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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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
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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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