Interface design between a probe and a spacecraft requires not only technical considerations but also management planning and mission analysis interactions. Two further aspects of importance are the flyby versus the probe trade-off, and the relay link design and data handling optimization

Casani, E. K.

English

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PROBEINT  ACEDESIGNCONSID  TION
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Jet Propulsion Laboratory
The subject of my talk is "Probe Interface Design Consider-
ations," a rather nebulous subject. Before I get into the subject,
I would like to discuss some of the soul searching that I went
through in coming up with this presentation. I think maybe I
handled it the right way. Of course, when one first thinks about
the interfaces between a probe and a spacecraft, the immediate
thing that comes to mind is the technical considerations that are
involved. I have done considerable work in both probe design and
interfacing of probes to spacecraft; my original approach to this
presentation dealt with the technical aspect of the interface.
After some initial work on the subject, I realized that my ap-
proach was altogether wrong. At that point, I sat back and re-
flected on some of the designs with which I have been involved
over the past ten years. My thoughts went back to the early
Mariner design, which some of you in the room may remember, at
that time we were designing probes of the Discoverer shape for
entry into an 80 milibar Mars atmosphere; I thought of many sub-
sequent designs and up through the current designs we have done
where we have looked most recently at the interfacing of this
Ames probe to a Mariner Spacecraft. In the process of this his-
torical thinking, I isolated what I think are three aspects of
that interface design which are worth talking about today.
o Management
o Mission
O Technical
Those three aspects are: first, the management interface;
secondly, the mission design interface which I feel, on this
particular mission, the outer planet missions, will be more
difficult than anything we have ever dealt with previously;
and finally, some of the technical considerations which we have
heard about today. I will talk in general about those as we move on.
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https://ntrs.nasa.gov/search.jsp?R=19750012304 2020-03-19T20:50:35+00:00Z
Let me now address the management considerations.
o Center Responsibility
o Science Inputs
Two of the most significant considerations are, first of all,
the center responsibility. We have designed missions where we have
had both the responsibility for the project, the probe and the
spacecraft assumed to be at one center; we have also designed mis-
sions where the responsibility for the project and the responsi-
bility for the probe is at one center while the responsibility
for the spacecraft is at another center. The distribution of
these responsibilities is going to be a major influence in the
way we go about designing the interface and handling the technical
considerations. It is important that before we progress too far
into the technical design decisions, that we are sure we understand
the management relationship between the participating centers.
The other point, of course, which will be important is how
we organize to get the science inputs into the design.
I think that the current MJU Science Advisory Committee which
is chaired by Dr. Van Allen has been very influential in our
technical thinking. And when we move into a project, it is going
to be of paramount importanee that we continue this type of activ-
tiy and that we maintain a good working relationship between the
scientific community and the actual technical implementation of
the project.
I reflected a little bit on Dr. Rasool's comment earlier
today when he attributed the high success rate of the planetary
exploration to the fact that we do have such a closeknit inter-
action between the science and the engineering aspects of a pro-
ject.
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I will now move on to the next subject, I would like to touch
on some of the considerations of the mission desi@n.
o Organization
o Flyby vs Probe
o Relay Link Design
We have seen today some specific technical presentations
which have shown some point designs for specific missions. I
don't think that we have come anywhere near scratching the sur-
face of the complexity of this mission design. I think that
first we have to address ourselves properly to make sure that we
do come up with a mission design team in a management sense,
which is properly represented by both the people who are design-
ing the spacecraft as well as the people who are designing the
probe, and as well, a good way to get the science input into the
de sign.
Two further aspects of importance are the flyby versus the
probe trade-off and the relay link design.
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If we look at the flyby versus the probe question, there has
always been, and I am sure there is going to be even more, a dif-
ficult decision making process in determining whether the prior-
ity should be put into the probe mission or whether the priority
should be put into the flyby mission. There is definitely going
to be a conflict of interest in what those two mission designs
are going to require. And from time to time we have attempted
to say, "Well, why don't we just forget about the flyby mission
because we are doing other flyby missions and minimize the flyby
requirements and optimize the probe mission." Now that may be
the easier way out but I don't think it will yield, necessarily,
the overall optimum design or the most return for the investment.
The most return for the investment is going to be a design which
is optimized and adequately considers inputs on both of those
two, what I look at as conflicting flyby geometry.
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The relay link design is another interesting consideration.
At first blush we would tend to think that the relay link design
is merely a communications design problem where we are looking
at optimizing the parameters involved in the link design, which
are the antenna geometry on the spacecraft, the antenna geometry
on the capsule, the caracteristics of the range, range rate, range
accelerations, and the look angles between the spacecraft and the
bus. But that is really an oversimplification of what is actually
involved. I think a few of the papers today touched on bits and
pieces of that. In particular, I draw your attention to the pre-
Sentation that was made by Mr. Hyde where he showed flight time as
a function of flyby altitude at the planet versus injected weight.
Well, that ties immediately into some considerations that were
shown previously where we were trying to optimize the relay link
geometry for a certain flyby altitude. It now becomes apparent
that the relay link flyby altitude is really tied into the flight
time as well as to the injected mass and when we consider two-
planet flyby mission, then the flyby altitude at the first planet
is going to determine what we can do at the second planet. So
what was originally just a simple consideration of the link de-
sign has some overriding considerations in not only the launch
vehicle capability and the flight time but also the subsequent
planet mission performance capability.
I think that this interaction is going to be much more than
what we have seen on any previous mission. The Viking mission
has a rather interactive mission, spacecraft, capsule aspect,
but I don't think it is anywhere near as complicated as what we
are looking at here.
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Moving on to some considerations relative to the technical
design, which by no means is the simplist, but I feel possibly
one which we have done enough work that we at least understand
what are the real problems.
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o Relay
o Data Handling
o P ower
o Thermal Control
o Guidance and Control
The relay is going to be one of the overriding considerations
in this spacecraft probe interface.
One of the things that we have been discussing in this Mariner-
Jupiter mission with Ames is how the responsibility of that design
should be divided among the participating centers. At first glance,
it would seem that possibly the simplest thing to do would be to
have one center provide all of the equipment that is on the probe
and the other center all of the equipment that is on the space-
craft.
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Well, if you pursue that line of discussion a little further,
it turnsiout that the interaction between the receiver and the
transmitter is such that both of those pieces of equipment should
be designed and supplied by one center, and that the interaction
between the antenna and the spacecraft is such that the antenna
should be an integral design of the spacecraft. You then come
out with a distribution of hardware which is not what your initial
intuition might make you feel is the right thing to do. But in
overall sense, it may be the better way to implement that design.
I am not suggestiong that this is the proper solution, but only
that the solution is tied tightly to the management arrangement
of which I spoke earlier.
Data handling: This topic has been touched on by several of
the previous speakers. We have looked at this problem in a general
sense and feel that the ability on board the spacecraft to handle
the data that the probe generates is going to be rather straight
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forward compared to the kinds of data handling that we are used
to doing on the current Mariner class spacecraft.
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Power: This interface is one that is rather interesting be-
cause on one hand we look at minimizing the overall cost of the
project and say, "Well, the way to do that is to use as much of
the equipment that is on board the spacecraft to service the probe."
That is, for example, to have the capability to do the battery
charging on the spacecraft as opposed to on the capsule. While
such arrangement could be made, it isn't necessarily obvious that
it is the best arrangement in an overall sense because we have
turned around and made a more complicated interface between the
spacecraft and the probe. And we have also designed a probe which
can't be, by itself, tested in terms of its capability to charge
its own batteries until it meets up with a spacecraft, which puts
us in an untenable position that there could be a fundamental
design problem that doesn't get disclosed until later in the pro-
gram; whereas if the battery charger were part of the probe sys-
tem, then the interface between those two elements would be
checked out earlier in the design. I cite that as a subtle ex-
ample of the kinds of technical problems that we can get into if
we don't understand these things that I talked about previously.
Thermal Control: This is going to be another interesting
design interface because the probe is going to have to be con-
sidered a major part of the spacecraft in the overall thermal
design of the spacecraft. It won't be a simple appendage that is
not going to interact with the spacecraft design. And I really
don't have a good feel for the exact way in which that problem
is going to be handled. We have had several discussions on this.
And other _than saying we see it as an area that is going to re-
quire significant attention early in the design, I don't feel
that we have given this one as much attention as it deserves.
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Guidance and Control: We have looked at this interface and
it appears to be rather straightforward, particularly in our abil-
ity to satisfy the probe requirements on the delivery accuracy,
zero entry angle of attack and spinning the probe on the space-
craft. We have looked at specific designs where, as far as the
probe is concerned, the interface to Mariner is identical to
Pioneer.
In summary, I would like to say that in having thought through
these considerations, that they are much farther reaching than the
simple technical interface but that I believe that a continual
cooperative effort between the science and engineering aspects of
the design, in addition to the proper management attention early,
is going to make this a certainly doable interface design.
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Probe interface design consideration

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