The Mariner IV spacecraft performed the first close fly-by of the planet Mars on July 14, 1965. Approaching within 6,118 miles of the planet's surface, the spacecraft obtained the first ever close-up photographs of the Martian landscape, as well as collecting a great deal of information about the planet's atmosphere, magnetic field, and radiation fields.
The design of the spacecraft itself contributed greatly to the mission's success. An overview of the spacecraft's subsystems is presented below.
Structure
The main body of the spacecraft consisted of an octagonally shaped structure in which the majority of the instruments, computers, and avionics packages were housed. The skeletal structure itself was nearly eliminated to reduce the weight. Instead, the packages themselves were used for the structural foundation.
Four solar panels, totaling 70 square feet, were arranged around the structure. Each of these panels weighed 18.7 pounds. A fixed, elliptical dish high gain antenna was attached to the sun-facing side of the spacecraft, and a low gain omnidirectional antenna was attached to the end of a boom which contained the magnetometer and other instruments on the opposite side. The total spacecraft weight was 575 pounds.
Power
The spacecraft's primary power was provided by 28,224 photovoltaic solar cells on the four solar panels. The nominal power provided by these solar panels was, at maximum power voltage: 310 Watts at Mars and 610 Watts at Earth. A silver-zinc battery was also available for power when the solar panels were not facing the sun.
Propulsion
For the mid-course corrections, a single rocket motor, burning a liquid monopropellant (anhydrous hydrazine), provided 50 pounds of thrust. (During the actual mission, only one midcourse correction was required.) In addition to this, 12 nitrogen gas jets were used for attitude control. These jets released nitrogen from tanks pressurized at 2470 psi.
Attitude Control
The spacecraft maintained an attitude in which the high gain antenna continually faced the sun. When properly aligned, the beam of the antenna was adequately aimed at Earth for most of the mission. Attitude control was maintained by three gyroscopes, a Canopus star sensor, a primary sun sensor on the sun side of the spacecraft, and a secondary sun sensor on the shade side. The attitude of the spacecraft was adjusted primarily by the nitrogen gas jets.
In addition to this, an auxiliary attitude control system was used in which vanes positioned at the ends of the solar panels corrected for solar pressure imbalance. Each of these vanes consisted of seven square feet of aluminized plastic film made of Mylar. The sun's electromagnetic radiation exerted a force of approximately a millionth of a pound on each vane, allowing the vanes to balance the attitude of the spacecraft on the sun.
Since the nitrogen gas jets were not sufficient to control the attitude of the spacecraft during the midcourse corrections, deflecting vanes were positioned in the rocket engine's exhaust stream. These movable vanes allowed the attitude of the spacecraft to be stabilized during the midcourse correction burns.
Telemetry
Data was sent back to Earth by means of two 10 Watt transmitters: a triode cavity amplifier and a travelling wave tube. The data was transmitted by alternately sending 280 bits of data from scientific instruments and then 140 bits from 90 different points that registered the condition of the spacecraft. During the early part of the flight, data was sent at 33 1/3 bits per second, and was later switched to 8 1/3 bits per second.
Thermal Control
Thermal control of the main body of the spacecraft was maintained by thermal shields. In order to minimize heating by direct sunlight, the sun-facing side of the octagon was insulated by a shield composed of 30 layers of aluminized Mylar mounted to the high gain antenna support structure. This shield was covered with black dacron on top and Teflon underneath. The other side of the octagon, facing away from the sun, was covered by a similar blanket to retain the internal heat generated. In this way, the temperature inside the spacecraft was maintained between 55ûF and 85ûF.
The proper temperature in the electronics compartments was maintained by polished aluminum shutters which were activated by coiled bimetallic strips that acted as thermostats. In this way, the shutters could be opened to expose radiating surfaces underneath or closed to conserve heat.
Outside of the protected areas, temperatures were maintained using paint patterns and polished metal surfaces for passive thermal control. The backs of the solar panels were blackened to re-radiate absorbed heat and keep the solar cells within their operating range of 10 to 130ûF. Also, the high gain antenna dish was painted green to keep it at nearly room temperature during the Mars encounter, but within its upper thermal limit when closer to the sun.
References
Mechanical Engineering, Vol. 87, February 1965.
NASA Facts, Document No. NF-39/2-68.
Report From Mars: Mariner IV 1964-1965, Jet Propulsion Laboratory Publication.
Scientific American, Vol. 214, March 1966.
Sunday, 01-Aug-2004 00:36:47 CDT
CSR/TSGC Team Web
