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Aviation History
1969
1969 - 0481.PDF
FLIGHT International, 13 March (969 The main features of Mariners 6 and 7 are shown in these two views of the spacecraft. In the upper illustration Mariner is oriented in very nearly the attitude in which it would appear from Earth, with the low-gain antenna pointing towards the Sun. The lower illustration shows the underside of the spacecraft with the experiments mounted on the two-axis scan platform. The thermal blanket has been removed for clarity ing to 449W at Mars owing to diminishing solar intensity. The maximum power demand will be 388W at encounter. A rechargable silver-zinc battery provides spacecraft power during the launch, mid-course manoeuvre or whenever the solar panels are turned away from the Sun. It is kept fully charged and provides an emergency power source at planet encounter. Mariner is designed to operate throughout its flight without the need for ground signals, with the single exception of the trajectory-correction manoeuvre. This is made possible by the command function of the central computer and sequencer, which performs the timing, sequencing and computations for other systems aboard the spacecraft, and initiates events during launch, cruise, mid-course manoeuvre and encounter. The timing and sequencing pi events are programmed before launch but can be updated during flight if necessary. These events include, for example, switching the telemetry transmissions between low arid high bit rates, switching the transmitter between the two antennae, and resetting the Canopus sensor throughout the flight to take account of changing oriemtationi. Two-way communication between Earth and spacecraft is provided by two transmitters (each operating at either 10W or 20W) and a single receiver. The latter receives uplink commands and ranging signals on about 2115MHz from Earth stations of the deep space network. Discrimination between the two Mariners is achieved by employing two slightly different frequencies. Only one transmitter on each spacecraft operates at a given time; if a failure in this channel occurs a failure- detection logic circuit switches to the standby transmitter. Again, the four transmitters in the two spacecraft all operate on slightly different frequencies. All communication between the spacecraft and Earth is in digital form and three different types of command may be transmitted to the spacecraft. A direct command results in the closure of a switch, and 53 such commands back-up all the critical spacecraft functions. A coded command provides data to the computer and sequencer for the mid-course burn, and to update the flight programme. Finally, a quantitative com mand is used to rotate the scanning platform (on which are mounted the experiment sensors) by given increments. Information can be transmitted to Earth at five different rates: 8| and 33-J- bit/sec for telemetry; 66f and 270 bits/sec for the science data during, respectively, encounter and data storage playback; and 16,200 bits/sec for the high^rate science data (this rate will be used only in conjunction with the 210ft antenna at Goldstone). The significance of this will be appre- THE HUMAN FOIBLES which prevented the scheduled launch of Apollo 9 on February 28 from taking place (see Flight last week, page 384) cleared up in time for the March 3 option, and the most complex-yet American space enterprise got underway on that day at 1600GMT (1100 local time) at Cape Kennedy. Apollo 9, containing for the first time on any flight all the elements for the lunar landing, was injected into a circular initial orbit of 118 miles for its planned flight of nine days 22hr 44min. Once in orbit the three astronauts, Col James McDivitt, Col David Scott and Mr Russell Schweickart, went through a check of the spacecraft and its systems. During dated when it is remembered that each of the 21 Mariner 4 pictures, consisting of 240,000 bits, took over 8hr to transmit at 81 bits/sec. With two spacecraft, both taking over 100 pictures, each containing 3.9 million bits, but using the same transmission rate, the time taken to transmit all the pictures would be excessively long. With the data rate increased by a factor of nearly 2,000, the transmission time for a single 1969 picture will be about 4min at the fast rate. These increases in data-rate transmissions have been due primarily to the introduction of the 210ft Goldstone antenna, to improvements in low-nodse maser prei-amplifiers, and better antenna feed design. [To be continued launch a fault had developed in the guidance and navigation computer on the command module which gave incorrect height data. NASA was rather non-committal about the nature of this fault and its possible effects. The computer and its equip ment (indeed, all the systems on the CM) have been well- proven on the two previous manned flights, but it seemed for a time that Apollo 9, which had been planned specially to qualify the CM, might have to be abandoned. Fortunately the fault was soon cleared to the satisfaction of both NASA and the astronauts. Another fault concerned a pressure gauge in the propulsion system; this too was quickly cleared. APOLLO 9 VERIFIES THE LM
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