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Aviation History
1964
1964 - 2180.PDF
FLIGHT /nternotiono/, 30 )uly 1964 beam neutralization. In addition to the usual function of providing in-flight control of the spacecraft, the SERT I command system demonstrated the feasibility of transmitting radio commands to a spacecraft propelled by an ion engine. It is possible that RF generated by arc or plasma oscillations could either saturate the command receiver or cause a false command. However, through the design of the spacecraft receiver and use of a high-wattage transmitter this possibility is considered remote, and did not occur on this occasion. Thrust-detection systems continuously monitored the spacecraft spin rate. Thrust of the contact-ionization engine was expected to cause the spin rate to change about 0.14 per cent per minute, and that of the electron-bombardment engine about 0.67 per cent per minute. Only very low instrumentation accuracy would be necessary it thrust were continuously produced for the whole time (about 30min) alloted to each engine. However, SERT I carried instrumentation of the highest accuracy to assure thrust measurements. Two of the thrust detection systems were solar-cell spin-period detectors. Each used a silicon photocell housed behind a narrow slit. Mounted 180° apart, the slit and cell combination generated one pulse each time the spacecraft revolved and the detector swept past the Sun. For pure spin, the system error in spin-period measurement was expected to be less than 0.1 per cent in the observation of a single revolution. The third thrust-detection system was an accelerometer sensing acceleration due to spin. When the arms deployed, and the pod cover was blown off the Hughes engine, the spacecraft's e.g. shifted and its spin changed. But deploying the engine arms caused a precession which took some time to damp out. The accelerometer provided information on motion around all three axes, to describe any complex motion. Accelerometer output frequency for pure spin with no engine thrusting should have been about 200c/s. With the Hughes engine 197 2000 VOLTS ELECTRIC POWER GENERATOR ELECTRONS TANTALUM NEUTRALIZE! AT SPACECR, POTENTIAL Fig 2 Schematic diagram of Lewis electron-bombardment engine in operation, this accelerometer output should have changed some 0.008c/s/s, or a total of lOc/s over the total time of operation. This is equivalent to a change in accelerometer period of 0.195 micro- seconds every second. With the Lewis engine thrusting, the accelerometer output changed a little more than 0.04c/s/s, or a total of 58c/s over the total time of operation. The change in accelerometer period equivalent to this is 1.12 microseconds every second. The accelerometer subsystem also had an analogue output that, under ideal spin conditions, could give thrust measurements in real time. Sensitivity of the accelerometer, and the precision of the electronic counters at the receiving station, were such that thrust from an engine burst of as little as lOsec could be measured within an accuracy of 0.000051b. AIAA DISCUSSES COMSATS This diagram was prepared by US General Electric to indicate the layout of the TV satellite, based on the defunct Nimbus weather satellite, which the company has pro- posed. Basic particulars of this project are given in the text on the following page Approximately 8,000 members attended the first annual meeting of the American Institute of Aeronautics and Astronautics, held in Washington DC between June 29 and July 2. Among the 350 papers presented were several on communications satellites. The Early Bird satellite of the Communications Satellite Corporation was discussed in our July 23 issue; other comsat papers at the meeting were read by representatives of the National Aeronautics and Space Administration and US General Electric. Leonard Jaffee, director of communications and navigation programmes for NASA's Office of Space Science and Applications, declared that Syncom III, for which no launching date has yet been set, will be put into a near-equatorial orbit by a Delta rocket with thrust augmentation from three solid rockets strapped to the Thor first stage. At least five different flights, involving three different missions, are contemplated. All will use vehicles based on the Syncom design. "The first spacecraft will gather data on gravity gradients for use in satellite orientation in the future," said Mr Jaffee. "The 6501b spacecraft will be injected into a 6,500-mile circular orbit using an Atlas-Agena. It will give us data for a basic understanding of the theory and the perturbations necessary to come up with a design for a gravity-gradient stabilization system for future space vehicles." He continued, "The satellite will carry a TV camera to measure deflections of gravity-gradient booms with solar heating. It will carry a propellant system to adjust the satellite's orbital eccen- tricities and to perturb the satellite's orientation system so that the restoring period can be measured. Different damping mechanisms will be tried, and it is hoped that one launch and one back-up launch will result in enough data to permit extrapolation of the results to higher and lower orbits." The second phase of the programme calls for injecting a spin- stabilized satellite into a stationary orbit at 22,300 miles. An Atlas-Agena, plus an apogee-kick rocket being developed by the Jet Propulsion Laboratory, will be used to bring off the synchronous orbital feat. Syncom IV will be much like Early Bird, except that it will carry only one communications transponder. In addition to the important technology that will be checked out with respect to the spacecraft, the two Syncom IVs to be launched will conduct experiments having to do with meteorology, radiation, navigation, Earth sensoring and electronically and mechanically de-spun aerials.
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