Problem areas in probe science technology are discussed that require a solution before probe systems can actually be designed. Considered are the effects of the model atmospheres on probe design; secondly, the effects of implementing the requirements to locate and measure the clouds and, trade-offs between descent sampling and measurement criteria as they affect probe system design

Ledbetter, K. W.

English

NASA Technical Reports Server

Impact of Science Objectives and Requirements on
Probe Mission and System Design
MR. KENNETHW. LEDBETTER: You have heard from previous
speakers the basic objectives and rationale for outer-planets
probe missions. I would like to build on these basics by dis-
cussing some of the problem areas in probe science technology
that require a solution before the probe systems can actually
be designed.
There are three areas I would like to briefly discuss.
First, the effects of the model atmospheres on the probe design;
secondly, the effects of implementing the requirements to locate
and measure the clouds; and, third, trade-offs between descent
sampling and measurement criteria as they affect the probe sys-
tem design.
Composition is one of the basic objectives and although the
probe will measure the actual composition, engineers must have
a model with which to design subsystems. The model atmospheres
that have been used by both NASA and industry for various studies
that have been done are those in the NASA SP series of monographs
assembled under the cognizance of Goddard Space Flight Center.
The authors for the atmospheric sections were primarily Neil
Divine and Frank Palluconi of JPL.
Figure 2-24lists some of the variant properties of the mono-
graph model atmospheres for Saturn and Uranus. The document
numbers are given in the footnotes on the figure. The corres-
ponding number for the Jupiter monograph is NASA SP-8069. Some
of the major differences are apparent. Since helium cannot be
identified directly from the spectrum, the models are necessarily
quite variable in Helium content. It varies extensively at both
planets, ranging at Uranus from about 4 percent in the warm to
60 percent in the cool. Adding to this, the variability of
methane from a negligible amount at Saturn to 9 percent in the
Uranus cool, the resulting molecular weight is between 2.1 and
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4.6. Trying to design a probe to this range of atmospheres is
extremely difficult and unrealistically restrictive.
The second-most important item on Figure 2-24 is the tempera-
ture differential between models at ten bars. It extends from
about i14 ° (Kelvin) in the Uranus cool to over 400 ° at Saturn;
and the Jupiter monograph models show a maximum of about 470 ° .
If you recall Arv Kliore's graph shown earlier, his Pioneer 10
data, extrapolated down to ten bars at the bottom of his graph,
would give a temperature on the order of 900 ° to 1000 ° . There-
fore, there could be as much as an order of magnitude of dif-
ference in the final temperature to which a truly common probe
must be designed. This, of course, is very significant to
both thermal control and to the life of various components of
an entry probe.
Figure 2-25 shows the effect of these variations upon entry
probe design for Saturn and Uranus with the same set of model
atmospheres. Note that the entry ballistic coefficient and the
descent ballistic coefficient were essentially constant for all
six models. The values are typical for non-parachute probe
descents. The slight difference in the descent value is due to
the different amounts ablated from the entry heatshield. The
peak decelerations vary from a little over a hundred to about
six hundred with the entry angles shown. Note that there is a
five-degree difference in the entry angle. This allows the
design peak G's. specifically about 585, to be about the same
for each planet. This flexibility in entry angle permits the
designer to account for some of the differences between planets.
A Saturn entry at 35 ° would have greater than 650 peak G's.
Instrument deployment parameters are also shown in Figure 2-25.
This particular design was for a non-parachute probe where the
instruments were deployed slightly above a hundred millibars in
pressure. At three G's descending plus twenty seconds the tem-
perature gauge is deployed, the mass spectrometer opening pyros
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i are fired, and the nephelometer cover is removed. Again, there
are variations in the time from entry, the mach number at deploy-
ment, and the altitude above one bar.
The bottom line on Figure 2-25 lists thetime to reach ten
bars which is also very important for a probe design. It
varies from about 27 minutes to 74 minutes; a very large factor
when considering thermal control and especially when consider-
ing the communications link. The data must be relayed to the
spacecraft before it passes out of range of the probe. Also,
descent time is important for sizing some of the subsystems, par-
ticularly, the power subsystem. In fact, since some components
must be designed to the minimum time (e.g. memory dump data rate)
while related components are designed to the maximum time (e.g.
total battery power) resulting conflicts yield an inefficient
design.
It is interesting to'note from bothFigure2-2_ and 2-25 that the
differences between models for a given planet are greater than
the differences between planets for a given model, pointing out
our overall ignorance as to the real atmosphere.
Of course, we all know we need better models. What can be
done to obtain them? Pioneer i0, has changed the essence of
these models for Jupiter. In fact, it might be better to discard
the old models and start over again. In addition, when progres-
sing from Jupiter to Saturn and Uranus the majority of models
that have appeared in the literature have utilized extrapolations
from Jupiter. Therefore, when the Pioneer 10 data are fully
applied to Jupiter, .the results should be extrapolated to Saturn
and Uranus.
Secondly, statistical means can be used to reduce some of the
uncertainty. Starting with a given nominal model and the various
3-Sigma possibilities for each Of the individual parameters that
comprise the model atmosphere, Gaussian-type distributions can
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Lbe constructed around that nominal and the extremes decreased.
This has been done for Jupiter by W. S. Cook at Martin Marietta.
He has a paper appearing in the July, 1974 issue of the Journal
of Spacecraft and Rockets which uses the nominal atmosphere from
the Jupiter monograph and performs Monte Carlo probabilistic sta-
tistics to establish warm and cool limiting models. The results
show that Cook's limiting models are less extreme than those in
the monograph. This is largely because the monograph models were
established with the intent of being worst-case models, therefore,
the effects of all worst-case parameters were added together.
This means that if a probability distribution were superimposed
upon the monograph models, the actual probability of the cool or
warm model existing would be near zero since the probability of
all parameters being the maximum worst-case value in the same
direction at the same time is near zero.
The second topic of discussion is the impact of the basic
objectiv_ to locate and measure clouds. Figure 2-26 shows the
pressure location of the clouds as given in the NASA monograph
model atmospheres. The three models are represented by ver-
tical lines as indicated by the abscissa, where for each
modeled cloud,the cloud top and the cloud base are shown. The
solid lines are smooth fits through the three points, repre-
sentinq the cloud top and the cloud base. The reason for this
method of presentation is to emphasize the point that there is
only one cloud and that its location is very uncertain, even in
these models which the Pioneer i0 data may replace. For ex-
ample, the water cloud base at Saturn is located between two
bars of pressure in the warm and well beyond a hundred bars in
the cool.
The dashed line on Figure _26 represents the end of a 38-min-
ute mission with a ballistic coefficient of 160 kg/m 2. Note that
the probe will just penetrate the cloud base of the second cloud
in the nominal atmosphere at about 7 bars. Since the clouds tend
to appear higher in the warm models and lower in the cool, the
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probe penetrates well past the cloud base in the warm but does
not reach the cloud tops in the cool. To penetrate the entire
cloud in the cool model is prohibitive.
Therefore, this implies a philosophy of designing to a
constant time rather than a constant pressure. This eliminates
the problem mentioned earlier of designing to different times
for communications, thermal control, and power subsystems. It
is also more compatible with the atmospheres themselves since
the probe penetrates deeper into the atmosphere in a cool model
as do the clouds. The time to reach a given pressure, is a func-
tion of ballistic coefficient. The end-of-mission line on Figure
2-26 would basically just move up and down for different ballistic
coefficients at different times. (Although for large changes in
B, the line would tilt.)
Another important consideration is the difficulty in measur-
ing the high clouds. In the Uranus warm model, the methane
cloud is up near a tenth of a bar. The probe has a high velocity
at this altitude and low density, and as the atmospheric density
increases, it slows down. Figure 2-27showsthat with the indicated
ballistic coefficient, the probe spends about seventy-four sec-
onds inside that Uranus cloud. A mass spectrometer with a 1 to
40 amu scan might be lucky to get one measurement inside. For
a temperature gauge, to make one measurement per kilometer, the
sampling interval would be on the order of about five seconds.
Figure 2-27 also shows similar information for the other Uranus
modeled clouds.
Thus, a re-evaluation needs to be made of the requirements
for measuring the high clouds in any of the outer-planet atmospheres
to determine if it is realistic to impose stringent requirements
upon the instruments to sample those clouds when the basic objec-
tive is to look at the total atmosphere.
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Figure 2--28 shows the overall trade-offs and related para-
meters involved in descent sampling. The descent profile,
indicated in the left box, is essentially the ballistic
coefficient or the rate with which the probe falls into the
atmosphere. The sampling criteria or performance in the bottom
right-hand box has two meanings: it is criteria before the
mission a_d it is measurement performance after a simulated
mission and, hopefully, the performance is equal to or greater
than the criteria. The top box is the instrument sampling time
or more correctly, the interval between measurements during a
descent. It is constrained primarily by the data rate, since
there is a maximum amount of data rate available from the power
system onboard the probe. If the criteria is fixed and states
that the probe must make a given number of measurements in a
given altitude differential, the probe can descend fast and have
a short sampling time or descend slower and have a longer time.
These factors all interplay.
One point to be made from this is brought out by Figure 2-29
and it is that good criteria are needed with which to design.
The design criteria directly reflects upon the ballistic coef-
ficient, data rate, and power subsystem. This figure shows three
that Martin Marietta has used during contract performance. The
first line is one that was used with contract NAS2-7488 with Ames
Research Center in 1973 entitled, "Study of Adaptability of Exist-
ing Hardware Designs to a Pioneer Saturn Uranus Probe." The
second line is a set of criteria that was obtained from a panel
of science consultants that Martin regularly convenes. The third
is a set of criteria that was used for Contract JPL 953311 en-
titled, "Outer Planet Entry Probe System Study" performed for the
Jet Propulsion Laboratory in 1972.
For the temperature and pressure gauges, the requirement from
set 1 is five kilometers per measurement, that is, one measurement
every five kilometers. From the 3rd set, the pressure require-
ment is one measurement every half a kilometer. There is an order
of magnitude of difference between these two requirements. It
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is about a factor of six for the mass spectrometer and, sur-
prisingly, for the nephelometer the requirements are almost
identical, when translating a typical scale height.
An improved set of criteria desperately needs to be de-
veloped. Perhaps it would be money well spent to employ
those principal investigators that will actually receive the
data, to determine, perhaps statistically, how close together
in the atmosphere the points really have to be measured in
order to make a realistic interpretation of the data returned.
The next two figures show additional details of the de-
scent parametrics. Figure2-30graphically shows that the measure-
ment performance for a fixed ballistic coefficient and instru-
ment sampling time increases with depth into the atmosphere.
This increase is more pronounced with either smaller ballistic
coefficients or lower instrument sampling times.
The effects of ballistic coefficient and sampling time
variations on performance at a given point in the atmosphere
are better shown in Figure 2-3L It displays measurements per
kilometer at cloud tops in each of the Saturn model atmospheres
versus ballistic coefficient. This is the range of ballistic
coefficients for a non-parachute probe. The parachute regime
is off the graph to the left and these curves become very much
steeper. The third parameter is the instrument sampling time or,
again, the interval between samples. Note that with a given
ballistic coefficient, changes in sampling time make a signifi-
can.t effect on performance. The solid lines are for the nominal
atmospheres; the dashed and dotted lines represent the extremes.
The lines indicating four second sampling times illustrate the
effect of the three NASA monograph model atmospheres on per-
formance.
The last Figure (2-32) then summarizes the items I feel are
important to emphasize. For the model atmospheres: whenever
possible extrapolate the Pioneer i0 data to Saturn and Uranus to
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see what effect this would have on the atmospheres that are
currently being used. Secondly, use statistical analysis to
reduce some of the model uncertainties to arrive at the best
nominal atmosphere possible. Then use statistical analysis
and physical relationships in a manner such that the various
parameters do not contradict each other when warm and cool at-
mospheres are derived.
Concerning cloud location measurements, the instruments
must search during the entire descent because, for a given
cloud, its location is uncertain even in the models currently
being used. Also, the measurement of high clouds is costly
in design. For descent measurement performance, a set of cri-
teria need to be accurately determined. This, of course, is
related to model atmosphere improvement and requires at least
a good nominal model atmosphere before this can be satisfactorily
done.
Lastly, in descent design philosophy, we recommend designing
for a maximum time in the nominal atmosphere, which may be the
time to ten bars, but that the overall probe design shouldn't
be penalized by going to identical pressures in all models. The
requirements should be based on the nominal model and then con-
sider extreme model atmospheres as 3-Sigma limits.
DR. RASOOL: I think Ken made a very important point that we
need, much more than ever, communications between the scientists
and the people who are designing the mission and, even more so, with
thi third person involved in between, the model maker. It is not
necessarily the scientists who make the models. Usually, there is
a time lag of a year and that's very bad because, these days, as
you saw, the measurements are being made at a very fast rate.
Toby Owen showed some slides which are very interesting, but by
the time they get reflected in the model, it's a year or two
years. So, we need interaction between the scientists making
measurements, the model maker, and the design maker.
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LMR. HERMAN: Just one comment. At the MJU meeting, A1 Cameron
stated that it was vital that we reduce the various uncertainties
of these models. He felt that these models are unnecessarily un-
constrained, which present unrealistic and very complex require-
ments for the probe design. The models are unnece§sarily and
unrealistically restrictive and the variables can be reduced.
DR. RASOOL: Ken made another important point; that we have
three models of Jupiter and now we have entirely different meas-
urements; and that we should reflect this into Uranus and Saturn.
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Impact of science objectives and requirements on probe mission and system design

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