NASA Spacecraft Window Design Expert Lynda Estes discusses the evolution of windows that give humans a unique view into space.

Lynda Estes: SpaceX, for instance, is toying with a large dome window for a 360-degree unimpeded viewing on their Crew Dragon. And a couple of our other commercial partners are looking into very, very large windows, giant aquarium-sized windows.

We’re moving to more engineering-friendly materials for our windowpanes such as the plastics, like acrylic and polycarbonate.

You can go from an all-glass window to an all-plastic window and save about 75 percent of the weight.

Deana Nunley (Host): Welcome back to Small Steps, Giant Leaps, a NASA APPEL Knowledge Services podcast where we tap into project experiences to share best practices, lessons learned and novel ideas.

I’m Deana Nunley.

Spacecraft windows provide astronauts a view that may be needed to navigate space or carry out their exploration mission with visual data. Windows on future spacecraft will be lighter, cheaper and more structurally sound than those on previous spacecraft.

Our guest today, Lynda Estes, is a NASA subsystem expert in structural window design at Johnson Space Center.

Lynda, thank you for joining us on the podcast.

Estes: Thank you. This is interesting.

Host: How would you describe the significance of spacecraft windows for the crew and for the rest of us?

Estes: Spacecraft windows symbolically represent the fact that we’re putting humans inside spacecraft. You don’t need windows on spacecraft that don’t carry humans. You can put your cameras outside and that kind of thing. But windows provide this visibility through the spacecraft itself for the crew to observe what’s going on in their environment. So, they can do inspections of their own spacecraft from the outside by looking through the window, or they can just observe the general environment, spaceflight environment, the Moon, or the Earth, or space station for instance has a major Earth observation priority that they use the windows for.

But for those of us on Earth, we can use the windows as well, because the crew will photograph and take video through these windows. And we can sort of vicariously live through the crew and our exploration down here on Earth using these photographs and videos through the windows that we get to share and observe. So, it’s a big reward for someone like me who does design, because whenever I see a photograph or a video through a window, I know, ‘Hey, my work enabled that result.’

Host: What is your role with spacecraft window design?

Estes: Well, I’m one of only two NASA engineers who specialize in this human-rated spacecraft window design and subsystem management. Our role together is to facilitate, guide, consult and enable successful and reliable, preferably lightweight, low-cost windows for all human-rated vehicles. So, that includes spacecraft, rovers, habitats, et cetera. Depending on the contract type, we offer different levels of rigor on these tasks. For instance, since we’re the responsible NASA technical hardware owners, for contractors that are building spacecraft that’s owned by NASA — that’s the traditional contract NASA has had — we’re responsible for the technical direction of the contractor when necessary and for closely monitoring the design, development, test, manufacturing, integration, build, maintenance, the qualification and certification. Then, in the end, the flight readiness of the hardware.

Sometimes, like in the case of Orion, we get to actually directly participate in some of these tasks, which is much different than the way we would conduct these contracts in the past. For Orion, we actually were able to host a number of the window qualification tests here at JSC, which allowed a lot of our next-generation engineers some real hands-on, stand-there-and-watch-it kind of experience, which is fabulous for that next engineering generation.

Now, for companies that are building their own spacecraft for use by NASA, such as Commercial Crew, our role is essentially very similar, where we facilitate, guide and consult, and also are monitoring their successful, reliable spacecraft window development. But we provide this expert input into the program regarding issues that affect safety and risk to the crew of the vehicle.

The task is really similar. It’s just that now we’re going through a different loop, if you will. While we do talk directly to the contractor, a lot of times, if there are impasses with the contractor, we get the Commercial Crew Program or the chief engineers involved in it to resolve or negotiate and mediate. In this arrangement, the programs use their influence at that point to ensure the safety of flight for issues that we, the engineering base, bring to their attention. We expect the partners in these arrangements to take more ownership of the safe design and development and maintenance. But in this new spaceflight management philosophy, what we found on Commercial Crew is that the partners don’t always have the breadth of understanding of all the technical issues associated with a window design to perform that task really efficiently.

So, if you look at the differences between their experience and our experience at NASA, we’ve learned from our experience, from all of our past programs, that’s Apollo, Shuttle, ISS, Orion and now both Commercial Crew companies, as well as others that are coming forward and asking really interesting questions about how they do designs because they’re taking new approaches. So, at NASA, we have an overview of all these different programs and their window designs and where the pitfalls were. I call it falling into a ditch, right? And so we know where all the ditches are and where the slippery roads are, where you’re going to fall into that ditch and we also know how in the past folks have gotten out of that ditch. But if you’re an individual designer and an individual contractor, all you know is that one window design that you’re working on. You don’t have the benefit of the history. So that’s where they benefit from NASA being available to them. And in these cases, we become sort of impartial consultants to these designers.

Over the years, we have been developing and refining the methodologies that we use to do these designs and make them safe and most importantly, reliable. We do continuously try to look for ways to better ensure the reliability of the windows. So, we have written our requirements documents for glass windowpanes that provide a roadmap to designers on how to ensure the proper reliability long-term. And also, my colleague and I work very closely with the contractor or partner to design and ensure the whole health of the system is designed in and maintained.

Host: What’s involved in designing spacecraft windows?

Estes: Spacecraft windows usually start with requirements, requirements for, these days we have requirements to actually have a window. At that point, with the help of the crew, the designers will decide what size and shape and what direction that window will face to see whatever it is they feel like they need to be seeing through that window. And once they establish that, they will essentially design the vehicle to accommodate that window and that shape and that size. At that point, it becomes handed over to the engineering group where I work, and the actual engineering on the design will evolve from there. And windows on spacecraft are always multiple-paned because of a lack of reliability for a number of the materials that we could use for windowpanes. Glass, for instance, which has been the traditional go-to material is a brittle material, which has issues with engineering because the strength of the glass is never really known at any time. It’s being entirely controlled by flaws on the surface of the glass.

And we know that when we stick a window into a spacecraft that it’s going to get damaged during its service time. So whatever flaw population it has down here on Earth that we’re satisfied with is capable of surviving for the entire mission life of the vehicle may not be a static and set number. If that pane gets a ding during service, it now loses life. And so we don’t know about that damage that occurred from a micrometeorite or crew interaction or whatever it was. And therefore we don’t know what its strength is or what it could be. And the second issue that we have to engineer for is the fact that glass will lose strength over time. So, if you put it in space on a space station, you load it up for 30 years and let’s say you never get a new damage on it ever. It doesn’t matter. You’re still losing strength over time. If you can imagine those tiny little flaws, the invisible flaws essentially, that were there when you manufactured the pane, what’s happening as they’re under load, they’re growing, they’re growing deeper. And at some point, they will do what we call ‘go critical,’ which means break the window.

And so we have to engineer to make sure that whatever the mission life is, we don’t have that situation where flaws can grow and get to their critical growth phase and fail the window. And we never have in the history, but in order to protect for these problems, we require windows’ panes to be a lot thicker than the surrounding structure that they’re mounted into. And we also require multiple pane redundancies. In other words, for the pressure pane, it needs a redundant pressure pane in case the primary pane fails. And for the thermal panes occasionally, well, we do have a requirement for thermal redundancy as well, but we do allow our designers some creativity on how they want to meet that. So it doesn’t always call into the necessity of a second redundant thermal pane, but it could, and some vendors choose to design that way.

So, there are many steps along the way to verify that you don’t have these flaws that can fail during the mission life. We primarily do proof testing, but there’s specialized ways in doing that. Apollo found all the ways that probably aren’t a good idea. And so we have learned our lessons from them and created a process and a methodology to engineer it so that we don’t recreate those problems and basic engineering misses in design. And then the other thing that we’re doing is we’re moving to more engineering friendly materials for our windowpanes such as the plastics, like acrylic and polycarbonate—something like what you might see in aircraft or even in these giant million-gallon aquariums that you can go visit. Those guys are using acrylic walls to hold that water back.

Host: So, is that the direction NASA is going, or NASA already has adopted?

Estes: That is the direction that we are going. Yes. And we’re in the middle of that. Orion has what we call a hybridized window. So, they were the first to use acrylic as their major primary pressure panes. So, windows typically in spacecraft are multi-paned because you need different materials to perform different tasks for the mission. The first pane, the pane that’s on the crew cabin, we call the pressure pane, meaning that it holds the pressure in the cabin for the crew environment. And so, since glass loses strength over time, it would be nice to not use glass at that point, in that job. So, we have looked at things like acrylic, which don’t have so much that same type of problem. It has its own problems, it brings its own problems to the table. However, those problems, when you’ve been dealing with glass, those problems are much easier to deal with than the problems of glass. When they come to design windows, most folks come from a background of designing with aluminum.

So going from aluminum to a plastic is more difficult because plastic does have issues that you don’t have to account for in aluminum, but it’s not as striking of a difference as going from glass to plastic, much harder to engineer material to a plastic. So, Orion has a hybrid window design, meaning it’s got both plastic panes and glass panes. So, the glass pane is the outer most pane, and that’s being used for the reentry heating. Other vehicles that don’t reenter in the future probably don’t need that glass for that purpose. Space Station uses an all-glass window, Shuttle used all-glass windows with the exception of one small window pane in their hatches, where that was an acrylic pane. And we have one commercial crew company that’s all plastic panes. They have multiple panes, but they’re all made from plastic. So, we’re seeing that evolution happening now and folks starting to tackle these new little quirks that plastic brings to the table.

Host: Yeah. Let’s talk more about that. How has spacecraft window design evolved since the beginning of human spaceflight?

Estes: Early on in the early NASA programs, windows were multiple paned of different types of glass. They could use, you might see fused silica or borosilicate glasses. Shuttle used a tempered aluminosilicate glass. And at the time that tempered aluminosilicate glass was the largest monolithic tempered piece of glass that was ever manufactured in the US. So they were pushing boundaries on engineering and manufacturing back then. On shuttle, we had two acrylic panes that were in our hatches and the hatch that went out to the payload bay from the crew cabin and in the hatch that went to the space station in the airlock. Both of those were small little acrylic panes. And I had the fortune of being asked to serve on a crew survival committee group that was investigating the possibility of improving survivability after the Columbia accident.

They realized I was not just the crew cabin subsystem manager at the time, but I was also the window person. And they were like, ‘Well, anybody can do the aluminum, but Lynda has to go do the windows because she’s the only one that knows all this stuff about the windows.’ So, I was looking at the windows from the Columbia debris, and I noticed that all of the glass in the windows was just gone. It had broken out somewhere in the breakup and the re-entry, and it was just gone. But in the hatches, in those silly little airlock hatches, that little acrylic pane was still there. Now, it didn’t look great. You couldn’t see through it, but it was still there. And I pushed on it a little bit and it offered a resistance to me. So, I was like, OK. And then I looked at the whole entire hatch, and I noticed that the hatch made from aluminum during that re-entry environment was so trying for that hardware that the metallic of the hatch plane had been aerosolized, it was just gone. You could put your whole entire hand through that part of the debris, but that acrylic pane is still there. And you can’t put your hand through it. It would resist you if you tried. And so, I was like, ‘Well, geez, that doesn’t make any sense to me because plastics have a very low melting point — lower than the metal.’ So I took my observation to our materials group and our thermal group who calculates thermal temperatures of things during re-entry. And I asked them, I said, ‘Why did this result this way? Does this make sense to you?’ And they all said, ‘No, it really is really curious.’ And so, our thermal group at the time said, ‘You know what? This is really interesting and so if you can get us some samples, we will do some testing in our Arc Jet and see what’s going on there.’

And when we did that, we basically found that the reason that acrylic on Columbia was there was that when you expose it to this reentry environment, it actually does not melt. It will ablate instead. And many folks already know that we have in the past used ablaters for things like heat shields. It’s nice to have an ablater where you’re reentering and the material, the top surface of the material, just ablates away rather than melts because it protects the temperature of the substrate below it. So acrylic surprised us in that test by ablating away. And so what has happened with the results of that test, which was offered to everybody, Orion contractors and the Commercial Crew contractors, is that one of the Commercial Crew contractors took that knowledge and that conclusion, and did a little bit more testing on their own and decided to make their thermal panes out of acrylic.

So, they have now freed themselves of the burdens associated with designing with glass, and they have a robust window for entry that can be very, very light. Most folks are not aware that the plastic materials have half the density of glass. So in addition to getting away from the need for redundant pane, you also have half the weight. So, you can actually save, you can go from an all-glass window to an all-plastic window and save about 75 percent of the weight by doing that.

 Host: From a structural engineering point of view, how challenging are these windows?

Estes: They’re very challenging. We have to do everything everybody else has to do when you structurally design something, plus. OK, so we have to also account for not only will it survive just from an immediate structural point of view, but also will it survive long-term life using a material that actually loses strength over time, when they’re using glass. Now, the plastics have a similar feature that can lose strength over time in creep on plastics, but that takes much longer. And it’s easier to handle than the loss of strength over time from glass, because the plastics are not intolerant of fresh damages. So, with the glass, we have to worry about damages from the crew and the micrometeorites affecting the strength and then losing strength from that point. But with the plastics, we just have the straight-up loose strength over time. But if you make it thick enough, you won’t have to worry about that.

Host: At what point in the concept development or design phase does the windows subsystem start to get attention?

Estes: So that’s an interesting question because early on, the crew will come in and say, we have to have this window, and it has to be this big. And so somebody will put a hole in their original spacecraft, pressure vessel design. But what I’ve noticed is that that hole tends to sit there and be empty for much of the rest of the spacecraft design process. And folks don’t actually get back to the idea that maybe they should plug that hole with something before they fly until pretty late in the flow. So it’s usually a real frantic design event when they suddenly realize, ‘You know what? There’s a whole entire book on designing with glass and of requirements that we need to be in compliance with.’ And they’re looking at their launch schedule and going, ‘I don’t even know if I can do this.’

It was interesting to me that our friends at SpaceX actually realized this problem in their early window design. They had a glass and a plastic pane in their design at the time. And they took a look at the requirements they needed to meet and the engineering they needed to do on the glass, and literally, a year before their first flight, their first demo flight, they changed their window design from glass to plastic. They’re very quick and nimble at that company, so they were able to actually make that design change and get it qualified in less than six months, which I was thoroughly thrilled for them and excited to see. And it was just really exhilarating to be associated with a team that was that nimble in terms of engineering turnaround. And then I was really excited to see them basically verify design that uses all plastic window panes, but because they went from glass to plastic in there, they were able to do that very quickly.

Host: Lynda, where do the design ideas for spacecraft windows originate?

Estes: This is very much a ‘necessity is the mother of invention’ problem, like many engineering designs are the same way. And so basically, where the window designs are formulated have to look back at the mission. What is this spacecraft going to do? Is it roving around on a lunar surface or is it floating around in space forever like a space station or does it have to go up and then come back home and experience the re-entry environment? So the mission and the mission phases and what this vehicle has to withstand during its mission basically drives the engineers to certain solutions. So that’s how these conceptual designs are basically made is by looking at the mission for the vehicle and what does it need.

Host: Do you talk with astronauts to find out what they want or perhaps hear about the difference windows made for them during a space flight or an expedition?

Estes: So, we don’t get a lot of communication to and from the crew. We do know that they are famously attached to large, high-quality optical windows in their vehicles. But we the engineering team don’t communicate back and forth to the crew very much just because by the time we, the engineering team, get to the point where we’re ready to engineer a window, the size and shape and location is usually already set because they’ve gone in and worked with the human engineering team to figure out what they want to use that window for, which way it needs to face and how big and what shape it is. So we usually take it from there.

Host: How would you compare spacecraft window design with window design for ships or airplanes? Are there lessons learned and shared experiences among window designers for aviation, nautical and space environments?

Estes: So, my window design colleague and I like to say that we know the secret in why the Navy does not put windows in submarines. It’s because it’s hard and putting a hole in your pressure vessel is a bad idea, but like John Kennedy said, ‘We do these things because they are hard.’ And so we figured it out. Last year in February 2020, she and I were actually invited to be guest speakers at a submersibles conference to discuss the similarities and differences between submersible and aircraft windows. And that was really fascinating to us. Their loads are much higher than ours because they’re going under water in the water pressure. Ours is just one atmosphere. But they do worry about some of the things, same things we worry about from a material point of view, creep and damage from the environment. These are all things that we worry about. For the submersible windows, as well as those on aircraft, we note that the users have the enviable ability to withdraw the vehicle from its environment and check on it structural health.

We had that too with shuttle because the shuttle reentered and came home, and we would be able to inspect our windows and remove and replace them when we needed and then send it back up. But those days are over now. We are now in a phase of spaceflight exploration, where we’re sending vehicles up and they’re up. They’re not coming back for a health check, for any kind of check, really. So we have to look far into the future and estimate what the spacecraft environmental effects are going to have on these materials. And it’s not simple. And it’s fraught with a lot of preconceived conclusions, which we’re trying to dispel or verify by test data. And we try to let the data lead us. So I guess it means we have to be a window whisperer, so to speak.

Host: OK, window whisperer, what do you see as the future of spacecraft window design?

Estes: I’m really excited to be working right now because I see the future of windows actually coming over the horizon now. There are a number of things that we have done over the last decade or so to encourage that blossoming of new approaches. And we’re seeing our contractors and our partners actually doing that. So SpaceX, for instance, is toying with a large dome window for a 360-degree unimpeded viewing on their Crew Dragon. And a couple of our other commercial partners are looking into very, very large windows, giant aquarium sized windows. So these are almost like storefront windows. So, my colleague and I look at that and with eyes wide going, ‘Oh my goodness, here we go.’ But this is the time where we’re seeing these new designs being looked into. So, these folks are now undergoing testing on these large windows, for instance, to see if it’s actually feasible to put that into space for a long period of time.

So thankfully, some of these designers are using our expertise to ensure they’re on a successful path. And my feeling has always been that as a federal employee paid by the taxpayers that my expertise belongs to America. So I’m happy to share that with them and help them stay out of the ditches, if you will. So I’ve been honored to be bestowed this responsibility and for 35 years, take it quite reverently. Folks across the world call all the time and we get calls also from the Navy as well, to help with windows and aircraft companies at the same time. So it’s pretty exciting to see the evolution going on out there and the influence that we’ve had on it is really exciting to see. It’s nice to know that when we leave this great Earth, we have left a mark.

Host: Wow. What an inspiration. Lynda, thank you so much for taking time to join us today on the podcast. This has been absolutely fascinating.

Estes: Thank you. I really enjoyed it.

Host: Links to related resources, Lynda’s bio and a transcript of today’s episode are available at APPEL.NASA.gov/podcast.

If there’s a topic you’d like for us to feature in a future episode, please let us know on Twitter at NASA APPEL – that’s APP-el – and use the hashtag Small Steps, Giant Leaps.

As always, thanks for listening.