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Aviation History
1978
1978 - 0970.PDF
1806 RIGHT International, 10 June J978 Nasa plans Skylab re-activation NASA is preparing to re-activate Skylab's attitude-control system in an attempt to prevent the space station from re-entering the Earth's atmos phere and showering debris over populated areas. This is the latest in a series of operations designed to extend Skylab's orbital life beyond the date when the Shuttle Orbiter can carry a Teleoperator Retrieval System (TRS) to the space station. The TRS, a remotely controlled propulsion sys tem being developed by Martin Marietta, could be used to boost the space station to a higher orbit or return it through the atmosphere on a trajectory calculated to impact an unpopulated area. With a length of 30m and a weight of 85 tons, Skylab poses a not inconsiderable threat to urban areas, and large sections of the station are expected to survive the heat of re-entry. Skylab was launched into a circular 442km orbit inclined 50° to the equator on May 14, 1973, by the last of 13 Saturn V launch vehicles. The station was manned during three missions by teams of three Apollo astronauts, and when the last crew returned to Earth on February 8, 1974, Skylab was in a 440km-high orbit. Since that time, sunspot activity has increased the density of the upper atmosphere, and for the past 18 months Nasa has been studying the repercussions in an attempt to predict accurately the orbital decay date, E-SYSTEMS' Garland (Texas) Division has shipped two of its improved 11m- diameter earth-station systems to Uru guay and Surinam in South America. The stations, part of a new generation of small-diameter Intelsat (B) earth stations, will initially link North and South America via satellite to the World Cup soccer games being held in Argentina this month. They can however be upgraded for use with orb iting satellites of the Intelsat sys tem. "These are just two more examples of our shrinking world and the rapid expansion of satellite communications among emerging nations," notes John Dixon, chairman and president of E- Systems. One earth station was ship ped to Montevideo under contract to the Government of Uruguay. The second was shipped in April to Para maribo, under contract to the Govern ment of Surinam. "Satellite communications techno logy has reached the point where these advancements can now be eco nomically introduced into the more sparsely populated and dispersed re gions of the world, such as parts of Africa, South America, Canada and Alaska and many island nations," says originally believed to be 1983 but now thought to lie in a 12-month period beginning a year from now. Nasa believes that it can save Skylab by carrying a TRS up to the space station, but first the station must be placed in an attitude which will reduce atmos pheric drag and possibly add some months to its orbital life. Any such manoeuvre would require re-activation of Skylab's onboard systems, which were switched off on February 9, 1974, a day after the last crew returned. The first operation in the planned re-activation sequence was performed on March 6, when engineers at the Bermuda tracking station sent a series of commands to activate the radio receiver and speaker systems. It was hoped that if the tele metry system had not deteriorated during its five years in space it would automatically send information on temperatures, pressures, angular orientation in orbit and remaining gas in the thruster control system. The first command brought a response for 2min, followed one orbit later by a carrier signal with no data. Flight con trollers then successfully used a back-up communications system. Other commands were sent to recharge Skylab's batteries with energy collected by the solar panels. Finally, the onboard flight computer was checked out on March 13 and found to be working. Having estab lished the feasibility of Skylab systems Dixon. "E-Systems mini earth stations, used in conjunction with the techno logical improvements achieved with Intelsat and other satellite systems— such as greater efficiency in signal- beaming and power output and in creased orbital accuracy—can ad vance worldwide communications sig nificantly," he emphasises. The new earth station can receive and send voice, data and video signals via satellite. It is designed for rela tively quick, easy erection in remote locations, and installation cost is claimed to be 10 per cent that of lar ger antennas. The system is well suited to domestic applications, in which a series of these small anten nas, interconnected for switching pur poses, could link together a number of smaller, widely dispersed towns. This would be much less costly than linking remote cities with telephone lines or with a series of microwave relay towers or signal /'boosters". E-Systems has already installed more than 40 earlier versions of 10m and llm-diameter antennas for satel lite communications systems in Al geria, Canada and other nations, as well as 36 larger Intelsat (A) earth- station antennas. re-activation, Nasa issued two nine- month contracts, each valued at $125,000, to Martin Marietta and McDonnell Douglas Astronautics for studies on the possibility of using the space station for engineering analysis of lengthy exposure to the space environment. From a makeshift control room at the Johnson Space Centre, formerly used to direct operations with Apollo Lunar Surface Experiments Packages (Alseps) left on the Moon, engineers in late April activated Skylab's coolant loops, batteries in the airlock module and the telescope mount and the tele scope mount's telemetry system. This provided valuable information from which the next sequences could be planned. Re-activation of the Thruster Attitude Control Subsystem (Tacs) and the Control Moment Gyroscope (CMG) stabilisation system is important because this is the only method by which engineers can suc cessfully delay premature reentry until a Shuttle is launched with the Teleoperator Retrieval System. Tacs comprises two modules of three thrusters each, located at the rear of the space station, and was used during manned operations for major attitude changes. The CMGs are a group of three 181kg gyroscopes designed to spin at 9,000 r.p.m., using the reactive force of a torque motor to tilt Skylab in the desired direction. These are the only systems capable of manoeuvring the station. One of the three CMG wheels failed early in the last manned mission and a second behaved erratically, threatening the continued stability of the station. In a sequence of commands now being prepared at the Johnson Space Centre, engineers hope to use the Tacs to place Skylab's longitudinal axis parallel with the earth's surface. The two remaining CMGs will then, it is hoped, maintain that attitude and so reduce drag. After the last crew returned from Skylab, the internal pressure was bled to 140gm/cm2; within two months it had evacuated to a vacuum. Sufficient oxygen and nitrogen, used to pressurise Skylab, remained for a further 200 days of manned opera tions, and there was sufficient water for 90 days. Nasa has no intention of using these "consumables" for future manned activity, which, if performed, would be limited to inspection by space-suited crew members of a Shuttle mission. If the re-activation sequence is performed as planned, a decision on Skylab's fate will be made within the next few months. Nasa would prefer to use a Shuttle-launched TRS to raise Skylab's altitude and so preserve it for inspection on a later flight. The TRS boost mission could come as early as October 1979. World Cup soccer via satellite
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