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Aviation History
1962
1962 - 1303.PDF
FLIGHT International, 26 July 1962 145 Spacecraft executing a typical trajectory correction command: moving high-gain antenna away from midcourse motor flame; rolling; pitching; receiving velocity increment from midcourse motor; attaining desired Venus trajectory; repositioning antenna; and re-acquiring cruise attitude by locking solar-panels on to Sun and high- gain antenna on to Earth The microwave radiometer is mechanized so that it can scan Venus during the fly-by. Initially, it will have a fast scan search. When it detects the planet, the radiometer will adopt a slow-scan mode. The infra-red experiment is attached to the rim of the dish- shaped microwave device and will scan with the larger instrument. The Mission The launch vehicle for the Mariner is an Atlas- Agena B. The Atlas and the Agena boosts Mariner to an altitude of 115 miles and an orbital speed of 17,450 m.p.h., after which the craft is placed in its Venus trajectory from a "parking" Earth orbit. During the launch phase, the Mariner spacecraft is protected against aerodynamic heating by a shroud. After Atlas cut-off, approximately 5min after lift-off, the shroud is jettisoned and the Agena B separates from the Atlas. The Agena B then pitches down from an attitude almost 15° above the local horizon to become almost level with the local horizon. In this horizontal attitude the Agena B fires for the first time and bums for almost 24min to reach orbital speed of 17,450 m.p.h. After this burning time, Agena B shuts down and coasts in a parking orbit for more than 13min until it reaches the optimum point in its orbit at which to fire for the second time. This second Agena firing injects the combined Agena/Mariner on an escape trajectory at a velocity of about 25,820 m.p.h. Injection occurs approximately over Ascension Island in the South Atlantic Systems checkout of Agena 8 second stage by Lockheed technicians, with Mariner protective nose shroud being fitted to the stage Ocean and approximately 23 to 34min after launch, depending on time of launch. A little more than 2min after second-burn cut-off or injection, Mariner is separated from Agena by spring-loaded bolts. The Agena then yaws 140° in the local horizontal plane and performs, a retro-manoeuvre which reduces its velocity and changes its trajectory. This is designed to prevent the Agena from impacting Venus, and from following Mariner too closely so that the space craft's optical sensors might mistake reflected sunlight from Agena B for the Sun or Earth. Separation from the Agena will cause the Mariner to begin a tumbling motion, which is corrected by the yaw, pitch and roll gyros acting on the gas-jet stabilization system. Mariner now is on a trajectory that will take it fairly close to Venus. The omnidirectional antenna is working and radiating the radio transmitter's full three watts of power. Before and during launch, the transmitter had been kept at about 1.1 watts. This is required during the period in which the launch vehicle passes through a critical area between 150,000 and 250,000ft, where a tendency exists for devices using high voltages to arc over and damage themselves. The following sequence of events is planned for Mariner:— The first command is issued by the CC&S 44min after launch. Explosive pin-pullers holding the solar panels and the radiometer in their launch position are detonated to allow the spring-loaded solar panels to open and assume their cruise position, and to unlock the radiometer. At launch plus 60min, the CC&S turns on the attitude control system and the Sun acquisition mode will begin. The Sun sensors, linked to the valves controlling the gas jets, orient the spacecraft so that its long axis is pointed at the Sun, thus aligning the solar panels with the Sun. Both the gyros and the Sun sensors can activate the gas jet valves, and a back-up radio command capability is provided to initiate the CC&S function and Sun acquisition. In order to conserve gas, the attitude control system permits a pointing error towards the Sun of 1°, or 0.5° to each side. The mixing network in the attitude control system is calibrated to keep Mariner slowly swinging through this 1° of arc pointed at the Sun. The swing takes approximately 60min. It is calculated that the gas jets will fire for one-fiftieth of a second each 60min to keep the spacecraft's solar panels pointed at the Sun. When the Sun has been acquired, the gyros are turned off to conserve their life and to lower the power demanded of the solar panels. The Sun acquisition process is expected to take less than 30min. When it is completed, the secondary Sun sensors on the backs of the solar panels are turned off to avoid confusion caused by light from the Earth. (To be concluded)
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