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Sturgeon's House

Toxn
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1. Introduction

 

Stealth is one of those buzz-words that everyone knows. Stealth makes aircraft invisible to radar, allowing a stealth plane to sneak up and punch other aircraft or SAM sites with impunity. Stealth is widely acknowledged as one of the most fundamental technologies that all new combat aircraft need to have. Stealth is also, like SH's old friend NERA, mostly completely misunderstood. This thread will attempt to change that, at least a little.

 

HUGE DISCLAIMER: I know, at best, the basics of what is essentially one of the darkest arts in an already black magic-heavy field (radio and radar engineering). I'll be relying heavily on others to correct my obvious mistakes, but this is and will be the lies-to-children version of the field, as told by another child. Still, given the state of knowledge out there, it's probably better than nothing.

 

2. The most basic basics

 

Okay, you ask, what is stealth then if we're all misunderstanding it? Here I think that the best analogy is that stealth is like camouflage, but for aliens. Camouflage famously entails the 5 (sometimes expanded to 7 with speed and spacing) S's: Shape, Shine, Shadow, Silhouette and Sound. Each operates on somewhat different principles, and can be more or less important in different scenarios, but are united in terms of how human senses work. We are pattern-finding creatures with passive senses, so anything that breaks up visual or auditory patterns, blends one into the background, or limits the amount of noise or reflected light one gives off will make you harder for another human to spot.

 

Radar, however, is generally not passive. Instead, a radar set sends out a beam of electromagnetic energy and looks for an echo. There's a huge amount of complexity in how this can be done (what frequency to use, how to generate and send the beam, how to track the returns and so on), but that's radar at it's most basic. So, like camouflage, ways to avoid radar will be united in trying to trick or defeat this basic mechanism. These principles are, roughly: Absorption, Redirection, Scattering and Emissions. Finally, and just to complete a fun acronym, there's also the side issue of radio-related Shenanigans. Taken together, these measures can significantly reduce how easy an aircraft is for a radar to "see" from certain angles.

 

Absorption is simple in concept: if something eats up the radar waves before it can get reflected, then the receiver doesn't get to pick up a signal and the plane doesn't get found. There's a whole realm of sneaky material science that goes into this, but from my understanding the two most common techniques currently being used are non-metallic structural components (which can be more or less transparent to radar) and foams or paints with nanomaterials in them (the famous grey stealth paint job generally being a weather coating instead of the magic material itself). These can be used in all sorts of clever ways: for instance, by making the forward edge of your wing out of radar transparent composite and then packing the area behind it with cones made out of radar-absorbing foam. Absorption can't make a non-stealthy design stealthy, however. It's more of a "cut 10% off our already-low radar return" sort of strategy.

 

Redirection is one of the single biggest reasons why stealth aircraft have their characteristic look. Generally the principle here is to make as many surfaces on the aircraft as parallel as possible, in order to direct the majority of your return to one or two places rather than scattering it all over the sky. Since the most common place you don't want returns to come back to is directly to the front, this also means that swept wings and tails are a must. It's also why flat bottoms are preferred: if someone is looking at your aircraft from below, then a flat bottom is the one shape guaranteed not to provide a good return until you are right above them.

 

The major enemy of this approach is the dreaded corner reflector, which is where any right-angled surface will reflect a return straight back to it's source. This is why stealth aircraft all have angled fuselages, hard chines and cranked tailplanes, and also why even things like landing gear hatches and bomb bay doors end up with saw-tooth profiles (note: not 90-degree saw teeth if you can help it, because corner reflector). The other major enemy of this approach is aerodynamics, which inherently prefers rounded frontal profiles that are great at reflecting returns back along an entire wing or fuselage segment. So stealth aircraft also tend to have aerodynamic features (sharp-nosed, flat-bottomed airfoil profiles, for instance) that make them a bastard to fly.

 

Scattering: if you're doomed to reflect something in an unwanted direction, then it helps to make the surface convex in order to disperse the return. This is seen in the shallow, curved fuselage profiles of stealthy aircraft which, along with their beaky fronts and hard chines, gives them a sort of alien bird quality. It's also really useful when designing air intakes for the engines you've sensibly buried inside the fuselage (seriously, the front of a jet engine is like a disco ball for creating noticeable radar returns): an S-shaped intake reflects about half as much energy as a straight intake with a similar profile.

 

Emissions are more or less self-explanatory: if you're trying to hide in the dark, then don't bring a flashlight with you. This means no big radio sources or old-school radar sets that a receiver can easily pick up on. I've heard that modern AESA radars are harder to spot for {electronic black magic} reasons, but the principle still stands.

 

Shenanigans are what you resort to in the corner cases where one or the other approaches described above are not possible. These usually make use of unintuitive electromagnetic wave-specific physics like half-wave resonance. The intake screens on the F117, for instance, seem to be sized so that the radar wave "sees" it as a solid surface and bounces off while still allowing at least a trickle of air in to feed the engines. These tricks tend to be fiddly, however, and can go very wrong when faced with radar systems that use frequencies much higher or lower than the ones that they were designed to counter.

 

3. Artists are dumb and wrong

 

So, having learned the barest minimum about how stealth works, let's point and laugh at the mistakes of artists who ape the form of stealth without understanding the content. Note: it's now almost impossible to grab high-resolution images off of websites, so you'll just have to google these things if you want to see them in any sort of level of detail.

 

Example 1: the F-19 from model kits in the late 80s

 

A fictional stealth plane from the time where people could be forgiven for not knowing a damn thing about stealth. The top-mounted air intake and engines are a good idea, but the rounded wings/fuselage profile and anhedral wingtips look like a great way to get returns from every direction. 5/10 for effort at a time when nobody knew what stealth really was.

 

Example 2: F/A-37 Talon from the movie Stealth

 

Considering that the damn movie is called "Stealth", the Talon is a remarkable example of a bunch of artists googling stealth aircraft and then adding enough greebles so that the result is neither stealthy nor much of an aircraft. It has intakes everywhere, a bunch of curves but few parallel lines, a swing-wing setup that I can only imagine puts a bunch of nooks and crannies into the airframe that reflect well, random greebles off at right angles and on and on. 0/10, the aircraft plays Incubus when it's angry and is therefore canonically a moody teenager.

 

Example 3: XA-20 Razorback from Tom Clancy's giant, throbbing brain

 

It's an F-20 that's inexplicably been converted into a CAS aircraft (presumably because, in the dark future of 2020, transaircraft rights are now government policy). That idiocy aside, it's more or less fine. Turns out that when you crib directly off of someone else's work you won't fuck things up too badly.

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Something that I should also emphasise about stealth is that small fuck-ups have big consequences. A completely un-stealthy aircraft has an average RCS of something like a few square metres. Meanwhile, a well-shaped stealth aircraft supposedly has an average RCS of around 0.1 square metres. The area of a landing gear well for such an aircraft might be something like 0.5x1m, if cunningly made. This means that, if you fuck up the stealth shaping of a wheel well (by, for instance, making it out of something radar-transparent or forgetting to shape the doors correctly) you can increase the cross-section of the entire aircraft by as much as five times! This sort of issue is all over the place, and is one of the reasons why simple, clean external lines are preferred - the more structures you have hanging off the fuselage, the more chances there are for something fucky to happen and for an accidental radar reflector to be formed. And, of course, the smaller you try to make your cross-section, the bigger the relative effect of any fuck-ups.

 

Getting a very small RCS across even a very tightly specified arc is accordingly a process of almost obsessive attention to detail. And getting a small RCS across a large arc is the sort of thing that you need very specific and potent computer-aided design tools to accomplish.

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One thing that urgently needs to be mentionned is that just like aerodynamics, or Armor vs shell behaviour, RCS is never ever to be evaluated by eye. There are so many details going up about how EM waves scanner or get absorbed that not even a general guess can be made when it comes to radar stealth vehicles.

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8 hours ago, W. Murderface said:

 

Now I'm not an expert in any of this, but this guy seems to know what he's on about. 

Thanks, that's a decent find.

 

Edit: this one of theirs might be even better as an intro:

 

 

And this one explains the issue of detection range not scaling linearly with RCS, as well as some of the other, more fiddly aspects of reducing cross-section:

 

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10 hours ago, eggs benedict said:

One thing that urgently needs to be mentionned is that just like aerodynamics, or Armor vs shell behaviour, RCS is never ever to be evaluated by eye. There are so many details going up about how EM waves scanner or get absorbed that not even a general guess can be made when it comes to radar stealth vehicles.

True, and as mentioned above getting stealth designs right is as much a matter of computing power and special software (and obsessive attention to detail) as anything else.

 

But there are some general principles that can be gleaned by eye. You don't need complex simulations to see that a B-52, for instance, is going to have a massive radar signature while a B2 is not. Similarly, debunking the idea that the Ho. 229 was a magical stealth plane doesn't require one to make a mock-up and put it into a radar chamber. A look at the intakes shows that any stealthy features were more or less a happy accident rather than the result of intent.

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9 hours ago, Gauntlet said:

Not completely sure, but I believe that the air intake of the Naval Strike Missle also contains some features in order to reduce the RCS.

Also i quite liked the book, Skunk Works.

From the few pictures available of the inlet, this does indeed seem like the case.

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55 minutes ago, Lord_James said:

Would decoys or active countermeasures (like chaff) be counted as stealth, since they make it more difficult to gain a target? Or is stealth, in this context, not being seen in the first place? 

I think certain forms of ECM count. But my understanding is that stealth is about minimizing signature rather than hiding it.

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