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Heat Shield Heating: Friction or Compression
Name: Bob
Status: Educator
Grade: Other
Location: AL
Country: United States
Date: June 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:
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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Update: June 2012
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