 Subunit 6.4, James Webb Space Telescope Examples and Unit Summary. My role during the early trade studies of the telescope was to work with all of the engineers and talk about the details of how everything would work, what the effect would be on science. But I have enough ability to work with engineers that I can understand what they're telling me even if I can't tell them that it would be better to do it some other way. So I can listen to them and draw out the information that we all need to think about. Mostly in the end I have to say, well, they know what they're talking about, but let's talk together. So in the early days I was very much part of the concept of how to build this observatory. I remember there was a day when we had engineers and draftsmen and scientists all in a room together with a big whiteboard and we started sketching out how the observatory had to look like. You need the mirror here, you need the sun shield here, you need to be in such an orbit to protect the observatory from the sun and the earth. So I was very much part of that. That goes in line with my particular visionary personality according to Charlie Pellerin's book. So in the early days it was at the very conceptual level and so I was very much involved in all of the trade studies then. As time went along, then I was able to step back and say, those decisions have all been made. Now let's just go make them happen. The other significant trades involved the instrument complement, what instruments we'd fly and what they would do, but perhaps one of the biggest ones was on the thermal architecture of James Webb. And I've been telling folks this from my stint here. James Webb is about six and a half metric tons, 6,500 kilograms. 4,000 of those kilograms have to be cryogenic, have to be below 50 degrees Kelvin. That's never been done before. And coming up with an architecture that can suitably do that has been probably the biggest challenge, not just in its design, but in figuring out how to test it. One of the bigger trades, the early trades that we did was not only on the thermal architecture, but the fact that we needed to have warm electronics on the cold side of the observatory. There were a lot of reasons for that and we did some real soul searching in that trade because anyone who's done a cryo mission will tell you, you never want to put warm electronics near a cryogenic box. But James Webb has literally thousands of wires connecting the warm electronics to the sensors. And had those boxes not been in proximity, we would have been in the situation where we might have been launching something that was tested without having those connections in their final configuration. That made a lot of us leery. The other thing that made us leery on that was all the great signal to noise that the instrument and the telescope was acquiring to detect these faint first light signals might have been washed out by electric noise had you had 10 or 15 meters of electrical connection between the instruments and the electronics that processed the data. So that thermal architecture was probably one of the biggest trades, most significant early trades that we did other than the aperture and the instrument set itself. So we had two different possible ways to go for the materials to use. One would be metals, metals such as Brillium or Aluminum which have different versions of wonderful properties but they all shrink a fair amount when you cool them down and they have different amounts of distortion when you cool them down. The other process would be based on glass of some sort or ceramic. We have some of those materials which are very close to not shrinking at all and not distorting much. So we had different ways to make lightweight versions of those. For instance, you can make a sandwich that has two glass-like sheets separated by honeycombs. Imagine that you take a piece of a bee's honeycomb and you slice through it so you get hexagonal cells. Then imagine putting that between two sheets of glass. Then this becomes very light and very rigid. Just as we have, as a standard material for spacecraft, we make it out of aluminum. Well, if you could do it out of glass or the right kind of glass, this would be really, really light and be pretty easy to polish up. So we tried that and it was the second, it was the runner-up candidate but it just didn't hold its shape when we cooled it down. So that was very disappointing because it might have been and if it had succeeded in that, it would have been so much easier. So our other alternative was Brillium. Brillium is element number four in the periodic table. It's extremely light. It's extremely stiff. It's almost as good as diamond in that domain. So it's a wonderful material but it's tricky to use. It's tricky because it's very hard to polish and it's tricky because the powder that it makes when you cut it is dangerous to human beings. So we have to do the cutting in the milling machine inside a hermetically sealed milling machine with negative pressure so every single particle of Brillium is captured and sent back to the factory. So nevertheless this was the method of choice and we're very proud of the results that we have. All of the mirrors that we got are now the right shape and they do have the right shape when they are cold at the very low temperature that the telescope will operate. It's a very long and tricky process but it worked. In this video lecture you learn that trade studies are common decision support tools that are used throughout the project lifecycle to capture and help assess alternatives. We covered the steps in the trade study process which are to define the objectives of the trade study, review the inputs including the constraints and assumptions, choose the evaluation criteria and their relative importance, identify and select the alternatives, assess the performance of each option for each criteria, compare the results and choose an option, document the trade study process and its results and you saw some examples of trade trees which are graphical tools that help you manage multi-variable options. Hey, congratulations! You've completed the final unit of the Space Systems Engineering course so that's great. You've kind of made it, you survived all the way through. I hope you enjoyed the class. Now there are some things to finish out here so don't give up yet. First click on the link to take the Unit 6 quiz. The Unit 6 quiz is 10% of your grade so make sure you study for it like you have for all the other ones. This will be your final quiz. Finish up your Mars sample return project if you've chosen to do that and make sure you share it with your peers using the Mars sample return project tab on the course page and I'll be looking in as well to see how people are doing so please if you've chosen to do that make sure you finish your Mars sample return project and get it posted by the April 18th deadline if you'd like it to be eligible for the prizes that you've heard described so please make sure you work on it and again I'll be checking in to see how things are going. Last thing to do before you finish up the course is make sure you take the final exam. It'll cover all the units of the course and it'll be worth 40% of the grade. So again congratulations for completing the course. It's been very enjoyable to have all of you as students I wish I got to meet you all but again congratulations.