Heat Shield Heating: Friction or Compression

name         Bob
status       educator
location     IA

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?"
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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.
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Bob Erck
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