Heat shield materials were tested under intense radiation in a gas dynamic laser. The laser is described and test results are presented

Lundell, J.

English

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TESTS OF HEAT SHIELD MATERIALS IN INTENSE LASER RADIATION
John Lundell
NASA Ames Research Center
MR. LUNDELL: As shown above, I have changed the title of the
talk from what's listed in the program for several reasons. First
of all, I don't think in fifteen minutes we can review the work
that's been done on the behavior of graphitic materials in intense
heating environments. Secondly, I thought you might be more in-
terested in some very recent results we got testing heat shield
materials under intense radiation in our gas dynamic laser.
Figure 6-9 schematically presents our gas dynamic laser. The
facility was funded by Paul Tarver, at Headquarters, several
years ago when it became apparent that the only way we would get
radiative rates of interest for planetary entry - particularly
Jovian entry - was to have a laser. It is a gas dynamic laser
in which we burn CO to CO 2. It lases at I0.6 microns and produ-
ces a continuous output at powers up to about 45 kilowatts. For
the test .I'll describe today we focused the beam with a one and
a half meter focal length mirror and simply re-imaged its focal
point on the target, which is sitting out in a room environment.
We did have a nitrogen jet blowing in front of the target. It
was spaced away from the target such that it was not impinging
on the target to cool it. The motive here was to try to blow
the plume away.
In some early work we did on graphite in the laser, we found
that at low intensities the plume could effectively block about
two thirds of the incident radiation, so we wanted to blow it
away and let as much radiation get to the target as possible.
Thus, the beam impinges on the target, and what we do is measure
the time from the •moment it impinges until it first burns thru.
That is, we are measuring burn-thru time. We do that with either
TV or movie cameras, and we also measure the surface temperature
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Figure 6-10 shows the test conditions. We looked at three
different materials: ATJ graphite, which is a representative,
fine-grain graphite typical of what's being used for ballistic
missile nose tips today; Carbitex I00 is a carbon-carbon composite
which is made by Carborundum Corporation. We found in our prelim-
inary survey of a lot of different materials, in the laser, that
Carbitex was the best carbon-carbon composite that we tested. The
third material is a phenolic carbon. This is representative of
what's being used as a heat shield material on ballistic mis-
siles today. These models were furnished by McDonnell Douglas,
St. Louis. Carbon phenolic is simply made by stacking up layers
of carbon cloth and then, essentially, gluing them together with
a phenolic resin.
We placed the models in the laser beam at a point where we had
about a third of a square centimeter irradiated spot. We had to
go to that small a spot in order to get intensities of interest.
So, what we did, then, was to leave the models at the same point
in the beam and vary the output power of the laser from essentially
four to 35 kilowatts. If we divide these power numbers by the
area of the irradiated spot, we come up with the indicated average
intensities: from ten to 92 kilowatts per square centimeter; in
English units, from 9,000 to 81,000 BTU's per square foot per sec-
ond. Now I want to emphasize that these numbers are the average
intensity. The laser does not have a spatially uniform output
beam; it's more Gaussian. So, the peak intensity may be a factor
of two or more above the average intensity; at this time, I don't
know the ratio of the peak to average intensity. You should note
that the burn-through time is probably more closely related to the
peak intensity than the average intensity.
Incidentally, we selected these conditions so that the lowest
intensity would represent entry into Jupiter using the warm at-
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mosphere, at about a six-degree angle. The intermediate intensity
represents a nominal atmosphere, going in at about seven degrees;
and the highest intensity represents the cold atmosphere, going
in at about nine degrees.
Figure 6-11 shows the results we obtained at the lowest
intensity, namely, an average intensity of 9,000 BTU/FT2-sec.
What I am plotting here then is, essentially, the target thick-
ness against the burn-through time. For each of these materials
we ran three or four different thicknesses from about an eighth
of an inch up to in excess of a half inch. As you can see, the
curves, then, to obtain a burn-through velocity or the velocity
at which the beam penetrates into the material.
We find that for this condition ATJ graphite has the lowest
penetration velocity, about an eighth of an inch per second; and
the carbon phenolic was in excess of a half inch per second; and
the carbitex fell in between.
Figure 6-12 shows the results we obtained at the interme-
diate intensity. Here I am plotting the same coordinates. The
relative ranking in the materials is the same: ATJ has the lowest
velocity, then the Carbitex, and then the Phenolic carbon. Note
that we are up to penetration velocities in the order of one to
almost two inches per second.
Figure 6-13 shows the results for the highest intensity; up
around 81,000 BTU/FT2-sec. The relative ranking in the materi-
als is still the same: ATJ is the lowest and phenolic carbon the
highest. However, you will note now that the materials are all
kind of coalescing together as far as performance goes. We
have penetration velocities from 2.2 up to about two and three
quarter inches per second. For the carbon phenolic point, for
example, the thickest model was 1.08 inches and the beam pene-
trated that in about .39 seconds.
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I thought it might be of interest to show you very briefly
a film clip of the test of that particular model to give you an
idea of what these things look like when they get hit with very
intense radiation. (Film clip shown)
MR. LUNDELL: We shot these pictures at 600 frames per sec-
ond and they are being projected at 24, so we are slowing it down
by a factor of twenty five.
The film indicates that the carbon phenolic puts on quite a
fireworks display at this intensity level. The other materials
give you about the same amount of plume, but you don't see as
much evidence of particulate mass loss as you see with carbon
phenolic.
Figure 6-14 summarizes the results in terms of mass loss
rates. The quantity we were determining from the previous slides
was the recession velocity. If you multiply that by the density
of the material, you can get a mass loss rate. So, that is what
we have here for the various average intensities and the three
different materials: ATJ, Carbitex and carbon phenolic. As you
can see, at the lowest intensity we've got almost a factor of
four to one difference in the mass loss rate between the graphite
and the carbon phenolic. When we get to the intermediate inten-
sity, this ratio drops to about 1.5. They got about 50 percent
more mass loss rate for the carbon phenolic. And when we get
to the highest intensity, they are all pretty comparable: from
about 18 to 21 ibs/ft2-sec, which was a pretty good mass loss
rate. To give you an idea of what that compares to in our con-
vective tests, I think the highest ablation rate I ever obtained
in a convective test on graphitic materials was about a half pound
per square foot per second.
These results are shown graphically on Figure 6-15, where
I'm plotting the mass loss rate against intensity. As you can
see, and as I noted before, down at the lowest intensity we have
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the largest difference on a relative basis between the materials;
and when we get up to the highest intensity, they are all running
about the same.
The thing to note here, however, is that the two all-carbon
materials are performing better than the phenolic carbon; and this
isn't too surprising. A predominant heat accommodation mechanism
under these severe heating conditions is sublimation and, in that
case, you want as much carbon up front as you can get. These
curves do turn out to be linear and if you take the slope of this
curve for ATJ you will come up with an effective heat 6_' ablation
of about 4,000 BTU's per pound, which is about half the _eat of
sublimation if one assumes that the specie being sublimed is C 3.
The curvature in the phenolic carbon curve, I think, is
probably due to the fact that we've got the phenolic there compli-
cating things when it pyrolyzes.
In conclusion I'd like to say that it does appear as though
the heat shield problem is going to be rather severe for entry into
the outer planets but, with the laser and the up-coming arc-jet
facilities which are going to be developed here at Ames and which
Howard Stine will describe shortly, I think we will be able to do
a pretty good job of simulating entry into the outer planets and
we will be able to determine why these materials perform the way
they do under these intense heating environments. Then, we will
be able to design the flight heat shield with a great degree of
confidence.
UNIDENTIFIED SPEAKER: Because of the linear relationship in
your last chart there it seems fair for an actual entry case where
the heating intensity reaches a peak and then comes down to just
integrate the area under it and make the thickness proportional to
that?
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MR. LUNDELL: Yes, I think that would be a pretty reasonable
thing to do, for a first approximation, based on what we know now.
In other words, I think even though the heating rate is varying
very rapidly with time, you are• going to stay pretty close to
thermal equilibrium.
UNIDENTIFIED SPEAKER:
meas uremen ts ?
MR. LUNDELL: Yes, we did.
Rankine; that's about 4100°K.
Did you get any surface temperature
They are running about 7400 °
UNIDENTIFIED SPEAKER: Do you consider your monochromatic
results reasonably applicable to the real case?
MR. LUNDELL: That's the real question in using the laser as
a simulation facility for planetary entry. In a planetary entry
case we expect radiation in the visible and the UV and, of course
with the laser we are way out in the infrared. In answer to your
question, I think it's okay for graphitic materials, or black
materials. It certainly would not be for the reflective materials.
UNIDENTIFIED SPEAKER:
of a world's record?
Is that 2.2 inches per second some sort
MR. LUNDELL: It is for me.
DR. NACHTSHEIM: The next speaker will be Bill Congdon from
Martin Marietta and there is a slight discrepancy in the program:
he will be describing Dave Carlson's work, which is the applica-
bility of the Pioneer Venus hardware to Saturn probes, and he will
also be discussing Martin's efforts on the development of silica
heat shields. So, in his talk he will essentially make two
talks, and make the transition from the evaluation of heat shield
materials to the development of heat shield materials.
VI-26


Tests of heat shield materials in intense laser radiation

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