Materials Engineering
how do you figure out what types of metal won't brake up or disintegrate in the atmosphere
Kyle - There are a number of factors which play into selecting materials for any engineering purpose. For example, two major issues associated with structural failure on re-entry are frictional heating and aerodynamic drag. The two are actually related, in that the drag and the friction are both caused as the re-entry vehicle passes through the atmosphere and runs into particles of air. Let's talk about drag first. Drag is, in its most basic form, the force necessary to push the air in front of a moving body (e.g. a re-entry vehicle) out of the way as an object moves past. The more dense the material, the more drag results. Thus, a sphere moving through air will experience less drag than a sphere moving through water. Drag is also as function of the shape of the moving object. For example, a square block with side length D has a drag coefficient of about 2 for speeds less than the speed of sound. For an oval that is D tall and 4D long (in the direction of flow) the drag coefficient is only about 0.25 for the same range of speeds. However, as you approach the speed of sound, these drag coefficients climb rapidly. This means that the force on the object moving through the air climbs quickly as well. Pushing the air out of the way also creates heat from friction between the air and the moving object. In fact, the surface of most re-entry vehicles becomes so hot that it would melt if made out of ordinary materials. This gets back to your question about breaking up. As most materials get hotter, their structural properties (strength, stiffness) change. Generally, they decrease, This means that, just as the strength is needed the most in a re-entry vehicle, it is beginning to decrease due to frictional heating. So, you need to choose a material that has several qualities: - It must shed heat rapidly to avoid literally burning away. This could also be achieved by using a material with a very high melting point. - It must have adequate strength to support the loads created by moving through the air, even at substantially increased temperatures. - If the re-entry vehicle is more-or-less like an aircraft, the material must be light enough that the resulting re-entry vehicle won't fall like a stone instead of moving through the air smoothly like a gliding bird. Well, a very *big* and sometimes ungainly bird. As a result of these requirements, most re-entry vehicles are composed of a variety of materials, each selected for a given purpose based on its particular features. For example, the leading edge of a space shuttle wing is formed from a carbon composite, which has very high heat resistance. The leading edge of a wing (as well as the nose) can experience substantial heating due to friction, especially depending on the angle at which the wing moves through the air (the "angle of attack"). Since the surface of a space shuttle experiences extremely high heat demands as well, the surface is covered in heat-resistant ceramic tiles. Both these tiles and the carbon leading edge are intended to protect the more vulnerable inner structure of the shuttle from the extreme temperatures. The inner structure is built of relatively light-weight (compared to the tiles) aluminum. This is not to say that because the aluminum is light that it is weak. Some aluminum alloys can be as strong as steel, but weigh only about 1/3 as much. Metals, however, generally suffer quite severely from the loss of strength with increased temperature I mentioned earlier. As a result, the aluminum airframe must be protected from the extreme heat of re-entry so as to maintain its desired properties. As you can see, this is quite a complex topic, and one which NASA and other space agencies have studied for quite some time. For more information about the space shuttle, take a look at NASA's website which talks about the processing (preparation for launch) of shuttle Discovery for a future mission. http://www.nasa.gov/missions/shuttle/f_omdp1.html I hope this helped answer your question. Charley