The ship, a fragment detached from the earth,
went on lonely and swift like a small planet.
-- Joseph Conrad
The design of the Magellan spacecraft was driven by the need for a low-cost, high-performance vehicle. The spacecraft for the earlier VOIR mission was to have been custom-designed and built, but the Magellan Project saved many of those costs by taking advantage of an inventory of mission-proven technologies and spare components (see Table 4-1).
Table 4-1. Equipment from Other Spacecraft
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Component Source
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Medium-gain antenna Mariner Mars 1971
High- and low-gain antenna Voyager
Equipment Bus Voyager
Star-scanner design Inertial Upper Stage
Radio-frequency traveling-wave tube assemblies Ulysses
Attitude-control computer Galileo
Command and data subsystem Galileo
Thruster rockets (small) Voyager
Electric-power distribution unit Galileo
Power control unit P-80 satellite
Pyrotechnic control Galileo
Solid-rocket motor design Space Shuttle payload
assist module (PAM)
Propellant-tank design Space Shuttle auxilary
power unit
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Magellan's simpler design also meant that some components would perform more complex tasks. For example, instead of using separate antennas for mapping and telemetry, the craft's primary antenna will perform both of these functions.
The team that designed Magellan worked within the stringent budgetary and performance requirements and produced a spacecraft that complied with the Project's fiscal reality and one that has our highest confidence in its ability to carry out the objectives of the Magellan mission.
The Magellan spacecraft (see Figure 4-1) that was loaded in the cargo bay of the Space Shuttle Atlantis weighed 3,453 kilograms (7,612 pounds) and consisted of
The parabolic, dish-shaped, high-gain antenna (HGA) dominates the top of the stack. The dish is made of strong, lightweight, graphite-epoxy sheets mounted to an aluminum honeycomb for rigidity. This antenna, further described in the telecommunications section of this chapter, functions as the primary antenna for radar operations, transmission of radar data, receipt of radio signals from Earth, and transmission of engineering health data to Earth.
The medium-gain antenna (MGA) is the cone-shaped structure mounted to the top side of the equipment bus. The low-gain antenna (LGA) is mounted on a platform held by struts above the HGA. Both of these antennas augment the HGA and are useful when the HGA cannot be pointed directly at Earth.
The altimeter antenna (ALTA) is mounted on the side of the forward equipment module (FEM), extending forward from beneath the HGA dish. It is used exclusively for radar altimetry. During the mapping part of each Venus orbit, the ALTA is pointed vertically down at the planet to provide one-dimensional readings of the heights of surface features. The 1.5-meter- (5-foot-) long aluminum structure has an aperture of 0.6 x 0.3 meter (2 x 1 feet) and weighs 6.8 kilograms (15 pounds).
The FEM houses the radar electronics, radio telecommunications equipment, certain attitude-control equipment, batteries, and the power- conditioning unit (see Figure 4-2). The boxlike housing measures 1.7 x 1.0 x 1.3 meters (5.3 x 3.3 x 4.3 feet) and is made of aluminum panels on a framework of square aluminum tubing that has been chemically milled for weight reduction. Two sides of the FEM have louvers for thermal conditioning. Mirror-surfaced covers shield the louvers from the intense sunlight at Venus.
Immediately below the FEM is the 10-sided equipment bus, built originally as a spare for the Voyager Project. The bus is a bolted aluminum structure with aluminum cover plates. It measures 42.4 centimeters (16.7 inches) high and approximately 2.0 meters (6.6 feet) across. Each of its 10 compartments is a 42- x 47- x 18-centimeter (16.5- x 18.5- x 7-inch) enclosure for electronics. An opening in the middle of the ring of compartments holds the hydrazine fuel tank for the liquid-propulsion system.
The bus compartments contain the flight computers, the input/output interface between the computers and Magellan subsystems, tape recorders, solar-array controls, solid-state bulk memory, and pyrotechnic control electronics.
The two square solar panels, shown in Figure 4-3, measure 2.5 meters (8.2 feet) on a side and together can supply 1,200 watts of power. With the arrays deployed, Magellan spans 10 meters (32.8 feet) from tip to tip of the panels. The light-colored lines visible on them are solar reflectors that keep the temperature of the arrays below 115 degrees centigrade (239 degrees Fahrenheit), even in full sunlight at Venus. Approximately 35 percent of the front surface is reflective mirrors, and the back surface is 100 percent mirrors.
The panels are hinged for stowage in the shuttle and were deployed while Magellan was in Earth orbit. During interplanetary cruise and in orbit around Venus, they rotate to follow the Sun. Solar sensors on the panel tips and a control package in the equipment bus maintain the panels' sunward orientation. The honeycomb aluminum backing structure, arms, and oversized joints are designed to enable the panels to withstand the force produced by the rocket burn that will insert Magellan into Venus orbit.
The propulsion equipment shown in Figure 4-4 includes a 24-thruster liquid-propulsion module and the solid-rocket motor (SRM) used for orbit insertion. The propulsion-module structure provides precisely aligned attachment of the SRM, as well as the liquid-propellant thrusters and associated plumbing, which are needed for trajectory/orbit corrections, attitude control during orbit insertion, and other functions.
The propulsion module also provides the attachment points for the IUS adapter structure. Both structures are made of graphite-epoxy trusses with sculptured titanium end fittings. Explosive bolts released the adapter, along with the IUS, after IUS burnout.
The spacecraft equipment can be grouped into several functional subsets: the radar sensor and altimeter antenna; telecommunications, including radio equipment and antennas (except the altimeter antenna); spacecraft attitude control and solar-panel articulation control; electrical power; propulsion and pyrotechnic control; thermal control; and structure and mechanisms. The radar equipment and functions are described in Chapter 5. The remaining subsets, which comprise the engineering subsystems, are described below.
Acquiring detailed knowledge of Venus' surface depends as much on Magellan's ability to send large amounts of data to Earth as it does on the radar equipment itself. Most of the communications components for sending, receiving, and decoding radio signals are located in the FEM. These components include a redundant set of receivers, command detectors, transmitters, data encoders, data modulators, exciters, control units, and switches used to interconnect them in various combinations with each other and with the externally mounted communication antennas. A receiver, command detector, exciter, and low-power amplifier are packaged together into an assembly called a NASA-standard transponder, of which there are two on Magellan. In addition to the low-power amplifiers in the transponders, there are two high-power amplifiers called traveling-wave tubes. The NASA-standard transponder and traveling-wave tube assemblies enable Magellan to transmit at a peak rate of 268.8 kilobits per second. In comparison, the Viking Orbiter in 1976 transmitted its detailed images of Mars at 16 kilobits per second.
Magellan uses two different transmitters that operate on the S and X frequency bands for communications with Earth. S-band, with a frequency 2,000 times higher than the AM radio broadcast band, is used for transmitting engineering data to Earth and for most command transmissions from Earth to the spacecraft. The high S-band frequency enables the HGA to concentrate the spacecraft's low-power (5-watt) engineering signals so that they can be detected on Earth from distances up to 257 million kilometers (160 million miles). X-band transmission, at a frequency rate almost four times greater than that of S-band, enables the HGA to transmit an even more concentrated signal to Earth. The use of X-band and a higher signal power (20 watts) enables the high data rates used for transmitting radar data to Earth.
From Venus orbit, this system will send engineering data about the spacecraft's condition to Earth at 1.2 kilobits per second through the HGA via S-band and simultaneously transmit the radar data at 268.8 kilobits per second via X-band. Backup data rates of 40 bits per second for engineering telemetry and 115.2 kilobits per second for radar data are available for certain circumstances or emergencies.
Working with the radio transponder, modulators, and amplifiers to make up a complete telecommunications subsystem are the high-, medium-, and low-gain communication antennas.
The 3.7-meter- (12-foot-) diameter HGA is critical to all aspects of the mission. It transmits and receives the mapping radar pulses, collects radiant energy emitted by Venus (for the radiometry experiment), sends science and engineering data to Earth, and receives commands from Earth that direct spacecraft activities. The HGA has a total beamwidth of 2.2 degrees at S-band and 0.6 degree at X-band.
The MGA is used primarily for sending commands to and receiving engineering data from Magellan during the Venus orbit-insertion (VOI) maneuver and during portions of the 15-month cruise period. Because its 18-degree beamwidth provides telemetry capability without the precise pointing required by the narrow-beam HGA, the MGA is also used for emergency situations when spacecraft pointing may not be correct.
The LGA, mounted on the top of the HGA, is placed so that no part of the spacecraft can interfere with its broad beam. Its design allows commands to be received from any direction within 90 degrees of its central axis. This hemispherical coverage pattern greatly reduces the need to precisely point the spacecraft during an emergency. An example would be a solar flare that produces energetic particles strong enough to alter the computer program used by the attitude-control subsystem. This type of anomaly would be sensed by the spacecraft, recognized as having a potential effect on pointing the HGA, and cause the spacecraft to begin corrective actions, including switching to the LGA for command reception from Earth. Thus, ground controllers would be able to augment the spacecraftÕs corrective actions, if required.
Magellan is a three-axis-stabilized craft, but it is required to perform frequent changes of its orientation in space as it orbits Venus. Keeping track of its precise orientation at all times via gyroscopes, this maneuvering is performed with reaction wheels controlled by one of two ATAC-16 computers located in the equipment bus.
During each orbit of Venus, Magellan will rotate four times: away from the planet to aim the HGA earthward for data transmission, toward space to scan stars for precisely determining any spacecraft-orientation errors, again toward Earth to resume data transmission, and back toward the surface of Venus for mapping. Throughout each elliptical orbit's mapping pass, the spacecraft continuously maneuvers in small increments to adjust the pointing of the HGA as the distance to the surface of the planet changes.
Throughout the mapping phase of the mission, there are 1,852 orbits requiring 7,408 major attitude changes in 243 days. If these attitude changes were performed solely with rocket thrusters, there would be more than 14,800 thruster burns for each of the spacecraft's three control axes (one to start a maneuver, another to stop it, and periodic thruster burns to control the rate). Indeed, Magellan would need immense fuel tanks.
However, Magellan is miserly with the fuel in its single, small, propellant tank. The repetitive attitude changes are instead accomplished with reaction wheels that use the principle known as Newton's Third Law: for every action there is an equal and opposite reaction. An illustration of this is a child jumping from a wagon. If the wagon is initially at rest, and the child jumps out the back of the wagon, the wagon moves forward as the child moves backward.
When it is desired to turn Magellan in a
particular direction, an electric motor inside the spacecraft is commanded
to spin a reaction wheel (a rotatable mass approximately 36 centimeters
[14 inches] in diameter) in the opposite direction. By Newton's Third
Law, the spacecraft turns in the intended direction while the reaction
wheel spins in the opposite direction. The spacecraft turn is stopped
by commanding the motor driving the reaction wheel to stop. Three reaction
wheels, one for each possible axis of rotation, are located in the FEM.
Projecting from one side
of the FEM is the barrel of the star scanner (see Figure 4-3). This highly
accurate attitude-sensing device is used to periodically correct errors
due to drift of the gyroscopes. Once a day during cruise and once an
orbit during mapping, the spacecraft performs a star scan. This involves
turning the spacecraft to a preset starting position, after which a single
rotation is used to sweep the optical star scanner across two known reference
stars that are 80 to 100 degrees apart. The stars' apparent positions
are calculated by the attitude-control computer using the gyroscope inputs,
and these positions are compared with the stars' true positions, which
are contained in data previously stored in the computer. The difference
represents how much the gyroscope drift has affected the computer's knowledge
of its own attitude since the last star scan. This error has typically
been less than 0.l degree per day during the cruise period. The computer
autonomously updates its attitude, as well as its own drift-compensation
model, based on the determined error.
Rounding out the complement of attitude-control
hardware are sun sensors and solar-array drive motors, which keep the
solar panels pointed toward the Sun. The sun sensors are located on the
outboard tips of the solar panels and feed information to the computer
about the current position of the Sun. The computer responds by commanding
the solar-array drive motors until the solar panels are as close to pointing
at the Sun as possible. Because the panels can be rotated around only
a single axis while the spacecraft can rotate about three axes, it is
not always possible to point the panels directly at the Sun. However,
most of the time the spacecraft attitude is programmed to keep the solar
panels' rotational axis perpendicular to the sunline, which allows the
panels to point exactly at the Sun.
Magellan operates
on 28 volts fed through a power-conditioning unit in the FEM. The power
source is the solar arrays, a pair of nickel-cadmium batteries, or the
solar arrays and batteries used simultaneously. Either battery could
support the mission with only a moderate amount of data loss should the
other fail.
The solar panels directly supply all power required by the
spacecraft during cruise and the data-transmission periods during mapping
operations in Venus orbit; this includes recharging the batteries. The
batteries augment solar-panel power during mapping, when the radar is
drawing maximum power. When Venus occults the Sun from the spacecraft,
the batteries supply the entire spacecraft power load.
The command and data subsystem (CDS) decodes, stores, and distributes
commands received from Earth to control spacecraft activities. These
include commands to the attitude-control subsystem that regulate the position
of Magellan and its back-and-forth changes between data gathering and
transmitting. Other commands control radar-operating parameters and sequence
other spacecraft subsystems through their operational states, as required.
Most commands for controlling the spacecraft are stored for later distribution.
The CDS can execute commands immediately upon receipt, however, if that
is required.
The CDS' second function is to gather the engineering and
radar data, format it for readability on Earth, pass it to the telecommunications
subsystem for immediate transmission in the case of engineering data,
or pass it to a tape recorder for storage and later transmission in the
case of radar data. Engineering data can also be stored on tape if there
is no communication with Earth at the time the data are gathered.
Redundant
tape recorders, called the Data Management Subsystem (DMS), each provide
storage for 1.8 gigabits of data. During the orbital mapping phase, the
DMS is used almost entirely for radar data, but some spacecraft engineering
data are stored there also. In addition to tape storage, the CDS bulk
memory is used to hold 5 kilobytes of sampled engineering data when real-time
transmission of telemetry from Magellan is interrupted, for example, when
the spacecraft is behind Venus. These sampled data are read out to Earth
immediately after the data interruption is over.
The 24 multipurpose
liquid-propellant (hydrazine) thrusters provide several functions: spacecraft
attitude control, trajectory/orbit correction, and reaction-wheel desaturations.
Positioned in the middle of the 10-sided equipment bus is the single
propellant tank that, at launch, contained 132.5 kilograms (293 pounds)
of monopropellant hydrazine. A helium tank is attached to the struts
of the propulsion-module structure and will be used, if necessary, to
offset a drop in the pressure of the hydrazine system, a drop that would
reduce thruster output level. The helium pressurant will be used if Magellan's
interplanetary trajectory requires a major corrective firing of the thrusters,
which, in turn, would drop the system pressure.
At each of the four outboard
tips of the propulsion structure is a group of six thrusters: two of 100-pound,
one of 5-pound, and three of 0.2-pound thrust. The large 100-pound thrusters,
aimed aft, are used for large midflight course corrections, large orbit-trimming
corrections, and controlling the spacecraft while the SRM burns during
VOI. The 5-pound thrusters, aligned perpendicularly to Magellan's centerline,
keep the spacecraft from rolling during those same maneuvers.
For the
duration of the interplanetary cruise and mapping phase, the tiny 0.2-pound
thrusters provide thrusts to desaturate the reaction wheels; they can
be used for attitude control, if required. Eight 0.2-pound thrusters
point aft and four are positioned for roll control. The aft-facing thrusters
are also used for small course corrections and orbit trims.
The SRM used
for orbit insertion at Venus is the Star 48B, the same motor used to send
commercial communications satellites into geosynchronous orbit around
Earth. The "B" denotes a motor using a carbon-phenolic nozzle, rather
than the newer carbon-carbon nozzle. The motor weighs 2,146 kilograms
(4,731 pounds), of which 2,014 kilograms (4,440 pounds) is propellant.
The
motor's thrust will reduce Magellan's speed for transfer from the spacecraft's
interplanetary trajectory into an orbit around Venus. The motor is aligned
with the spacecraft's center of gravity to within 0.25 centimeter (0.1
inch) to provide sufficient balance of mass during the SRM burn to allow
the 100-pound thrusters to maintain stability and prevent the spacecraft
from tumbling.
Attached to the underside of one equipment
bus compartment is a box containing the control electronics that arm,
disarm, and fire detonators to activate various explosive bolts, pin-pullers,
and other devices. These enable release of the solar panels from their
stowed position after deployment from the shuttle, actuation of propulsion
valves, ignition of the SRM, and separation of the spent SRM after orbit
insertion.
Magellan will be subjected to sunlight approximately
twice as intense as that which reaches Earth, potentially for several
years. On the other hand, shaded exterior spacecraft temperatures can
plunge to -204 degrees centigrade (-400 degrees Fahrenheit). Throughout
the mission, the constant maneuvering of the craft will subject nearly
every exterior surface to those ranges of heat and cold. Special effort
was required in thermal control to keep electronics from overheating and
moving parts from freezing, while minimizing weight and the need for electrical-heater
power.
Electronics housings are wrapped in multilayered thermal blankets
(see Figure 4-5) that insulate and reflect light. The outer layer of
all external blankets is a material called astroquartz. It is similar
to glass-fiber cloth, but is better able to withstand intense solar radiation.
In fact, chemical binders normally used in astroquartz to control flaking
had to be baked out when tests showed that the light intensity at Venus
could discolor them and eventually cause a buildup of heat.
The HGA, ALTA,
MGA, LGA struts, and propulsion-module structure are painted with a special,
inorganic water-based paint, developed at NASA's Goddard Space Flight
Center, to withstand and reflect intense solar radiation while minimizing
discoloration. Electronics compartments in the FEM and the equipment
bus have louvers that open or close automatically to regulate the dissipation
of heat from inside the spacecraft. Covering these openings, and also
lined up in strips on the solar arrays, are thin mirrors to reflect sunlight.
The mirrors have been etched to diffuse reflections that could bake some
other exterior part of the spacecraft.
The net effect of these materials
makes the craft tend toward cold temperatures rather than hot. To assure
that some cold-sensitive components do not become too cold, flexible electrical
heaters have been installed inside housings or wrapped around such fixtures
as the solar-panel articulation bearings.
Structural
design is crucial to an efficient spacecraft design, and Magellan was
no exception in this regard. Structures must be strong enough to withstand
the g forces of launch and orbit insertion and to withstand the tremendous
acoustic environment that exists in the shuttle bay during launch. Critical
alignment among optical sensors, inertial sensors, and antenna boresights
cannot be allowed to suffer significant changes from deformations caused
by these environments. All of this has been accomplished on Magellan,
without exceeding weight limits imposed by the shuttle or the IUS performance
limits, through the efficient use of various structural materials, including
aluminum, titanium, beryllium, aluminum honeycomb, and graphite-epoxy
composites.
Mechanisms used on Magellan include the hinges used to fold
the solar panels in the shuttle bay for launch, retention and release
devices used to hold the solar panels during launch, and the solar array
articulation joints, including the cable-wrap assemblies, that allow the
solar panels to rotate nearly 360 degrees without the use of slip rings.
Also used on Magellan are bolts and explosive-actuated release nuts used
to hold the spacecraft to the IUS adapter and to hold the SRM to the spacecraft
until separation. At separation, such devices as springs and guide pins
ensure enough distance between the separated bodies to prevent subsequent
collision.
Magellan's brains are two ATAC-16 computers
located in the attitude-control subsystem and four 1802 microprocessors
in the distributed CDS. All computers are in a redundant configuration
as insurance against a breakdown, are fully reprogrammable, and are modified
equipment from the Galileo Project.
Magellan is programmed to do a lot
of its own "thinking" if problems arise. Past space missions often produced
huddled experts exploring ways of working around a malfunctioning spacecraft.
Magellan takes advantage of advances in fault-detection software design
to analyze problems that occur and carry out a series of alternative remedies.
Minor
or slowly developing problems revealed by telemetry will be managed by
ground personnel. However, time-critical or mission-critical malfunctions
will be detected, analyzed, and dealt with by two onboard fault-protection
software systems: one for attitude control and the other for the rest
of the spacecraft.
Problems with the attitude control are treated "holistically"
in a full-system health analysis to ascertain the integral cause and remedy.
Other spacecraft malfunctions are managed on an individual basis by software
in the CDS.
Although some of the spacecraft attitude-control software
was inherited from the Galileo mission, most is new because of differences
between the control systems and the missions. Ninety percent of the 6,000
lines of code in the attitude-control software is new, including 2,000
lines for fault protection. Of the 18,000 lines of code for the CDS,
45 percent is unmodified Galileo code, 20 percent is new, and 35 percent
is modified Galileo code. The fault-protection software resident in the
CDS totals 1,500 lines.
Spacecraft operation is controlled for several
days at a time by commands sent from Earth and stored in the CDS. During
mapping, this method requires accurate navigational data that are updated
as frequently as three times a week.
Control of the radar system is performed
with data generated by the Radar Mapping Sequencing Software (RMSS) located
on Earth. Almost all significant command sequences are stored in a simplified
form on mission-control computers at JPL. For most command sequences,
engineers simply select from that set and add parameters. Ground computers
then convert the sequences into the Magellan command bit patterns for
transmission to the spacecraft.
During the interplanetary cruise, these
commands will span up to three weeks of activity. During mapping, up
to eight days of commands are sent at a time. An extra day is included
in every upload to provide a safety buffer if there is a delay in commanding
the next upload.
As you can see, the Magellan spacecraft is a marvel of
high technology. Its single payload, a radar sensor, and the synthetic-aperture
method of radar mapping are both products of equally sophisticated technologies,
as we shall see in Chapter 5.
Did you know...
After orbiting Venus for more than a dozen years, there will be
sufficient onboard propellant only to desaturate the reaction wheels.
There will not be enough propellant to prevent the orbit from decaying,
which will result in Magellan eventually entering the atmosphere of
Venus and burning up.
Electrical Power
Command and Data Handling
Propulsion
Pyrotechnic Control
Thermal Control
Structure and Mechanisms
Computing and Software
Chapter 5 - The Radar System
The Magellan's Venus Explorer Guide
