Atmospheric structure of outer planets as determined by pressure, temperature, and accelerometers is reviewed and results obtained from the PAET earth entry are given. In order to describe atmospheric structure, entry is divided into two regimes, high and low speed. Acceleration is then measured: from these measurements density is determined as a function of time. The equations of motion are integrated to determine velocity, flight path angle, and altitude as a function of time. Density is then determined as a function of altitude from the previous determinations of density and altitude as a function of time. Hydrostatic equilibrium was assumed to determine pressure as a function of altitude. Finally the equation of space applied to determine temperature as a function of altitude, if the mean molecular weight is known. The mean molecular weight is obtained independently from either the low speed experiment or from the composition experiments

Sommer, S.

English

NASA Technical Reports Server

COMPARATIVEATMOSPHERESTRUCTUREEXPERIMENT
S. Sommer
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MR. SOMMER: We have heard quite a bit about pressure, tem-
perature and accelerometers being used for probes for the outer
planets. I thought I would take this opportunity to just review
very briefly how we use these instruments to determine atmosphere
structure, spend a few minutes to review very briefly the results
that we have obtained from our PAET earth entry, and to then des-
cribe, again very briefly, some of the instruments that we have
flown on PAET, that we will fly on Viking, and that we hope to
fly on Pioneer Venus.
As indicated on Figure 8-20, in order to describe atmosphere
structure determination, we divide the entry into two regimes,
high speed and low speed. We measure acceleration and from the
acceleration we determine density as a function of time. We
integrate the equations of motion to determine velocity, flight
path angle, and altitude as a function of time. Then we determine
density as a function of altitude from the previous determinations
of density and altitude as a function of time. We assume hydro-
static equilibrium to determine pressure as a function of alti-
tude. Finally, we apply the equation of space to determine tem-
perature as a function of altitude, if we know the mean molecular
weight. We ob:tain the mean molecular weight independently from
either the low speed experiment or from the composition experi-
ments.
During the low speed portion of the flight, and by low speed
I mean somewhere around mach one or two or where you can deploy a
temperature sensor without destroying it, we measure pressure,
temperature, and again, acceleration. We correct pressure and
temperature to ambient values. We solve the equations of hydro-
static equilibrium and vertical motion, and o_tain altitude and
velocity as a function of time and mean molecular weight.
We compute pressure and temperature as a function of altitude,
and we apply the equation of state to obtain density as a function
of altitude.
VIII-46
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Figure 8-20
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The next figure, Figure 8-21, indicates what we hope
to obtain if we flew more than one probe at the same time, as
Pioneer Venus does. I added molecular weight to the chart be-
cause this independent measurement can be used to compare
with measurements made by the composition experiments. We hope
to be able to get some insight on circulation of the global scale.
We intend to be able to make some vertical wind determinations.
We will attempt to measure atmcspheric turbulence in the lower
part of the atmosphere. If any of the four probes on Pioneer
Venus survive impact, we hope to make some seismic measurements.
Now what I would like to do is run through some of the re-
sults that we have obtained from our PAET experiment. The first
is a trajectory determination. Plotted on Figure 8-22 is velocity as
a function of time from lift-off. The dots shown here are experimental
points determined from the method that I showed you on the first
slide, and is compared to radar tracking data obtained both
from Bermuda and from Wallops. I have indicated the division be-
tween the high speed experiment and the low speed experiment.
Velocity up to about this 576 seconds was determined solely from
acceleration and from about 576 seconds on, from acceleration,
pressure, and temperature measurements.
You will note that we have reasonably good agreement. The next
Figure 8-23 shows altitude as a function of density. This is one
of the primary measurements. The region above about twenty-six
kilometers, where we reached a mach number of about two and deployed
our temperature sensor, density was determined solely from the accel-
erometers whereas at lower altitudes, density was determined by using
accelerations, pressures and temperatures. You will notice that the
data covers over five decades of density. Since this is a log plot,
we have plotted the difference between the measurements and
meteorological data on the right hand side of the figure. Although
local differences approach 20 percent, it turns out that meteoro-
logical data has much more uncertainty than this particular experi-
ment.
VIII-48
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•"ADDED EXPERIMENT CAPABILITY
• ATMOSPHERIC MEAN MOLECULAR WEIGHT
• CIRCULATION AT GLOBAL SCALE
• VERTICAL WIND DETERMINATION
• MEASURE ATMOSPHERIC TURBULENCE
• SEISMIC MEASUREMENTS
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VIII-49
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The last of the data plots, •Figure 8-24, from PAET is de-
duced temperature as a function of altitude, where temperature
is determined from readings of the accelerometers. Essentially,
from 26kin up, the temperature data is deduced solely from the
accelerometers and is compared to meteorological data from Viper
Dart firings made about one hour before and one hour after PAET.
Notice the similarity between the two sets of data and the
almost perfect agreement with the meteorological data where direct
measurements of temperature and pressure were made.
Let me spend the rest of my few minutes comparing the in-
struments on the three missions that the Ames group has been,
and is involved in; PAET, Viking, and Pioneer Venus. Figure 8-25
is a comparison of atmospheric temperature sensors for the three
missions, comparing type, range, accuracy, and weight.
For PAET, we used chromel-alumel thermocouples, the range
from 200 to 660 degrees kelvin. We had an accuracy of about one
degree. We had two sensors that deployed through the heat shield,
each weighing about six tenths of a pound.
Viking is carrying two temperature sensors for us, and the
one that I am describing here is the one that comes out through
the aeroshell before separation. It is also a chromel-alumel
thermocouple with a range from 100 to 700 degrees kelvin. The
accuracy is three and one half degrees plus the one percent of
reading, and it weighs about one pound.
On Pioneer Venus, we are planning to use a resistance thermo-
meter. The range, again, is very similar - 200 to 800 degrees
kelvin. The accuracy requirement is much more severe. We feel
that the temperature differences around the globe are small, and
we are trying to determine what those are, thus the 1/4 degrees
accuracy requirements; total weight is about 1/2 pound.
The way we plan to deploy the temperature sensor for Pioneer
Venus is illustrated in the next two figures. Figure 8-26 shows a
VIII-51
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plan view of a deployable arm that is located within the after-
body cover. The arm comes out and bends down so that the sensor
sees the flow around the body outside the boundary layer before
it comes to the afterbody. This is one of the concepts that we
are contemplating.
The other is illustrated on Figure 8-27, which is the con-
cept that we have used on PAET that will be used on Viking and
could be used on Pioneer Venus and/or any of the other planets.
Next, Figure 8-28 contains a similar comparison of the pres-
sure sensors. For PAET, we used a vibrating diaphragm pressure
sensor which measured pressures from a .001 to one atmosphere
with an accuracy of about one percent of reading. There again,
we carried two sensors, each one weighing about seven tenths of
a pound.
On Viking, for the entry vehicle, we are carrying a stain-
less steel, conventional type diaphragm pressure sensor. The
pressures to be measured are from .001 to only .15 atmosphere.
Accuracy is about 2 percent of reading and weighs very close to a
pound.
For Pioneer Venus, we are planning to carry a number of
miniature silicon diaphragm diffusion-bonded wheatstone bridge-
type sensors. They are sensors about a quarter of an inch in
diameter, weighing a few grams. We are contemplating carrying
anywhere from six to twelve in order to cover the range from
30 millibars to about 100 atmospheres. The goal is an accur-
acy of about 1/2 percent of reading. The weight of that entire
system, including electronics, is on the order of 0.8 pound.
Figure 8-29 illustrates how we intend to sample the pressure,
either through the heat shield at the stagnation point or through
tubing opening adjacent to the temperature sensor. When the tem-
perature sensor is deployed, then that pressure sensor will make
its readings starting at that time.
VIII-55
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Finally, Figure 8-30 compares the accelerometers. Again,
I have compared the PAET, Pioneer Venus, and Viking sensors. They
are type sensors. The electronics are not
The used on PAET, which was designed
for a range of about 80 G maximum on the axial accelerometer, was
capable of up to several hundred G's. It weighed about 0.4 pound.
The Viking instrument, where the maximum acceleration expected is
less than 22 G's, is shown in the middle figure. Since) this in-
strument is already developed, it is the leading candidate for
Pioneer Venus. The people who have built, designed, and flown
these instruments have been working for about the past year and
a half on a sub-miniature instrument that has exactly the same
capabilities, weighing about fifteen grams. When this instrument
is qualified, it will be a leading candidate for planetary entry
acceleration measurements.
Figure 8-31 shows a blow-up of the Viking instrument. It
has over one hundred parts including alnico and magnet housing.
I want to compare that to a schematic (Eigure 8-32) of what the
accelerometer manufacturer calls the model eleven, that has about
nine parts. The primary reason for the simplicity, they say, in
this is in the magnet, It is made out of a rare earth material,
samarium cobalt. An instrument of this type has been built, and
is ready for test.
In conclusion, I would like to say that instrumentation for
atmosphere structure determination is available with very little
modification for application to outer planet exploration.
QUESTION: ['_at is the name of that vendor with the super-
light instrument?
CHAIRMAN: Bell Aerospace
QUESTION: What is the altitude range you hope to get tur-
bulence measurements on?
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MR. SO_MER: Anywhere during terminal descent for Pioneer
Venus, from around 70 km to as close to the surface as we can go.
That kind of turbulence measurements we hope to make are really
statistical measurements. In other words, we are going to try
to count the number of times that the vehicle will feel acceler-
ations above pre-selected values. We will sum those up over a
period of time, transmit those back, and then analyze the data.
That is the only kind of data capability that we have available
for that experiment.
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VIII-63


Comparative atmosphere structure experiment

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