A NASA engineer in a cleanroom looks at one of the sunshield layers with a grid pattern of ‘rip-stops’. Photo: Nexvolve.In order to protect the mirrors and instruments aboard the James Webb Space Telescope (JWST) from solar radiation, NASA and its industry partners have come up with a lightweight, five-layer sunshield that is a complex and innovative feat of material science and engineering. Each layer is made from a unique composite material, each has a specific thickness and size, and all the layers must be precisely separated in space. There are even special seams and reinforcements to limit meteorite damage.
The sunshield consists of five layers of a material called Kapton. Each layer is coated with aluminum, while the sun-facing side of the two hottest layers (designated layer 1 and layer 2) are also coated with doped-silicon (or treated silicon) to reflect the sun's heat back into space. The sunshield is a critical part of the JWST, which is due to be launched in October 2018, because the infrared cameras and instruments aboard must be kept very cold and out of the sun's heat and light if they are to function properly.
Kapton is a polyimide film that was developed by DuPont in the late 1960s. It has high heat-resistance and remains stable across a wide range of temperatures from -269°C to 400°C, and does not melt or burn at the highest of these temperatures. On Earth, Kapton polyimide film is used in a variety of electrical and electronic insulation applications.
The sunshield layers are coated with aluminum and doped-silicon to take advantage of their optical properties and longevity in the space environment. The doping process involves mixing in a small amount of another material during the silicon coating process to make the coating electrically conductive. This is done so that the layers can be electrically grounded to the rest of the JWST and to ensure they will not build up a static electric charge across their surface. Silicon has a high emissivity, which means it efficiently emits heat and light, and so acts to block the sun's heat from reaching the infrared instruments that will be located underneath it. The highly-reflective aluminum surfaces also bounce the remaining energy out of the gaps at the edges of the sunshield’s layers.
The kite-like shape of the sunshield and the number of layers both play an important role. Each of the different layers are positioned and separated with precision to accomplish their function.
"The shape and design also direct heat out the sides, around the perimeter, between the layers," explained James Cooper, Webb telescope sunshield manager at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "Heat generated by the spacecraft bus at the ‘core’, or center, is forced out between the membrane layers so that it cannot heat up the optics."
"The five layers are needed to block and re-direct enough heat to get the telescope down to required temperatures, with margin," Cooper added. "The fifth layer is mostly for margin against imperfections, micro-meteoroids holes, etc." The gap between the layers provides an additional insulating effect.
Each layer of the sunshield is incredibly thin. Layer 1, which will face the sun, is only 0.05mm thick, while the other four layers are just 0.025 mm thick. The silicon coating is around 50nm thick, while the aluminum coating is around 100nm thick.
The layers are all slightly different sizes and shapes. Layer 5 (just under the primary mirror) is smallest and layer 1 is largest; layer 1 is relatively flat and layer 5 is more curved. The layers are closer together at the center and further apart at the edges to direct heat away from the center and toward the outside of the layers.
The Webb telescope optics (like the infrared camera and mirrors) must always be protected from direct exposure to any hot objects. So the membranes are sized and positioned such that the mirrors only have a direct line of sight to the cold layer 5, while the sun only directly shines on layer 1 no matter which way the telescope is pointed.
The layer material is tough, but it could still get a small tear or hole, which could become much larger. So there are areas where each layer is melted together, called thermal spot bonds (TSB). In addition, reinforcing strips of layer material are thermal spot bonded to each of the five layers every six feet or so, forming a grid pattern of ‘rip-stops’.
"This has been shown through testing to arrest a tear and keep it from extending outside of a given grid area," said Cooper. This means that if a meteoroid, or small meteor, punches a hole in a layer of the sunshield, the size of the damage can be limited. These rip-stops are not intended to stop a meteoroid, merely to contain the area of damage.
This story is adapted from material from NASA's Goddard Space Flight Center, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.