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“Houston We Have a Podcast” is the official podcast of the NASA Johnson Space Center from Houston, Texas, home for NASA’s astronauts and Mission Control Center. Listen to the brightest minds of America’s space agency – astronauts, engineers, scientists and program leaders – discuss exciting topics in engineering, science and technology, sharing their personal stories and expertise on every aspect of human spaceflight. Learn more about how the work being done will help send humans forward to the Moon and on to Mars in the Artemis program.

On Episode 211, Mike Salopek goes in depth on the International Space Station’s power systems and the new solar array technology that will continue to power experiments and modules for years to come. This episode was recorded on August 16, 2021.

Transcript

Gary Jordan (Host): Houston, we have a podcast! Welcome to the official podcast of the NASA Johnson Space Center, Episode 211, “Station Solar Arrays.” I’m Gary Jordan, and I’ll be your host today. On this podcast, we bring in the experts, scientists, engineers, astronauts, all to let you know what’s going on in the world of human spaceflight. In case you’re unfamiliar, the International Space Station’s power systems are in the process of getting an upgrade. Over the past few years, the nickel hydrogen batteries that once stored energy gathered from the station’s basketball court-sized solar arrays were replaced with lithium-ion batteries. And most recently, astronauts installed brand-new solar arrays to augment the station’s power supply, meaning both the original and the new arrays will collect solar power for the same power channel. So, why do we need these power upgrades? Why are the arrays being augmented? How has technology improved? All these questions and more being answered by resident expert Mike Salopek, the International Space Station power augmentation project manager, on this episode. So, without further delay, let’s get right into it. Enjoy.

[ Music]

Host: Mike Salopek, thanks for coming on Houston We Have a Podcast today.

Mike Salopek: I’m happy to be here and talk with you today.

Host: All right. It’s all about station solar arrays today, and I know we are — well, we already have started installing new ones. All that’s part of a greater plan, so I really want to get into not only those greater plans, but the technical details of what it means to augment these solar arrays. First, I want to understand a little bit more about you, though, to understand what it takes to manage a project such as this. So, Mike, tell me about yourself.

Mike Salopek: Sure. So, let’s see. I started working at Johnson Space Center back in 2007 after I graduated from college. I went to Embry-Riddle Aeronautical University in Florida, and I got an aerospace engineering degree there. I actually started out on the flight operations side, so the flight controller for the space shuttle for a couple of years at the end of the program, and then after that I moved over and did flight control for space station. And then, around 2016, I became a civil servant, and I was looking to expand my horizons a little bit, and work on some actual projects. And I had the opportunity to come to the ISS vehicle office, and at that time I was working on life support systems, exploration life support systems. That was my technical background in flight control. And then starting this summer, I had the opportunity to take over solar arrays, and actually docking systems as well for, for the space station program, and it’s been an interesting learning experience so far. What’s, what’s nice about it is that I got to kind of hone the decision-making skills and the management skills with the technical background that I was used to. And I had touched these things peripherally in, in my background as well, especially on the power side. I mean, all of my systems used this power, so had to be familiar with the architecture. And then, now I get to take the things that I learned managing stuff that I was familiar with, and now I can do and start managing things that I’m less familiar with. So, I’m looking forward to the challenge. It’s a very exciting project. Obviously, it’s really important. We’ll talk a lot about that later, is the why this project is important, and what it’s going to do for the space station.

Host: I want to dive into the power system, though. This is a very interesting topic. We’ve never really dove this deep, I think, into understanding just a single system that makes the International Space Station work. So, what’s interesting, though, is if you’re looking at the space station, one of the main features of the shape of the space station are the gigantic solar arrays, and these are the ones that we’re augmenting. So, tell us about, a little bit more about the original arrays. What we’re, are we working with to augment? What are these legacy arrays?

Mike Salopek: Sure, yeah. And you’re absolutely right, and I was about to even mention it. It’s one of the most iconic features of the space station, are these very large solar arrays. So, a solar array is, it exists on something called a, we call it a photovoltaic module, or PVM, and that contains the structure that is mounted to the rest of the truss. It contains the mechanisms that allow the array to, to rotate and point toward the Sun. It also contains the batteries and the other electronics that control the mechanisms, and also regulate the power and things like that. The array itself is a part of that PVM, and it kind of consists of three major parts. There’s two blanket boxes, which are the solar arrays themselves are what we, they’re referred to as blankets. They’re kind of a hinged blanket. We’ll talk a little bit more about that later. There’s one on each side of the array, so if you’re looking at one of the arrays, there’s kind of the two sides that are, like, blue and orange, and then there’s, in the middle, there’s the mast, and that’s the structural stiffening mechanism that keeps the arrays deployed. And that mast canister, when it’s folded up, that mast is all folded up in there, and it’s, that’s the third part of a particular array. The PVMs are actually only launched one at a time. It took many shuttle flights, obviously, to build the space station, so it took at least four to assemble all the solar arrays. The last set, actually, not even arriving until 2009, which is coincidentally one of the first shuttle missions that I worked whenever I started here. The blankets themselves, they’re very large, right? So, if you look at the space station, the space station’s very big. The blankets are 115 feet long and 38 feet wide. So they’re, they’re huge, and the way they’re kind of broken up, so on the legacy arrays they contain over 32,000 individual solar cells, and those are grouped together in what we call power strings. Those power strings are distributed on the 82 panels of the blankets. As I said, the blanket is kind of like a hinge. It’s kind of like a hinged blanket, and it retracts into that blanket box, or unfurls when it folds out, almost like hoisting a sail, if you can imagine, imagine that.

Host: Well, the blankets — you’re drawing comparisons to like a sail. You’re using the word “blanket,” but is this, so is it like a — when you’re talking about these solar cells, and the actual arrays themselves, is it a softer material, or is it a rigid structure?

Mike Salopek: It’s fairly rigid. It’s not, there are things called rigid solar panels. I think Orion is using those, and they kind of deploy, and work a little differently, but the blanket is a bit of a flexible material. You know, the new arrays, the iROSA (ISS Roll-Out Solar Array), is a much more flexible material than the old arrays, and the blankets themselves consist of multiple different layers. So, you have the actual, like, solar cells on there, and then you have, like, varying other layers that provide the structural stiffness, and then provide protection from the space environment as well, with the temperature. There’s atomic oxygen, so it’s a very corrosive compound that exists in low-Earth orbit that we have to make sure that the external parts of the space station can withstand exposure to that, and that’s actually one of the reasons why the, we had to go build new solar arrays is that the legacy solar arrays were degrading a lot faster than we had originally predicted. I think one of the key areas that they looked at was they just didn’t quite understand the degradation of that atomic oxygen, how much of a factor it was. You know, the other thing, too, it obviously took a long time to build the space station. Again, a lot of these solar arrays came up very early on. I think the first one was launched in 2001- or 2002-time frame, and as we were building the space station, you know, we had a lot of shuttle flights coming up. One of the things that also can contaminate arrays, and we do a pretty good job of protecting them, but, like, propellant plumes and things like that. So, there’s a lot of stuff in that low-Earth orbit environment, and when you have visiting vehicles coming to and from, there’s other opportunities for contamination as well. Early on in the shuttle program when we were docked to space station, then we’d have to do water dumps and things like that while attached to ISS. None of that is really great for the external environment of the ISS as well. So, you know, there’s a variety of factors why the legacy arrays ended up degrading faster, and that’s, I’m sure those are all related.

Host: Yeah, and that’s just one component, right? So that’s the actual arrays. Now, what’s interesting is, the, the arrays themselves, their job, right, is to draw power from the sun.

Mike Salopek: That’s right.

Host: That’s the main purpose of them, is to draw that power. Now, when they draw that power, it gets put through something, I believe, called a power channel. What is that, and what is the downstream of that?

Mike Salopek: Yeah, so a power channel is, is we’ve divided up the space station into these eight power channels, and they’re all the components working together to get power from the outside to the inside, and the users on the inside of the station, or the users on the outside as well. So, the arrays would start to collect that from the sun. Once the power is generated in the array, it kind of goes to three different places. It’s either shunted back to the solar array — so there’s something called a sequential shunt unit, SSU, and those are actually components that we’ve had a lot of challenges with over the years. They’ve failed prematurely on us, and ended up in the, we ended up with a couple of close, almost shortfalls of having spares of that unit. But what that does is, that is used to regulate the voltage coming out of the array. So the array produces, we, we produce about 160 to 170 volts from the array, and that SSU regulates that voltage, because if the downstream users don’t require that much power while the array is still pointed at the sun, still generating all that power, so we shunt the power back into the array, and it gets dissipated as heat. The way the downstream loads could go is, it could also, we would store power into batteries, right? So, we’re not always pointed at the sun, because we have eclipse time frame once, you know, every, about half of our orbit is spent in the shade, we’re not generating power there, and we have to store power for that time frame. So, what’s not shunted back into the array either gets diverted to the power, or to the batteries, and then the rest of it gets diverted downstream to the downstream users. And those downstream users could be external components in payloads, things like the core systems, the pumps that keep everything cool, or the fans that blow the air out. Or it could be a payload rack, like the combustion integration rack, or even the lights inside ISS, right? So those are all examples of downstream loads, and the way that that occurs is, there’s a couple of other components. So, one, with the batteries, there’s a battery discharge controller, I think it’s a BDCU (battery discharge control unit), and that regulates when the battery needs to start discharging to keep downstream users powered, or when it needs to cut off charging of that battery so we don’t damage it that way. The power going from the batteries or from the arrays to the downstream users also go through something called an MBSU, main bus switching unit, and what that does, that allows us to direct the power from the arrays down to these different other sub-components called DDCUs (DC-to-DC converter unit), which are DC current converter units. So, as I said, we collect the power at around 160 volts. Our users use it at 124 volts. So, we collect it a little bit higher, because we have to transport it a fairly long distance from the array down to the truss, and then to the inside of the vehicle. So, we have to collect at a little higher voltage, so that way we’re guaranteed to get the 120 volts at the, at the user level. Those MBSUs allow us to cross-tie power channels if we need to. So, if you remember back, I said, hey, we launched these power, these solar arrays one at a time, and for a long time we only had one solar array wing on the space station, so two sides. So that gave us two power channels, but we had a bunch of components inside that would be on future power channels that don’t exist yet. So, the MBSU allows us to kind of cross-tie all of those, and allow, like, one array to power different power channels than it would normally be set up for. And that was obviously vital to the early parts of the space station. We don’t really do a lot of that now unless it’s an off-nominal situation, mainly because you have to be careful if you do that so you don’t overload the power channel. So, if you all of a sudden, most of our power channels are pretty well-subscribed, there’s a lot of users on them, and if we were to just all of a sudden cross-tie one power channel to another, you could also end up taking out that power channel, because the voltage will drop too low because there’s way too many users downstream, almost like, you know, plugging too many things into your socket in your kitchen. You’re going to blow a fuse. So, something similar like that would happen on ISS.

Host: Yeah.

Mike Salopek: We don’t, we don’t do that too often, unless it’s a dire situation.

Host: Yeah, there’s only so much power that can be subscribed, and you got a lot of people signing up for that power. So yeah, it’s all about management at that point. All right, who really needs it? What are the critical systems?

Mike Salopek: Exactly. Exactly. And, you know, to kind of simplify it, if you kind of think about how power gets to your house, right, so you have the solar array, which is like the power plant. You have those distribution substations; those are like your MBSUs. The transformer on the power pole out in front of your house, that’s like the DDCU. You have the wire going to your house, and then inside, or at the user level, there’s also something called a remote power controller module. And those are like your individual, like your circuit breaker panel outside, and that allows us to turn off, turn on and off power to individual users. So, any, like, even, like, the lighting is on its own RPC, as we call it, remote power controller, which is, it’s just a switch that opens and closes power to that particular unit. There’s also an on or off button, so again, it’s kind of like you have your circuit breaker outside, but you can flip your light switch on and off, as long as that circuit breaker’s closed.

Host: Who’s flicking the breaker? Is it flight controllers, or is it the astronauts, or is there redundancy?

Mike Salopek: It’s primarily flight controllers. You know, the crew, we try to keep them as busy as possible actually running the scientific payloads. You know, there’s a lot of stuff that they have to do hands-on. So, any time we have the ability to send a command to the space station to configure the equipment for them to use, we would do that. They can do it, you know, they have to interface to it, and in some contingency situations they need to, but also for some normal situations they may elect to do it as well. But typically, the Mission Control Center, MCC, handles most commanding of that.

Host: And you said, back in the day, you were a flight controller mostly with life support.

Mike Salopek:That’s right.

Host: So, you were working with, what’s the guy? The ETHOS (Environmental and Thermal Operating Systems) —

Mike Salopek: ETHOS, yeah, that’s right. I was an ETHOS for, for eight years, yeah.

Host: — wow, OK. Yeah, so the guy next to you was, I guess power is SPARTAN (Station Power, ARticulation, Thermal, and Analysis)? Is that the role?

Mike Salopek: SPARTAN, that’s right, and we would work a lot together, because the life support system is one of the main power hogs of the, at least the internal part of the space station, right? I mean, our carbon dioxide removal system uses almost a kilowatt of power. Our oxygen generator uses almost a kilowatt of power. We have heaters all over the place that, you know, they add up to using a couple of kilowatts of power if they’re all on at the same time, and stuff like that. So, we would work very closely with SPARTAN to help offload and reconfigure our systems, since maybe we had things on a different power channel that we can swap over to that equipment. Or we didn’t need some stuff on, like you don’t always need the heaters on, and things like that. And that kind of goes to that management. You know, the SPARTANs are making sure that we don’t overload one power channel. What’s interesting about ISS, too, is at certain times of year, of the year, we have more power availability than other times of year, and it’s because of the angle of the space station to the Sun. We refer to it as the beta angle. If you think of it like, on Earth, during the winter, it gets dark earlier, and it stays dark longer. Same thing kind of happens on ISS, and we’re at a pretty high inclination, right, 56 degrees. So, you know, when, couple of times a year we either have a lot of sunlight, or we don’t have as much sunlight and we have to power down systems so we don’t overload the power channels. Now, that’s gotten a lot better in recent years. You know, we did a big battery upgrade where we replaced the old, I think they were nickel hydrogen batteries with lithium-ion batteries. So, they hold a lot more, and they aren’t degraded, right. We all know that rechargeable batteries degrade over time, and then with the iROSA solar arrays coming up, we are now able to produce a lot more power when we are in sunlight. So those batteries are going to charge faster, they’re going to hold more charge, and that eliminates a lot of our future issues with power management and planning, so we can run more stuff at the same time. The other interesting thing, too, and it’s kind of relevant to the new arrays, when you start trying to figure out what they’re actually doing for us, is twice a year, during the orbital solstice, so when the Sun is directly head-on, it’s a beta of zero with the ISS, so completely head-on just like the solstice here on Earth, where the Sun, you know, is everything, day and night are the same amount of time. That’s when we can really gauge the exact performance of our arrays, because we don’t have to do any kind of beta angle offset with them. We have full sun on the arrays. We can really see what they’re doing for us, and I think the next solstice is coming up within the next couple of weeks here on ISS. So, we’ll actually get a good gauge of what the new solar arrays are doing for us then.

Host: So essentially, the sun is shining directly on the arrays. You don’t need to move them in any sort of way to get it. Lowest atmospheric interference or something, so it’s a pretty reliable measurement. Is that the logic?

Mike Salopek: Exactly. Yeah, it’s the most amount of power that they’re going to produce, right, because we don’t have to angle the arrays at the Sun. And, you know, with the way that the ISS is configured, it’s hard to point all of the arrays directly at the Sun all the time. There’s other challenges structurally with doing that. It’s a very complicated issue called longeron shadowing. So those masts that we have that support the arrays, if you think about…deep space is very cold, so when you’re facing deep space, it’s very cold. When you’re shaded, it can also get very cold. But obviously, if you’re in direct sunlight, since there’s no atmosphere or anything to help regulate the temperature, the metals heat up very, very quickly as well. So, if the arrays shade each other just the right way, it can actually structurally deform the, the mast, to the point where they could break. Now, we’ve prevented that with software controls, where we bias the array angle such that that shading doesn’t happen, and it’s a very complicated analysis. I’m not even all that familiar with it, but what it does is it prevents one array from adversely shading those longerons on the other array, or itself, so that you don’t get that temperature difference that’s going to cause the metal to bend. If you think of like a bi-metallic metal strip, right, it’s similar to that, only it’s all one metal. And it’s just because one side is colder, the other side’s hotter, it’s going to warp it, and that can cause a major structural failure if you’re not careful. So that’s one of the major challenges that we’ve had to deal with, with these legacy arrays, and it goes a lot into the planning and management as well. And that’s one of the reasons why we don’t always get our desired power generation that we want, because we have to kind of bias these arrays, so they don’t, we don’t end up with that shading.

Host: Yeah. So the beta zero allows the least amount of shading, most reliable power draw, with, you know, you’re limited on the shading, and it sounds like even if it is shading, the mission is not just collecting as much sunlight as possible. There are thermal constraints that are part of your decision-making as well.

Mike Salopek: That’s exactly right, and because and, you know, the other constraints that we have to work too is around what we call dynamic events, so vehicle docking, undocking, robotic ops [operations], any kind of —

Host: Because you have to, like, lock it, right? You have to lock —

Mike Salopek: — we have to lock the arrays. That’s right, and, you know, there was, one of the concerns when we had that loss of attitude control a couple weeks ago was what damage could we have done to the arrays with those thrusters firing, because they weren’t, fortunately, the SPARTAN acted extremely quickly. I’m not exactly sure who was on console. I don’t know who works over there anymore, but they acted very quickly to get the arrays in as safe a config[uration] as possible. And fortunately, we’ve done a bunch of external surveys and we haven’t uncovered anything that I’m aware of yet. We also looked at the iROSA array, just to make sure it’s [unintelligible] attached a little differently, and there hasn’t, the before-and-after pictures look almost, they look identical to us, and we’re still getting good performance out of everything. But, you know, that was between the station moving differently than what it should be, right, that is going to put stress on the structure. When they’re very large, you have large moments and everything, and then, also, with the thrusters firing kind of unexpectedly in an unanalyzed, they’re pointed, those thrusters were not meant to do that. They’re not supposed to do it. We have analysis to show what they would do to our external components. So, you know, we’re still sorting through the impacts of that, but it kind of ties into how difficult it is to manage these really large arrays. There’s a lot that you don’t necessarily think about right away that goes into it, because as you pointed out, it’s not just necessarily about collecting power.

Host: Right, yeah, all these different constraints, structural, thermal, all of that, and then, on top of that, you said, you know, early on in your description of what these power channels are, you talked about that the legacy arrays, the one that we’ve talked about — and we’ve alluded to the new ones, iROSA, I think that’s the main thing we want to talk about — but you said that there’s been degradation. Now, what is that?

Mike Salopek: Yes.

Host: And you said there’s some environmental things. You know, it sounds like this is something that might just happen over time. What exactly is happening to the legacy arrays that they are degrading?

Mike Salopek: So, you have atomic oxygen reacting with the materials to kind of degrade the cell’s ability to generate power from the Sun. There’s also space debris, micrometeoroid orbital debris, MMODs, they can create small perforations in the arrays, and hit cells, and completely take those out. We have a couple of actual just failed power strings, which is a collection of solar cells, on each array, so that all leads to degradation. Any kind of deposits from thruster firing or other, other contamination will deposit on the arrays, and, you know, block some of that solar energy from activating those solar cells as well. All solar arrays degrade, right, it’s just something that comes with using it, because, you know, the components — they don’t really wear out, but they get deposits on them, the harsh environment of space changes the, the electrical generation properties of the solar cell material, and all that leads to less power generation over the course of a lifetime of array. And when we write requirements and we build these systems, we often levy what we call an end-of-life power generation requirement, and that allows us to size the array for the power that we need it to generate after X number of years in service, right? So, I think the legacy arrays were originally designed for a ten or 15-year service life. We’ve obviously gone well beyond that, especially for some of the early ones that have been up there, and we plan on going even further. The new arrays, the iROSAs, are also designed for ten-year service life, and they were levied the requirement to generate 20 kilowatts of power per array at the end of its service life. So you have to take into account these predicted degradation rates, which we’ve learned a lot more since the legacy arrays, and we would apply that to the iROSA design to ensure that we don’t get some, the unexpected higher degradation than what we were planning before. We obviously apply some sort of conservative factor of how much you’re willing to risk the end-of-life power generation, and that’s how you size how many power strings you need, and how big the array needs to be.

Host: And so, why is it important that we continue, you know, you have this normal degradation. You know, why not just deal with the degradation, and just, you know, pick and choose which things you want to cover? What’s important about maintaining continued power levels on the station?

Mike Salopek: Yeah, great question. So, you know, the primary purpose of ISS is to perform scientific experiments for a variety of our customers, including, we’re getting ready to have a commercial module come to ISS within the next couple of years as well, and we’re going to be providing a lot of the power to that module. So, we have all of our existing payload facilities that are on board that have been running for a number of years, that we wanted to continue to run going forward. We constantly have new payloads coming up wanting to use, you know, more power and things like that. We’ve expanded laboratory space on ISS. We have some additional facilities, what we call racks, EXPRESS (EXpedite the PRocessing of Experiments to the Space Station) racks, to allow us to run more payloads. All of that has increased the amount of predicted power we’re going to need to provide, so that way we can continue to run as many experiments as we run. Yeah, you could manage by saying, well, you know, on this many times a year, we’re going to have to power down half of our experiments on ISS in order to make sure that we can keep the lights on and the air conditioner running, right? If you think of ISS like an actual, like, office building, or a high-tech laboratory facility, you have just the power you need to run that facility, but then you also have the power that you need to run your experiments as well. Well, we obviously can’t compromise on running the facility, so we’d have to power down some of the experiments. That’s a big impact to a lot of our customers. Some of these experiments, if you shut them off, you could lose valuable data. Some of them, it may ruin the experiment. It may make it more difficult for them to plan, and if we’re trying to make ISS more useful to a wider variety of customers, we don’t want to impose additional constraints on managing, saying, hey, you know, every October we’re going to have to shut you off for two weeks, because we don’t have enough power, right? That may cause someone to second-guess on flying an experiment to ISS. So we’ve done this big power expansion project, so that way we can support more customers going forward, including commercial module, which will be a huge upgrade to the space station, and a huge step in the direction of commercializing low-Earth orbit, which is a major priority for, for the agency. They’re going to have their own laboratory space. Now, they’ll be able to run more experiments, so, but and it also makes it easier for, you know, universities, and other companies that want to fly payloads to ISS, and not have to worry about trading off between I get to be powered versus you get to be powered, and things like that. So, and if we want to continue to use ISS going forward, you know, I think we’re looking at expanding it to at least 2028, the legacy arrays are only going to degrade more, and that’s just less stuff that we’ll be able to do when we have more users, you know, waiting, waiting in the wings to use ISS.

Host: Lots of people want to use ISS, lots of cool experiments. The commercial module you’re alluding to is Axiom Space. They want to attach their own module to the forward port, and they won an award for that. So, there’s a lot coming up, and that’s a lot of justification to say, hey, this is important. We want to continue to provide power. We don’t — exactly as you’re saying, we don’t want, you know, all these experiments that are running, collecting valuable data, to just stop because we can’t provide the power, right? So, we just —

Mike Salopek: That’s right.

Host: We want to keep providing the power. Now, you mentioned iROSAs. Now, let’s get into that. This is that new solar array. This is the solar array that is going to be providing that power, and it’s part of a larger plan. But let’s start with an iROSA. What is this new solar array?

Mike Salopek: Yeah, so iROSA stands for ISS Roll-Out Solar Array. So, this company out in California called DSS (Deployable Space Systems) — I think they were bought by another company called Redwire within the last year – but they, this is their intellectual property, right? Like, this is their design. They’ve designed these roll-out solar arrays. And we actually did an experiment on ISS, I think, back in, like, 2016 or 2017, where we kind of tested out a proof of concept of this design when we were getting ready to select how we want to upgrade our solar arrays, and obviously it performed well. And we ended up selecting to go build six of these solar arrays to upgrade our legacy arrays. What’s nice about them is that the original solar arrays, oh, man, I mean, they were probably designed back in the early ’90s, right, with the best technology available at that time. Well, solar array technology and materials technology, all these things have progressed hugely since, you know, the early ’90s time frame, including the ability to use more composites, things like that. Solar cell density is a lot higher than it used to be. They’re more reliable. They last longer, things like that. So, all of that is incorporated in the design, but the kind of, the secret sauce of it is the way that they are deployed and stored, right? So there’s, if you think of kind of a wrapped-up, like, paper towel tube or something like that, the array blanket is completely wrapped around that, as with the structural support boom, which is like a composite-like carbon fiber boom that deploys, and that provides the structural rigidity. So instead of metallics and the little beams that we have, the longerons on the mast canister, it’s now a carbon, carbon fiber composite boom, and there’s a twist to it to help provide the structural stability and rigidity that we want for the solar arrays. So, because they’re able to be rolled up, and they’re smaller because we have better density of solar and things like that, we’re able to launch them on a Dragon now. We don’t need a big space shuttle to launch it. We also, a lot of the existing infrastructure with the SSU, the MBSU, the DDCUs, the batteries, that’s all existing on ISS. So, in order to fly these solar arrays we only needed a mod[ification] kit, which is just a structural attachment point to the existing solar arrays, and the iROSAs to fly up. And we were able to fly two at a time, and the first two, you know, were earlier this summer. So, you know, again, like I kind of referred to the old solar array deployment as more of, like, hoisting a sail; these are just like unrolling a sheet of, well, like unrolling your paper towel tube, or unrolling wrapping paper, something like that, right? And it stays rigid once it’s unrolled.

Host: See, that’s nice, and I like the paper towel analogy. That, that really helps me to understand sort of how this thing is rolling out, and what’s cool is, if you see it in the Dragon, the unpressurized trunk of the Dragon, it’s kind of like you got this paper towel, and then the way I imagine it is, it’s like if you break that into two pieces, and fold it in half, you can shove two of them in the trunk of the Dragon, which is pretty remarkable considering that the legacy arrays were flown on the shuttle. Now we’re taking up a lot less space, right? You’re folding them up and putting them in the trunk of the Dragon. You mentioned DSS, which was bought by Redwire. I know Boeing has the contract for the solar arrays, and with Redwire as the sub to make and deliver these. And they’re going to produce a decent amount of power, now that they’re unrolling, right? So how does it work? Now, I think, let me ask you this, because what’s interesting here is, it’s not a solar array replacement. We’re not putting this in a new, a new area. It’s a solar array augmentation. So, what does that mean?

Mike Salopek: Right, so the existing legacy solar array, we’re still using about half of it. So, the iROSA, it attaches to the mast canister. There’s a structural attachment that I referred to already as the mod kit. It’s just a couple of support beams that are attached at angles with the mast canister that we’re able to bolt on. There’s a couple of wire harnesses that allow us to kind of jumper between the new solar array and accept power from the old solar array, and route that into that SSU to control the, the voltage that the arrays produce. And then that, the iROSA actually rolls out, and it covers about half, a little bit more than half, of the existing solar array. And those power strings are essentially deactivated. So, the existing, you know, I think I said the existing solar array is 115 feet long. The iROSA is 60 feet long. The old solar array was 38 feet wide. The iROSA is only 20 feet wide. So, it’s, it’s smaller. It does contain less strings, so there’s only 48 power strings on the iROSA, but that replaces the 48 power strings that we’re shading, actually, we shade — yeah, I think we shade about 48 power strings on the, on the old array as well. So, total I think, from a power string perspective, there’ll be about the same as what we had before, but these arrays are designed to produce 20 kilowatts at the end of their life, with only 48 solar cells, right? So, the old arrays were also designed to produce 20 kilowatts at the end of life with, you know, many more strings, strings on them. So, it kind of goes to show you how far the technology has really, you know, really advanced there. And as I said, you know, we’re able to get some power out of the existing arrays. It varies on which one, because of some of those other factors I talked about before. The older arrays are more degraded than the ones that came up, you know, back in 2009. Some arrays have more failed strings on them than others, and things like that. But yeah, we’ll be able to produce much more power from each of these power channels once these, all the iROSAs are up there. Yeah.

Host: That’s fantastic, yeah, because what’s cool, I think, is the way you described it was perfect. It’s essentially, it rolls out right in front of the other ones. And it sounds like you’re deactivating whatever is shaded, and then whatever this small amount of space, like slightly more than half of the original solar array is generating about the same, which is, which is remarkable. But it’s kind of cool, because you said how simple this design is. You have the roll-out solar array, and you’ve got basically a modification kit as a mounting bracket to attach it to the mast canister. Now, it’s really just doing all the same stuff as the arrays. You’ve got — there’s no changes, it seems like, to how you control the solar arrays. You can pretty much do the same stuff. It’s just collecting more power.

Mike Salopek: That’s right, yeah. Because again, we were able to take advantage of the existing infrastructure that we already had on ISS, right? So, we have the beta gimbal assembly, the BGAs, and that’s what rotates and points the arrays toward the sun. And then we have the big solar alpha rotary joint, the big gear that rotates all of the arrays 360 degrees around every orbit. So that rotates very slowly. Those BGAs are used to bias the arrays toward the Sun, but also help us with that longeron shadow, shading issue. But yeah, so it’s really, we were able to drastically increase the power output with a relatively, I mean, relatively speaking, simple modification, because of how much other stuff we were already able to take advantage of. And that’s why Boeing, you know, Boeing integrates all this stuff for ISS. They’re the prime contractor for ISS, so they’re familiar with all the other hardware. And they were able to find the right vendors to go build the components that we needed to make this work. And it’s not just the solar arrays: there was the mod kit. There was different companies that went and built that, and those fly separately. We’re actually getting ready to install mod kits for the next set of arrays. The EVAs (extravehicular activity) for those, that mod kit is next week. I don’t know when this will air, but on the 25th of August is when the next set of EVAs will be for installing the mod kits. [Editor’s note: those spacewalks were postponed.] And then, our next set of arrays, wings three and four, will arrive, they’re slated for SpaceX 26 next summer. And they’ll be delivered to KSC (Kennedy Space Center) before that, but there’s some other work that has to be done. Part of what it was, was what we call the flight support equipment, FSE, the carrier that actually the arrays are installed to, so they can fly up safely in the Dragon trunk, and they don’t get damaged due to the launch vibration environment, and things like that. And then, that also allows that has all the grapple fixtures, so we can extract it from the trunk with the robotic arm and position it to the right work site for the crew. So, these are all the other components that the Boeing project had to go design, and oversee, and integrate all together. And a lot of that work is done at KSC after the sub-vendors kind of build all these other components.

Host: Yeah, we saw a little bit of that during those, during those spacewalks. Essentially, they pulled out the flight support equipment with all the right stuff on top of it, and you’re right, they put it right near the work site, so astronauts can do a spacewalk. And that was essentially their garage, where they went and got all the tools necessary, all the pieces, so they can eventually carry it over to where it was ultimately going to be installed. The installation was pretty interesting itself. We had, there was a little bit of trouble to aligning the solar array on the mounting bracket, and they came up with some pretty cool workarounds to eventually get that thing secured. Do you remember what that was?

Mike Salopek: Yeah, so we’re actually addressing that issue on the next four arrays, and we’ve figured out what we need to go do to fix it. So hopefully, that will, it actually won’t happen again because we’re actually also going to test it on the ground. But essentially, what happened was, so again, if you imagine the way that the array is packaged, it’s like a tube but it’s hinged. It’s cut in half, and there’s a hinge. So, and that’s the way it, it flies up. So, when we install it on orbit, we install one side to the mod kit, and that’s what’s called the soft capture system. And it’s basically just like a latch, and there’s a slot on the iROSA side, and a latch on the mod kit side, kind of almost like a door latch, right? And, and, one, the crew installed one side. That went fine, because that was a straight-on installation. And then when it was time for us to open up the hinge and swing it over to soft capture it to the other side of the mod kit, so now we have the whole array kind of unfolded from the middle – it’s still rolled up, but the roll is now, both sides of it would be attached to the mod kit – as the hinge was swinging over, the slot on the solar array side was not wide enough to, when you, the angle in which the hinge was coming in on the latch, on the mod kit side, we didn’t necessarily fully account for how that was going to hit the side of the slot. So, we basically figured out we just need to make the slot a little wider, which is easier said than done on existing hardware that has all the structural analysis and the stress analysis, but we figured it out. So, what happened was that that latch was interfering with the slot. Now, fortunately, there was enough kind of play in the slot on the other side and the hinge to allow us to kind of strap it and pull it over, so we can get it latched, and I think this is something that we’ve all kind of fundamentally experienced. If you’ve ever tinkered with anything around your house with a hinge on it, and how, like, you make all these measurements, and you think you’ve got it lined up, and then you go and move the hinge over, and you’re off by a little bit. Because of the way that the angle of the hinge, the sweep angle of the hinge, doesn’t quite line up with something that’s, you know, you’re trying to mount with something head-on. And that’s essentially exactly what happened on orbit. Again, we figured it out. We actually just modified wings three and four, where we sent them back to the machine shop, and they machined out the slot a little bit wider. And we’re actually going to do a test of it when the array is completely built. One of the last things we’ll do is, we’ll test it. They have a test stand out at DSS that will allow us to swing open the hinge, and make sure it latches appropriately with the mod kit side, and we won’t have that problem anymore. Which is great, because you never want to launch hardware that you have to rely on the crew to come up with something on the fly to work around. It happens almost all the time, especially when you’re interfacing hardware built by different people and at different times, existing legacy hardware as well. I mean, it’s actually kind of amazing how few problems we end up having, and that really goes to show that we — everything is very well-documented, it all comes down to having very good, strict interface requirements where the critical dimensions are well-known, and things like that.

Host: Right. Yeah, absolutely. You got me chuckling over on this side for a bit, because you were talking about, you know, just something that happens around the house, and, Mike, I am, like, I’m one of those people that, when I try to hang a picture, you know how you could do everything in your power to make sure the hooks are aligned just right. You bring out the level, and make sure it’s, you know, perfectly level, completely parallel to the wall, or the floor, or whatever. And then you put it up, and it’s crooked. And you just roll your eyes. Yeah, that level of aggravation, I totally feel for the engineers, if it’s anything like hanging a picture, probably a little bit more complicated, but —

Mike Salopek: Little bit more complicated, but similar frustration each period.

Host: — yeah, I’m definitely one of those people, so I do not envy them. But this is part of a larger strategy. You already alluded to a couple; we’re already starting the work to get future solar arrays to the space station. Now, the interesting part about this is, there are eight solar arrays that are part of the legacy makeup, and that picture of how you imagine the space station. But we’re only going to do six of them, or we’re only going to augment six of them. Now, why is that?

Mike Salopek: Mm-hmm, yeah, so that’s a really good question as well, and I think it kind of gives good insight into the decision-making process and the kind of job we do over on the program office side. So, you know, starting back, I think they started looking at wanting to do an ISS power augmentation back in, like, 2016, 2014 time frame, because, you know, there’s multiple organizations across the program were looking at the current rate of solar array degradation. What are our near-term power usage targets? What are our long-term power usage targets, things like that? And we realized, hey, we’re going to have a problem in, you know, X number of years, so we need to start doing something about it. And so, then we just, so now, you have to go look at, all right, so now we’ve identified a problem, we realize we have to go do something about it, now you got to figure out how much of the problem you want to address, and do you have a need to completely address the entire problem, or can you only, you know, do we need or can we accept some smaller fix? Because obviously, that costs less money, and it allows us to use that money for other things that we might need to go do. So, it becomes a priority trade at that point. So, when they were looking at the degradation rates of the solar arrays, and also the power draw requirements on each power channel, they’re not all exactly equal. Some power channels are used to specifically power the ISS core systems, the things like the lights, the coolant pumps, the fans, all that stuff that keeps the crew cool, comfortable, and able to work. There’s other power channels that are specifically dedicated just for payload. There’s, you know, each module is powered by different power channels. What are our commercial partners, what power channel are we going to give to them? Our international partners, what power channels do they get from, what are their long-term needs? So we’ve looked at all of that as a big detail assessment, and we identified that the first two power channels that we absolutely need to go do are the ones that we did, you know, this, this summer, which were the 2B and 4B power channels. 2B is the core ISS power system. 4B is used a lot for scientific payloads and things like that, both internal and external. That was our oldest solar array, so it was the most degraded. So, we kind of get the most bang for our buck there. That one was a no-brainer. Hey, we’re definitely going to go build two solar arrays. Now it becomes a, well, if we’re building two solar arrays, do we, are we able to increase the amount of power on some of these other channels? And if we’re already building two, do we want to build four? Six? Eight? What’s the right number, right, for, do we get some sort of, is there, now that we’ve sunk in all this cost of design, how many more do we want to go build to help solve our future problems? And you’re balancing that against, again, how much budget do you have, when do you have the budget, what are other things that you need to go do? It obviously takes a lot of time and effort to go install these solar arrays with multiple EVAs. It takes upmass on visiting vehicles. These are all very complicated factors that you eventually have to address. So then, we kind of narrowed it down to, we definitely need to do, that’s how we identified with the six arrays, then. We said, hey, we definitely needed to do 2B and 4B. You’ll have to forgive me, because I forget which two we’re going to do next year right now, but that was, like, the next in line, like we definitely want to upgrade these. And then, the last two, they’re primarily being upgraded because that will allow us to really make sure that we could give enough power to our commercial, to the commercial module coming up. So that was kind of the priority order and decision-making process. After that, the last two that are the newest solar arrays, those power channels maybe weren’t as heavily loaded, and we feel like that they could last, you know, as long as we need to for the space station. So, again, it’s kind of a, you know, that was a high-level kind of explanation of all the variety of factors we look into. I’m sure there’s other things that I’m forgetting, but it’s never just as easy as, hey, let’s go build this many things because this is how much power we’ll need, right? You got to go look at all of the other factors, and where you want to spend your time, effort, money, other resources that are not necessarily obviously, like upmass and things like that.

Host: Yeah. And then there’s the redundancy factor, too, right? So, you said each of the power channels, you sort of prioritized it, but I’m sure there’s, into it is built, you know, we’re providing more than necessary, just in case something goes wrong, we still have enough power to carry us through whatever. I’m sure that’s built into it as well.

Mike Salopek: Yeah, that’s right as well. So, you know, the internal systems, there’s redundancy there. There’s parallel DDCU sets that allow if one power channel’s having an issue the other power channel can kind of pick it up, and things like that. And those are a lot on our payload channels. So that was also looked at as well, and we kind of determined it’s not, while it would be useful to maybe go upgrade those last two power channels, it’s not the best cost to benefit ratio.

Host: Yep. Now, I think, I think another benefit here and this is, I know we’re running out of time here, so I’ll make this my last question, is the ROSA technology is, you know, we’re using that. You said you went through a lot of tests to make it work for the needs of the International Space Station program, and all this great stuff we want to do. But I think what’s really cool is that that same technology is going to be used for future NASA missions. ROSAs are going to be on the Double Asteroid Redirect Test mission that’s going to go on a probe, and it’s going to crash into an asteroid. And then there’s the Gateway that’s going to use them. They’re going to be pretty big on the Gateway, but that’s, you know, you’re going to need power when you’re in orbit around the Moon. So, it’s pretty cool that we’re testing this technology now in low-Earth orbit to prepare us for all this great stuff we have coming up.

Mike Salopek: Yeah, that’s exactly right, and, you know, that really is another key goal of the ISS, right? So not only do we want to facilitate science and research for industry and educational institutions, and other agencies, and things like that, but as an agency we want to go beyond low-Earth orbit. We want to try to develop the technologies and techniques that we need to go do that. So, the ISS is an extremely valuable test bed for doing that, and the fact that we were able to go run a flight test of an iROSA for, it was just a ROSA at that time on ISS to validate that technology, and now we’re able to operationally use them. We’re going to learn more from that, that those lessons can then be applied to these future programs as well. I mean, it’s just, I can’t stress enough how valuable it is to be able to use ISS continually for things like that. And even in my old job, when it was the life support project that I was running, we were building life support systems for future exploration missions beyond low-Earth orbit, and we were flying them to ISS and testing them and they’re going to replace the core system there. It’s almost very similar, it’s like an augmentation to some of the life support systems on ISS. It’s almost exactly like what we’re doing with these solar arrays as well, and it’s just, you know, the ISS has been an amazing asset in order to do that. And it’s one of the ways that we’ll make sure that we’re successful exploring deeper into space. And again, not only for human spaceflight but for, you know, non-human spaceflight, or the probes that we send out, right? If we’re, some of these technologies with solar arrays or computer systems and things like that, can be applied to other parts of the agency as well that aren’t necessarily directly related to human spaceflight. So, it’s very exciting.

Host: Absolutely. Mike, what a perfect way to end right there. This was absolutely fascinating, Mike, I got to say, because me personally, I’ve worked on these arrays for, for years now, just getting ready and putting out the messaging, making sure that we had an understanding of what the overall plan was, and communicating that. And then, eventually, executing the spacewalks, and this is by far more information than, than I’ve uncovered in those years, you know, just in this short amount of time. So, I really appreciate you taking the time to walk me through the, all the aspects of what’s gone into making the solar array and power augmentation plan a reality. So, really appreciate your time, Mike. Thanks for coming on.

Mike Salopek: Thank you very much. I enjoyed it as well, and happy to come back and talk more at any other time. [Laughter]

Host: I’ll hold you to that. Awesome.

Mike Salopek: All right.

[ Music]

Host: Hey, thanks for sticking around! I hope you learned something new today, because I certainly did from Mike. It was his descriptions of the station solar arrays were absolutely amazing, and I’ve been following the solar arrays for quite some time. So, I hope you did learn something today. If you want to know what’s going on aboard the International Space Station, whether it’s the solar augmentation project, or something else aboard, even the science experiments that Mike pointed out, there’s a lot that’s coming up, and it’s all available at NASA.gov/iss. We’re also on NASA.gov/podcasts. You can find us there and listed along with many of the other podcasts that are across NASA, the entire space agency. Make sure you check them out as well. Houston We Have a Podcast, though, is on social media under the NASA Johnson Space Center pages of Facebook, Twitter, and Instagram. Just use the hashtag #AskNASA on your favorite platform to submit an idea for the show, and make sure to mention it is for us at Houston We Have a Podcast. This episode was recorded on August 16th, 2021. Thanks to Alex Perryman, Pat Ryan, Norah Moran, Belinda Pulido, and Jennifer Hernandez. Thanks again to Michael Salopek for taking the time to come on the show. Give us a rating and feedback on whatever platform you’re listening to us on and tell us what you think of our podcast. We’ll be back next week!