Multimedia Mania/abacus-multimedia-mania.iso/dp/0115/01158.txt

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$Unique_ID{bob01158}
$Pretitle{}
$Title{Pioneer
Chapter 3: The Pioneer Spacecraft}
$Subtitle{}
$Author{Fimmel, Richard O.;Allen, James Van;Burgess, Eric}
$Affiliation{Ames Research Center;University Of Iowa;Science Writer}
$Subject{spacecraft
spin
earth
power
antenna
axis
pioneer
data
direction
scientific
see
pictures
see
figures
}
$Date{1980}
$Log{See RTG*0115801.scf
}
Title:       Pioneer
Book:        Pioneer: First To Jupiter, Saturn, And Beyond
Author:      Fimmel, Richard O.;Allen, James Van;Burgess, Eric
Affiliation: Ames Research Center;University Of Iowa;Science Writer
Date:        1980

Chapter 3: The Pioneer Spacecraft

     The Pioneer spacecraft were designed to fit within the 3-m (10 ft) shroud
of the Atlas-Centaur launch vehicle.  Each spacecraft was stowed with its
booms retracted and its antenna dish facing forward (i.e., upward on the
launch pad).  Basically, the two spacecraft had to be extremely reliable and
lightweight; their communications systems had to transmit information over
extreme distances; and each had nonsolar heat sources to supply electrical
power.

     Each Pioneer spacecraft comprised several distinct subsystems:  a general
structure, an attitude control and propulsion system, a communications system,
thermal control system, electrical power system, navigation system, and most
important to the scientific mission, a payload of 11 sophisticated instruments
for scientific observations and measurements.

     To communicate over the great distances from the outer Solar System, the
dish-shaped antennas of the spacecraft had to be pointed toward Earth.  The
simplest and least expensive way to do this was to spin-stabilize each
spacecraft, keeping its spin axis pointing toward Earth.

General Structure

     From its cone-shaped, medium-gain antenna to the adapter ring that
fastened the spacecraft to stage three of its launch vehicle, each spacecraft
was 2.9 m (9.5 ft) long.  The structure of each spacecraft centered around a
36-cm (14-in.) deep, flat, equipment compartment, the top and bottom of which
consisted of regular hexagons with sides 71 cm (28 in.) long.  Attached to one
side of this hexagon was a smaller "squashed" hexagon compartment that carried
most of the instruments for the scientific experiments.

     A 2.74-m (9-ft) diameter, 46-cm (18-in.) deep, parabolic, dish-shaped,
high-gain antenna of aluminum honeycomb sandwich material was attached to the
front of the equipment compartment.  Its feed was topped with a medium-gain
antenna mounted on three struts which projected about 1.2 m (4 ft) forward.  A
low-gain, omnidirectional antenna extended about 0.76 m (2.5 ft) behind the
equipment compartment mounted below the dish of the high-gain antenna.  Two
three-rod trusses, 1200 apart, projected from two sides of the equipment
compartment.  At their ends, radioisotope thermoelectric generators were held
about 3 m (10 ft) from the center of the spacecraft.  A third single-rod boom,
1200 from the two trusses, projected from the experiment compartment to
position a magnetometer sensor about 6.6 m (21.5 ft) from the center of the
spacecraft.  All three appendages were extended after launch.

Attitude Control and Propulsion

     A starlight sensor on each spacecraft provided a reference on the bright
southern star Canopus, and two sunlight sensors provided a reference to the
Sun.  Attitude was calculated from the reference directions to Earth and the
Sun, with the known direction to Canopus provided as backup.  Before Pioneer
11 was launched, the gain and threshold settings of its starlight sensor were
modified to improve performance on the basis of experience gained during the
first few months of Pioneer 10's flight.

     Three pairs of rocket thrusters located near the rim of the antenna dish
were used to direct the spin axis of each spacecraft, to keep it spinning at
the desired rate of 4.8 rpm, and to change the velocity of the spacecraft for
in-flight maneuvers.  The system's six thrusters could be commanded to fire
steadily or in pulses.  Each thruster developed its propulsive jet from the
decomposition of liquid hydrazine by a catalyst in a small rocket thrust
chamber to which the nozzles of the thruster were attached.  Attitude and
velocity were changed by two thruster pairs mounted on opposite sides of the
rim of the antenna dish.  One thruster of each pair pointed forward, the other
aft.  To change attitude, the spin axis of the spacecraft was precessed in the
desired direction by firing two thrusters, one on each side of the antenna
dish.  One thruster was fired forward, one aft, in brief pulses of thrust at a
precisely timed position in the cycle of rotation of the spacecraft.  Each
thrust pulse, timed to the rotation, precessed the spin axis a few tenths of a
degree until the desired attitude was reached.

     To change velocity, the spin axis was first precessed until it pointed in
the direction along which the correcting velocity had to be applied.  Then two
thrusters, one on each side of the antenna dish, were fired continuously, both
in the same direction (i.e., forward or aft, to apply the correcting velocity
in the desired direction).  For example, if the spacecraft's spin axis were
aligned to its flightpath, the correcting velocity would be applied forward to
increase its velocity along its flightpath and aft to decrease it.

     To adjust the spin rate of the spacecraft, two more pairs of thrusters,
also set along the rim of the antenna dish, were used.  These thrusters were
aligned tangentially to the antenna rim, one pointing against the direction of
spin and the other pointing with it.  Thus, to reduce spin rate, two thrusters
were fired against the direction of spin.  To increase spin rate, they were
fired with the spin direction.

Communications

     Each Pioneer spacecraft, in its journey to explore the giant outer
planets, carried two identical receivers.  The omnidirectional and medium gain
antennas operated together and were connected to one receiver, while the
high-gain antenna was connected to the other.  The receivers did not operate
at the same time, but were interchanged by command or, if there was a period
of inactivity, they were switched automatically.  Thus, if a receiver had
failed during the mission, the other would have automatically taken over.

     Two radio transmitters, coupled to two traveling-wave-tube power
amplifiers, each produced 8 W of transmitted power at S-band.  The
communications frequency uplink from Earth to the spacecraft was at 2110 MHz,
the downlink to Earth, at 2292 MHz.  The turnaround ratio, downlink to uplink,
was precisely controlled to be compatible with the Deep Space Network.

     The data system of each spacecraft converted scientific and engineering
information into a specially coded stream of data bits for transmission by
radio to Earth.  A convolutional encoder arranged the data in a form that
allowed most errors to be detected and corrected by ground computer at the
receiving site of the Deep Space Network.  There were 11 data formats divided
into scientific and engineering data groups.  Some science formats were
optimized for interplanetary data, others for the encounters with Jupiter and
Saturn.  Engineering data formats specialized in data handling, electrical,
communications, orientation, and propulsion data.  All formats were selected
by command from Earth.

Thermal Control

     Temperature was held between -23 degrees and 38 degrees C (-10 and 100
degrees F) inside the scientific instrument compartment, and at various other
levels elsewhere so that the scientific equipment onboard the spacecraft
operated satisfactorily.

     The system of temperature control was designed to adapt to the gradual
decrease in solar heating as the spacecraft moved away from the Sun, and to
those frigid periods when the spacecraft passed through Earth's shadow soon
after launch and when it passed through Jupiter's or Saturn's shadow during
planetary encounters.  The temperature control system also controlled the
effects of heat from the third-stage engine, atmospheric friction during
launch, spacecraft thermoelectric power generators, and from other equipment.

     Equipment compartments were insulated by multi-layered blankets of
aluminized plastic.  Temperature-responsive louvers at the bottom of the
equipment compartment, opened by temperature-sensitive bimetallic springs,
controlled the amount of excess heat allowed to escape.  Other equipment was
individually thermally insulated and was warmed as required by electric
heaters and twelve 1-W radioisotope heaters fueled with plutonium-238.

Electrical Power

     Nuclear-fueled electric power for the Pioneer spacecraft was derived from
SNAP-19-type radioisotope thermoelectric generators (RTGs), developed by the
Atomic Energy Commission, similar to those that had been used successfully to
power the Nimbus-3 meteorological satellite.  These units converted heat from
the radioactive decay of plutonium-238 into electricity.

[See RTG: Four radioisotope thermoelectric generators (RTG's) provided
electrical power in each Pioneer spacecraft.]

     The RTGs were located on the opposite side of the spacecraft from the
scientific instruments to reduce the effects of neutron radiation.  Mounted in
pairs on the end of each three-rod truss, these four RTGs developed about 155
W of electrical power for each spacecraft at launch.  By the time each
spacecraft reached Jupiter, the power output had decreased to about 140 W.  It
continued to decrease, but at a slower rate, as Pioneers 10 and 11 proceeded
on their long journeys after Jupiter encounter.  The depletion of power was
not caused by the isotope source itself, but resulted from a deterioration in
the junctions of the thermocouples which converted heat into electricity
within each unit.  The RTGs supplied adequate power for the mission because
each spacecraft needed only 100 W to operate all its systems and experiments.
The scientific instruments consumed only 25 W.  Any excess power from the RTGs
not required by the spacecraft was dissipated into space in the form of heat
by a shunt resistor radiator.  Alternatively, any excess power was used to
charge a battery that automatically supplied additional power for short
periods when the spacecraft required more than the output of the RTGs.

Navigation

     Throughout the mission, the axis of the highgain antenna was slightly
offset from, but parallel to, the spin axis of each spacecraft within close
tolerances.  Except during the early stages of the flight near Earth and when
adjustments were made to realign the spacecraft to make course corrections,
the spin axis of each spacecraft always pointed toward Earth, within a
tolerance of 1 degree, to provide best communication.

     Analysts used the shift in frequency of the radio signals from the
spacecraft together with angle tracking by the antennas of the Deep Space
Network to calculate the speed, distance, and direction of the spacecraft from
Earth.  The motion of the spacecraft away from Earth caused the frequency of
the spacecraft's signals to drop and their wavelength to increase.  This
effect known as the Doppler shift allowed the speed of the spacecraft to be
calculated from measurements of the change in frequency of the signal received
at Earth.  As the spacecraft continued outward, angle tracking became less
important.  Pioneer's path was calculated by use of celestial mechanics, and
the radio data were used to determine just how close the spacecraft was to its
calculated path.  Residual Doppler data (i.e., the difference between the
Doppler shifts expected and those observed) provided information to keep the
trajectory updated and to determine the masses of planetary bodies the
Pioneers encountered.

     The radio beam to Earth was offset 1 degree from the spin axis of the
spacecraft.  As a result, when the spin axis was not directed exactly toward
Earth, uplink signals received by Pioneer from Earth varied in intensity
synchronously with the rotation of the spacecraft.  A system on the
spacecraft, known as conical scan (CONSCAN), was originally intended to
automatically change the attitude of the spacecraft in a direction that would
reduce such variations in signal strength, thereby returning the spin axis to
align with the direction of Earth to within a threshold of 0.3 degrees.
However, flight operations personnel developed and used a direct command
technique that allowed them to conserve the gas supply of each spacecraft so
that the mission could be extended beyond the encounters with Jupiter and
Saturn.

Scientific Payload

     The Pioneer spacecraft, as they moved through interplanetary space on
their way to Jupiter and Saturn and beyond, were to investigate magnetic
fields, cosmic rays (fast-moving parts of atoms from the Sun and from the
Galaxy), the solar wind (a flow of charged particles from the Sun) and its
relationships with the interplanetary magnetic field and cosmic rays, and any
interplanetary dust concentrations they might encounter in the asteroid belt.

     At Jupiter and Saturn, Pioneer investigated their planetary systems in
four main ways:  by measuring particles, fields, and radiation; by spin-scan
imaging the planets and some of their satellites; by accurately observing the
paths of the spacecraft and measuring the gravitational forces of the planets
and their major satellites acting on them; and by observing changes in the
frequency of the S-band radio signal before and after occultation to study the
structures of their ionospheres and atmospheres.