An overview of the problems of entry aerodynamics and heating is given with emphasis on survival of the probe, predictability of performance, and reliability of performance. Technological challenges to performance prediction are considered and include: turbulent heat transfer, radiation blockage, chemical state of the shock layer, afterbody heat transfer, asymmetric ablation, and real-gas aerodynamics. It is indicated that various obstacles must be overcome in order to achieve technology readiness. These obstacles are considered to be: extrapolations from ground tests to flight; lack of flight experience; lack of parametric data; and uncertain knowledge of atmospherics

Olstad, W.

English

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SESSION V - ENTRY AERODYNAMICS _ND HEATING
Dr. Walter Olstad, Chairman
NASA - Langley Research Center
MR. VOJVODICH: We are very fortunate in having Dr. Walter
Olstad of Langley to chair the entry aerodynamics and heating
panel. I am not going to go into Walt's background. He is well
published in this area and without further delay, I will turn the
proceedings over to Dr. Olstad.
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DR. OLSTAD: Yesterday we heard some discussion about tech-
nology for the probes being pretty much in hand. Today we have
some surprises for you. The technology isn't all that well in
hand, and we have some genuine concerns about which you will
be hearing today.
Before launching into the talks by the panel, I would like to
give a brief overview of some of these problems.
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Looking first at the problem of entry aerodynamics and heat-
ing, Table 5-1, we ask: What are we supposed to do? The first and
obvious answer is to assure survival of a probe, which gets us in-
to the heating problem. But, beyond that, mere survival of a
probe isn't sufficient. It doesn't guarantee any data coming back;
or if data does come back, it doesn't guarantee that you can
interpret that data. So it is very important that we be able to
predict performance and that performance be reliable.
Figure 5-1 presents some of the challenges to making predic-
tions for aprobe entermg a severe environment. We always have
the problem of transition from laminar flow to turbulent flow.
And, as those of you who know anything about the transition prob-
lem are aware, the only way to learn about it is through experi-
mentation. It is not something you can calculate. Unfortunately,
our ground facilities don't provide the conditions that will be
encountered during entry in the outer planets. And so, we have
to extrapolate from experiments and ground facilities.
Furth_,rm_, we must be _ble tn Dre_ct the turbulent
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heating. Turbulent heating is also an area where empiricism is
necessary. Once again, we have to extrapolate from ground facil-
ity experience, and that is a long and uncertain extrapolation.
The third area is one that I have labeled radiation blockage.
The ablation products which are injected from the vehicle's sur-
face tend to absorb some of the radiant energy from the shock
layer. This is generally a beneficial effect which heats up
those ablation products which are then swept into the wake. But
we have a difficult time predicting how much absorption or block-
age we get. One of the big problems is that we don't know the
radiative properties of some of the heavy molecules which are
constituents of the ablation products. Further, we don't know
really what the chemical state of the ablation layer is. We
don't know if it is in chemical equilibrium or not. That makes
quite a difference in any calculation.
As you will hear a little later in this session, there is
some question about the chemical state of the shock layer itself,
and this, again, relies on experimentation. Fortunately, we can
do a good bit of the necessary experiments in shock tubes.
Another problem area is that of afterbody heat transfer.
Generally, it is not large enough to significantly affect the
design of a probe but the greater confidence we have in predict-
ing afterbody heating, the less will be the margin of safety we
have to put into heat shield design and the more weight can be
allotted toward increasing the science payload or enhancing sys-
tem reliability.
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Asymmetric ablation may be something of a problem. It can
affect the aerodynamics for the rather blunt vehicles that we
are talking about. Our intuition tells us it is not too much of
a problem. There is some experience which shows that it can be
a rather severe problem for slender vehicles. It is an area
that hasn't been looked at very carefully, as yet, for blunt
V-4
vehicles and requires some attention if we are to have full con-
fidence in our ability to predict the performance of a probe.
The last area is real-gas aerodynamics. We have lots of
wind tunnels, lots of ground facilities in which we can study
aerodynamics, but generally we don't get real-gas effects which
can play an important role during planetary entry.
So these are some of the technical challenges that still
remain. They are being worked on, and I am reasonably confident
that we will have the right kind of information at the right time.
But it is not all in hand right at the moment.
On Table 5-2 I have listed some of the major obstacles that
must be overcome to achieve technology readiness. We have to
extrapolate our experience from ground facilities to the flight
environment, and that extrapolation is very lengthy and uncer-
tain in terms of heating rate experience; it is an order of
magnitude or more that we are extrapolating. I am sure you will
hear more about this problem in the second session this morning.
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There is a lack of flight experience. The flight experience
that we have now is in the regime of Apollo entry. With Pioneer
Venus we will gain some flight experience at more severe condi-
tions. But when you talk about outer planet entries, even the
Saturn and the Uranus entries, we are talking about potential
heating rates, an order of magnitude larger than the Venus heat-
ing rates. So we will be lacking any real flight experience, and
there is bound to be some kind of risk associated with undertaking
a mission without it. At the present time, I am not sure we know
how to assess that risk. It is important that we be able to
assess it and to quantify if as best we can so that the mission
planner can then make his decision as to how much of a risk he
is willing to accept.
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There is a lack of parametric data, as well. If you look at
the information available to a probe designer, it is limited to
a rather small family of sphere-cone vehicles and a small family
of spherical segment vehicles, like an Apollo shape, and that is
about it. And as you will hear a little later, even that infor-
mation leaves a lot to be desired, at least in terms of predic-
tions of heat transfer.
Finally, lets address the area that was talked about yesterday,
the uncertain knowledge of the atmospheres. I heard what I thought
were two stories that were somewhat conflicting. I heard one story
that said the upper and lower bound atmospheres, or the cold and
warm atmospheres, were probably too far away from the nominal;
that if you applied some statistics and asked about three sigma
errors and things like that, you could close in on the nominal
atmosphere. But then I heard that the nominal atmosphere wasn't
necessarily the most probable atmosphere. We also heard a good
bit about the Pioneer 10 results, and the question which has
arisen as to how to interpret those results and what they mean
in terms of an atmospheric model. Think back to our experience
with the Martian atmosphere; what we know as the Martian atmos-
phere now falls completely outside of the bounds that we had
placed on the Martian atmosphere prior to any information gained
from Martian orbiters. So I am not all that confident that we
can squeeze down on the nominal atmosphere because I am not all
that sure the nominal atmosphere is the proper one.
We need some good information on what really are the bounds
of the atmosphere. Obviously, the scientists can't tell us pre-
cisely what the atmosphere is. That is one of the reasons we
are going there. But anything they can tell us about what really
are the upper and lower bounds on the atmosphere will be very
helpful in probe design.
I wish to elaborate a bit more on the lack of flight exper-
ience, and what it really means. This Figure 5-2 is labeled as
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the current OSS mission model. That is the model which Dan Herman
came up with yesterday. Let's look at what kind of flight ex-
perience will be generated by the current series of proposed mis-
sions. The schedule shows the Pioneer Venus multiprobe mission
with launch in May, 1978, two Mariner Jupiter/Uranus spacecraft
(possibly with Uranus probes) with launch late in 1979, two
Pioneer Saturn probes with launch late in 1980, two Pioneer
Saturn/Titan spacecraft (possibly with Titan probes) early in
1982, and two Pioneer Jupiter probes with launch early in 1984.
At first glance this may appear to be a reasonable sequence in
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(roughly) increasing order of difficulty. However, when trip
times are considered the sequence becomes rather distorted. The
first probes to enter are the Pioneer Venus probes late in 1978,
only one year prior to the Mariner Jupiter/Uranus launches. The
next probes to enter are at Saturn in early 1984, only a few
months before the Pioneer Jupiter launches. All other probes
enter the target atmospheres after 1984. As a result, the only
real flight experience which can impact outer planet probe de-
sign must be gained from the Pioneer Venus multiprobe.
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So with this kind of schedule, we face the possibility of
committing ourselves to a series of probe experiments without
really gaining any flight experience. This may be all right,
but we have to assess the risk associated with this kind of
operation. I don't think we have as yet. Instead, we rather
hopefully claim that the technology is in hand. As I said earl-
ier, I think you will hear this morning that it is not that well
in hand.
I'll now introduce our first speaker, Donn Kirk of Ames
who will discuss the effect of initial conditions on the de-
duced atmosphere for Uranus and Jupiter entries. This relates
to our ability to reconstruct an atmosphere based upon the data
we get from a probe considering the uncertainties in entry con-
ditions and aerodynamics.
V-9


Entry aerodynamics and heating

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