Cluster II spacecraft shake, bake and roll
28 April 1999The road towards final acceptance for a Cluster II spacecraft can be ratherrough. Having completed its assembly at the Friedrichshafen plant ofDornier Satellitensysteme, the first Cluster II flight model (FM6) is nowundergoing a rigorous series of systems and environmental tests at IABG (Industrie Anlagen Betriebsgezellschafte) near Munich. For the next few months, the drum-shaped spacecraft willundergo all sorts of indignities - vibrations, temperature changes, rapidrotation and magnetic monitoring.
The first stage of this endurance trial - the acoustic test - has already been successfully completed. During the last week of March, FM6 was placed inside a special 8 metre square, sound-proof chamber. There it was subjected to a mixture of sound frequencies (from 20 to 8,000 Hz) in order to mimic the shaking and loud rumbling noise generated during a rocket launch.
"No-one is allowed inside during this test," said Hans Bachmann, the Cluster II Assembly, Integration and Verification Manager. "The noise is so loud that anyone in the chamber would be killed instantly."
The next set of tests on FM6 is now nearing completion. These are designed to check the spacecraft's ability to withstand the harsh environment of outer space. Over a period of nine days (20-28 April), FM6 is placed inside an airless vacuum chamber while its temperature is varied from 2:C to 27:C.
"Although we use four giant pumps, it still takes 2= to 3 days to remove all the air," explained Bachmann. "To change the spacecraft's temperature, we use dozens of small, battery-powered heaters attached to the platform, science instruments and subsystems. For maximum temperature, we turn all of its heaters on, then switch almost all of them off to reach the minimum operational temperature."
Once the engineers have checked that no damage has been caused in the thermal-vacuum chamber, FM6 will be placed on a rotary table for its spin-balance test. Engineers will watch for any minute wobble or imbalance as the cylindrical spacecraft is rotated at up to 30 revolutions per minute.
"It's very important to ensure that the satellite's centre of gravity is in the correct place," said Bachmann. "If it isn't, we have to place small lead weights in different positions on the spacecraft structure until we get it right."
Finally, FM6 will be scanned to find out how magnetically clean it is. This is important because any spacecraft magnetism could affect the scientific measurements of the Earth's magnetosphere made during the mission.
"We compensate for magnetic anomalies by adding small bar magnets, about 2 cm long and 5 mm square. These cancel out any magnetism on the spacecraft," explained Bachmann.
So how is the long, drawn-out procedure progressing? "We have a long way to go," said Bachmann, "but we're making excellent progress."