September 29, 2010
Engineers have created a unique engineering structure called the Integrated Science Instrument Module (ISIM) that recently survived exposure to extreme cryogenic temperatures, proving that the structure will remain stable when exposed to the harsh environment of space. The material that makes up the structure, as well as the bonding techniques used to join its roughly 900 structural components, was created from scratch.
The ISIM will serve as the structural "heart" of the James Webb
Space Telescope. The ISIM is a large bonded composite assembly made of a lightweight material that has never been used before to support high-precision optics at the extreme cold temperatures of the Webb observatory.
Imagine a place colder than Pluto where rubber behaves like glass, and where most substances that are gases on Earth are liquid. The place is called a Lagrange point, and is nearly 1 million miles (1.6 million kilometers) from Earth where the Webb telescope will orbit. 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 -411° Fahrenheit (-246° Celsius), 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 from NASA's Goddard Space Flight Center in Greenbelt, Maryland.
The passage of those tests represents many years of development, design, analysis, fabrication, and testing for managing structural-thermal distortion.
The ISIM structure is unique. When fully integrated, the roughly 7-foot (2.2 meters) ISIM will weigh more than 2,000 pounds (900 kilograms), and it must survive more than 6.5 times the force of gravity. The ISIM structure holds all of the instruments needed to perform science with the telescope in very tight alignment. Engineers at Goddard had to create the structure without any previous guidelines. They designed this one-of-a-kind structure made of new composite materials and adhesive bonding technique that they developed after years of research.
The Goddard team of engineers discovered that by combining two composite fiber materials, they could create a carbon fiber/cyanate-ester resin system that would be ideal for fabricating the structure's 3-inch-diameter (75 millimeters) square tubes. This was confirmed through mathematical computer modeling and rigorous testing. The system combines two currently existing composite materials, T300 and M55J, to create the unique composite laminate.
To assemble the ISIM structure, the team found it could bond the pieces together using a combination of nickel-iron alloy fittings, clips, and specially shaped composite plates joined with a novel adhesive process, smoothly distributing launch loads while holding all instruments in precise locations. The metal fittings also are unique. They are as heavy as steel and weak as aluminum, but offer low expansion characteristics, which allowed the team to bond together the entire structure with a special adhesive system.
"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 said. "By incorporating all of our lessons learned into the final flight structure, we met the requirements, and the test validated our building-block approach."
Scientists performed a 26-day test to specifically see whether the car-sized structure behaved as predicted as it cooled from room temperature to the frigid — very important because the science instruments must maintain a specific location on the structure to receive light gathered by the telescope's 21.3-foot (6.5 meters) primary mirror. If the contraction and distortion of the structure due to the cold could not be accurately predicted, then the instruments would no longer be in position to gather data.
The test itself also was a first for Goddard because the technology needed to conduct it exceeded the capabilities offered at the center at that time. "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 from Goddard — photogrammetry is the science of making precise measurements by means of photography. Making the measurements in the extreme temperatures specific to the Webb telescope was another obstacle the NASA engineers had to overcome.
Despite repeated cycles of testing, the truss-like assembly designed by engineers did not crack. Its thermal contraction and distortion were precisely measured to be 170 microns when it reached -411° Fahrenheit (-246° Celsius), 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.