January 01, 2007
The Environmental Test Laboratory, affectionately known as the "shake and bake" lab, includes "shake tables," which move large objects rapidly in horizontal and vertical directions, exposing them to the forces they will experience during launch. Recently, the lab's vertical table held the spare flight model of the antenna for the reflector structure assembly, one of the radiometer's two subsystems. Attached to the antenna was a test model of the electronics structure assembly, the radiometer's other subsystem. The test model weighs exactly the same as the flight instrument.
Although the shake test was only for the antenna with its lightweight graphite composite support structure, engineers included the electronics structure assembly model so they could observe how the antenna will behave when fully integrated into the reflector structure assembly.
All spacecraft instruments undergo rigorous testing to ensure they can survive the tremendous forces they'll be subjected to during launch. After all, if an instrument is unable to withstand the shakes, rattles, and rolls of a rocket launch, it will never get the chance to be proven in space. According to Alex Nicolson, the contracts technical manager for the Ocean Surface Topography Mission, most of the force generated during launch is from shaking and from the acoustic noise generated by the launch rockets.
The ~23 kg (50.2 lb) reflector structure assembly antenna is held up by extremely strong graphite legs, which support the entire structure. (Figure 4) Each leg can carry a load of more than 5 tons.
For the recent shake tests, the antenna was shaken vertically, in what engineers call a "3/4-g swept sine" test. In this process, the shake table moves, or sweeps, through a series of frequencies from 5 to 140 Hz while a constant acceleration of 3/4 g is applied. At the lower frequencies, you can actually see and hear the structure shaking. But at some point, the structure begins to shake so fast that, to the human eye, all motion appears to have stopped. However, wires from the shake table to the antenna structure (Figure 5) record the antenna's motion and display the results on a computer screen.
In addition to the swept sine test, the antenna underwent two other shake tests. In a "sine burst" test, the antenna was shaken at only one frequency (30 Hz), over an amplitude range that has the shape of a teardrop. During the "random" test, the antenna was shaken through random frequencies from 20 to 2000 Hz. Each of the three tests was performed along each of the instrument's three directional axes for a total of nine tests per instrument component. Testing at the various frequencies helps engineers better understand the instrument's behavior in launch conditions.
After the shake tests, the antenna had to endure the thermal vacuum chamber (Figure 6), the baking component of "shake and bake". The antenna was baked at 80° C (175° F) in a complete vacuum for 96 hours to remove any gaseous materials embedded in the solid structure (Figure 7). When solid structures release gases when exposed to heat or vacuum, the process is known as outgassing. If a spacecraft instrument isn't thoroughly "cleaned" before launch, the vacuum of space could cause the instrument to outgas, making for some potentially serious problems.
Following trials in the acoustic chamber, the antenna was integrated on the reflector structure assembly for another series of environmental tests. When all the tests were complete, engineers deemed that nothing moved significantly or changed shape as a result of the tests. The results were good news. The instrument components appear to be flight worthy and flight ready.
Jason-2 is on schedule for a June 2008 launch from Vandenberg Air Force Base in California. Following in the tracks of TOPEX/Poseidon and Jason-1, Jason-2 will extend the time series of ocean surface topography data to two decades. This additional information will allow scientists to improve our understanding of ocean circulation and heat transport leading to better climate models and predictions.
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