The feasibility of an outer planet common atmospheric probe is studied with emphasis on entry heating rates and improved ephemeris. It is concluded that a common probe design is possible except for Jupiter; the basic technology exists except for Jupiter heat shielding. A Mariner class bus provides for better bus science and probe bus communications than a Pioneer class bus

Ellis, T. R.

English

NASA Technical Reports Server

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TEN BAR PROBE TECHNICAL SUt_--MARY
T. R. Ellis
DYNATREND INCORPORATED
MR. ELLIS: I am going to start with the conclusions of the
study. That way, if Tom pulls out the hook and removes me from
the podium, at least the major points will have been covered.
In preparation of this report, we read and reviewed a stack
of material done by most of the people in the room over the past
five years or so, a stack about six feet tall, when piled up,
and tried to, in 25 words or less, summarize this material, to
provide a management-level technical review and summary.
The major conclusions that we reached, after digesting all of
this material, are shown on Figure 4-i. This set of conclusions
was reached prior to the Pioneer i0 mission and there are some
modifications that must be made to them, as a result of the
Pioneer 10 data.
The most significant conclusion was that a common probe de-
sign looks quite possible for the five bodies we were consider-
ing; that is, Jupiter, Saturn, Neptune, Uranus, and Titan, except
possibly for Jupiter since the design for Jupiter is quite a bit
heavier. The heat shield fraction is so large that it didn't
really make good sense to try to combine Jupiter with the other
planets in a common probe mission.
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A similar kind of thing, at the other end of the spectrum,
could be said for Titan; that is that Titan doesn't quite require
the heat shield fraction that is required for Saturn, Uranus and
Neptune, and you are paying a penalty in trying to go to Titan
with a common probe. But it looked to us that in that case, it
was probably worth it, rather than going to a completely new
design.
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The Probe weight for the common probe was in the 250 pound
class. We did look at the two bus concepts, and I classify them
here as Pioneer and Mariner. I am really talking about a spin-
ning bus versus a 3-axis stabilized bus, of which the Pioneer and
Mariner are the prime samples.
The Pioneer bus produced a lighter overall spacecraft, able
to be launched using smaller launch vehicles. The Mariner class
provided slightly better probe communications and a more stable
platform for the bus science.
Another significant conclusion, contrary to much of the
work that had been done prior to this review, was that staging
during entry appeared unnecessary except possibly, again at Ju-
piter.
A common science payload (Figure 4- 2) appeared consistently
throughout most of the study work. It included the five instru-
ments that have become quite familiar to everyone, pressure sen-
sor, temperature sensor, accelerometer, neutral mass spectrometer
and nephelometer. The science objectives are shown and each in-
strument is related to the particular science objective that it
would primarily accomplish by the deltas on the chart. The cases
where an instrument is a secondary instrument for a particular
science objective are noted by the X's on the chart.
A couple of other instruments were examined very briefly.
One of them was the solar radiometer. It appeared from most of
the work that had been done, that the sun angle during probe de-
scent was quite poor in practically every case. And, therefore,
while it was a very desirable instrument, perhaps as a replace-
ment for the nephelometer, it was not included.
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Figure 4- 3 reviews, basically, the sampling rate and shows
how the various instruments are sampled during entry and descent.
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:_/(: design we have in our report is basically the McDonnell-Douglas
conceptual design as it most nearly approximated the character-
istics necessary for this mission.
In reviewing the communications geometry, Figure 4-4, the
communications range at entry and end of mission shown here are
the maximum conditions of any of the various missions from all
the reports, with the exception of a few where there were special
requirements. There are a few missions flown at extremely high
spacecraft flyby periapsis, that exceeded these ranges, but most
of the missions were within the constraints shown here; also true
of the maximum range of probe look angle excursion of 60 degrees
and the maximum bus look angle excursion of 45 degrees.
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These conditions set the tone for the communicati©ns system
and the major trades, Figure 4- 5, which showed up in the various
studies that were done. To a large degree, I think these trades
have been covered by previous speakers.
The bus relay link antenna for the 3-axis stabilized bus,
is a dish, in the typical design the dish had a 40-degree half
angle pencil beam with about 12 db gain.
In the spinning spacecraft, you have a choice between try-
ing to duplicate that pattern with a despun antenna, which is
just about impossible to integrate into the spinning spacecraft
design, or using an axisymmetric antenna, as shown in the base-
line design. It has a gain of about one and a half db and a
50-degree half angle. This makes the spinning spacecraft appear
to have like a i0 1/2 db deficiency in comparison to the 3-axis
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stabilized spacecraft, but about three and a half db is recovered
because of the difference in the planet noise received. If a
dish antenna is looking right at the planet, the entire planet
disc is within the beam width of the antenna and a much higher
planet noise contribution is received, whereas the axisymmetric
pattern looks all the way around the spacecraft; only a small
bit of that antenna pattern intercepts the planet disc and the
planet noise contribution in the receiver is much less. So that
the net difference is about 7 db between the two.
Many of the studies were done at 400 megahertz, and others
were done at 860; a few were done at 1,000; and here and there
there were some S-band systems. But the principal case could be
made for the 860 megahertz frequency and the 400 megahertz fre-
quency. The principal difference here was related, again, to
the spacecraft configuration and the spacecraft antenna size.
There is a set of communication design link charts in the re-
port that compare the spinning spacecraft with a 400 megahertz
communications system with the 3-axis stabilized spacecraft at
860 megahertz, and basically demonstrate that either of these
systems can do the job within the design constraints that I
showed two slides ago.
Also, in the modulation technique area, both PSK PM and FSK
systems were used and, again, both can do the job. There are
some advantages and disadvantages to each, mostly relating to
the fading conditions that are assumed for the atmosphere. And
these are probably not too significant if you consider only the
upper atmosphere of these planets, becoming most significant if
you try to enter into Jupiter's atmosphere.
In terms of staging, there appeared to be quite a difference
when we started looking at the different staging designs and one
of the things that emerged very quickly was that some studies
were using a staging altitude that was basically trying to reach
IV-10
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some low G-level descending; that is, to exit from entry above
the tropopause. Others were trying to reach some G-level at a
particular velocity; typically, something like Mach .7 above i00
milibars pressure. And when you start looking at what these dif-
ferent ground rules mean on the different planets with the dif-
ferent model atmospheres that have previously been discussed,
the design conditions for exit from entry become quite different.
For example, all of these shown on Figure 4-6 are i00 milibar
altitudes in kilometers; that is, reference altitude in the
model atmospheres. The pressures, if you started talking about
coming out above the tropopause, are quite a bit higher.
In trying to compare the results of these studies using
different ground rules, we ran into a lot of apple-and-orange
problems. As shown in Figure 4-7 , we did conclude that, with
the exception of Jupiter, staging was probably not required.
Staging does provide a better science mission in that you can
use one ballistic coefficient to arrive at some pressure alti-
tude prior to exposing most of the main science instruments,
and then change the ballistic coefficient for descent and opti-
mize the time you spend in the atmosphere, optimize the data
sampling rate for the various instruments, and optimize your
communications geometry and communications time perhaps a little
better. But that is quite a penalty to pay to gain these small
improvements.
Unstaged entry turns out to be lighter, in most cases, and
we are basing these numbers on our 250-pound probe, by about 15
or 16 kilograms in weight, :and removes all of the complexity
associated with the parachute design, heat shield jettisoning,
and all of the associated mechanisms.
Staged entry accommodates the conflicting ballistic coef-
ficient requirements better. It improves the ability to expose
sampling inlets after entry, and while these are advantages,
they certainly don't outweigh the advantages of unstaged entry.
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Now, in terms of heat shield, Figure 4" 8 summarizes very
briefly the entry conditions we found at the various planets,
and the ranges of these planets. I won't dwell on this because
it is all in the report.
Figure 4-9 shows the principal reason for excluding
Jupiter prior to the preliminary information from the Pioneer i0
encounter. Without the ability to go to very shallow entry
angles and with the atmospheric model that had been projected
prior to Pioneer i0, the Jupiter heat shield mass ratio is just
completely out of tune with the heat shield mass ratios for the
rest of the missions.
Also, the ability to simulate those heating conditions is
quite limited. The heating conditions associated with Jupiter
entry as shown on the convective heating and radiative heating
plot of Figure 4-10 and the simulation capability shown reveal
the very limited simulation capability that exists and this also
led us to the feeling that Jupiter should be postponed.
I think I will move ahead to the last, Figure 4-]L (The only
thing that I am skipping is the spacecraft interplay, and that
was covered very thoroughly just a few minutes ago.)
The impact of the Pioneer l0 data on our conclusions has to
a degree been covered already. The potential change in atmos-
pheric model should reduce the entry heating rates. The improved
ephemeris should allow a much shallower entry and further re-
duce the heating rates. And the fact that the radiation environ-
ment is now better known should improve the ability to design
both the probe and the bus for a Jupiter mission.
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MR. CANNING: Are there any questions that would be other
than lead to revisions to the Ten Bar Probe Summary?
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MR. HE_M: Not a question, but a comment. What I have
seen on the charts indicates why, up to the Pioneer I0 encounter
we did not plan a Jupiter entry program until 1985; primarily,
because test facilities did not exist in the United States to
simulate the entry conditions. And one key issue of this work-
shop, and subsequent studies, would be another assessment: is a
Jupiter entry probe at a shallow entry angle conceivable, from
a commonality standpoint, with that of a Saturn and Uranus probe?
MR. CANNING: Yes, I think that you would find that the
commonality would be less expensive than indicated by the earlier
study.
MR. HERMAN: But is it real? I am still skeptical.
MR. CANNING:
be !'non-common. "
It is likely that a Jupiter probe would still
°-
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IV-19


Ten bar probe technical summary

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