<< Page 47 Page 48: Page 49 >>   Stealth "Low Observability" technologies   This section will explain some of the design elements of stealth aircraft. Stealthy airplanes definitely look unusual, and here I will discuss the design principles that lead to these unconventional shapes. First, though, it is important to put stealth into a historical perspective, and to realize that making an aircraft stealthy is much more complicated than trying to make it not show up on radar screens (as if that were not complicated enough). While stealth technology is often associated with modern aircraft that are almost invisible to radars, the truth is that military aircraft have been trying to evade detection since they were first used in combat. When the primary means of detection were visual and auditory, those were the “senses” from which aircraft were designed to hide. This could be achieved through paint schemes that mimicked the colors of the terrain under the aircraft or of the sky around it, or sometimes through special lights, shiny coatings, and other techniques. But with the advent of radar, combat aircraft suddenly were exposed to detection unless they flew between hills, inside canyons, or no more than a few meters off the ground, and even then they could be detected once radars were developed that could determine the relative speeds of everything they saw (using the Doppler effect or timed-pulse techniques). Additional systems, for enemy detection and for weapons guidance, were developed around heat-seeking technologies – these sensors could “see” the infra-red “glow” of the hot parts of the aircraft and of the engine exhaust ejected out the back. As will be explained in this section, at first the evasion of radar and infra-red sensors was thought to be highly impractical at best. Radar reflections and infra-red emissions would have to be reduced by factors of over a thousand before sensors would fail to detect an aircraft within striking distance. Billions of dollars could be spent trying to build such “stealthy” aircraft, but it might simply not be possible to make them stealthy enough. Well, billions of dollars WERE spent. Through a few ingenious insights, aircraft design features were developed that drastically reduced radar reflection. Other features were developed to hide the radiation of the airplane’s heat from infra-red sensors. These features were tested and developed into the stealth technologies we see today. However, these developments still have to be balanced with stealth from human senses: sight and hearing. An airplane that cannot be detected by a radar or infra-red sensors can still easily be shot down if it can be seen, or if it can be heard easily enough for other sensors and weapons to be aggressively directed its way. So it is still as important as ever for combat aircraft to evade human spotting. It is true, though, that evading radar and infra-red sensors is more important, since these work at longer range. So we should start by describing the older and simpler techniques of evading human sight and hearing, and then move on to the more important and more technologically complex design features that hide an aircraft from radar and infra-red sensors.   - Acoustic Stealth - Visual Stealth - Infra-Red Stealth - Radar Stealth (This is the really interesting one!)   Assuming you have read the four pages linked to above;   Conclusion, Summary, and Further Reading: Stealth This section went through all the known techniques to make airplanes hard to pick up by a variety of means. This involves the well-known unique attributes of stealth aircraft – flattened fuselages, straight parallel edges, sawtooth doors and panels, non-rectangular air intakes, slit-like nozzles, radar-absorbent materials, special paints – as well as other, less well-known, mostly less-important considerations. These descriptions of stealth technologies come from public material, so some of them might be inaccurate. Some of it might be misinformation put out by the secretive military folks in charge of the stealth programs. Some of it makes wild guesses about future possibilities, guesses which might be way off or which might already be secretly in use. When talking about something that still has secret aspects to it, one can never be too sure of anything. But the points I described here make sense, and non-classified materials (and just empirical observation and common sense) seem to support them. So there you go. I hope you enjoyed reading it. If you have any comments, corrections, new information I’m missing, etc, please do email me. Of all the things I said, which are most important? Here’s a helpful summary: - Acoustic stealth: The least important aspect of stealth technology deals with trying to evade human ears. The fact that an airplane (or a mission profile) is “quiet” usually derives from attempts at reducing drag, increasing engine efficiency, or flying at speeds and altitudes that keep an aircraft outside the reach of enemy sensors. The exceptions to this – the only aircraft designed to be quiet before anything else – are the X-24, Q-star, and YO-3. - Visual stealth: Camouflage attempts to blend an aircraft into its surroundings. This originally meant the color of the terrain (brown and green, or blue for naval planes) or the color of the sky (light blue, white). Attempts at making aircraft harder to see by making them shiny (so they reflect their environment) or transparent (so you see through them) failed, but the use of lights to illuminate aircraft surfaces can be effective at visually blending the aircraft into the background. Low-visibility grey is the current norm: Light grey for low-flying aircraft (attack helicopters, tank-killers, troop transports, etc), dark grey for high-flyers (bombers, interceptors, refuelers), black for REALLY high-flyers (spyplanes) or for high-visibility trainers. Grey aircraft are usually light grey in the underside (the part seen against the sky) and darker grey on top (the part seen against the ground). Modern low-viz paint schemes attempt to reduce contrast by painting in darker colors the areas that are most illuminated by the sun (e.g. flat areas on top) and painting in lighter colors the areas usually not so brightly illuminated (e.g. the vertical sides, the insides of the intakes). - Infra-Red stealth: Engines can be placed on top of the airplane so heat is radiated up, not down. Engines can be placed between the tails so that the tails hide most of the heat radiation. Paints can be used that emit little IR radiation, or that emit IR in frequencies most easily absorbed by the air (so that the air acts like a fog). Cold air (like from the air intake) can bypass the engine and be mixed with exhaust gases to make them cooler. Most importantly, the nozzles can be flattened so that the hot exhaust gases mix more quickly with ambient air. (A flat sheet of exhaust created by a slit-like nozzle will have a higher surface area – and thus interact with more air – than a cylinder of exhaust crated by a round nozzle). - Radar stealth: “Shape, shape, shape, and materials”. Radar-Absorbent Materials (RAM) absorb a lot of radar energy thus keeping it from being reflected back to the radar. Carbon absorbs radar, iron creates magnetic fields out of it, so carbon ferrite (“iron balls” suspended in some kind of plastic) work best. The skin of the airplane can be made of such materials that radar waves of a variety of wavelengths penetrate them just a little, and bounce internally taking a path that is half a wavelength longer than that taken by radar waves bouncing at the surface: You get wave cancellation from the out-of-phase radar waves when this half-wavelength path difference is achieved. Wave cancellation and RAM play a small part in modern stealth, though: The most important thing is a shape that reflects most radar energy away from where it came, unless it comes from a few narrow directions. Waves bounce at the same angle they hit the surface, so only waves that hit the surface perpendicularly (sent from a direction at 90 degrees to the surface, give or take a few degrees) get sent right back. A plane with a faceted surface and with parallel edges has few directions perpendicular to it (while a spherical surface has some point on it that is perpendicular to ANY given direction). Chines also make the surface perpendicular to fewer directions than if the plane had a cylindrical shape. Most planes have round shapes that reflect a little radar energy in every direction, while stealth planes are flattened and have straight edges so as to reflect most radar energy in only a few directions (the plane will only be detected if the radar is in one of those directions, and then only briefly since the plane is always moving so any spot does not stay for long in one direction relative to the airplane). Surfaces at 90 degrees to one another reflect radar right back to where it came (so rectangular engine intakes, and vertical and horizontal tails, make an airplane easy to pick out on radar, and wings tend to be smoothly blended into the fuselage rather than meet at a sharp “armpit” angle). The fan blades at the front of the engine reflect a lot of radar – they can be hidden inside serpentine (S-shaped) intakes so they cannot be seen from the outside. In general, reducing the radar returns in the frontal quadrant (minimizing the reflections of radar sent from in front of the airplane) is best, since the enemy is ahead of the airplane until it is fired at, so minimizing the nose-on radar reflections delays detection. Finally, curvy surfaces with non-constant local radii of curvature means that the radar reflection fluctuates quickly in intensity, making it hard to pick out against the background static, and hard to get a lock on. The images below summarize why a faceted surface (like the F-117, F-22) can only be detected on radar from a few specific angles (curved surfaces reflect radar in every direction), why a flattened fuselage with chines and a rounded top and bottom (like the B-2, X-45, Blackbird…) reflects most of the radar away (it acts like a wedge), and why straight-line parallel edges (like the B-2, X-45, X-47, F-22, F-35) make an airplane harder to detect except from a handful of directions (you can only detect it if you’re parallel to an edge, with is ALWAYS with a curved-edge airplane but almost NEVER with a straight-edged airplane). Almost all of this, I learned from reading Bill Sweetman’s books: “Lockheed Stealth” goes through most of these ideas, and “Inside the Stealth Bomber” covers most of the remaining ones. His books on the design of the F-22 and F-35 also contain many of these ideas. Much of this material – including the stories of the X-24 and YO-3 super-quiet airplanes, the jeeps and planes painted pink by the British, etc – can be found online on enthusiast sites like mine or on Wikipedia. So for more in-depth information about these technologies, and stories about their development, just Google the right keywords!