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DESTINATION MOON: A History of the
Lunar Orbiter Program
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- CHAPTER VI: THE LUNAR ORBITER
SPACECRAFT
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- A General
Description
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- [111] Before surveying
the design and development phases of the Lunar Orbiter Program, it
will be useful to describe the spacecraft which Boeing built for
Langley. In the final design the Boeing Orbiter weighed about 385
kilograms and was 1.7 meters tall and 1.5 meters in diameter at
its base, without including the solar panels and the antennas.
Structurally the spacecraft had three decks supported by trusses
and an arch. On the largest deck the main equipment was mounted:
batteries, transponder, flight programmer, photographic system,
inertial reference unit (IRU), Canopus star tracker, command
decoder, multiplex encoder., and the traveling-wave-tube amplifier
(TWTA), together with smaller units. Four solar panels and two
antennas extended from the perimeter of this equipment
deck.1
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- Above it, the middle deck supported the
velocity control engine (the 100-pound-thrust Marquardt rocket
motor), the fuel tanks, the oxidizer tank for the velocity control
engine, the coarse Sun sensor, and the micrometeoroid
[112]
detectors. Above this the third deck contained the heat shield to
protect the spacecraft from the heat generated by the firing of
the velocity control engine. In addition the four attitude control
thrusters were mounted on its perimeter. This uppermost deck was
part of the engine module, which could be detached for test
purposes. Directly under the engine was the high-pressure nitrogen
tank, which provided pressure to feed fuel to the velocity control
engine and to operate the attitude control
thrusters.2 This tank was one of the critical units; if
anything caused it to lose pressure, the spacecraft could not
maneuver, and an entire mission could be ruined.
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- These and other items of spacecraft
equipment formed subsystems of the whole spacecraft system.
Working together they performed the Lunar Orbiter mission. The
Eastman Kodak photographic subsystem has previously been
described.3 Electrical power was provided by a power system
which operated in two modes: 1) solar panels converted solar
radiation into electric current, and 2) batteries powered the
spacecraft systems for short periods of occultation from the Sun.
In periods when the solar panels would receive radiation from the
Sun, the power supply would [113] run from the
panels through the output voltage regulator to the other
spacecraft systems (mode 1). This happened for the major part of
the mission. At the same time power generated by the panels would
also be directed into the battery charge controller, and from
there a charging current would flow into the batteries as they
could accept it. When no sunlight fell on the panels, the
batteries would supply power to the output voltage regulator, and
this would direct its flow to the spacecraft subsystems (mode
2).4 In addition the power system had regulators and
controllers to reduce unusual fluctuations to a minimum and enough
solar cells to allow micrometeoroid damage to some without
dangerous reduction in the capacity of the solar panels to
generate electricity.
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- The attitude control subsystem served as
the navigator for Lunar Orbiter during an entire mission. Composed
of Sun sensors, the Canopus sensor, the inertial reference unit,
and the thrusters, the system controlled the spacecraft's attitude
in space in reference to the Sun, the star Canopus, and the Moon.
The Sun sensors would "see" the Sun, produce signals which
activated the attitude control thrusters, and these would align
the spacecraft's roll axis with the sun. Once this reference was
established the spacecraft could maneuver off the reference and
the IRU would remember [114] the original
reference. If the need arose to move the spacecraft back to that
reference, the IRU would signal the thrusters to correct the
attitude. However, the IRU simply remembered reference points; it
did not establish them.
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- Attitude control was directed by the
flight electronics control assembly (FECA) and the Flight
Programmer, which received data from all sensors and then informed
ground control monitors, who could update the Programmer for
future attitude maneuvers. The FECA and the Flight Programmer
controlled the spacecraft's attitude around its X (roll), Y (yaw),
and Z (pitch) axes by activating the thrusters. They also governed
the orientation of the photographic subsystem's camera lenses in
relation to the surface of the Moon. Commands from Earth would
make the spacecraft rotate through an angle around each axis
according to the task to be executed, and the outputs of the gyros
in the IRU would tell the Flight Programmer when the new attitude
had been achieved. The Flight Programmer would stabilize and
maintain the spacecraft in the new attitude relative to the three
reference directions, and the IRU would tell it when there was any
deviation from the established attitude.5
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- [115] The Atlas-Agena
D launch vehicle placed all five of the Lunar Orbiter spacecraft
in parking orbits around Earth. The Agena with the spacecraft
would remain in the parking orbit until the time to begin the
translunar trajectory maneuver in which the Agena, would fire out
of Earth orbit toward the Moon. Once the spacecraft separated from
the Agena there remained the task of correcting its initial
trajectory and then of deboosting it into lunar orbit. The
velocity control subsystem held the responsibility for this task
and had to execute any changes in trajectory and speed.
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- The heart of the system was a
100-pound-thrust rocket whose hypergolic, fuel and oxidizer
ignited when the Flight Programmer commanded the intake valves to
open. A burn to change the spacecraft's velocity would then occur
and continue until the valves closed. Duration of any burn would
be determined by information from the accelerometers in the IRU
compared with prestored data in the Flight Programmer. The rocket
engine was gimbaled to provide thrust vector control in order to
accommodate center-of-gravity offsets and thrust asymmetries. The
IRU accelerometers provided inputs for thrust vector control, the
purpose of which was to keep the thrust of the velocity control
engine through [116] the spacecraft's center of
mass.6
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- A nominal mission would provide for two
midcourse maneuvers to bring the Orbiter's trajectory precisely in
line with an imaginary point where it would be deboosted into
orbit around the Moon. At this predetermined point the velocity
control subsystem would fire to slow the spacecraft and allow it
to go into an initial orbit around the Moon. Ground personnel
would then check out the spacecraft's orbital behavior and its
various subsystems before making' any decision to transfer to
another orbit. Once they found the spacecraft's subsystems to be
operating correctly, they would make a decision to inject it into
a photographic orbit.7
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- Receiving and transmitting data to and
from the spacecraft was the job of the communications subsystem,
many of whose components had been flight-proven in the Ranger and
the Mariner programs. This complex assembly could operate in four
modes: 1) tracking and ranging, 2) command, 3) low power, and 4)
high power. The communications system could send and receive data
simultaneously while also transponding velocity and ranging
signals for the Deep [117] Space Network's
tracking system.
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- The spacecraft's low-gain antenna picked
up all incoming signals from the NASA-JPL Deep Space
Instrumentation Facility stations. Commands from DSIF were routed
to the command decoder and stored. The spacecraft would transmit a
command from Earth back to Earth for verification before ground
controllers sent an "execute" command. Upon receiving the execute
command the communications subsystem would advance stored commands
from the decoder to the Flight Programmer to be carried out.
Photographic data with performance, environmental, and telemetry
data would be transmitted to Earth by the high-power
mode.8
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- Photographic data were transmitted in a
different way than telemetry data were. The spacecraft had two
antennas that operated in the S-band at the frequency of 2295
mega-cycles. Normally, when photographic data were transmitted to
the ground receiving stations, the communications subsystems
operated in the high-power mode and transmitted via the
one-meter-diameter parabolic high-gain antenna. Simultaneous
transmission of photographic and telemetry data was carried out as
follows:
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- [118] The 50-bit/sec
telemetry data train is phase modulated onto a 30-kc subcarrier,
which is then combined with the video data that have been
transformed to a vestigial sideband signal. That signal is created
by amplitude modulating the data on a 310-kc subcarrier by means
of a double balanced modulator. This suppresses the carrier and
produces two equal sidebands. An appropriate filter is then
superimposed on the double sideband spectrum, essentially
eliminating the upper sideband.
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- Since the missing subcarrier must be
reinserted on the ground for the proper detection of the vestigial
sideband signal, provision for deriving such a subcarrier signal
is made by transmitting a pilot tone of 38-75 kc. That pilot tone
is exactly one-eighth of the original 310-ke subcarrier frequency,
and is derived from the same crystal oscillator. Multiplying the
received pilot tone by 8 in the ground equipment provides a proper
subcarrier for reinsertion.9
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- Lunar Orbiter photographic data were never
encoded; instead, data were transmitted as frequency-modulated
analog signals. All other data from the spacecraft were encoded
and sent on the subcarrier frequency as described above. The
temperature control subsystem protected all of the spacecraft's
other subsystems from the extreme temperature variations of the
deep space environment. Heat from the Sun could warm external
parts of the spacecraft to 120°C while areas not exposed to
solar radiation would cool down to -160°C. These extremes
were beyond the temperature [119] levels which
most components could endure. The temperature control system
established an environment ranging from + 2°C to +30°C
for the operation of all subsystems. A few components were exposed
to direct sunlight: the four solar panels, the two antennas, the
bottom of the equipment deck. The solar panels were designed to
withstand temperature variations of +120°C to -160°C
without cracking or buckling from severe expansion and contraction
over a long period of time.10
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- Beginning at the uppermost deck a heat
shield insulated the spacecraft from the rocket engine's heat
while the entire area down to the lower deck was enshrouded in a
thin-skinned aluminized mylar and dacron thermal blanket that
covered all equipment except the Canopus star tracker's lens, the
camera thermal door, and the components mentioned above. The
bottom of the equipment deck, which faced the Sun most of the time
during all five missions,, was coated with a special paint having
a high heat emission-absorption ratio. Small electric heaters were
installed on the spacecraft inside the thermal blanket to raise
the temperature if it fell below +2°C. The arrangement
maintained everything under the thermal blanket at an average
temperature.11
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- [120] The photographic
subsystem had the most rigid temperature restrictions. Film could
withstand heat only up to about 50°C, and moisture In the
photographic subsystem would condense below 2°C, fogging the
camera's two lenses. Eastman Kodak designed the system to be
biased cool and warmed with little electric heaters. The "bathtub"
housing the system did not touch the equipment deck but was
affixed by four legs. Heat transfer between the "bathtub" and the
equipment mounting deck was largely radiative, making heat
absorption and dissipation a slower, more even
process.12
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- One other component of the temperature
control system was added after the original design to protect the
photo-subsystem. This was the camera thermal door. Thermal tests
showed that without any cover over the camera's lenses, the lenses
would be more susceptible to extreme temperature variations and
stray light leaks inside. The major purpose of the camera thermal
door was to reduce or eliminate the possibility that through
heating the lenses could expand and alter the focal length so that
distortions would result in the photography. The door would also
help to control the internal temperature of the photo-subsystem so
that it would not become too cold during periods of occultation
and allow moisture condensation on the lenses. The door was added
as one of the last components of the [121] spacecraft
before final design configurations were fixed. It was not part of
the Eastman Kodak camera subsystem, and Boeing took the
responsibility of designing, fabricating, and testing
it.13
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