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.
The midcourse propulsion system provides trajectory correction for close approach to Venus.