<< Page 50 Page 51: Page 52 >>   Infra-Red Stealth   Ah, now we’re getting to the really interesting stuff. All right, how do heat-seeking sensors work? Well, when something is hot, it emits electromagnetic radiation – that is, light. When something is REALLY hot, you can see that light just using your eyes, such as when metal gets red-hot, or when it melts by being yellow-hot or even white-hot. Light bulbs also glow because they are heated by an electric current. However, even if something is not hot enough to glow visibly, it still glows in the infra-red part of the spectrum. Humans can’t see infra-red light, but it’s not too hard to make an electronic camera that can. In fact, many electronic night-vision devices work not only by amplifying the low level of ambient visible light, but also by seeing infra-red light that is emitted by warm things. (You have probably seen videos taken from police helicopters where criminals running away appear as white outlines against a dark background). Some of these devices even come with infra-red flashlights which emit light that is invisible to people who are not looking through the device. That way you can “light up” a room without this being visible to anyone else. Some Sony digital cameras and camcorders have this “nightshot” technology and can be used in (what appears to the naked eye to be) absolute darkness. Anyways, the hotter something is, the more infra-red light it radiates. So, if you could see infra-red, then a jet engine would glow very visibly, as would the hot exhaust coming out of it. In addition, the leading edges of the airplane (the front of the wing and the nose, where air first hits the airplane and can be compressed and heated during high-speed flight) also become quite hot. How can an airplane hide this heat from the enemy’s sensors? One way is to have the engine sit on top of the airplane, rather than underneath, such as on the B-2 bomber, Global Hawk UAV, and a few other airplane types. This makes the heat radiate out the top of the airplane, rather than out the bottom, making detection by ground-based sensors more difficult (the B-2 also flies higher than most airplanes). In addition, the back of the engine (the hottest part) can be “hidden” between two tails, which means an infra-red sensor will not see this hottest part unless it is looking at the airplane directly from behind. The A-10 Warthog and the Global Hawk UAV have the tails hiding the backs of the engines in this manner, making it harder for heat-seeking missiles to track these aircraft except directly from behind. These techniques only block your radiation from being seen from a few directions, though, so you better place your engines such that the tail and/or the rest of the airplane blocks the radiation from going in the direction where you think the airplane is most likely to be attacked from. Another way to hide your heat is to cover (i.e. paint) your plane with a material that does not radiate easy-to-detect radiation. This can be done in two ways. One, some materials have lower emissivities than others. This mean that the relationship between how hot something is and how much heat it radiates is not the same for all materials. So if you find a material that does not radiate much heat at the temperatures your plane will be at, you should cover your airplane in that material. The other way to keep your airplane’s infra-red radiation from being picked up is by radiating at wavelengths that are more easily absorbed and scattered by the air. “Infra-red” just means light with a wavelength too long to be seen by the human eye; The wavelength of “infra-red light” can vary from just a little longer than red light, to billions of times that. So there is a whole range of infra-red light wavelengths, like with visible light. Some of them slip through the air fairly easily and can be seen for tens of miles, but some of them are not quite as penetrative, as they get absorbed or scattered by the air. Now, as you can probably deduce, the relationship between how hot something is and what wavelength of light it emits is not the same for everything (not everything is a Black Body that follows Wien’s Law). At a certain temperature, for example, some metals may be red-hot, some may be yellow-hot at that temperature, some may be white-hot, and some may barely give off a visible glow at that same temperature and emit in infra-red wavelengths instead. So different materials emit radiation in different wavelengths at different temperatures. Once you know the temperature your airplane skin will probably be at (given things like how much it compresses the air around it, how hot the engine gets, etc.) then you should find a material that, when heated to that temperature, emits infra-red light in a wavelength that does not penetrate the air very well, but instead in a wavelength that gets absorbed or scattered by the air. That way, if you cover your airplane in that material, the air will act like a fog to cover your heat, and thus keep infra-red sensors (and heat-seeking missiles) from detecting you unless you’re extremely close to them. You might think that just using some magic paint that emits infra-red radiation a little differently will not make a significant difference (especially if you studied heat-transfer infra-red radiation, black bodies, Wien’s Law, etc). But you’d be wrong. While developing the F-117, Lockheed painted an airliner with different materials that they thought would reduce the infra-red radiation emitted. One of these materials reduced the detectable infra-red emissions of the airliner to about 10% of what they were before! The third and last way to hide your heat from infra-red sensors is the one most commonly associated with stealth aircraft. The concept is a simple one, but implementing it (in such a way that your engine’s thrust is not ruined and your engine nozzle doesn’t melt) is extremely challenging, and was in fact one of the most troublesome features during the F-117’s development. The concept is this: Your jet engine ejects out the back a large quantity of hot gas, all the time. This massive plume of hot air behind your aircraft can make life very easy for your enemy if he has an infra-red sensor (especially if he has a heat-seeking missile). You can imagine an airplane flying forward and leaving behind a very long cylinder of hot air. On the surface of this cylinder, where hot engine air meets the cold outside air, the hot cylinder air is cooled and mixes with colder air. However, the inside of the cylinder stays warm for a while, making it easy to detect. The obvious solution to this problem is to flatten the cylinder of hot air into a sheet, so that ALL the hot air is cooled quickly as it immediately mixes with cold air. This means that, instead of having a round nozzle on the back of your engine, you need a flattened nozzle, one that goes from a round tube to a thin, narrow slit. (Some hair dryers actually have nozzles like this, if that helps you visualize what I’m talking about). The problems with this are that the air gets slowed down a little bit on its way out, making you lose some thrust, and that the nozzle gets very hot with all that surface area exposed to hot air, so you probably need Space-Shuttle materials to keep it from melting. This will also make the back of your airplane very hot, so you need to manage its infra-red emissions by using the methods described in the previous paragraphs. Despite these problems, a flattened exhaust nozzle is a common characteristic of stealth aircraft, and the only way to keep heat-seeking missiles from easily detecting the air coming out of your engines. Many stealth aircraft even pump some ambient air into the exhaust ducts, to make the exhaust start cooling even before it leaves the airplane. And that's how you keep your airplane from being picked up by FLIR sensors and from being shot down by heat-seeking missiles.