01 October 2010
01 October 2010
The composite structure of the Webb telescope has recently survived exposure to extreme cryogenic temperatures, proving that the structure will remain stable when exposed to the harsh environment of space.
The ISIM, or the Integrated Science Instrument Module flight structure, will serve as the structural heart of the James Webb Space Telescope. The ISIM is a large bonded composite assembly made of a light weight material that has never been used before to support high precision optics at the extreme cold temperatures of the Webb observatory.
The Webb telescope will orbit at a Lagrange point nearly one million miles from Earth, a place colder than Pluto where rubber behaves like glass and where most gasses are liquid. At this point in space, the Webb telescope can observe the whole sky while always remaining in the shadow of its tennis-court-sized sunshield. Webb's components need to survive temperatures that plunge as low as 27 Kelvin (-411 degrees Fahrenheit), and it is in this environment that the ISIM structure met its design requirements during recent testing. ""It is the first large, bonded composite space flight structure to be exposed to such a severe environment,"" said Jim Pontius, ISIM lead mechanical engineer at NASA's Goddard Space Flight Center in Greenbelt, Md.
When fully integrated, the roughly 2.2-meter (more than 7 feet) ISIM will weigh more than 900 kg (nearly 2000 lbs) and must survive more than six and a half times the force of gravity. The ISIM structure holds all of the instruments in very tight alignment, using a carbon fibre/cyanate-ester resin system for the structure's 75-mm (3-inch) diameter square tubes.
""We engineered from small pieces to the big pieces testing all along the way to see if the failure theories were correct. We were looking to see where the design could go wrong,"" Pontius explained. ""By incorporating all of our lessons learned into the final flight structure, we met the requirements, and test validated our building-block approach.""
The Mechanical Systems Division at NASA Goddard performed a 26-day test to specifically test whether the car-sized structure behaved as predicted as it cooled. The test was a first for NASA Goddard because the technology needed to conduct it exceeded the capabilities then offered at the centre. ""The multi-disciplinary (test) effort combined large ground-support equipment specifically designed to support and cool the structure, with a photogrammetry measuring system that can operate in the cryogenic environment,"" said Eric Johnson, ISIM Structure Manager at NASA Goddard. Photogrammetry is the science of making precise measurements by means of photography, but doing it in the extreme temperatures specific to the Webb telescope was another obstacle the NASA engineers had to overcome.
Its thermal contraction and distortion were precisely measured to be 170 microns when it reached 27 Kelvin (-411 degrees Fahrenheit), well within the design requirement of 500 microns. ""We certainly wouldn't have been able to realign the instruments on orbit if the structure moved too much,"" Johnson said. ""That's why we needed to make sure we had designed the right structure.""
The same testing facility will be used to test other Webb telescope systems, including the telescope backplane, the structure to which the Webb telescope's 18 primary mirror segments will be bolted when the observatory is assembled.