THE SPACECRAFT
In the 11 brief months which JPL had to produce the Mariner spacecraft
system, there was no possibility of designing an entirely new
spacecraft. JPL’s solution to the problem was derived largely from the
Laboratory’s earlier space exploration vehicles, such as the Vega, the
Ranger lunar series, and the cancelled Mariner A.
Wherever possible, components and subsystems designed for these projects
were either utilized or redesigned. Where equipment was purchased from
industrial contractors, existing hardware was adapted, if practicable.
Only a minimum of testing could be performed on newly designed equipment
and lengthy evaluation of “breadboard” mock-ups was out of the question.
Ready for launch, the spacecraft measured 5 feet in diameter and 9 feet
11 inches in height. With the solar panels and the directional antenna
unfolded in the cruise position, Mariner was 16 feet 6 inches wide and
11 feet 11 inches high.
THE SPACEFRAME
The design engineers were forced to work within the framework of the
earlier spacecraft technology because of the time restrictions, but
Mariner I and II could weigh only about half as much as the Ranger
spacecraft and just over one-third as much as the planned Mariner A.
[Illustration: _Mariner spacecraft with solar panels, microwave
radiometer, and directional antenna extended in flight position.
Principal components are shown._]
ROLL AXIS
OMNIANTENNA
MAGNETOMETER SENSOR
PARTICLE FLUX DETECTORS (GEIGER TUBES)
RADIOMETER REFERENCE HORNS
MICROWAVE RADIOMETER
INFRARED RADIOMETER
ION-CHAMBER
COSMIC DUST DETECTOR
EARTH SENSOR
SOLAR PANEL
COMMAND ANTENNA
SOLAR SAIL
ATTITUDE CONTROL GAS BOTTLES
SOLAR PLASMA DETECTOR
DIRECTIONAL ANTENNA
The basic structural unit of Mariner was a hexagonal frame made of
magnesium and aluminum, to which was attached an aluminum
superstructure, a liquid-propelled rocket engine for midcourse
trajectory correction, six rectangular chassis mounted one on each face
of the hexagonal structure, a high-gain directional antenna, the Sun
sensors, and gas jets for control of the spacecraft’s attitude.
The tubular, truss-type superstructure extended upward from the base
hexagon. It provided support for the solar panels while latched under
the shroud during the launch phase, and for the radiometers, the
magnetometer, and the nondirectional antenna, which was mounted at the
top of the structure. The superstructure was designed to be as light as
possible, yet be capable of withstanding the predicted load stresses.
The six magnesium chassis mounted to the base hexagon housed the
following equipment: the electronics circuits for the six scientific
experiments, the communications system electronics; the data encoder
(for processing data before telemetering it to the Earth) and the
command electronics; the attitude control, digital computer, and timing
sequencer circuits; a power control and battery charger assembly; and
the battery assembly.
The allotment of weights for Mariner II forced rigid limitation in the
structural design of the spacecraft. As launched, the weights of the
major spacecraft subsystems were as follows:
Structure 77 pounds
Solar panels 48 pounds
Electronics 146 pounds
Propulsion 32 pounds
Battery 33 pounds
Scientific experiments 41 pounds
Miscellaneous equipment 70 pounds
Gross weight 447 pounds
THE POWER SYSTEM
Mariner II was self-sufficient in power. It converted energy from
sunlight into electrical current through the use of solar panels
composed of photoelectric cells which charged a battery installed in one
of the six chassis on the hexagonal base. The control, switching, and
regulating circuits were housed in another of the chassis cases.
[Illustration: _This hexagonal frame, constructed of magnesium and
aluminum, is the basic supporting structure around which the Mariner
spacecraft is assembled._]
[Illustration: _Plan view from top showing six magnesium chassis
hinged in open position._]
VIEW LOOKING AFT ASSEMBLIES HINGED IN OPEN POSITION
SCIENTIFIC EQUIPMENT ASSEMBLY I
COMMUNICATIONS ASSEMBLY II
DATA ENCODER AND COMMAND ASSEMBLY III
ATTITUDE CONTROL AND CC AND S ASSEMBLY IV
POWER ASSEMBLY V
BATTERY ASSEMBLY VI
The battery operated the spacecraft systems during the period from
launch until the solar panels were faced onto the Sun. In addition, the
battery supplied power during trajectory maneuvers when the panels were
temporarily out of sight of the Sun. It shared the demand for power when
the panels were overloaded. The battery furnished power directly for
switching various equipment in flight and for certain other heavy loads
of brief duration, such as the detonation of explosive devices for
releasing the solar panels.
[Illustration: _Mariner spacecraft with solar panels in open
position. Note extension to left panel to balance solar pressures in
flight._]
The Mariner battery used sealed silver-zinc cells and had a capacity of
1000 watt-hours. It weighed 33 pounds and was recharged in flight by the
solar panels.
The solar panels, as originally designed, were 60 inches long by 30
inches wide and contained approximately 9800 solar cells in a total area
of 27 square feet. Each solar cell produced only about 230
one-thousandths of a volt. The entire array was designed to convert the
Sun’s energy to electrical power in the range between 148 and 222 watts.
When a later design change required the extension of one panel in order
to add more solar cells, it was necessary to add a blank extension to
the other panel in order to balance the solar pressure on the
spacecraft.
In order to protect the solar cells from the infrared and ultraviolet
radiation of the Sun, which would produce heat but no electrical energy,
each cell was shielded from these rays by a glass filter which was
nevertheless transparent to the light which the cells converted into
power.
The power subsystem electronics circuits were housed in another of the
hexagon chassis cases. This equipment was designed to receive and switch
power either from the solar panels, the battery, or a combination of the
two, to a booster-regulator.
CC&S: THE BRAIN AND THE STOPWATCH
Once the Atlas booster lifted Mariner off the launch pad, the digital
Central Computer and Sequencer (CC&S) performed certain computations and
provided the basic timing control for those spacecraft subsystems which
required a sequenced programming control.
The CC&S was designed to initiate the operations of the spacecraft in
three distinct sequences or “modes”: (1) the launch mode, from launch
through the cruise configuration; (2) the midcourse propulsion mode,
when Mariner readjusted its sights on Venus; and (3) the encounter mode,
involving commands for data collection in the immediate vicinity of the
planet.
The CC&S timed Mariner’s actions as it travelled more than 180 million
miles in pursuit of Venus. A highly accurate electronic clock
(crystal-controlled oscillator) scheduled the operations of the
spacecraft subsystems. The oscillator frequency of 307.2 kilocycles was
reduced to the 2,400- and 400-cycle-per-second output required for the
power subsystem.
The control oscillator also timed the issuance of commands by the CC&S
in each of the three operating modes of the spacecraft.
A 1-pulse-per-minute signal was provided for such launch sequence events
as the extension of the solar panels 44 minutes after launch, turning on
power for the attitude control subsystem one hour after launch, and for
certain velocity correction commands during the midcourse maneuver.
[Illustration: _The spacecraft used two antennas for communication.
The omni-antenna (top) was utilized when the directional antenna
(bottom) could not be pointed at the Earth._]
[Illustration: _This command antenna (on solar panel) was used to
receive maneuver commands._]
A 1-pulse-per-second signal was generated as a reference during the roll
and pitch maneuvers in the midcourse trajectory correction phase. One
pulse was generated every 3.3 hours in order to initiate the command to
orient the directional antenna on the Earth at 167 hours after launch.
Finally, one pulse every 16.7 hours was used to readjust the
Earth-oriented direction of the antenna throughout the flight.
TELECOMMUNICATIONS: RELAYING THE DATA
The telecommunications subsystem enabled Mariner to receive and to
decode commands from the Earth, to encode and to transmit information
concerning space and Mariner’s own functioning, and to provide a means
for precise measurement of the spacecraft’s velocity and position
relative to the Earth. The spacecraft accomplished all these functions
using only 3 watts of transmitted power up to a maximum range of 53.9
million miles.
A data encoder unit, with CC&S sequencing, timed the three phases of
Mariner’s journey: (1) In the launch mode, only engineering data on
spacecraft performance were transmitted; (2) during the cruise mode,
information concerning space and Mariner’s own functioning was
transmitted; and (3) while the spacecraft was in the vicinity of Venus,
only scientific information concerning the planet was to be transmitted.
(The CC&S failed to start the third mode automatically and it was
initiated by radio command from the Earth.) After the encounter with
Venus, Mariner was programmed to switch back to the cruise mode for
handling both engineering and science data (this sequence was also
commanded by Earth radio).
Mariner II used a technique for modulating (superimposing intelligent
information) its radio carrier with telemetry data known as phase-shift
keying. In this system, the coded signals from the telemetry
measurements displace another signal of the same frequency but of a
different phase. These displacements in phase are received on the Earth
and then translated back into the codes which indicate the voltage,
temperature, intensity, or other values measured by the spacecraft
telemetry sensors or scientific instruments.
A continually repeating code, almost noise-like both in sound and
appearance on an oscilloscope, was used for synchronizing the ground
receiver decoder with the spacecraft. This decoder then deciphered the
data carried on the information channel.
This technique was called a two-channel, binary-coded, pseudo-noise
communication system and it was used to modulate a radio signal for
transmission, just as in any other radio system.
Radio command signals transmitted to Mariner were decoded in a command
subassembly, processed, and routed to the proper using devices. A
transponder was used to receive the commands, send back confirmation of
receipt to the Earth, and distribute them to the spacecraft subsystems.
Mariner II used four antennas in its communication system. A cone-like
nondirectional (omni) antenna was mounted at the top of the spacecraft
superstructure, and was used from injection into the Venus flight
trajectory through the midcourse maneuver (the directional antenna could
not be used until it had been oriented on the Earth).
A dish-type, high-gain, directional antenna was used at Earth
orientation and after the trajectory correction maneuver was completed.
It could receive radio signals at greater distances than the
nondirectional antenna. The directional antenna was nested beneath the
hexagonal frame of the spacecraft while it was in the nose-cone shroud.
Following the unfolding of the solar panels, it was swung into operating
position, although it was not used until after the spacecraft locked
onto the Sun.
The directional antenna was equipped with flexible coaxial cables and a
rotary joint. It could move in two directions; one motion was supplied
by rolling the spacecraft around its long axis.
In addition, two command antennas, one on either side of one of the
solar panels, received radio commands from the Earth for the midcourse
maneuver and other functions.
ATTITUDE CONTROL: BALANCING IN SPACE
Mariner II had to maintain a delicate balance in its flight position
during the trip to Venus (like a tight-wire walker balancing with a
pole) in order to keep its solar panels locked onto the Sun and the
directional antenna pointed at the Earth. Otherwise, both power and
communications would have been lost.
A system of gas jets and valves was used periodically to adjust the
attitude or position of the spacecraft. Expulsion of nitrogen gas
supplied the force for these adjustments during the cruise mode. While
the spacecraft was subjected to the heavier disturbances caused by the
rocket engine during the midcourse maneuver, the gas jets could not
provide enough power to control the attitude of the spacecraft and it
was necessary to use deflecting vanes as rudders in the rocket engine
exhaust stream for stabilizing purposes.
The attitude control system was activated by CC&S command 60 minutes
after launching. It operated first to align the long axis of the
spacecraft with the Sun; thus its solar panels would face the Sun.
Either the Sun sensors or the three gyroscopes mounted in the pitch
(rocking back and forth), yaw (side to side), and roll axes, could
activate the gas jet valves during the maneuver, which normally required
about 30 minutes to complete.
The spacecraft was allowed a pointing error of 1 degree in order to
conserve gas. The system kept the spacecraft swinging through this 1
degree of arc approximately once each 60 minutes. As it neared the limit
on either side, the jets fired for approximately ¹/₅₀ of a second to
start the swing slowly in the other direction. Thus, Mariner rocked
leisurely back and forth throughout its 4-month trip.
Sensitive photomultiplier tubes or electric eyes in the Earth sensor,
mounted on the directional antenna, activated the gas jets to roll the
spacecraft about the already fixed long axis in order to face the
antenna toward the Earth. When the Earth was “acquired,” the antenna
would then necessarily be oriented in the proper direction. If telemetry
revealed that Mariner had accidentally fixed on the Moon, over-ride
radio commands from the Earth could restart the orientation sequence.
PROPULSION SYSTEM
The Mariner propulsion system for midcourse trajectory correction
employed a rocket engine that weighed 37 pounds with fuel and a nitrogen
pressure system, and developed 50 pounds of thrust for a maximum of 57
seconds. The system was suspended within the central portion of the
basic hexagonal structure of the spacecraft.
This retro-rocket engine used a type of liquid propellant known as
anhydrous hydrazine and it was so delicately controlled that it could
burn for as little as ²/₁₀ of a second and increase the velocity of the
spacecraft from as little as ⁷/₁₀ of a foot per second to as much as 200
feet per second.
The hydrazine fuel was stored in a rubber bladder inside a
doorknob-shaped container. At the ignition command, nitrogen gas under
3,000-pound-per-square-inch pressure was forced into the propellant tank
through explosively activated valves. The nitrogen then squeezed the
rubber bladder, forcing the hydrazine into the combustion chamber.
[Illustration: _The midcourse propulsion system provides trajectory
correction for close approach to Venus._]
FUEL TANK
NITROGEN TANK
JET VANE ACTUATOR (ONE OF FOUR)
THRUST CHAMBER
Hydrazine, a monopropellant, requires a starting ignition for proper
combustion. In the Mariner system, nitrogen tetroxide starting or
“kindling” fluid was injected into the propellant tank by a pressurized
cartridge. Aluminum oxide pellets in the tank acted as catalysts to
control the speed of combustion of the hydrazine. The burning of the
hydrazine was stopped when the flow of nitrogen gas was halted, also by
explosively activated valves.
TEMPERATURE CONTROL
Mariner’s 129 days in space presented some unique problems in
temperature control. Engineers were faced with the necessity of
achieving some form of thermal balance so that Mariner would become
neither too hot nor too cold in the hostile environment of space.
The spacecraft’s temperature control system was made as thermally
self-sufficient as possible. Paint patterns, aluminum sheet, thin gold
plating, and polished aluminum surfaces reflected and absorbed the
proper amount of heat necessary to keep the spacecraft and its
subsystems at the proper operating temperatures.
Thermal shields were used to protect the basic hexagon components. The
upper shield, constructed of aluminized plastic on a fiberglass panel,
protected the top of the basic structure and was designed for maximum
immunity to ultraviolet radiation. The lower shield was installed below
the hexagon; it was made of aluminum plastic faced with aluminum foil
where it was exposed to the blast of the midcourse rocket engine
exhaust.
[Illustration: _Methods used to control the temperature of the
Mariner spacecraft in flight._]
CHROMATE CONVERSION COATING
UPPER THERMAL SHIELD
POLISHED ALUMINUM
LOUVERS
GOLD PLATE
BLACK PAINT
LOWER THERMAL SHIELD
WHITE PAINT
The six electronics cases on the hexagon structure were variously
treated, depending upon the power of the components contained in each.
Those of high power were coated with a good radiating surface of white
paint; assemblies of low power were provided with polished aluminum
shields to minimize the heat loss.
The case housing the attitude control and CC&S electronics circuits was
particularly sensitive because the critical units might fail above 130
degrees F. A special assembly was mounted on the face of this case; it
consisted of eight movable, polished aluminum louvers, each actuated by
a coiled, temperature-sensitive, bimetallic element. When the
temperature rose, the elements acted as springs and opened the louvers.
A drop in temperature would close them.
Structures and bracket assemblies external to the basic hexagon were
gold plated if made of magnesium, or polished if aluminum. Thus
protected, these items became poor thermal radiators as well as poor
solar absorbers, making them relatively immune to solar radiation.
External cabling was wrapped in aluminized plastic to produce a similar
effect.
The solar panels were painted on the shaded side for maximum radiation
control properties. Other items were designed so that the internal
surfaces were as efficient radiators as possible, thus conserving the
spacecraft’s heat balance.
THE SCIENTIFIC INSTRUMENTS
Four instruments were operated throughout the cruise and encounter modes
of Mariner: a magnetometer, a solar plasma detector, a cosmic dust
detector, and a combined charged-particle detector and radiation
counter. Two radiometers were used only in the immediate vicinity of
Venus.
These instruments are described in detail in Chapter 8.