Gas properties which can affect outer planetary entry probe radiative heat transfer are reviewed in order to predict the effect of processes such as radiative relaxation, radiative cooling, and equilibrium radiation intensities on entry. These processes are quantified in order to avoid overestimating the radiative transfer by an over simplified approach to the problem. By reducing the uncertainties in the knowledge of these processes, the heatshield weight is minimized by reducing safety factors and performance limits

Leibowitz, L.

English

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RADIATIVE RELAXATION RATES AND INTENSITIES DURING OUTER
PLANET ENTRIES
Dr. L. Leibowitz
Jet Propulsion Laboratory
DR. LEIBOWITZ: This morning I would like to give you a re-
view of the gas properties which can affect outer planetary entry
probe radiative heat transfer.
The goal is to be able to predict the effect of processes such
as radiative relaxation, radiative cooling, and equilibrium ra-
diation intensities on entry. The purpose is to better quantify
these processes in order to avoid overestimating the radiative
transfer by an over simplified approach to the problem. By reduc-
ing these uncertainties in the knowledge of these processes, we
hope to minimize the heatshield weight by reducing safety factors
and performance limits that might otherwise have to be put in.
Figure 5-7 is a schematic diagram that roughly shows flow
regions for an outer planetary entry probe. The atmosphere of
the outer planets, as you know, is molecular hydrogen and helium,
for the most part. Through the shock layer these gases are trans-
formed into hydrogen atoms, ions and electrons. You can basical-
ly think of the shock layer in terms of three regions, neglecting
the boundary layer. First we have a weakly radiating non-equili-
brium layer. In this layer the shock heated gas undergoes chem-
ical reactions and is transformed as it flows into the ionized
species. Then we have the equilibrium layer where the gases are
considered in local thermodynamic equilibrium and the radiation
transfer can be calculated accordingly. Finally we have a high-
temperature radiative cooling region where the hot gas radiates
much of its energy away into the outer flow and by loosing that
energy the temperature falls and it, therefore, radiates consid-
erably less energy to the wall, thus causing lower heat transfer.
These three regions represent areas of separate topics of
study. The non-equilibrium layer is the one that we have been
emphasizing. In this region the radiation is proportional to the
electron concentration. The electron concentration is initially
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zero at the shock wave and then as the reactions take place it
increases to an equilibrium value; therefore, when the electron
concentration is much below the equilibrium concentration the
radiation is much below the equilibrium radiation. So, in the
case where the relaxation distance is long compared with the
standoff distance you have a large region of virtually radiation
free gas.
This is considerably different from the case of non-equilib-
rium radiation for Earth and Venus, where the non-equilibrium
overshoot of molecular species behind the shock wave resulted in
an increase in radiation over what the equilibrium theory would
indicate.
Our approach has been to develop shock tubes which produce
conditions as close as possible to entry, then to make measure-
ments of the radiative and kinetic properties of the shock heated
gases and finally, the experimental data is applied to flow field
calculations in order to obtain entry heat flux. Data has been
obtained both in a conical arc driver, shown in Figure 5-8 and a
newly-developed annular arc driver, called ANAA shock tube. The
ANAA shock tube deposits energy of a capacitor bank into a flow-
ing gas which then immediately expands and cools before it can
lose energy to the walls of the shock tube while it waits for a
diaphragm to open. With this new shock tube, Jupiter and Saturn
entry velocities and pressures, for the most part, can be simu-
lated.
In the diagram of Figure 5-8, we see a capacitor bank which
discharges a spark into a gas. The heated gas then rushes down
the tube driving a shock wave ahead of it. The radiation emitted
behind the shock wave, then, is measured by a series of spectro-
meters and monochromators. Hydrogen line and continuum channels
are detected, including the profile of the H Beta line using a
fiber optics slit system which can beused to get electron den-
sities and temperatures directly.
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Figure 5-9 is our latest trace obtained last Friday and it is
our closest attempt at simulating out e r planetary entry condi-
tions. This is for an initial pressure four torr and 26 kilo-
meters per second. This roughly approximates peak heating for
a•Saturn entry. We have measured here the intensity of the H Beta
line as a function of time. This is a magnified version. Inten-
_ ' , initially at the shock arrival, the intensity issity is down so
virtually zero; then, as the chemical reactions take place and
the electrons begin to be formed, the intensity suddenly jumps
and then rapidly reach an equilibrium value. The relaxation dis-
tance is the distance between the shock arrival and when equi-
librium is achieved. It is rather substantial: four centi-
meters compared with standoff distances. We will see that a little
later.
Figure 5-10 is a plot of relaxation distance times the initial
pressure in the shock tube as a function of the shock velocity.
The dark points are the higher pressure data obtained with the
ANAA shock tube. The solid line is a curve fit obtained from
numerical integration of the ionization and dissociation reac-
tion kinetics. By adjusting rate parameters one can see that
there is rather good agreement on the dependence on the part of
both the data and the calculations. • The squares represent data
obtained at a much lower pressure in the conical driver and while
the data agrees very well at the higher shock velocities, it di-
verges somwhat at the lower velocities which seems to indicate
the possibility of test time limitat_0ns in these low velocities.
With the kinetic data obtained by fitting the experimental
results we can apply the kinetics program to the flow field case.
This is the subject of the next talk=by Dr. Kuo. It is with
data_shch _as this tha_we will _ be able _tO quantify t/%e non-equi-
librium effect for outer planet entry conditions.
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We have made a rather comprehensive comparison of equi-
librium hydrogen and helium radiation measurements with theory.
We have covered ranges of temperatures from i0,000 ° to 20,000 °
Kelvin and electron densities that cover the full range of
Saturn and Jupiter entry conditions. Figure 5-11 is a sample
of some of the typical agreements that we have obtained. This
is hydrogen line radiation and these are hydrogen continuum
channels over a wide range of temperatures. As you can see,
for the equilibrium calculations, we are very well able to pre-
dict what we measure in the shock tube. Throughout the full
range of all conditions that we have covered we get a twenty-
five percent agreement with the theory.
Concerning radiative cooling measurements, we've just begun
to use the capabilities of the ANAA shock tube for this study.
Radiative cooling could result in up to a seventy percent reduc-
tion in radiative heating during portions of Jupiter entry tra-
jectory. Initial experimental data is in reasonable agreement
with simplified calculations. This work is now being continued.
In conclusion, due to recently improved simulation facili-
ties that are able to produce Jupiter and Saturn entry conditions,
and the development of the non-equilibrium flow programs, we are
in a good position now to accurately assess the effect of each
of these radiative processes on the entry trajectories themselves
and on the heatshield requirements.
UNIDENTIFIED SPEAKER: On that final chart, the curve you
labeled as a function of lambda. It goes eight tenths, point
three, and point sixty-five. Is that a peaking situation and, if
so, what would cause that peaking?
MR. LEIBOWITZ: The top curve is line radiation which is con-
siderably more intense than the continuum. The bottom two traces
are continuum which increases with decreasing wavelengths.
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UNIDENTIFIED SPEAKER: Lew, you said something about a
seventy-percent reduction in radiative heating for Jupiter; would
you expand on that a little bit: for what conditions?
MR. LEIBOWITZ: The question was under what conditions do you
get a seventy-percent reduction in radiative heating due to the
radiative cooling effect. That's a rough number. That would
correspond, probably, to close to a worst case. I think that's
a rather severe entry of, like, entry angles of greater than
ten degrees. I don't have the exact numbers.
I don't claim that that would be an integrated value.
UNIDENTIFIED SPEAKER: I see; because the thing that strikes
me is that Jupiter is such an energetic entry and the tempera-
tures are so high and I think it would drive us towards equili-
brium much better than the other planets.
MR. LEIBOWITZ: This is a different phenomenon when we talk
about radiative cooling. That's not non-equilibrium. It's true
that we expect the non-equilibrium effect to be much more signi-
ficant, I think, at Saturn than for Jupiter.
LD_IDENTIFIED SPEAKER: Could you make some kind of a comment
about the sensitivity of this to the presence of those heavy ele-
ments we heard about yesterday.
MR. LEIBOWITZ: Yes. I haven't looked into that personally.
Some work has been done here, I think, by Bill Page. His
data that I have seen seems to indicate that it's not that sensi-
tive. We haven't gone through this but our physical intuition
seems to indicate that the heavy elements should be at the lower
altitudes and one wonders whether it percolates up to the altitudes
of severe entry.
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UNIDENTIFIED SPEAKER: I thought Jupiter's peak heating was
located at about the same height as the Pioneer 10 occultation
data controversy.
MR. LEIBOWITZ : Yes?
UNIDENTIFIED SPEAKER: Howard, what is the pressure level at
which peak heating occurs?
MR. LEIBOWITZ: All I know is about 107 dynes per square cen-
timeter if that tells you anything. I think there is a two-fold
problem. We can answer that question in a shock tube. The work
has already been started at Ames on that. This can be continued.
It is fairly easy to make shock tube measurements of what a lit-
tle bit of one thing and another does. As I say, the initial
indications are that it may not be that important. Hydrogen has
always been an impurity that causes more problems in measurements
of other compounds.
MR. SEIFF: Here is a comment. I have been working on this
problem actively as I think everybody knows. There are two things
that these gases can do. In the first place, I think their pre-
sence was a presumption. If they are present, they can do two
things. One of them is they can absorb energy by dissociating -
in trace constituents that Will not be an important effect. The
other thing that they can do is introduce line radiation in
other locations than those that are being studied here. Again,
with minor constituents, this should not be an important effect.
MR. LEIBOWITZ: I think all these species are present, pro-
bably, as ablation products, in much higher concentrations in
the shock layer, than they would be in the atmosphere.
MR. OLSTAD: For the case of the Jupiter entry, a steep en-
try into a cold atmosphere which is the worst case in terms of
heating rate, the shock layer is essentially optically thick. If
you put any other radiators in there it doesn't matter unless it
affects the temperature. The trace constituents won't affect
the temperature too much. In that case, they shouldn't be too
severe. It can have some effects on the non-equilibrium chemistry.
As Lew mentioned, there have been some tests here at Ames which
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have introduced trace amounts of methane and ammonia. They have
found, essentially, no effect on the amount of heating. But
these were really trace amounts. I think there is some evidence
that, in the Uranus atmosphere at least, they may be more than
just trace amounts.
UNIDENTIFIED SPEAKER: At pressures of less than about a tenth
of a dyne per square centimeter you are above the photochemical
level and you will just have a hydrogen atmosphere, basically.
There isn't even any methane to make photochemical products.
MR. SEIFF: Could we see that chart again that shows the
relaxation lengths? (Figure 5-10)
I presume those were relaxation lengths - that would be the
products of pressure and the relaxation lengths. That capital L
there is the distance behind the shock wave? How was that defined?
Is that when the radiation peaks?
MR. LEIBOWITZ: Yes. It is defined on the sample oscillo-
graph. It's the distance to approach of equilibrium.
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MR. SEIFF: For example: • at one torr ambient pressure,
at 32 kilometers per second, you might expect to get, say, one
centimeter of relaxation distance?
MR. LEIBOWITZ: That's right. A flow-field case will be
shown in the next talk for an entry velocity of 28 kilometers with
a calculated length of four centimeters.
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Radiative relaxation rates and intensities during outer planet entries

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