Shuttle Entry Heating
Craig Fink
posted a while ago in sci.space.shuttle. It really didn't generate much
discussion, so I'll repost it here.
http://groups-beta.google.com/group/sci.space.shuttle/msg/51168e065533f96b
Well, I think what you've said here is not quite right. Columbia wasn't
like the remaining three Orbiters. It's bottom surface was rougher, and
the boundary layer would trip to turbulent sooner that the others. One
wing, or the other. On one mission both wings tripped to a turbulent
boundary layer quite early during entry and survived. A turbulent boundary
layer increases surface temperatures, but not beyond what the tiles can
handle.
Also, the really hot temperatures (5000-10000F) "in" the bow shock, not
the flow behind the shock. My understanding of the problem is that as the
air enters the shock wave, it's compressed, heating it to the extreme
temperatures. At these extremely high temperatures, the molecules of the
air cannot stay together. So, after some very short time period, the
Oxygen and Nitrogen decompose from O2 and N2 into O and N ions, creating a
plasma. This takes energy, and the temperature decreases until it drops
below the temperature necessary to split the O2 and N2 molecules.
So the plasma is a mixture of O2, N2, O and N, and it is at equilibrium.
At the beginning of entry, there is more energy, so more O2 and N2 will
decompose and create a larger percentage of O and N ions. With the
equilibrium temperature behind the shock around 3000F. Later in entry,
there is less energy so less O2 and N2 will decompose, but the temperature
is still the same. Kind of interesting that no matter how fast your going,
the temperature won't change much until all the O2 and N2 has decomposed.
The O and N ions can be thought of as storing potential energy, as long as
the ions can be kept from recombining or recombining with something else.
That's were the problem with letting the plasma flow into Columbia's wing
comes in. At one of the CAIB meetings, someone from NASA talked about
materials and the plasma entry environment.
They basically said some materials are catalytic and increase the
temperature of the plasma. To me catalytic implies, that the material
would be causing O an N ion to recombine back into O2, N2 and NOx without
being used up. I'm not sure catalytic is the right word. Burning might be
a better word to describe what is going on. An example is the aluminum in
the wing. The introduction of Aluminum into the plasma make the formation
of another molecule possible, Aluminum oxide. Aluminum, which is a fuel in
solid rocket motors, and used in fireworks to produce the "hot" color
white, burn really hot and release lots of energy. Also, it would have a
different, probably higher decomposition temperature than Oxygen and
Nitrogen.
So, as the plasma enters the wing, the hot O ions burn with the Al
releasing energy and raising the equilibrium temperature. More O2
molecules decompose making more O ions available to burn with the more
aluminum. Maybe this is what they are calling catalytic.
Additionally, with a large enough hole in the wing the bow shock can move
to the inside the wing, where the extremely high temperature could be
right next to catalytic materials.
Noncatalytic materials, are really material that have already been burned.
Like Silicon-Oxide, or Silicon-Carbonate. Although Carbon can withstand
extremely high temperatures, it will burn at much lower temperatures, so
it must be protected from the O ions. It really makes me wonder about the
wisdom of mixing ablative patches with reusable tiles, because if any of
the material in the ablative is catalytic, or increases the equilibrium
temperature, the patch could actually increase temperatures on the
reusable tiles behind it. But, I would hope NASA has considered that.
Does a heat shield patch need to be smooth or not protrude into the the
airflow? Well, the answer is yes and no. It depends on where the patch is.
The entire leading edge of the wing can be considered a really large
protuberance into the airflow. If we follow some air around the wing,
it'll help everyone understand what's going on.
First the air hits the bow shock, where it slows down to subsonic flow,
increasing pressure and temperature. The air decomposes into a plasma and
reaches it's equilibrium temperature, a temperature that the Carbon-Carbon
leading edge can handle. At the stagnation point on the leading edge, the
air comes to a complete stop then either goes up above the wing or down
below the wing. We'll follow the air down.
At the stagnation point, the wing leading edge is perpendicular to the
free stream airflow. As, the air flows around the leading edge, it begins
to accelerate and expand, reducing the pressure and temperature of the
plasma. At some point is passes though and expansion shock and is moving
supersonic again, and the pressures and temperatures continue to drop as
the flow continues around the leading edge. The O and N ions are now below
there equilibrium temperature and will start to recombine releasing energy
and increasing the temperature. But, this reaction is slower that the
decomposition reaction, so the temperature of the flow drops as the flow
expands.
The Shuttle flies entry at an angle of attack of 45 degrees. This means
the flow has been expanded from 90 degrees at the stagnation point to 45
degree on the bottom of the vehicle. An expansion fan of 45 degrees. This
is the environment that the tiles find themselves in. Air that has been
compressed (temp increased 10000F), decomposed into a plasma (temp
decreased to 3000F) and then expanded thru a 45 degree expansion fan
(wave) which cools airflow further (2200F).
The difference between a laminar boundary layer and a turbulent boundary
layer is an additional decrease in temperature due to conduction from the
hot plasma to the RCC/Tiles which radiate some of this heat, further
reducing the temperature. While nice to have, it isn't necessary as
Columbia has survived entries where the boundary layer has tripped to
turbulent quite early. Also, the temperature decrease will be moderated
(won't be as great as you would expect), because some of the O and N ions
recombining releasing energy. As the temperature decrease due to the 45
degree expansion won't be as great as you would expect.
If a large protuberance due to a patch is place on the tiles, a new
compression shock forms, with the extremely high temperatures next to the
tiles. (Note, the elevons do move down creating a compression shock,
increasing the temperature on the elevon tiles, and they survive)
But, this is not true of the leading edge. A large protuberance can be
added to the leading edge, as long as it has a large diameter. The bow
shock will form in front of the protuberance/patch and the environment of
the patch should be the same as that of the rest of the leading edge. Even
a really thick patch (a foot thick) could be applied to the leading edge.
As long as the patch only creates expansion fans(wave) under the vehicle.
Which is exactly what happens when the thick patch ends at some point
under the vehicle.
So tile patches need to be smooth and not create large (bow) compression
type shocks, and leading edge patches need to wrap around the stagnation point
and can be as thick as you want. And if your worried about changing the
aerodynamics of the vehicle due to the big thick patch, put an equivalent
patch on the other wing to keep the aerodynamics symmetric.
Comments and corrections welcome,
--
Craig Fink
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