According to NASA, keeping an infrared telescope at very cold operating temperatures isn’t an option, it’s an absolute necessity; therefore the James Webb Space Telescope is kept at temperatures of under -370°F. Otherwise, sunlight would warm the telescope, swamping the very faint astronomical signals it detects, effectively blinding the telescope’s eye.
The five-layer sunshield was constructed as a radiation blocker, and its job is to abolish nearly 100,000 thermal watts of solar heat, and reduce it to one tenth of a watt on the cold side; practically a million to one reduction. In addition to the complex designing process, another major problem NASA’s engineers face is measuring the sunshield’s effectiveness. Since there is no cryogenic chamber on the planet big enough to hold the telescope – and building one doesn’t make sense from a budget and practical standpoint – an indirect solution was found. The Webb engineers have constructed a 1/3-scale model and a test facility to perform the critical thermal test of the sunshield system.
To conduct the thermal test the team set itself two main goals: to verify that the sunshield design can actually block and redirect the sun’s energy before it reaches the telescope and to verify the accuracy of computer thermal models used to predict how the full-size sunshield will perform. “The flight sunshield will be deployed and visually inspected prior to flight, but only a computer simulation of its thermal performance will be used to determine if it’s ready to launch,” explains Keith Parrish, Webb telescope Sunshield Manager at NASA’s Goddard Space Flight Center, Greenbelt, Md.
This approach, though innovative, was implemented in the past in other fields. “This is very similar to wind tunnel testing of large aircraft,” Parrish notes. “Most aircraft, especially large commercial airliners, are simply too large to undergo full-size testing. Computer models, which extrapolate the test data from smaller scale model wind tunnel tests, are used to verify final design and predict the full size aircraft’s performance.”
Just like the telescope itself was resized for simulation purposes, so was the sun. The sun’s heat was simulated by electrical heater plates placed very close to, but not touching, layer 1, the warm sun-facing layer. The 1/3-scale model was placed in a thermal vacuum test chamber at lead contractor Northrop Grumman’s manufacturing facilities in Redondo Beach, California, where power to the heaters was steadily increased until layer 1 reached similar temperatures as those expected in flight, well over 100 degrees Celsius.
In order to measure how the sunshield reacts, approximately 400 temperature sensors were placed all over the sunshield. “We also keep an eye on the chamber’s gaseous helium-refrigerated shroud temperatures and liquid helium cooling plates; these cooling plates simulate the cold background temperature of space at the orbit of Webb, which is around 7 Kelvin (-446.8°F),” says Parrish. “We can’t get these plates all the way down to 7 K, which is pretty close to absolute zero. The plates typically get down to the 15 to 25 K (-434.4°F. to -414.4°F) temperature range, so exact knowledge of their temperature is critical to understanding the sunshield’s performance.”
A crucial instrument used is the radiometer; it was mounted around the sunshield and during trial tests it measured the heat radiation bouncing around and between the sunshield, the cold plates, and the chamber walls. Since this kind of effect doesn’t occur in space, the team tried to understand how the bouncing heat impacts the test results.
Other factors taken into account include the extreme environment the telescope resides in. Seven different testing conditions were used to gather temperature data, and these test conditions were tailored so that engineers can study how the sunshield performs in space under a variety of conditions. Some test conditions exaggerated or increased temperatures and heat flows in specific areas of the sunshield. Even though these test conditions do not simulate flight conditions, they’re designed to isolate and better define particular variables used in computer thermal simulations.
“One specific test condition used a mechanism in the chamber to change or warp the sunshield’s shape,” Parrish explained. “Since proper shape is critical to the sunshield’s performance, this test condition gave engineers important data so they could see if computer models can actually predict the thermal impact of shape changes.”
After the temperature data was gathered, the next phase commenced. The team ran computer models over and over again with small changes to mimic the actual test conditions, trying to better match the temperature data from the sensors on the sunshield to the computer models. “This is really the critical part in the whole testing process,” notes Parrish. “Gathering the test data was just the beginning. Understanding that data and how it applies to the flight sunshield’s predicted thermal performance is the critical step.”
All the careful planning and rigorous procedures paid off, as the test showed successful results. Currently the data is being analyzed, and the scientists are checking if the test temperatures accurately reflect the thermal performance of the flight sunshield. According to the team, the completion of this stage is scheduled to these very days.
TFOT also covered the James Webb Exhibition at AAS displayed in 2007, as well as a short video reviewing the historical designing of the James Webb Space Telescope. Another related TFOT story is the successful launch of the Interstellar Boundary Explorer, designed to help scientists understand the way in which solar wind helps protect Earth from dangerous cosmic rays.
For more information about the Webb Telescope’s Sunshield, see the official press release.