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Name: Bob
Status: Educator
Age: N/A
Location: IA
Country: United States
Date: March 2006


Question:
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?"



Replies:
Excellent question! It may help your class to have a basic understanding of supersonic flow (nozzles) and shockwaves before answering. It could be a good segue into the subject. These are relatively short chapters in most thermodynamics and gas dynamics books.

The great speed of entry produce shock waves, which are abrupt transitions in the thermodynamic properties of the air. Air goes from essentially zero speed, low pressure, and ambient temperature to high speed, high pressure, and very high temperature very quickly. Lots of energy is added before the air even gets to the vehicle. That said, the wave of a reentry vehicle is only a short distance from the vehicle itself, so the air is deflected around the heat shield, generating further heat through direct friction. This is not that much compared to the heat generated before the vehicle gets to it, but it is still significant.

So much of the heating is done by the compression through the shock, but there is also friction.

There is also another effect of reentry that this explanation doesn't go into. At the hypersonic reentry speeds of a spacecraft, the heat generated through the shock actually ionizes the air and disassociates the molecules into a plasma. Remember the radio blackouts with the apollo missions? They could not transmit through the interference caused by the ionized layer of air they were generating. There are all sorts of implications from dealing with a plasma, but I don't know what effect it would have on the heating.

David Brandt


There are a couple of questions here: 1. The need for heat shields on re-entry but not lift-off. The speed of the spacecraft on launch is slow compared to the density (and hence friction) of the atmosphere. As the spacecraft gains altitude, and hence speed, the density of the atmosphere decreases dramatically, thus reducing friction and the need for heat shields. The reverse is true upon re-entry. Then, the speed of the spacecraft compared to the earth and its atmosphere is large and as the spacecraft descends, the density of the atmosphere increases, thus increasing air resistance and friction (heat). In some cases, the trajectory of the spacecraft is adjusted so that the craft "skips" along the dense atmosphere like a flat stone can be made to skip across a lake rather than "boring into" the atmosphere. This of course is a trickier maneuver. In everyday experience we do not experience the large differences in speed that cause significant heating of an object moving fast through the atmosphere, but at several thousands of miles/hr this becomes potentially hazardous heating due to friction.

Vince Calder



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