The role of computers in space explot:ation
C. R.GATES
and
W. H. PICKERING
let Propulsion Laboratory
California Institute of Technology
Pasadena, California
craft, with respect to the economy and· precision of its
design; the performance of the navigation system, in
the accuracy with which the spacecraft can be guided
to the moon or Mars; and the performance of the data
gathering system with respect to the amount of data
which can be gathered and the speed and. accuracy
with which it can be analyzed.
In my discussion I will describe some of the more
prominent ways in which computers have been used
in the space program.
INTRODUCTION
The modern digital computer has been fundamental
to the space exploration program. Computers have profoundly affected almost every aspect of space technology, including spacecraft design, celestial mechanics,
mission control, and the gathering and processing of
data generated by the spacecraft. Indeed, the evolution
and growth of computer technology is suggestively
parallel to the growth in space technology.
Perhaps the most revealing way to examine our indebtedness to computers is to recall conditions before
computers were available. Fifteen years ago our main
project at JPL was the Corporal, a ballistic missile with
a range of some tens of miles. The calculation of its
trajectory took three weeks of intense labor; now we
can compute the trajectory of a spacecraft to Mars in
a few minutes. Formerly, solving a problem in control
stability or structural design involved many hours of
hand calculation and was likely to require extensive
laboratory experimentation; now, many problems in
these fields are solved more cheaply, more accurately,
and more quickly by means of the computer.
In the last analysis, the effect of modem, high-speed
computers on the space exploration program has been
to increase performance-the performance of the space-
TRAJECTORIES AND
CELESTIAL MECHANICS
The most dramatic usage of computers in the space
program is probably in the field of celestial mechanics.
Celestial mechanics is the oldest of the sciences, and
since a spacecraft in flight behaves like a miniature
planet, our trajectories and navigation are based on this
ancient and ancestral science.
Mathematically, celestial mechanics is· concerned
with the solution of Newton's equations. From Newton's time until very recently, solutions were obtained
only with prodigious labor and by means of amazing
and ingenious mathematical contortions. The digital
computer and the impetus of the space program have
profoundly affected celestial mechanics. Computer calculation of the trajectory of a spacecraft such as Mariner IV is quite straightforward, even though the
numerical techniques used are sophisticated. In the
selection of trajectories for Mariner, many hundreds
of trajectories were calculated and analyzed in order
to determine those trajectories which give the best accuracy, payload weight, and picture quality at Mars.
The computer also greatly increases the accuracy
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From the collection of the Computer History Museum (www.computerhistory.org)
34
The Role of Computers in Space Exploration
with which a spacecraft such as Mariner can be
guided. For the guidance of Mariner, the orbit is determined from radar data, and a small maneuver to
be executed by the spacecraft is calculated. The computer permits these calculations to be carried out swiftly
and accurately while the spacecraft is in flight; without the computer, approximations of much less accuracy would have to be used.
SPACE FLIGHT OPERATIONS:
SPACECRAFT COMMAND AND CONTROL
The largest use of computers at the Jet Propulsion
Laboratory is in the real-time processing of spacecraft
data for command and control purposes; we refer to
this process as Space Flight Operations.
Our Space Flight Operations Facility, or SFOF, will
shortly contain three "strings" of computers, each string
consisting of three computers, an IBM 7040, 7044,
and 7094. Into the SFOF come spacecraft data obtained from tracking stations located throughout the
world. The data may be from the spacecraft, concerning
either the condition of the spacecraft or conditions in
space, or the data may be navigational, referring to the
position or velocity of the spacecraft. These data are
sent in digital form over telephone circuits to the
SFOF, where they are routed into the computers.
After processing, the reduced data are presented to
scientists and engineers for analysis and interpretation.
The engineers conducting these analyses have access in
parallel to the computer and are able to request in real
time various programs, options, and methods of display.
Real-time command and control centers are also
used extensively in the manned space flight program.
The control center for Mercury was at Cape Kennedy,
supported by facilities at Goddard Space Flight Center. The control center for Gemini and Apollo is at the
Manned Spacecraft Center in Houston.
Two basic types of computer processing are carried
out in Space Flight Operations. In the first type, the
real-time data stream is simply separated, translated
into convenient units and symbols, and presented directly to the engineer or scientist. This is the "real-time
mode," and it has been the backbone of spacecraft
analysis. In the SFOF at JPL, one computer per string
is devoted to this function, which includes sorting, decommutating, formatting, and preparing printer and
plotter output.
In the second type of processing, a quantity of accumulated data are analyze~ and certain parameters extracted. An example of this type of processing is
orbit determination, in which several hours or days
of tracking data are analyzed in order to determine
the orbital parameters of the spacecraft.
We have been one of the pioneers in time-shared multiple-channel computer usage, in which a number of
users can simultaneously communicate with the machine and receive output. At times I think we would
have gladly relinquished the pioneering honor to someone else; the problems in such a system-the interaction
between programs, the difficulty in diagnosis, the intelligibility of a program only to the programmer who
wrote it, the difficulty in reproducing a condition in
which a failure occurred, the problems of meeting
schedules and keeping adequate documentation-are
well known. However, the system has carried out its
flight mission reliably and successfully, the experience
gained is valuable, and most of these difficulties appear
to be behind us.
SPACECRAFT DESIGN AND
SPACE TECHNOLOGY
In a spacecraft such as Ranger, Mariner, or Surveyor,
subsystems derived from a variety of technical disciplines must all function together with tightly knit
precision and harmony. Guidance and control, communications, science, structural, thermal, and propulsion subsystems must work together. Furthermore, in
spacecraft design it is necessary to consider almost all
of the physical properties of these subsystems-electrical properties, weight, mass distribution, heat generation and conduction, reflectivity, etc. Also, there are
overall constraints of weight and volume placed on
the spacecraft. And for a planetary spacecraft there
is a totally inflexible constraint of time; the spacecraft
may be launched only at infrequent intervals, necessitating accurate and reliable scheduling procedures.
The use of computers has increased the accuracy
and speed with which the spacecraft and its subsystems
can be designed. Using computer techniques to explore
a whole spectrum of design possibilities, design parameters can be selected quickly and precisely. The result is a materially increased performance. Without
sacrifices in cost, reliability, or schedule, the applicati~n of computers results in a spacecraft which will
send back more and higher-quality scientific data regarding conditions in space.
As an incidental aside, it is interesting to note that
our Mariner-Mars spacecraft contained four digital
computers. These were used for science data handling,
engineering data handling, command processing, and
command sequencing.
The impact of computers on engineering technology,
in fields such as structural design, circuit design, heat
From the collection of the Computer History Museum (www.computerhistory.org)
The Role of Computers in Space Exploration
transfer, etc., has generally followed a pattern of great
interest and importance. Initially, computers were used
to solve problems in their traditional form. For example, the solution to a differential equation would
be obtained by hand in series form, and the series was
then evaluated on the computer. In the next stage of
development, the computer was given the differential
equation directly, which was less work for the engineer
and usually yielded a more accurate solution. Today,
it is common practice to state the problem to the computer in terms of the end result desired, and the computer both formulates and solves the differential equation. Thus, the computer has been applied further and
further upstream.
CONCLUDING REMARKS
In summary, we find that the computer has touched
almost every phase of the Space Exploration Program.
Spacecraft design is now based on computer analyses,
and the result is a better spacecraft; furthermore, a
steadily increasing portion of the design process is
being carried out by the computer. In celestial mechanics, the effect of computers ..,has been dramatic.
And in space flight operations, all data gathered by the
spacecraft are processed by computers Indeed, the number of computers we will shortly have serially in the
data stream is startling; counting computers in the
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spacecraft, at the tracking sites, in the communication
link from site to SFOF, and at the SFOF, we will have
up to 10 computers handling the data.
The role of computers in semitechnical and nontechnical areas has also been vital. Computers are
used for configuration management, budget preparation, inventory control-the list is endless.
Turning to the future, we are looking forward to the
attractive features offered by the new generation of
computers-the much-needed increases in capacity,
speed, memory, reliability, and ease of programming.
However, the impact of computers on basic technology
is likely to have the most significant future effect on the
space program. When I was a student, a large part of
the technology was devoted to problem-solving methodology. Today much of this methodology is becoming
obsolete. The new methodology, based on fundamental
principles and the computer, makes possible the ready
treatment of larger systems. Instead of having to analyze
the relationship between resistor A and resistor B, the
engineer may treat the relationship between circuit A
and circuit B, or system A and system B. The viewpoint is less microscopic, and more macroscopic. For
example, we are beginning to treat system characteristics, such as cost and reliability, as design parameters.
Thus the symbols by which the technology is represented, and hence our way of thinking, are being
changed by the modern digital computer.
From the collection of the Computer History Museum (www.computerhistory.org)
From the collection of the Computer History Museum (www.computerhistory.org)