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On Mars: Exploration of the Red Planet. 1958-1978

[244] Jim Martin and his colleagues hoped the lander mission would follow the ideal schedule: Final prelaunch activities begin 56 days before launch with the terminal sterilization of the entire lander system within its bioshield. The craft must survive a 40-hour sterilization cycle, during which temperatures will reach a maximum of 112°C. During this preparation period, the lander is functionally passive except for its two mass-spectrometer ion pumps. Following a checkout, the propellants, pressurants, and flight software are loaded, and the lander is mated with the orbiter. After the first prelaunch checkout, initiated by the orbiter under local control of the guidance computer, the spacecraft is encapsulated, transported to the launch pad, and mated with the launch vehicle, followed by the second and final prelaunch checkout. All major communication with the lander before separation is accomplished through the orbiter communications link.
During the launch and boost phases, only the power and pyro controllers, the data acquisition system, and the tape recorder are active. After the spacecraft separates from the launch vehicle, the orbiter commands the lander computer to separate the bioshield cap and begins the lander cruise state. During cruise, the lander is largely passive. Only the data acquisition system, ion pumps, and thermostatically controlled heaters on propulsion equipment, the biology instrument, and the inertial reference unit are powered. The heaters prevent the freezing of propellants and biology nutrients. Heat also controls viscosity of the gyro flotation fluids. The primary [245] housekeeping chore during the cruise phase is monitoring the thermal balance and the equipment when it is powered.
The tape recorder is activated about every 15 days to ensure its later performance. An update to the computer requires the activation of the computer and the command detectors and decoders. The portion of the computer memory used during prelaunch checkout procedures is modified during the cruise so that it can perform other operations during the mission. The gas chromatograph-mass spectrometer requires a venting-and-bakeout sequence to rid the analyzer section of absorbed gases. For bakeout, with its high peak-power demand, the lander batteries are first conditioned and charged using orbiter power; the computer, detectors, and decoders are powered up; and a six-hour bakeout sequence is commanded from Earth, followed by a week-long cooldown period to reestablish the proper thermal equilibrium. About five such cycles in two groups are required, each accompanied by mass-spectrometer readings, which are analyzed to determine the performance and health of the instrument. After each activity, the lander is powered back to cruise state and, after the final bakeout of the gas chromatograph-mass spectrometer, a cruise check is made and the batteries discharged. About 52 days before reaching Mars, the final conditioning and charge cycle is undertaken for the lander batteries.
Before the lander separates from the orbiter, a four-and-one-half-hour checkout verifies the lander systems' health. A group of orbit commands precedes this last check, during which local control is assumed by the lander computer and power is transferred from the orbiter to the lander. At checkout completion, the computer memory is read out, the batteries are recharged on internal power, and the computer reverts to standby. After cruise checkout, power is transferred back to the orbiter, which assumes control. The next events prepare the lander for its release. For eight hours, the radioisotope thermoelectric generators recharge the lander's batteries.
Twelve hours before separation-318 days into the mission-an orbiter commander turns on lander command detectors and decoders, placing the lander under the control of its own computer. Mission control commands update descent information and carry out checkout decisions made by the operations team. The commands are directed to the lander via its S-band receivers. A memory readout follows update, and the lander assumes a standby mode. This sequence is repeated three and one-half hours before separation. About two and one-half hours before separation, direct orbiter command starts the separation sequence. Final preparations begin with warming up the inertial reference unit to its operating temperature. At 37 minutes before separation, a final "go" is uplinked from Earth and received by the lander 15 minutes before separation. At this point, valve-drive amplifiers, pyrotechnic controllers, entry thermal control, and relay communications link are activated. A final check verifies that the inertial reference unit has transferred to the entry condition and that all systems are go. If these checks fail, the lander is powered down and transferred to the update mode. If the checks pass, the telemetry system is.... 

[246] Lander details

....switched to an entry mode, the bioshield base connectors between orbiter and lander are separated, and the lander-orbiter separation pyrotechnic devices are fired.
Immediately after separation, attitude control-deorbit propulsion is readied by opening the isolation valves. After inertial reference unit calibration, attitude control is initiated by orienting for the deorbit burn. The burn is delayed until the lander capsule is far enough away from the orbiter that the orbiter's solar panels will not be damaged or contaminated. The pitch yaw engines supply the deorbit impulse with a 23-minute burn. The control system ensures that the lander is in the proper position for the entry science experiments to function. The retarding-potential analyzer and the upper-atmosphere mass spectrometer collect data during the three-hour descent.
Entry and Landing
After orienting the lander in preparation for entry into the Martian atmosphere, the control system turns on the radar altimeter, which assumes the high-altitude search mode. On sensing 0.05 g with the longitudinal accelerometer, the attitude control system is adjusted, and the computer begins radar-altimeter data processing. Aerodynamic forces quickly trim the entry vehicle to about a - 11° angle of attack, corresponding to the lander's....

[247] Landing capsule system



[248]....offset center of gravity. Instruments collect additional entry science data for pressure and temperature during the remainder of the deceleration period.
At 5.5 kilometers above Mars, the computer begins parachute deployment based on radar range to the surface. Terminal-propulsion valve-drive amplifiers power tip, the aeroshell separates from the lander, and the terminal-roll-propulsion isolation valves open within about seven seconds after parachute deployment. Radar-altimeter changes occur with separation of the aeroshell, and a lander body-mounted antenna switches into use. The four-beam doppler terminal-descent and landing radar is also activated to sense velocity relative to the surface. The lander's legs are deployed from their stowed position.
At about 1.5 kilometers above the surface, the computer initiates another radar-altimeter mode change and shortly thereafter opens the terminal-propulsion isolation valves. The parachute-base cover assembly separates from the lander, and the lander descends toward the surface under three-axis attitude control. The control system and engines halt the horizontal velocity acquired while on the parachute by tilting the entire lander upwind. At the same time, residual vertical velocity is stopped. On sensing 610 meters to the surface, the radar altimeter switches to low-altitude mode; the low-altitude mode for the terminal-descent and landing radar begins at 100 meters. At about 50 meters, vertical navigation continues inertially, ignoring radar-altimeter data. At 17 meters, the terminal engine-shutdown switches are armed, and a constant velocity descent is initiated to maintain a speed of 1.5 meters per second until landing-leg touchdown. Velocity steering continues. using the terminal-descent and landing radar. On sensing closure of the terminal-engine-shutdown switches, the computer commands shutdown of the terminal propulsion system by closing a pyro-activated isolation valve, backed up by a software timer.
Landed Operations
The landed mission begins with several housekeeping chores, which include shutting down all descent guidance and control equipment except the computer and the inertial reference unit; the latter operates five more minutes to establish the local vertical altitude and the direction of north. This information is used to compute the direction of Earth so the high-gain antenna can be accurately pointed the following day. Protective devices are armed but not yet activated, the telemetry is set to the highest relay data rate mode of 16 kilobits per second, and the first real-time imaging sequence is begun. A multiple readout of about 25 percent of the computer's memory follows.
After deploying the high-gain antenna and the meteorology boom, opening the camera dust-removal valve, and opening the cover to the biology-processor and distribution assembly, all mission pyrotechnic events are completed. A second real-time imaging sequence begins and continues until the orbiter disappears over the horizon. The relay link fades [249] out about 10 to 12 minutes after landing and, at 15 minutes after, the transmitter is shut off. The meteorology instrument and the seismometry instruments are turned on, and the high-gain antenna is stowed to its normal rest position. Finally, the adaptive mission is begun by activating the mission sequence of events.
Before the Viking landers had the opportunity to perform this complex series of events on Mars, managers, scientists, and engineers faced a multitude of problems on Earth.