<< Page 8 Page 9: Page 10 >>   Sonic/Supersonic Shockwave Drag (Compressibility) Only a minor component of drag in airliners (which fly at 5/6 of the speed of sound) and not at all an issue in propeller-driven planes, this is a HUGE problem for supersonic jets (and rockets). Drag increases so much as you reach and pass MACH 1 that for a long time people wondered whether an airplane would ever break the so-called “sound barrier”. (No one ever wondered whether it was physically possible to go faster than MACH 1. That is a myth – the movie The Right Stuff, otherwise a wonderful, wonderful movie, is partially responsible for making people think that scientists thought supersonic flight was inherently impossible. Even during the 40’s, it was well known that bullets and some rockets flew faster than sound. The question was, could lift be generated at supersonic speeds? Could a wing work? Would an airplane stay in the air? This had not been proven conclusively, but as we know now, airplane wings can and often do work at supersonic speeds). To understand how this drag works, and why it is so severe, we must understand how a shockwave works. Sound waves are pressure waves moving through the air. They displace air because of differences in pressure. Similarly, air knows to move out of the way of a subsonic airplane because of the high pressure ahead of its nose and leading edges. In subsonic flow like the one above, there is no one place where the air is pushed. Rather, it is pushed out of the way smoothly and continuously, starting to move BEFORE the actual “arrival” of the nose (because the pressure waves that move the air are ahead of the plane itself). But, if a supersonic airplane is moving the air around it out of the way like this: then the next bit of air will only be pushed out when it is hit by the outwards-traveling pressure wave. But the airplane is traveling faster than that wave (the wave is not ahead of the plane anymore), so the second layer of air will be pushed out of the way “behind” where the first layer was pushed, because the airplane will have traveled forwards before the wave gets there: So each bit of air pushes the next bit of air, but it takes less time than it takes for the plane to travel forwards, so the air ends up doing this: The red line shows the shockwave. (There’s one in the bottom too, but I thought I’d leave it out so you can more easily see the abrupt change in the air caused by the shockwave). It’s the place where the air is suddenly shoved to the side. Because of this, a shockwave is a boundary between high-pressure air and low-pressure air, between hotter air and colder air. This pressure difference is strong and sharp enough to break windows (if a plane flies supersonic too low), and to refract light (like in The Matrix… well, not quite, but kinda like that), as can be observed in this windtunnel picture of an X-15 model, and in this picture of a supersonic T-38 taken with an extremely low exposure as it flew between the sun (background) and the camera. The best pictures I’ve ever seen of refraction due to shockwaves were taken at the last San Francisco Fleet Week, showing Blue Angel #5 in a MACH .99 pass (see bottom of page). In three dimensions, a shockwave extends out as a cone from the nose of the airplane, as well as from the wings, tail, and any bump or other feature which causes the air to change direction around it. When a shockwave hits, the air is compressed, heated, cooled (if it’s a low-pressure shockwave) and immediately displaced. The energy required to move so much air like this is what causes all the extra drag. A supersonic airplane will form a cone like this that is miles wide, as it flies forward at supersonic speeds. That means a LOT of air every second goes through that cone, and a LOT of energy goes into moving and heating it. Very large supersonic airplanes, like the Concorde, Tu144 and B-1, can fly several miles high and still form powerful shockwaves that break windows on the ground. Below, Blue Angel #5 does a fast pass at MACH 0.99. The shockwaves compress the air so much that the sharp pressure gradients cause the light to be refracted as it passes through the shockwaves: Compressibility drag and its effects (shockwaves) are most severe where the air is compressed/squeezed/”pinched” the most – at a bubble canopy, at the nose, and over the wings. This compression can be so severe at these places, it can cause the water vapor in the air to condense, especially in a humid day. So sometimes pictures of supersonic planes show condensation cones where this compression is strongest: The best way to minimize shockwave drag (other than not flying that fast) is to have pointy noses and leading edges, and thin/”skinny” fuselages. The air being pushed by this pointy, skinny nose (left) is undergoing less-severe changes (it’s pushed to the side less, and compressed less, and thus heated less) than the air being pushed by this blunt, fat nose (right): This is why supersonic airplanes have pointy noses, and sometimes even thin-edged wings. Pointy & skinny supersonic planes: The F-104, SR-71, Concorde, a couple F-5s, and the X-3 Stiletto, the first airplane to apply this idea. There are other tricks, when it comes to the shape of the wing and fuselage, that reduce supersonic drag. One is to sweep the wings. The effective MACH number over the wings is the real MACH number divided by a function of the cosine of the angle of sweep (roughly). Sweep the wings by 60 degrees and you get about half the supersonic drag! (But at low speeds you get a lot of induced drag and a lot less lift). Most modern jets’ wings are swept 35 degrees or so. An F-86, the first American swept-wing jet, and the much more modern B-52. Another trick is to reduce the cross-section of the fuselage where the wings are, so that the total cross section of the airplane stays roughly constant. This way, instead of pushing air out or pulling it in (because the airplane widens or narrows), air is merely being swirled around near the airplane, and shockwaves are less severe. This is called the Whitcomb area rule, and leads to airplanes with “coke-bottle fuselages”: Above: Three area-ruled airplanes; the F-106, T-38, and F-5. Below, a 1944 patent by Junkers showing a trans-sonic area-ruled forward-swept-wing design – check out page 140 for the related Junkers 287, the first FSW jet. Heating is also a big issue with compressibility drag (this is another reason, in addition to viscous drag, that supersonic airplanes become very hot, and why the Blackbird had to withstand temperatures no other airplane had ever encountered). When air is compressed by a shockwave and slowed down relative to the airplane, it heats up. The more the air is slowed down and compressed, the more it heats up. So a blunt nose (which slows down the air a lot by deflecting it a lot at the last minute) not only generates a lot of drag at supersonic speeds, it also gets very hot. The bottom of the Space Shuttle is not just blunt, it’s FLAT. When the Shuttle reenters the atmosphere from space, it is flying at about MACH 25. It needs to slow down as quickly as possible in order to be able to land. (Sure, it’s very high, but at 5 miles per second, it can descend very quickly and doesn’t have that much time). So it orients itself to fly belly-first into the air, at extremely high angles of attack. The air that hits the nose and belly is SEVERELY compressed, reaching pressures of 60 atmospheres! (That’s a LOT of pressure drag, which slows down the Shuttle quickly indeed). The temperatures go to the thousands of degrees. The shockwaves are so powerful that, when reentry occurs at night or early in the morning (as was the case with the Columbia), the shockwaves wake up people dozens and dozens of miles away. Heat management is the toughest part of designing a reentry spacecraft. When the Mercury modules were being designed (at the very beginning of the space program), no material could go through these kinds of temperatures without melting or coming apart. The way engineers dealt with this was by equipping reentry capsules with thick heat shields, made of materials that would be good insulators (so all the heat stayed near the surface) and that wore away during reentry, conveniently carrying away the heat. By the time those capsules opened their parachutes and hit the ocean, the bottom would be a lot thinner than before reentry, as most of it is worn away by the air and heat during reentry. The danger with this is, if you reenter too fast (too-high speeds, too much heating, heat shield wears too quickly or just comes apart) or too slowly (too much time in the air, heat shield wears all the way through, exposes capsule to heat and pressures), the heat shield fails and the astronauts are cooked. So a material that can withstand this heat and NOT wear away is better. But these material are brittle ceramics, so they cannot be melted or molded like metals and plastics – they are too brittle (crack rather than bend, like clay rather than rubber) and have too-high melting points – so the whole hull/shield cannot be made of one big piece. So the heat-resistant ceramics must be “sculpted” and applied in small pieces – thermal tiles. This had been developed by the time the Shuttle was designed, so reentry spacecraft have since been fitted with thermal tiles.