JWST's tennis court-sized sunshield

23 January 2018

Interview with

Scott Willoughby, Northrop Grumman

We’ve got our giant mirror to collect the light, our highly sensitive instruments to analyse this radiation. But we’ve got one big problem in the shape of a giant ball of exploding gas - our Sun. Just the slightest amount of heat, essentially infrared radiation, would be enough to distort any data collected by James Webb. Izzie Clarke spoke to engineer Scott Willoughby, Vice-president of Northrop Grumman... 

Scott - The sun shield is amazing. It’s probably one of the technologies that distinguishes Webb, along with this large mirror, which is like a diamond pattern, but imagine a big diamond pattern the size of a tennis court. As wide as a tennis court, as long as a tennis court. It can’t start off that big. It can’t start off that big because it will never get off the ground. It won’t fit inside the shroud so it has to start it’s life packaged like a parachute 

Izzie - This packaged up sunshield has been one of the biggest challenges for this mission. Not only does it have to protect the instruments from the heat of the sun… The sunshield has to weigh as little as possible, fold up into this small rocket and then spring to life once it reaches its destination.  But that’s not all…

Scott - It also consists of five layers of material. This one layer isn’t enough to block the heat. We are about plus 185 degrees farenheit in the sunny side and we go to minus 388 degrees fahrenheit on the cold side.

Izzie - For those of us who work in celsius… that’s 85 degrees celsius on the hot side right down to -233 degrees celsius on the cold side: This sunshield needs to get rid of a massive temperature difference of just over 300 degrees. Which it does thanks to the five layers, each the width of a human hair, of a material call kapton.

Scott - It’s a pretty remarkable design. It uses two fundamental concepts… first of all when you’re in space there’s not air so you don’t have to worry about the thermal getting to you. What happens is the light, as the wave of heat comes into you, it’ll hit the first layer and about 90% of that heat due to the surface of this thin layer will reflect right back into space, but that’s not good enough. But then there’s another layer… so that’s layer one.

Then there’s layer two and they’re spaced apart, say about 10 inches apart or something. Those will then trap a level of that heat and turn it into a wave and then radiate it out the side. But still, yet again, some of that heat makes it through layer two onto the other side of it.

And then we need layer three. It’s going to trap some in its internal reflections and vent it to deep space. And then we need layer four. Then it turned out we were able to stop at layer five. And trust me, we wanted the least amount of layers possible because every one of those layers has to get folded and stowed and deploy on orbit with lots of moving parts. It turns out that in our design it took five of these layers in order to block that much heat.

I had the greatest question from an elementary school when I was talking once and this little kid goes to me: why didn’t you just build one layer five times as thick? The root of that is because we couldn’t fold it if we made it any thicker. I gave him an example and he had a notepad in front of him. I said “take the front sheet off your notepad and make a paper aeroplane.” He started making one and I said “that’s pretty easy, you just stop that.” I said “rip the cardboard off the back of your notepad and now make me a paper aeroplane.” He’s like "oh, I can’t do it." I said “Well that’s our problem.” Once we made the paper too thick we were never going to be able to fold it up. So, literally, the thickness of these membranes, four of the layers are only one thousandth of an inch thick, and one of the layers is two thousandth of an inch thick - that’s all they are.

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