Diamonds, D&D, and Disco Balls... Demystifying Stealth

Sorcery?  Black arts?  Not quite.

One can not discuss fighter aircraft these days without mentioning its "stealthiness".  Stealth has become such an overwhelming issue that it has come to define the "Fifth Generation" of fighter jets.  Many even claim that a stealthy aircraft need not worry about other more traditional performance parameters like speed and maneuverability.  "You can't shoot what you can't see!" they say.  It is implied that stealth aircraft fly around completely unseen, like some sort of ninja in the sky.

So how does stealth work?

I am not going to pretend that this is a definitive thesis on the subject.  Modern stealth design is complicated stuff.  It is far from wizardry however.  

Radar

Radar is, in simplest terms, a form of electromagnet energy.  It is emitted from one source, radiates outward, then bounces off various objects in the atmosphere.  It is invisible to the naked eye, therefore it is a rather obscure concept to many of us.  When we think of radar, we think of big metal dishes, radiation, and microwave ovens. 

Since invisible radiation is a little abstract, we will instead imagine radar as a different form of electromagnetic energy:  Visible light.  

Visible light and radio waves differ simply by the frequency and wavelength.  While they do have different properties, they act the same.

Modern military radar installations work in much the same way as searchlights were used in WW2.  A "beam" is sent into the sky, searching for any shiny objects (aircraft) that do not normally belong there.  Energy from that beam is reflected from that object, back down to the ground to observers.  Radar does this invisibly, and at much farther ranges than visible light, but the principle is very much the same.  

P-61 Black Widow night-fighter

Radar absorbant materials

Making an aircraft hard to detect with visible light is relatively easy.  Paint an aircraft flat black, fly it at night, and you are done.  This method can be seen in some WW2 aircraft like the P-61 Black Widow.  

Making a radar hard to detect against radar, you have to be a lot more creative.  

Since radar reflects best off of metallic objects, an aircraft's radar cross section (RCS) can be reduced simply by reducing the amount of metal in an aircraft.  This can be done using materials such as carbon fiber, composites, and even wood.  The good news here is that many modern aircraft already use composites and carbon fiber, as these materials are lighter and stronger than traditional metals.  Metal still makes up a large part of modern aircraft, however, since you need it for wiring, engines, and other applications.  

The most desired method would be to replicate that flat black paint, only instead of absorbing visible light, it needs to absorb radar waves.  Modern stealth aircraft are covered with radar absorbent material (RAM).  Early applications of RAM used in the SR-71 consisted of microscopic ball bearings that absorbed radar energy, converting it into heat.  Modern applications have improved on this greatly.  

So why not cover a conventional aircraft with RAM coating and call it stealthy?

RAM coatings have serious drawbacks.  They are often expensive to make and difficult to work with.  It is also quite fragile and it does not work well in certain weather conditions.  The B-2 bomber needs to be kept in special air-conditioned hangers for this reason.  The original stealth fighter, the F-117 Nighthawk, had a RAM coating that was useless in wet weather.  RAM coatings are also not practical for certain parts of the aircraft, like jet-engine nozzles, canopies, or wing leading edges.

The biggest issue surrounding RAM coatings is that merely reduces, not eliminates, the radar energy reflected.  It also has to be tailored to a certain frequency.  This means that an aircraft covered in RAM material can be "seen" by radar simply by changing the frequency or increasing the power or the beam or sensitivity of the receiver.  

No matter how RAM is used, or how much nonmetallic materials are used in its construction, some radar energy will be reflected.  There is a way to deal with this.

I'm a nerd...  This is how I roll.

Facets

Like visible light, radar energy travels until it encounters an object.  It is then reflected.  How this energy is reflected is very much dependent on the surface of the object it is reflected from.  If it hits a flat object straight on, it is reflected back towards its origin, much like a reflection in a mirror.  If that flat object is tilted to an angle, then that energy is reflected at an angle.  By strategically placing those flat surfaces, or facets, one can control which direction incoming radar energy is directed.

Take a look at the dice above.  Those dice are used for playing Dungeons & Dragons, as well as other role-playing games.  Each dice has a different shape to facilitate a different range of numbers.  The one on the far left is a tetrahedral four-sided-dice or a "1D4".  Moving from left to right, we move on to the more familiar cube-shaped six-sided dice (1D6), the eight-sided (1D8), twelve-sided (1D12), twenty-sided (1d20) and finally the ten-sided 1D10 (two 1D10s are rolled together to each make a digit for 1d100).  

Imagine if we were to shine a light directly on a side of each of these dice.  The 1D4 would reflect light from one facet, while the other three sides would remain in shadow.  By contrast, the 1D20 would reflect light strongly from the facet hit, as well as the three facets surrounding it, and more still from the facets surrounding those.  As we angle the light, more sides become visible on the 1D4, but never more than three.  Once facet always remains in shadow.  As this is done, however, the facets that are exposed to light become dimmer.  As they are placed at an angle, they reflect light away, instead of towards the source.  Moving the light source on the 1D20 has seemingly no effect, as one facet simply begets the next.  

More facets, more sparkle.

This is why precious gems are cut into intricate shapes with plenty of facets.  The facets help reflect light, giving the gem its characteristic sparkle.  Gems are cut to maximize the amount of light reflected, giving them a distinct sparkle.  Taken to a larger scale, to a non-transparent object, we get a very familiar sight to anyone who has ever been to a dance club...

As the amount of facets increase, so does the scattering of the light, or other electromagnetic radiation.  The convex shape of a disco ball is highly visible, even when only a small amount of light shines its way.  

Picture the shape of a traditional, non-stealthy aircraft.  For the most part, aircraft are basically shaped like long tubes with wings and tails.  Various openings are scooped out for engine intakes and the like.  Control surfaces like elevators, ailerons, and rudders are built in, and these surfaces move about as the aircraft flies.  On military aircraft, missiles and bombs are carried on the bottom of the aircraft on pylons.  Many modern aircraft still use propellors, which reflect radar like a strobe light.

Lots of shiny metal, bulges, fins, and propellors.  The Tu-95 is the "Anti-Stealth Bomber".

With its rounded metal fuselage, odd bits sticking out, and thirty(!) contra-rotating propellor blades, the Tupolev Tu-95 "Bear" is just about the farthest from stealthy an aircraft can be.  All the RAM in the world would do little to reduce its RCS.  

When the USAF set out to build an attack fighter that could evade enemy radar, engineers at Lockheed and Northrop had their work cut out for them.  

Looks like a D&D dice, doesn't it?

The "Hopeless Diamond".

Lockheed's proposal was simple in theory.  Design an aircraft where just about every surface reflects from the source.  Its shape is similar to the eight-sided D&D dice pictured above, only flattened out.  The aircraft's engines would be buried inside the aircraft, and the cockpit and air intakes would be placed inside the "shadow" of the aircraft's top.  

This early design did not have a hope of flying, earning it the nickname "Hopeless Diamond".  With no proper wings or control surfaces, it would have made a terrible aircraft, but it did have a tiny RCS.    So tiny, in fact, that a seagull perched atop a mockup resulted in a significantly larger radar signature.

Lockheed's "Have Blue" prototype.

Lockheed tweaked the "Hopeless Diamond" design, carefully adding larger wings and inward canted tails.  It was still ungainly and dangerously unstable during flight, but the recent advent of fly-by-wire controls meant that a computer could lighten the pilots workload.  

This prototype, designated the "Have Blue", was the basis of of the now famous F-117 Nighthawk fighter-bomber.  The F-117 proved itself quite valuable over its lifetime, but it was very much a niche product.  It was not particularly fast, nor did it have impressive range.  Its payload was minuscule, even when compared to multi-role fighters like the F-16.  The F-117 was able to sneak in where other aircraft could not, however.  By clandestinely knocking out enemy defenses, it allowed other fighter-bombers to do their work easier.

F-117 Nighthawk

Hotspots

Simply put, stealth aircraft attempt to do two simple things:

1. Reflect as little radar energy as possible.
2. Control the reflection of what radar energy cannot be absorbed, so it does not return to the sender's receivers.

In theory, a stealth aircraft is easily capable of these feats.  In the real world, things get a little more complicated.

Might as well put a disco ball back there...

Subsonic stealth aircraft like the F-117 and the B-2 bury their engines deep inside the aircraft.  This keeps all of their metal bits safely away from prying radar.  Supersonic aircraft do not have this luxury, however.  The use of afterburner requires an external exhaust nozzle, as ignition of an afterburner inside an aircraft's hull would have fiery consequences.  

Both the F-22 and the F-35 utilize  external nozzles.  The F-22's uses a two-dimensional nozzle that blends smoothly with the flattened fuselage.  These nozzles help stealth in another way, discussed later.  The F-35, as well as the Russian PAK FA and Chinese J-20 and J-31, all use more traditional round exhaust nozzles.  These rounded, metal nozzles are a definite "hotspot" for radar reflection.  They cannot be covered with RAM, and their shape is more akin to a disco ball.  While their RCS is mitigated somewhat by being inset slightly and shrouded by bodywork, it still is not very stealthy.

The F-22 and its thrust vectoring nozzles.

Since a stealthy aircraft relies very much on its shape to reflect radar harmlessly away, it is very important for that aircraft to maintain that shape.  In flight, this is simply not possible.  Even while flying straight and level, an aircraft has to make minor adjustments using its control surfaces (rudder, elevators, ailerons, etc).  As these control surfaces move, the shape of the aircraft changes somewhat, possibly increasing its RCS.

Remember those thrust-vectoring nozzles on the F-22?  Not only are they properly shaped to help control radar reflection, but they can help make minor adjustments to pitch, reducing the use of the F-22's control surfaces.  

While small, minor maneuvers can be mitigated somewhat, more extreme maneuvers cannot.  Remember that four-sided dice from earlier?  From most points of perspective, the sides of the dice are all facing away from the observer.  Once that dice reaches a certain position, however, the observer is looking at a facet dead on.

Good stealth.

 From most angles, a stealth aircraft will reflect radar energy away from the source, rendering it useless to the radar operator.  The pilot of that stealth aircraft needs to be especially wary of the aircraft's position relative to the radar, however.  If the pilot initiates a maneuver that puts a large facet perpendicular to the radar, that facet will reflect energy straight back to the source, instead of harmlessly away.

Bad stealth.

Stealth aircraft operations thus far has emphasized a cautious, well planed flight path.  Known enemy radar installations are circumvented as much as possible, and pains are taken to limit the stealth aircraft's exposure to radar.  Heavy maneuvering is highly discouraged, as even a moderate turn could greatly increase the aircraft's RCS.

Boeing X-32 showing off its weapon bay.

A well known feature of stealth aircraft is the ability to carry some of its weaponry in internal weapon bays.  This reason for this goes well beyond aerodynamics or esthetics.  By carrying weapons inside of a stealthy fuselage, the weapons' RCS does not enter the equation.  

Most modern missiles and bombs follow a simple design feature; they are metal tubes with small fins attached.  Needless to say, a metal tube with fins sticking out is not a stealthy shape, especially when viewed from the side.  It becomes even less stealthy when it is attached to an aircraft's wing or fuselage by way of a pylon.  

AGM-158 "stealth" cruise missile.

It should be noted here that even "stealthy" weapons, like the AGM-158 or the Joint Strike Missile will significantly increase an aircraft's RCS when mounted externally.  The pylon and missile still add extra "facets" to the aircraft's shape, reflecting radar energy in various directions.  

The F-35's AN/APG-81 AESA radar.

Not only does a stealth aircraft have to deal with enemy radar, but they have to keep close tabs on their own emissions as well.  Going back to the WW2 spotlight analogy, all the flat-black paint in the world could not disguise an aircraft flying at night with its landing lights on.  

Much like a submarine, stealth aircraft need to "run silent".  This means that their own radio emissions need to be kept at a minimum.  This includes radio communications the use of radar.  Modern systems can mitigate this somewhat, with AESA radars capable of "steering" a beam to limit its chance of detection.  

Making the unstealthy...  Stealthy.

F-15SE Silent Eagle.  Not "stealth"...  But maybe "stealthy enough"?

Designing a truly stealthy aircraft requires an obsessive focus on that aircraft's materials and shape.  Even then, the aircraft needs to operate under strict parameters to avoid showing off "hotspots".  Knowing this, one can not simply take an existing design and make it into a true stealth aircraft.

It is possible to make an existing aircraft stealthier, however.

The F-15 Eagle is not a very stealthy aircraft.  It is big, uses lots of metal, and has a shape that emphasizes performance over all else.  Yet Boeing is now marketing the F-15SE "Silent Eagle" that promises to greatly reduce the F-15's RCS.  How can they make such a claim?

The most obvious change is in the way weapons are carried.  The F-15SE basically takes the F-15E's conformal fuel tanks (CFT's) than modifies them to carry missiles instead of fuel.  This effectively gives an armed F-15 the RCS of an unarmed F-15.  

Other modifications are more subtle.  For the F-15SE, the traditional flight controls are swapped out in favor of a "fly-by-wire" system.  This not only modernizes the F-15's flight controls, but it reduces the amount of metal in the aircraft.  Future aircraft may take this concept even further by utilizing a "fly-by-light" system that uses fiber-optic cable instead of metal wires.  

The F-15SE also utilizes RAM in key hotspots around the aircraft to reduce its RCS.  Special attention is paid on the foreword section of the aircraft, as this is the aspect that will be most likely "seen" by enemy ground radar.  

To a radar, this looks like a disco ball.

Imagine what a jet engine must look like to radar.  Lots of shiny metal bits, going in all sorts of directions.  In use, all those metal bits are spinning around reflecting radar energy in every direction.  The front of a jet engine is basically a rotating disco ball in the eyes of a radar receiver.  

While the exhaust nozzle is a little harder to hide, a jet intake can positioned in a way so that radar return is unlikely.  Baffles can be put in place, or the engine can be "tucked in" behind a serpentine air passage.

The Super Hornet's air intake.  

While the intake fan is visible in the Super Hornet above, it will quickly disappear if the viewing angle changes.    The shape of the intake itself is coated with RAM and angled so as to discourage radar energy from reaching the engine.  

The Super Hornet also utilizes a stealthier AESA radar, closer attention to panel alignment, and the elimination of "gaps" and other potential hotspots.

Changes like these help the Super Hornet boast of a smaller RCS than the smaller legacy F/A-18 Hornet.  

Despite being physically larger, the Super Hornet's RCS is smaller than its predecessor.

Similar attention to stealthiness was paid during the design of the Eurofighter Typhoon, Dassault Rafale, and Saab Gripen.  While none of these aircraft could be considered "stealth fighters", they are a great deal stealthier than older fighters like the F-16.  

It should be noted here that while fighters like the Super Hornet and Typhoon cannot boast of the same reduced RCS that fighters like the F-22 and F-35 can, they do offer reduced RCS without the added cost, maintenance issues, and performance sacrifices.

Much has been made about the F-35's decreased performance compared to older fighters.  Instead of revisiting that trope, we will instead compare the F-15 with the F-22.  Both fighters are roughly the same size and shape, but the F-22 has substantially more power.  Despite this, the F-15 has a faster top speed.  While the F-22 does boast of supercruise, this is mostly due to raw power over aerodynamics.  

Is it all worth it?

B-2 visible on IRST.

Even with the extra cost and performance compromises, stealth does promise to keep aircraft hidden from enemy detection.  "You can't kill what you can't see!" is the argument often used.  This is true, but only to a point.  

Like anything else, radar detection capability is improving over time.  The days of a radar operator hunched over a green screen searching for a "ping" are behind us.  Modern radar signals are interpreted by computer using software that is continuously updated to make the most out of it.  While a stealthy aircraft design is "baked in" during development, a ground radar's capability can be improved by something as simple as a software patch.  

There are other ways to detect aircraft besides radar, as well.  Infrared Search and Track (IRST) systems locate and track aircraft by the heat they produce.  While radar can be reflected harmlessly away or absorbed, it is much more difficult to reduce an aircraft's heat signature.  The very act of flying through the air produces friction, heating up the aircraft relative to the air around it.  From the rear, an aircraft's exhaust shines like a signal flare, especially while using afterburners.

"Not so stealthy now...  Are ya?"

Modern IRST systems do not currently have the range of radar, but they are improving.  As the use of stealthy aircraft proliferates, you can expect IRST systems to see even further enhancements.  The best part about IRST is that it is completely passive, meaning that it does not have to transmit a signal.  While a radar operator might potentially give away their position, the IRST operator does not.  

"I'm doing my part to help detect stealth bombers!"

One of the more interesting ideas on the horizon is the use of "Passive Radar".  Instead of using transmitters, passive radar takes advantage of all the ambient radar signals emitted by non-military sources.  Our atmosphere is full of signals generated by radio broadcasts, weather radar, and even mobile phones.  Passive radar installations simply monitor for radar reflections from this ambient radiation.  

In effect, passive radar forgoes the "spotlight at night" analogy altogether.  Instead, passive radar looks for aircraft using the light of day.  Since all of that ambient radar energy consists of different wavelengths and frequencies traveling in a myriad of different directions, current stealth designs would be of little use.  

Northrop Grumman's Next Generation Bomber concept.

Perhaps one of the biggest arguments against stealth is its own effectiveness.  

Current stealth designs do work.  They offer an unmatched first day of war capability in which they can "kick down the door" by invading enemy airspace and neutralizing air defenses.  With enemy radar installations, airfields, and surface-to-air missile batteries gone, the air battle is all but done.  

Once enemy air defenses are gone, stealthiness is pretty much irrelevant.  This is why non-stealthy aircraft like the B-52 bomber are still very much in use.  Despite being a much older design, the B-52 carries a larger payload and has longer endurance than the B-2.  The B-52 is also considerably cheaper to run.  

Despite this, it would seem as though stealth design in here to stay.  The USAF's NGB or "Next Generation Bomber" will be a stealthy design, much like the B-2.  Early concepts for a "6th Generation fighter" like the F/A-XX also appear to be stealthy.  Nations currently working on their own indigenous fighters, like Turkey, South Korea, and Japan also favor stealthy designs.  

While stealth is likely to remain a major design emphasis in military aircraft, time will tell if it will continue in its current form.  IRST, passive radar, and other improvements may make stealth too difficult to incorporate into the majority of fighters and bombers.  Instead, stealth may again become a niche capability much like it was with the F-117.