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Name: Bob
Status: Educator
Grade: Other
Location: AL
Country: United States
Date: June 2006

The class has been discussing why a heat shield is necessary for return rather than the entry of a spacecraft. We then raised a more fundamental question: "Is the heat caused by friction of air, which is the standard explanation--or more by compression of air--like happens in a diesel engine?"

The reason why a heat shield is needed for return rather than entry has been discussed before. Basically, a shield is not needed on ascent because the speed on the way up, where much air is present, is fairly low. But the spacecraft is going very fast on reentry and needs protection.

The question about compression and friction is a good one. If one does a search on "aerodynamic heating" one can read several good explanations. But....

A little background: molecules and atoms have energy that is proportional to the temperature. In a gas, the energy is mostly kinetic (motion). In a solid or liquid, the energy is mostly related to vibrations. The average energy is the same for all molecules that are at the same temperature. For example, the energy of each of the molecules in a table, in the air, and in a glass of water that are in a room are about the same, on the average. Some more and some less.

If you put a spacecraft into a hot oven (which has air molecules with more kinetic energy and moving faster), the oven molecules would transfer heat to the spacecraft until the spacecraft got hot. Now, instead of putting the spacecraft into the oven, blow air at it that is going VERY fast. The atoms of the heat shield are struck by air molecules which are moving very fast. Basically, it is equivalent to a high temperature, and so the heat shield atoms also are warmed up. This is certainly the mechanism in the case where there is little air and the air molecules can directly strike the spacecraft and cause to get hotter.

On the other hand, suppose there is more air and the air molecules bounce off each other a lot. Well, right next to the solid surface, the air molecules are moving slowly (relatively speaking). The fast-moving air that is flowing or impinging on the surface is being slowed down or brought to rest. Air is not a perfect fluid, but has some viscosity, and heat is generated as the kinetic energy is dissipated as the air is suddenly being slowed. In addition, one can look at this as the air being compressed, and one can imagine the air getting hotter by means of that mechanism. So in either case, the surface gets hot.

Of course, the air that is slamming into the solid surface tends take heat away too, and this balance between heating and cooling is called the "recovery factor."

Because airplanes fly through the air fast, one would expect that surfaces that are struck by a lot of fast moving air, like wing leading edges, to be also warmed up, and in fact they also get warm.

Bob Erck

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