cents:

There's the problem with knowing exactly where to put the radar, at what distance and what angle. And to do that miracle, you first need to know the flight path, and the altitude of the stealth plane itself to position them perfectly. Even if you gonna put them in perfect positions to do that. The angle and distance will constantly change because the stealth target doesn't stand still, it's constantly moving, especially if it's gonna have a radar warning reciever, you bet it's gonna try everything to fool the enemy's radars. So the transmitter and receiver antennas are gonna have to be able to constantly moving with it. This would require the 2 radars to have exceptional mobility to keep up with the target, a slight change in direction of the target would require you to instantly travel hundreds of miles (depending how great the distance is between the target and the 2 respective radars).

Knowing where to put the radars require some miracles, but to be able to put them where you want require some more miracles. There are 2 basic types of bistatic radars as far as I know: 2 antennas placed at such a position that the transmitting antennas radio wave gonna hit the aircraft, bounce back straight into the receiver antennas at the respective angle. This require not only to know the position in term of distance and angle of the aircraft, it also require to know exactly the angles of "spikes" radar return of the aircraft itself, pretty much know the aircraft as much as the its creators. This might be hard as the details of the rcs of an aircraft are kept in very tight security. And as I said, a shift of the aircraft in its direction would require the antennas and receivers to travel hundred of miles to keep a constant angle. A way to minimize this is to use multiple receivers scattered all over the area. This is what I called a system of system of system. And I believe by the time people can create such system, today stealth technology would be made obsolete by other much more convenient and technologically more realistic.

The second type of bistatic radar is called forward scatter radar, which means that the receiver isn't designed to receive the radar return that bounce back from the aircraft itself, but to receive the the aircraft' shadow (where the aircraft blocks the transmitter radio wave from being received by the receiver, creating a "shadow" on the receiver's screen). As the stealth aircraft works around its rcs, the "shadow" of it is uneffected by radar return. However, this require the receiver and transmitter antennas to be in a straight path. If you gonna put one on the ground, its counterpart antennas would have to be in a higher altitude than the intended stealth target at a 180 degree, because radio waves don't travel in an arc, it travels in straight path. This might work on low flying stealth aircraft (there are few to none, accept for stealth cruise missiles), but a high altitude stealth aircraft, it means you probably need one antennas end standing in space ;D. Another problem is that it's extremely vulnerable to attacks. In a conventional radar, where transmitter and receivers are located at the same place, you can stay behind your own defense. However, a bistatic radar would need one standing behind the enemy aircraft, exposed to the enemy offensive forces with nothing to protect it. Taking them out would be fairly easy, and once either the transmitter or receiver destroyed, the radar is compromised.

Hope my post has been helpful ;)
 

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Thanks for the huge effort donnage99 but if separated illuminator/receiver radar is so difficult how does SARH (launcher mounted illuminator/missile mounted seeker) do it? Though out of fashion for air to air missiles they are still the minestay of ship based SAM systems.

Cheers, Woody
 

donnage99

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Woody said:
Thanks for the huge effort donnage99 but if separated illuminator/receiver radar is so difficult how does SARH (launcher mounted illuminator/missile mounted seeker) do it? Though out of fashion for air to air missiles they are still the minestay of ship based SAM systems.

Cheers, Woody
Because they aren't trying to detect something inbetween them. What you are suggesting is totally different thing. The radar directly feed its information to the missile. Bistatic concept has the receiver indirectly receives the radar return from the enemy aircraft itself, thus its angles are influenced by the aircraft's position. I'll try to draw some picture so it would be easier to visualize it.
 

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hmmm...I could be totally wrong, BUT my thought was that bistatic radar can work in several ways, one of which i depict here:

what if you have 2+ radars in separate locations, each with a transmitter and receiver, and a datalink between the two.
At time 1, radar 1 emits a pulse. At time 2, all radars are in listening mode. Because radar 2 is offset from radar 1, it stands a chance of getting a return even against a target presenting its stealthy side to radar 1. This gives radar 2 at the very least a bearing. Say there is a third radar, and it gets a return as well. Because the three antennas are datalinked with the transmitter, you have enough information for a triangulation.

The number of radars, transmitters and receivers could vary (i guess the more, the better), but basically you can sequence them in time (fractions of seconds) so you are emitting from locations over a broad aspect, and receiving over a broad aspect.
It may not be enough for a targeting plot, but depending on the geometry involved, it will give you the approximate location.
 

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donnage99 said:
... it also require to know exactly the angles of "spikes" radar return of the aircraft itself, pretty much know the aircraft as much as the its creators.

Any other angle would be better than transmitting and receiving from exactly the direction your enemy has designed his aircraft to defeat. :p

donnage99 said:
Because they aren't trying to detect something inbetween them. What you are suggesting is totally different thing. The radar directly feed its information to the missile. Bistatic concept has the receiver indirectly receives the radar return from the enemy aircraft itself, thus its angles are influenced by the aircraft's position. I'll try to draw some picture so it would be easier to visualize it.

Sorry but I don't think that is how it works. For example, Vietnam era sparrow SARH missiles didn't have any means of communication with the illuminating radar other than receiving it's reflected radar energy from the target aircraft.

What I do concede is that homing in on reflected radar energy is a lot different from determining bearing, altitude, range, speed etc. For this the receiver would need to know the illuminator's position fairly accurately. More importantly it would need to know the timing and direction of the radar pulses for synchronization. This could be done relatively easily on the ground but would be more challenging in the air.

A powerful AWACS platform a long behind the action, might make an ideal transmitter, allowing the receiver fighter aircraft the full benefit of radar without any emissions of their own, thus maintaining stealth. And if multiple data-linked receiver aircraft pooled their antennas and operated as a single very large array... Perhaps I've said too much :)

I'm sure some of you radar gurus could sort this out in a second. Please enlighten.

Cheers, Woody
 

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Its far easier to increase wavelength to VHF or more where most X band optimised stealth (cm wavelength) with its associated shapes under 1 meter in lenhth or so have no effect on the VHF radar. This indeed is what the Russsians have done. The Serbian F 117 killer is reportedly a vhf radar either of marconi design modified or Russian depending on who you talk to and in Gulf War 1 those apaches on night one were taking out early warning Iraqi radar of vhf wavelength because they could detect not track (No advanced computer filtering, triangulation or active arrays) the F 117s intermittently. Traditionally large wavelengths have low accuracy but making them active phased array and positioning several together and using triangulation between several radar looking at same thing have produced almost X band levels of accuracy, certainly enough for midcourse. See Carlo Kopps site (yes I know hes biased on F 22 etc for a good primer on VHF and anti stealth radars. the move toweards large shaped rcs shaping in x 47b is a result of this threat. Curiously large aircraft like B 2 from a shaping standpoint are still largely immune but one has doubts on rear geometry of F 22 especially 2d nozzle regions. Not having a supercomputer or pole to check this is sadly speculation.
 

donnage99

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Woody said:
Any other angle would be better than transmitting and receiving from exactly the direction your enemy has designed his aircraft to defeat. :p
If it's eurofighter typhoon or super hornet you're talking about, with only rcs treatment on its frontal aspect, than yes! Any other angle accept for that certain "stealth" angles would do fine. But all aspect stealth aircraft are designed that they remain stealthy all around, albeit some angles have larger rcs than others. The whole aircraft is designed that the radar return would all focus on certain angles, on the radar testing screen, appearing as "spikes." This is where the technique "edge alignment" comes from. If the enemy radar is lucky enough to position itself face to face with where the spikes are, they'll get a faint return, but they immediately lose it as the aircraft is constantly moving. So that other angles you are talking about are the spikes. An all stealth aspect aircraft is designed to defeat all but certain very narrow angles or "spikes" of radar return. At least, we aren't talking about radar frequency band, yet, we are just talking about angles.

Sorry but I don't think that is how it works. For example, Vietnam era sparrow SARH missiles didn't have any means of communication with the illuminating radar other than receiving it's reflected radar energy from the target aircraft.
Oh, I see. My apology! I got it mixed up with something else. Like I said, I don't know that much to radars. What I said is just simple geometry logic. However, the way you link bistatic radar with SARH is still irrelevant, as the SARH sparrow receives its reflected radar energy from a NON stealthy aircraft, where the aircraft is technically lit up by enemy radar over a wide angle of radar return. We are talking about stealth aircraft, where there are only a limited amount of very narrow angle, which means the receiver has to be directly face to face with these angles.

A powerful AWACS platform a long behind the action, might make an ideal transmitter, allowing the receiver fighter aircraft the full benefit of radar without any emissions of their own, thus maintaining stealth. And if multiple data-linked receiver aircraft pooled their antennas and operated as a single very large array... Perhaps I've said too much :)
Using AWACS for detection and tracking so to limit the aircraft's own signal has been done since forever. However, if one day, this technology wonderment of system of system of system bistatic radar works, you can be sure that the stealth enemy aircraft can also calculate and figure out where the receiver end is at once they locate the transmitter one, as the receiver has to be in an respective angle and distance with the stealth aircraft and the transmitter.
 

donnage99

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ubiquitous08 said:
Its far easier to increase wavelength to VHF or more where most X band optimised stealth (cm wavelength) with its associated shapes under 1 meter in lenhth or so have no effect on the VHF radar. This indeed is what the Russsians have done.
This is what I refer to as more realistic counter stealth approach, something that has been and is being done. JThere are a handful of problems with low frequency band, such as easy to jam or poor accuracy. However, these problems are still not seemingly nigh impossible to overcome both technologically and tactically in the foreseeable future like bistatic radars. It is also needed to point it out that the downing of f-117 is also resulted from the excellent tactics of Zoltán Dani. When people think of downing a f-117, they think "oh, so he modified the radar" but they often forgot that just modifying the radar is only small factor of the whole issue, you got to need to able to avoid the electronic attack and HARM of the enemy (something that has completely paralyzed Saddam Hussein's forces hours into the war), learning the flight path of the enemy aircraft routes so as to place your battery accordingly.
 

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Yes I do the man disservice by not acknowledging his superb overall strategy. He truly is the David who fell the Goliath.
 

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Personally I would attribute much of it to poor flight planning on part of NATO. One of the worst thing about the incident is you have people constantly shouting that "steath aircraft don't work" etc. Nobody ever said they were invincible.

If the radar was modified it could probably detect the F-117A occasionally at certain angles. Yet in my opinion it certainly could not track or guide the missile to the target. That was done through other means. There were almost certainly spotters looking for aircraft, and night vision devices and low-light electro-optical systems could have been used.
 

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