How can a satellite the size of a loaf of bread take the heat of operating in the extreme conditions existing in space without overheating? In episode 56, we’re joined by Naia Butler-Craig from the Georgia Institute of Technology to discuss her open access article “An investigation of the system architecture of high power density 3U CubeSats capable of supporting high impulse missions,” which was published in November 2018 in Embry-Riddle Aeronautical University‘s open-access McNair Scholars Research Journal.
Websites and other resources
- Naia on Twitter
- Gizmodo article on SpaceX’s Starlink project
- JPL‘s page on Mars Cube One (MarCO)
- Macscientists article about Naia
- Interview with Naia on Ladies Love STEM
- Interview with Naia for Vanguard STEM
- Podcast on history and economics of CubeSats
An Investigation of the System Architecture of High Power Density 3U CubeSats Capable of Supporting High Impulse Missions
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Hosts / Producers
Doug Leigh & Ryan Watkins
How to Cite
Leigh, D., Watkins, R., & Butler-Craig, N.. (2019, August 20). Parsing Science – Taking Heat in Space. figshare. https://doi.org/10.6084/m9.figshare.9757793
What’s The Angle? by Shane Ivers
Butler-Craig: CubeSats are basically nanosatellites. If you think of a huge one-ton satellite, try to condense it to the size of a loaf of bread.
Leigh: This is Parsing Science the unpublished stories behind the world’s most compelling science as told by the researchers themselves I’m Doug Leigh.
Watkins: And I’m Ryan Watkins. Today, in episode 56 of Parsing Science, we’re joined by Naia Butler-Craig from the Georgia Institute of Technology. She’ll talk with us about her research into the thruster systems aboard miniature satellites known as CubeSats, along with her work protecting the electronics on board from overheating while operating in the extreme conditions that exist in outer space. Here’s Naia Butler-Craig.
Butler-Craig: My name is Naia Butler-Craig, and I am a recent graduate from Embry–Riddle Aeronautical University. I got my degree in aerospace engineering with two minors in computational and applied mathematics. I am currently a GEM fellow at Los Alamos National Laboratories in Los Alamos in New Mexico, doing some computational physics. And in the fall I will be joining Georgia Tech as a PhD student in the aerospace engineering program. And the lab that I was working in is actually called the High Power Electric Propulsion Laboratory, where I will be doing some in-depth study on plasma physics and hall thrusters, and ion thrusters, and anything having to do with ion propulsion.
What CubeSats are
Leigh: In 1999, Bob Twiggs of Stanford University wanted the challenge graduate students to engineer and launch satellites within the time and financial constraints of their degree program. As the story goes, Twiggs was inspired by the small plastic cubes used to display beanie babies, and proposed limiting the size of the spacecraft to a similar one. The first CubeSat was launched four years later. In just 16 years since that first launch more than a thousand CubeSats had been launched, often via the international space station for scientific missions, or as secondary payload on larger rockets if for commercial reasons. Ryan and I began our conversation with Naia by asking her to tell us more about what these satellites are used for, as well as how she got involved with the technology.
Butler-Craig: CubeSats are basically nano satellites. If you think of a huge one-ton satellite, try to condense it to the size of a loaf of bread. The way they’re measure is by Us, or units, so 1U is exactly 10 centimeters by 10 centimeters by 10 centimeters. So 2U is basically two cubes on top of each other. And what they’re used for right now is for a pretty low Earth orbit research. They’re definitely branching out into more complicated missions, and more high delta-v missions. And delta-v just means basically the fuel it takes to get somewhere, or the amount of velocity it takes to get somewhere, or to perform your mission. And so I think the most recent kind of big headline about CubeSats was the Marco CubeSat that actually went to Mars. Took some amazing images. That was pretty revolutionary for the technology as it’s the first time that’s been done. But I got exposed to them through my first internship with NASA Glenn, where I got to work on something called the ALBus CubeSat, and ever since I’ve been pretty fascinated and obsessed by the capability that is showing the potential to take us into some really interesting, cutting-edge deep-space research.
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Watkins: In physics, impulse refers to how well a particular rocket, jet engine, or thruster, performs. Ones with higher impulse allow for performing a greater number of maneuvers, but also require more propellant, and therefore more room for it’s storage. And that’s a scarce commodity on CubeSats. We asked Naia to explain what kind of missions can be carried out with the sorts of thrusters that are typically on CubeSats today.
Butler-Craig: So a low impulse mission is something that doesn’t require too much propellant within your thruster. So you don’t need a lot of thrust to get – or to perform – what you’re trying to perform in your mission. Some CubeSats are actually in little orbit, and they’re completely reliant on just their attitude control system …
Watkins: Just a quick interruption to explain that attitude control refers to systems which administer the guidance navigation and control of objects, including CubeSats, in three-dimensional space.
Butler-Craig: And they have very small thrusters that are just for attitude control so that they’re pointing in the direction in point. So those are typically low Earth orbit missions, but some utilize a little bit of a higher impulse thruster option. A higher impulse mission where you’re just kind of floating in space, and letting gravity do what it will with you. And those are probably cold gas thrusters, which just use cold gas, and push it out the back for some thrust. And that could be used for, you know, some very – just not complex – maneuvers in space.
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Leigh: While the price to buy space aboard the rockets used to launch commercial CubeSats can cost upwards of $100,000, smaller rockets – sometimes called sounding rockets or research rockets – can provide a much more affordable means of launching them. They also have a much shorter lead time than commercial rockets, for which substantial launch backlogs exist in many parts of the world. We asked Naia to tell us more about these alternative launch vehicles.
Butler-Craig: I would usually call them suborbital rockets, and these rockets they don’t really go to space – well, I think they’re capable of doing so – but they’re not like the shuttle-type of rockets. Their objective isn’t to get you fully immersed into space. They basically do suborbital flight, and they kind of hit the cusp of our atmosphere, and they come back down. They’re almost like 30-minute flights, and they’re much faster, and they come down, like, right away. But, that’s another cool part about these CubeSat is they have something called P-POD deployers, which stands for Poly Picosatellite Deployers, which can launch your CubeSat using just some springs from a suborbital rocket and get you into the orbit that you want, instead of requiring enough delta-v and impulse .., and basically fuel … to get you fully immersed in a space like a solid rocket booster would, per se.
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Watkins: In addition to this spring-loaded deployment system, CubeSats are also interesting in that they often feature origami-like designs to maximize the small form-factor of the platform. One of these is something called shape-memory alloys: materials that can be bent into almost any shape desired, and which snap back into their original shape when heated. As Naia used them in the design for her CubeSat, we asked what her experiences were using these types of metals.
Butler-Craig: My sister is seven, and I took her to NASA and they had it, and she was floored. But, but the first demonstration that I saw was a wire of it and it was bent up, bent up, bent up … and then the guy got a lighter and heated it back up, and it snapped back to his original position. And it was very useful for our hinges for our solar arrays. Our team – super intelligent people – they decided to use them for our hinges. So, you basically bend the shape memory alloys in a certain position for launch and deployment, and then send electrical current to them to heat them up, and then they snap your solar panels open. So that was one use of them and the other – I think it’s being published with patents I’m not sure I can talk too much about it – but basically cutting edge things that you can do with them that are a lot more reliable and testable than wire and other regular CubeSat technology that’s commercially available this moment. You can push them down and they can take whatever form you really want to, which is really cool. And so it’s pretty versatile technology and material.
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Ion thrusters and Hall thrusters
Leigh: There are several different types of thrusters that are common among CubeSats. Because of their small size weight and efficiency, two popular means of providing CubeSats with propulsion our Hall thrusters and ion thrusters. These electric propelled thrusters use a variety of techniques to generate and discharge the plasma that propel these satellites during maneuvers, as Naia discusses next.
Butler-Craig: Ion thrusters and the Hall thrusters are both electric propulsion technologies. So what that means is they’re using plasma to thrust. They’re both very cool technologies. It’s been said that Hall effect thrusters are a little bit easier to work with, because you’re leveraging a natural phenomenon. And so the way hall thrusters work is that they leverage something called the Hall effect, which is a natural physics phenomenon. Electrons, as we know, are typically much lighter than ions or protons. And so you have electrons moving around the inner channel of the Hall thruster; the propellant is xenon gas, and it’s ionized by energizing it with electrons, so you now have both positive xenon ions and negative electrons. And as it’s flowing around in a circle within the chamber, the xenon positive protons are too heavy to continue rotating, so they’re kind of thrown out, with a certain momentum depending on a few different parameters of your thruster. That’s the very basic-level explanation of how it works. It’s different than an ion thruster, because ion thrusters energize the gas in the chamber and they use something called a grid to accelerate the ions outside of the thruster and produce thrust in the opposite directions. And when they’re running they have this really pretty blue light, and it looks so sci-fi.
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Science missions aboard cramped CubeSats
Watkins: All of the electronics in a single CubeSat can take up no more than one usable liter of volume – that’s just 72 cubic inches – and weigh no more than 1.33 kilograms or 2.9 pounds. Because of this scientists may link multiples of these 1U devices together into a longer satellite. For example, Naia and her team tested a 3U system by linking three individual units together. Given the size constraints of these satellites, we wondered how scientists managed to fit their scientific payload – or telecommunications equipment – into a device that’s already packed with propulsion systems and other electronics necessary for maintaining orbit.
Butler-Craig: When it comes to design trade-offs your science mission is very limited the further you want to go from space, because your propellant tanks gonna need to be bigger, and you’re gonna need a thruster. And so that’s gonna limit your space for science and whatever other things you have with your CubeSat including something called an Attitude Determination and Control system. They’re available commercially, and they come in about the size of 1U. So that’s taken up if you just have a 3U CubeSat, which is what we were using in my research. And you lose 1U just for Attitude Determination and Control – which is very vital when you’re trying to get around space, because you need to know where you’re at and where you’re going. You lose so much space that your science has to be contained in 1U or less. But people have been so creative, which is why the CubeSats are just such fascinating things. Like, even high schools and colleges have jumped on this research problem and attack different science missions and just leverage, you know, the natural gravity gradient in space for attitude control. They’ve leveraged the magnetic field of Earth using magnet torkers to do their attitude control to make more space for their science mission. So it really depends what you’re trying to accomplish, but most CubeSats: they have something called duty cycles. So, not everything is running at the same time, which is another thing that you can play with in your design: what are your mission modes, and what needs to be on at all times, and how much power … what can only be on in the cold side of your orbit as opposed to the hot side? So all those things come into play when you’re making these decisions, which makes it that much more fun but extremely complicated.
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Avoiding overheating aboard CubeSats
Leigh: CubeSats have to displace both the heat generated internally by their on-board electronics, as well as the heat that they experience externally from solar radiation. So it’s critical to design various systems to ensure that the internal components of the spacecraft don’t overheat, as Naia explains after this short break.
Leigh: Here again is Naia Butler-Craig.
Butler-Craig: So there’s a bit of a temperature gradient. When we’re in our cold side of our orbit, as opposed to our hot side. And that was kind of the scope of my research in my role at NASA at the time, which was to conduct the thermal environment tests, or T-BAC tests. So our orbit trajectory – this was modeled by one of our thermal engineers … within our orbital environment we were looking to only get down to about negative 10 degrees celsius. And our higher point was about 35, and at most like 40 degrees celsius. Compared to our CubeSat which is generating about a hundred watts of heat. A little bit up to like 75 degree celsius. So space does play a huge part in radiating that heat. And because of our strategic placement of our, what is called a heat sink, which is really just a block of aluminum metal, which was meant to absorb all the heat and basically throw it out into the space environment … And, again, we were very strategic with putting it at the end of the chassis – and the chassis just the outer frame of the CubeSat – so that it would interface directly with space, but it wasn’t inside in the middle and that’s how we kind of controlled the flow of heat. So that temperature gradient can do a lot of damage to electronics and that’s what we decided to test during this T-BAC test, to make sure all of our flight hardware could survive it.
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Do xenon tanks create temperature changes?
Watkins: Some listeners may have noticed that propane tanks on gas grills tend to get cold when the gas is being used by the grill to create heat. Given that ion thrusters use xenon gas stored in a small tank to produce the plasma for propulsion, Doug and I were interested in whether temperature changes are brought about by the xenon tanks themselves.
Butler-Craig: Because xenon gas is a noble gas, and it doesn’t interact with much, I doubt it would get hot. Just because as long as nothing’s interacting with it, or it’s not interacting with anything. And it’s not charged; it’s still in the gas form. It’s pretty stable within your propellant tank. The only way it would get hot was through energizing it with those electrons, and because that’s happening – or should be happening – pretty far from your propellant tank. And, of course, there’s valves and systems in place within your xenon feed system that should prevent any backflow of electrons into your propellant, because then that would definitely cause issues, and probably erode your propellant tank. But, otherwise I don’t think that it would it would get hot or cold unless it was just being acted on by the external surrounding environment.
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Leigh: Naia’s research into heat dispersion was a technology demonstration project, meaning that she carried out her tests on actual CubeSat devices, but here on Earth, using some components designed to mimic the conditions that the satellites would experience in space. Ryan and I followed up by asking Naia what some of the specific sources of heat are on CubeSats. We were also curious to learn if the ion thruster is being researched and developed at NASA’s Jet Propulsion Laboratory also add to the heat which must be dissipated.
Naia: Our heat was coming solely from something in our spacecraft called a Discharge Board, which is just a bank of resistors and its entire purpose was to generate heat. But we did put some thermocouples on the rest of our electronic boards, and they were generating, you know, some very small margins of heat. Because of course we’re in the vacuum space, and all you have is radiation. And basically conduction through these interface points within the CubeSat that heat would spread to the rest of the CubeSat. So the point of these interface points, and really thermal conductivity points, was to basically evenly distribute heat instead of keeping it on the electronic board, because of course you don’t want to ruin your electronics. And so it really depends on the spacecraft, just what it’s using but the PCB stack-up – which is basically it could be like your the brain of your entire spacecraft – it’s capable of generating heat because it’s working so hard to function, or help everything else function. And it’s all electronics which, of course, uses electricity which can get hot. But in the case of having an ion thruster, there there is a bit of heat management that needs to be considered, because it’s straight energized plasma – or energized particles – that’s being generated within the CubeSat. And because, you know, we’re using metal for most of our parts those things can easily dissipate to the rest of your spacecraft, which would be very harmful. But the CubeSat that I’ve worked on do not have an ion thruster – I used something called engineering design units – which are basically some preliminary units; they’re not going to really be used or just kind of used to figure out how you want design your spacecraft. And then I use spare parts, and then some even dummy boards. So ,I just wanted to see if the thermal management system on the cubes that I was working on would be enough to handle an ion thruster onboard with it.
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Thermal vacuum testing chambers
Watkins: Most of us have a concept of what a vacuum chamber is. But thermal vacuum chambers are those in which the temperature inside of the chamber can be controlled for scientific testing purposes. We asked Naia how the chambers at the NASA Glenn research center were used in her research, and what it was like working with them to stress test the CubeSats.
Butler-Craig: Thermal vacuum chambers are instrumented with patch heaters, or a thermal shroud, to heat the inside of the chamber to whatever temperature you want. And to cool it down you use liquid nitrogen which will spew from one of the sides of the chamber and drop temperatures down to as low as, I think, when we were just kind of feeling it out we got as low as negative 100 degrees celsius. And the chamber that we use got down to about 10 to the minus 6 torr, that’s called vacuum rated. That’s, you know, very high vacuum. And they come in all different shapes and sizes I mean I’m the size of a table, to the size of a lunch box, to the size of a room, and a house. Glenn has some amazing vacuum technology capabilities which has made them one of the best in the world. They have something called Plum Brook Station in Ohio, and they have the biggest vacuum chamber in the world. And Avengers – I think the first Avengers – was actually filmed there, which is really cool. And I got the tour that place. So it’s fun. I would like to get my hands dirty like that. And I like to not just learn about the science but the engineering and even, you know, the mechanics of everything, and what it takes to create your test, or create a science setup for an experiment set up. So that included temperature instrumentation, like thermocouples. It included the vacuum pumps, so turbo pumps, cryogenic pumps, roughing pumps … which are usually used as auxiliary pumps. They get you down to a certain pressure level and then a more sophisticated vacuum pump will then kick on to get you down to the vacuum rated levels.
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Hurry up and wait
Leigh: While simulating the conditions of outer space sounds like a lot of fun, science can also be filled with long periods of waiting, as well as mishaps. With this in mind, Ryan and I followed up by asking Naia to tell us more about her experiences with testing CubeSats and those thermal vacuum chambers.
Butler-Craig: As cool as it may sound, you know, you have to wait for your – first of all your shroud – to heat up and cool down. But you also have to wait to get down to vacuum rated levels, and that can take a very long time depending on the volume of your chamber. Ours was – it was about the size of a big table and volume-wise, you know, they’re cylindrical … so extruded up. So it’s a pretty big tank, and it took some time to pump down. And when you mess up … if you mess up, or maybe you forgot to put something, or instrument something, on your CubeSat – which is sealed in a vacuum chamber – you have to first vent the chamber. Which means you have to let it get back to atmospheric pressure, which takes some time, because you can’t just overload it, you know, you don’t want to break anything or, you know, kill your CubeSat. You have to unseal that chamber, and fix whatever you have to fix, and then put it back and do the whole thing over again. And sometimes we would overshoot. And that’s why my experiments did not use any flight hardware, or anything that would be expected to go to space. So if we overshot and we went way too high on our temperature on the thermal shroud then, you know, we didn’t kill anything important. And when you go way too low you can freeze your electronics; and it can be a very dangerous thing as far as the safety of your hardware is concerned. So it does take some time in some careful preparation.
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Watkins: In May 2019 SpaceX’s Starlink project received a lot of criticism because of their deployment of 60 satellites – of 12,000 planned for orbit – in order to deliver high-speed internet to customers around the globe. In particular, astronomers objected because they believe that the satellites will severely impact their scientific observations, as briefly occurred during this Starlink deployment. Listeners may remember our discussion with Alice Gorman about space junk in Episode 6 of Parsing Science, available at parsingscience.org/e6. So we asked Naia to share her thoughts on whether we might be launching too many of these kinds of satellites.
Butler-Craig: I think it’s definitely prone to become an issue, or it can. Starlink just sent up just a multitude of satellites. And so it’s an incredible feat it’s really cool the way that they did it, but yes, it does pose a big risk and a safety issue for the things that are flying outside of Earth and flying in space especially, you know, our astronauts.
Watkins: One final interruption to explain that satellites can be classified by the height of their orbits. CubeSats tend to operate in low-Earth orbit – abbreviated as “leo” for the acronym LEO – or about 300 miles to 1,000 miles above the Earth. Then there’s geostationary satellites which follow the direction of the Earth’s rotation at about 22,000 miles above the equator, and are referred to as being GEO satellites.
Butler-Craig: There have been policies put in place by NASA to try and regulate this, but LEO is very densely populated with space junk. And I think if we keep it up so will GEO. But if Starlink is sending them up to geostationary orbit where it’s less populated and – you know, the ISS is not there – there’s not many schools sending anything up that high, because of course you need stuff like thrusters that not as commercially available as they will be in the future. So I think it’s okay in that sense, but I’m sure space policy makers will step in as it gets to be too much, because schools are becoming very privy to the technology. And everybody wants to send up a CubeSat because it’s pretty easy, and you can do a lot of cool missions and science with it. But I am definitely concerned about our LEO. Some junk has gotten as far as GEO, but I just think it’s less dense up there. Plus ISS is not really a – it’s not a threat to the ISS. Well it’s up there, but I know Amazon is looking to launch a few satellites for the same reason as SpaceX. Snd it’s cool because … it’s interesting that, you know, they’re trying to get wi-fi out to everyone but I just hope that, you know, they’re thinking about the considerations for, you know, our space environment.
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Racial and gender discrimination in science
Leigh: In June, Naia we did quote, “Today a man looked me dead in the eyes and told me my job prospects as a white man are much lower than yours is a black girl because there’s so few of you. It’s a real problem,” end quote. Since breaking down the discrimination that has long plagued the scientific community has finally become a topic that’s gaining global attention, Ryan and I thought it was important ask Naia about the challenges that she, as a young African-American woman, has overcome.
Butler-Craig: Yes, I did get that comment. And what was worse to me is that, kind of, the specific verb usage, you know, was something that we deal with a lot as black woman or really a minority in this field. And it’s microaggressions, so it’s not blatant sexism or racism, but you can very much tell that … you can taste the hints of it in certain situations. And so he referred to himself as a white man and me as a black girl, which was … which I found kind of dimunitive and patronizing just because, you know, in the workplace it doesn’t matter how much younger I am: we’re colleagues, and thus should, you know, be on a level playing field. That was, you know, one experience, and then him talking about his job prospects, and being very adamant about it; trying to back it up with research, that I’m not sure where he got but, you know … it just kind of enforces a very false idea that minorities are chosen to do great things in amazing workplaces to fill some kind of quota, and that their intelligence and background and experience they’re null or, you know, they don’t matter because they fill the quota. While he may have had good intentions – I really don’t know, I don’t think he was a bad person – his statement was really telling me to my face that I’m not smart enough to get here on my own merit but I was chosen because I’m a black woman. And, you know, to call it “a problem” too, which I thought was a problem …. and I think that companies are getting privy to the fact that this equality thing, like, it’s pretty hot right now. And I’m not really upset about that, but of course they should be doing so genuinely and not just token hires. But that’s definitely revealed when you’re hired. I don’t think anybody gets in job for being black or hispanic or a woman, and they can’t tell. The work I’ve got to do I can definitely tell I was chosen because I knew what I was doing, or at least could learn what I was going to be doing. So that experience is definitely not unique. It’s things that can, you know, fall like water off my back now because it’s so typical that it really doesn’t faze me as much. But what I loved about my interaction was how quickly and effectively my mentor stepped in to basically stop him and, you know, tell him that’s not true. Stick up for me in a place where, you know, you’re afraid to speak up a young student … you know, you don’t want to jeopardize your job on your first day. So she used her power and her authority to really be my advocate.
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Links to article, bonus audio and other materials
Watkins: That was Naia Butler-Craig discussing her paper “An investigation of the system architecture of high power density 3U CubeSats capable of supporting high impulse missions,” published in November 2018 in the McNair Scholars Research Journal. You’ll find a link to her paper at parsingscience.org/e56, along with bonus audio and other materials we discussed during the episode.
Leigh: Reviewing Parsing Science on Apple Podcasts is a great way to help others discover the show. If you haven’t already done so, head over to parsingscience.org/review to learn how to do so. Or, if you have a comment or suggestion for future guests or topics, visit us at parsingscience.org/suggest … or you can leave us a voice message, toll free, at 1-844-XPLORIT, that’s 1-844-975-6748.
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Preview of next episode
Watkins: Next time, in episode 57 of Parsing Science, we’ll be joined by Karen Macours from the Paris School of Economics. She’ll discuss her research into the problems with using the popular Big Five personality measure outside of western, educated, industrialized, rich, and democratic settings.
Macours: So once we have all these data sets we then can certainly compare … analyze to what extent the pattern in the data – the correlations within the data – coincides with what you would expect if these data indeed measure the Big Five.
Watkins: We hope that you’ll join us again.
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