The performance of silica heat shields in the outer planet atmospheres is analyzed and described in a set of differential equations. Results are presented and discussed

Howe, J.

English

NASA Technical Reports Server

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PERFORMANCE OF REFLECTING SILICA HEAT SHIELDS
DURING ENTRY INTO SATURN AND URANUS
John Howe
NASA Ames Research Center
MR. HOWE: I just want to take_a moment to orient some in
the audience who may not be familiar with the reflective heat
shield concept.
The idea is that you take a material that doesn't absorb
radiation, like a window (Figure 6-30) - the radiation will go
through it - and you pulverize it and you then use that as your
heat shield; that is, you put it back together somehow. But now
it's in finely divided particles, and voids, as Bill Congdon just
talked about. It still is not absorbing radiation significantly,
but it is also not transmitting it. It's back scattering it,
reflecting it. This is the whole idea.
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We've tried to analyze the performance of silica heat shields
in the outer planet environments. Very briefly, I want to show
you what's in •this analysis (Figure 6-31). This is a picture of
the front end of an entry probe, and one has incident radiative
flux and convective flux and the surface is ablating. One can
divide the radiation into an inward intensity and an outward,
backscattered intensity, and one can have a mirrored surface on
the back if he wants. For boundary conditions, we insulate the
back to see that no heat gets through. This system is described in
a set of differential equations: an energy equation, that is the
usual heat conduction equa£ion £ unsteady - with terms having to
do with absorption - where K is the absorption coefficient - the
absorption of the outward_i_ntensity and the inward intensity; and
the emission of radiation because the material gets hot.
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These intensities are obtained for each spectral band, "m"
for a pair of equations: one having to do with the outward in-
tensity and the other havingto do with the inward intensity. All
of the properties are temperature dependent; that is thermal con-
ductivity, density, specific heat, absorption coefficient, scat-
VI-56
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tering coefficient. The optical properties are not only tem-
perature dependent but they are wavelength dependent. So one
has to solve this mess. It's non-linear, and it's coupled,and
it's transient; and we've got a scheme for solving that.
I want to show you the res,L_.ts for that. First of all,
let me just tell you what some of the properties are that have
to go into this. The absorption coefficient is very important,
and we want to know what it is as a function of wavelength and
temperature. Figure 6-32 shows what we have been able to get
out of the literature for a material called Ultrasil. There is
data in the catalogs at room temperature. There is some data in
a narrow temperature band region near 1600°K by a man named Rupp
for certain wavelengths; and then Spivak, in the infrared end of
the spectrum, has some data that goes out fairly uniformly over
a broad temperature range. But, clearly, we need more data in
the intermediate range, and we need some higher temperature data
on absorption coefficient. You can see that the absorption var-
ies wildly with temperature - orders of magnitude - and this is
built into our code.
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The scattering coefficient shown in Figure 6-33 tells you
how much radiation is reflected. With high scattering, there is
high reflection. The bottom curve is the scattering coefficient
for a fibrous astroquartz laminate - fibre size of five or six
microns - and it's really not a very high scattering coefficient
- something like 40 reciprocal centimeters - at the most. The
upper curve is a slipcast silica, Glasrock. It's a commercially
available fused silica and it's not particularly good, either,
but we are going to use both of these and show the effects.
Theoretically, I think that one can come up with a scat-
tering coefficient that's about twice as good as this Glasrock,
depending on the void size, and so forth, as Bill Congdon just
talked about. You can see that the Glasrock is far better than
the fibrous material.
VI-59
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the front surface temperature rises rather sharply because some
of the radiation bands are absorbed right on the surface. So the
front surface temperature passes through a peak corresponding to
both the radiation and the convection pulse and then begins to
cool down so that when a subsonic mach number (of 0.7 here) is
reached, the front surface is really quite cool. It's being
cooled by the atmosphere flowing over it.
The interesting thing is that the rear surface temperature
doesn't really see any heat at time zero; but, when that radia-
tion pulse comes on, some of that gets through instantaneously
to the back surface. This is a finite thick slab. Not all of
the radiation is reflected, some of it is deposited there. So,
this shoots the rear surface temperature up to a little peak and,
as 'the light goes out, or the radiation diminishes, that begins
to drop down. But, then conduction from the hot front surface
finds its way through the material and the back surface tempera-
ture begins to rise again. For this particular case we lost
about a centimeter of material due to ablation for this 2 centi-
meter thick shell.
Figure 6-38 shows the corresponding temperature profiles
at various points in time. The peak temperature was at about
twelve seconds, and it's dropping at sixteen seconds; at twenty-
three seconds there is some heat flowing toward the front as well
as heat flowing toward the rear; and at forty-six seconds it's a
fairly uniform temperature - everything is over.
Figure 6-39 shows a thicker slab going into the same atmos-
phere, but I want to show you something. The previous two fig-
ures use the Glasrock scattering coefficient, and I noted that
there was just a small temperature rise at the back surface when
the radiation came on. But if we use the Astroquartz scattering
coefficient, which is nowhere near as good, the rear surface
temperature really shoots up. So, you see how important this is.
The rear surface temperature, essentially, designs the heat
shield. If you are going to have a low rear surface temperature,
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you have to have a high scattering coefficient. Going from
Astroquartz to Glasrock, effectively knocks that rear surface
temperature by a thousand degrees, Kelvin. So, this is really
great; you can get by with thinner heat shields with the higher
scattering coefficient. If we can, indeed, double the scattering
coefficient above Glasrock, we can go thinner yet.
Figure 6-40 shows a result for the nominal Saturn atmosphere
at a 15 ° entry; we saw the environment in Figure 6-36. This is
just a one and a half centimeter thick shell; not thick enough
it turns out, because the rear surface temperature goes up to
• _
almost 1700 ° or 1800 ° Kelvin. This is essentially due to the con-
duction from the front face through. On this one we lost about
three quarters or eight tenths of a centimeter of thickness due to
ablation.
A Jupiter case is shown in Figure 6-41. It is five centi-
meters (original thickness) shell, and you can see the case wasn't
quite finished. It's been finished since this slide was drawn.
The surface temperature goes up quite high - around 3500°K. This
is a 20" entry into Jupiter, which is very severe. The radiative
heating is something like a hundred kilowatts per square centi-
meter - up in the extreme range that John Lundell talked about -
so this is really a very hard entry. We lose about two and a half
centimeters of material. The rear surface temperature doesn't go
very high, something around 400 or 500 degrees Kelvin. So this is
thicker than we need.
We have made quite a number of runs for these three planets,
one entry angle for each planet. We haven't really run any ex-
tensive parametric studies as yet, but we have summarized the re-
sults of these studies on Figure 6-42.
As I mentioned, the backface temperature essentially de-
signs the heat shield. This Figure is for a heat shield density
of 1.49 grams per centimeter cubed which is about the same as the
carbon phenolic density that Sam Mezines discussed. The figure
is for the Glasrock scattering coefficient.
VI-70
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?The figure shows rather surprisingly, perhaps, that it is
easier to go into the Uranus cool dense atmosphere than it is
into the Saturn nominal atmosphere; the reason being, this is
essentially a radiative environment and this heat shield just
doesn't accept the radiation. It back-scatters it. So it is
pretty easy in terms of backface temperature rise. What it tells
us is that if we are limited to, say, 700 degrees Kelvin backface
temperature, (that corresponds to Sam Mezines' ._00 degree Fahren-
heit interface temperature), you can get into the Uranus cool
dense atmosphere with a less than two centimeters thick shell,
into the Saturn atmosphere with a little over a two centimeter
shell, and into Jupiter with four centimeters. That comes out
to about 1.56 inches of heat shield for the 20-degree Jupiter
entry. And that is really a severe environment. So silica
really looks very attractive for severe radiative environments.
I think one thing that this silica heat shield could do is
broaden the entry envelope into these planets, if that is of
any interest for other mission considerations.
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So these are the results so far, and we are busy trying
to extend these results to other entry angles, other atmospheres.
UNIDENTIFIED SPEAKER: The vitrification process, is that
as simple as surface melting that then propagates back through
the silica material?
MR. HOWE: We don't have that modeled in great detail.
What we have is a density change; that is, when we reach a cer-
tain temperature we say the material from then on is trans-
parent; it no longer scatters. So, we have that built in, but
we don't have it modeled in anygreat detail.
ORIGINAL PAGE IS
OF POOR QUALITY
VI-74
JUNIDENTIFIED SPEAKER: Isn't the temperature like 3200 °
or something like that? I thought maybe your peak temperature
would have melted the surface.
MR. HOWE: Oh, yes; there's a region at the front of it
that's melted and is no longer back scattering. The scattering
is being done in the depths.
UNIDENTIFIED SPEAKER: A question of clarification: Were"
you limited to 800°F temperature, or temperature rise, because
you have plotted there a temperature rise?
MR HOWE: That 800°F was a design temperature that McDonnell
Douglas used for the interface temperature between insulation and
the heat shield. The ordinate on Figure 6-42 is really absolute
temperature, I shouldn't have said temperature rise.
MR. LEIBOWITZ: Does the performance of this heat shield
change dramatically for a very intense Jupiter entry where the
peak radiation falls below two thousand angstroms?
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MR. HOWE: That is about 6 e.v. at the peak. It will reflect
effectively in wavelengths between about 0.5 and 6.0 e.v. Those
are the constraints for this. So, if it falls into the vacuum
ultraviolet - I guess that's what you are thinking?
MR. LEIBOWITZ: Yes.
MR. HOWE: Well, then, it's not going to reflect. Actually,
my own opinion is if you have radiation in the vacuum ultraviolet
the material won't see it anyway; it will be absorbed by all the
molecular species in the gas phase - in the boundary layer.
GEORGE DEUTSCH: I notice that you dealt with an appreciable
thickness above the liqueous temperatures of the silica; what's
to keep that from simply flowing away?
VI-75
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MR. HOWE: I think that a melt layer that flows along the
body would be very thin, George. Bill Nicolet took a look at
that in an earlier stage of the Saturn-Uranus studies and con-
cluded that it was really a thin melt layer. These temperature
profiles are quite sharp, and the material is eroding at a great
rate; so that the primary mass loss is due to the thermochemical
erosion normal to the surface.
UNIDENTIFIED SPEAKER: It looks like the equations you first
showed were spherical coordinates. Are all of these spherical
coordinates?
MR. HOWE: Oh, there's a little exponent in there. If you
set it to one it's a spherical geometry, and if you set it to
zero it's a slab.
UNIDENTIFIED SPEAKER: But it's either a slab or a sphere?
MR. HOWE: Yes, if we want to do a cone, those are essentially
slabs. That is, these are very thin shells, with large internal
radii. So, I think a slab would do us well anywhere except at
the stagnation point. And even there it is pretty accurate.
SAM MEZINES: Based on the fact that you got most of
the heat shield requirements from the shock layer radiation con-
tributions, would a shallow Jupiter entry with higher convective
heating require more heat shield than you have shown here?
MR. HOWE: I don't really know, Sam; it's a possibility.
We would like to try that seven and a half degree angle Jupiter
case to find out. These are not trivial things to run, I might
mention. In order to get one case, somebody has to stay up all
night - me. We are trying to improve that situation.
VI-76


Performance of reflecting silica heat shields during entry into Saturn and Uranus

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