Very low earth orbit satellites, known as VLEOs, have become important in a variety of surveillance and imaging applications. Trouble is, they’re so low, they don’t stay up very long without propulsion to keep them up. Eventually, they run out of fuel. That’s a problem the Defense Advanced Research Projects Agency is trying to solve with a program known as Otter. For details, DARPA program manager Sarah Popkin joined the Federal Drive with Tom Temin.

Interview transcript: 

Tom Temin Dr. Popkin, good to have you with us.

Sarah Popkin Thank you for having me, Tom.

Tom Temin And let’s begin with VLEOs. What does it mean? Are they different from regular LEOs, low earth orbit, and what are they generally used for? And are there a lot of them?

Sarah Popkin So, the biggest difference between, let’s say, other spacecraft and VLEO spacecraft is the fact that they cannot ignore atmospheric drag. It is something that they have to face constantly. They have to face it at magnitudes greater than a typical, let’s say, low earth orbit or LEO satellite. So, in order to overcome that drag, you need some kind of propulsion. You need some fuel to feed that propulsion system. But eventually, if you want to operate at durations, let’s say on the typical five- to 10-year timeline for satellites, you need a lot of fuel, so much so that it really becomes impractical. So, there are some VLEO satellites there, but they’re at somewhat the upper range of VLEO altitudes. And so they’re sort of starting to address the drag problem with onboard fuel. But so far, as far as I’m aware, there’s no other air-breathing electric propulsion system that actually harvests air like we’re trying to do.

Tom Temin Right. We’ll get to that. So the idea, though, is that to get them to stay up, as long as the electronics will last, you need enough fuel. So the fuel becomes the limiting factor. It becomes uneconomic, I guess, if they’re always crashing and burning in the atmosphere. You got to replace them a lot.

Sarah Popkin Correct. You would have to replace them quite frequently, so it becomes a cost problem. Basically, if you had to constantly throw up another one, you have to deal with launch costs, new satellites, it just, it starts to break the bank.

Tom Temin And there has been a lot of commercial activity in low earth orbit and VLEO satellite launches. But if DARPA is involved, it sounds like the government is also in this space. The space of space, let’s say.

Sarah Popkin Yeah, correct. So, DARPA is in the game of VLEO, but we also work with the Air Force Research Laboratory. So they are very interested in very low earth orbit as well.

Tom Temin All right. So your project, Otter, is a way to give these VLEOs engines that self-propelled based on the air that’s in the atmosphere at that height.

Sarah Popkin Yes. So, the way Otter is set up, it’s all about developing air-breathing electric propulsion so that we don’t have to carry our fuel, but we can actually collect the sparse ambient air that is around us and direct that air to a thruster, essentially an electric propulsion thruster. So that’s how Otter is set up, trying to collect the air and also feed it into a thruster that can survive the air environment. So, another challenge with VLEO is that there’s this nasty particle called atomic oxygen out there that loves to eat away at any component, especially if it’s a warm component. It just is eager to find a friend. And sometimes that bond is not a friendly bond and it starts to eat away at materials.

Tom Temin What we used to call rust.

Sarah Popkin Yeah, rust for space. Space rust.

Tom Temin All right. Well, air-breathing, I mean, every engine is air-breathing because you have combustion. You need oxygen. So what do you envision actually happening in this process to create propulsion without fuel?

Sarah Popkin So, I’ll make a clarification. So, in space electric propulsion there is no combustion. Air-breathing engines that we experience on the ground, such as with, you know, if you’re on a flight going across the country or if you’re a ramjet or a scramjet, then yes, you’re going to collect air, you’re going to mix it with fuel, and that air will sustain enough pressure to create heat. And then you get combustion and it comes out the back end. It makes thrust. There is just not enough air in VLEO to sustain any kind of what we would consider chemical combustion. So this air-breathing electric propulsion, it’s just that. It’s electric propulsion. So you provide the air into a thruster and you ionize it. So you turn it into pluses and minuses, and then once it’s ionized and you can start exerting, let’s say, a magnetic force or electric field on those ionized particles and it forces it out the back end. And that’s how you get thrust. The challenge with air is that if you’re familiar with a periodic table, it’s kind of high up. It’s nitrogen and oxygen. So it’s kind of high up on the periodic table, which means that its electrons are really close, which means that it’s hard to force those electrons off. So it’s harder to ionize it. Essentially, compared to traditional space propellants, which are krypton and xenon, they’re much lower, easier to ionize. So you don’t need to dump as much electricity into the air or into the propellant in order to get your thrust. So that’s another challenge of the air-breathing electric propulsion, is not only how do you collect the air, but also how you efficiently ionize it. Because it’s just physics. It’s hard to ionize.

Tom Temin Got it. We’re speaking with Dr. Sarah Popkin. She’s a program manager in the tactical technology office at DARPA. In every system has friction. And so you’re not trying to develop a perpetual motion machine in space, correct?

Sarah Popkin Correct. There is definitely an art to designing a low drag spacecraft. This is an interesting domain. Me being an aerospace engineer, I’m used to, or I was trained on Bernoulli’s principle, airfoils, things like that, continuum flow. But as you get higher and higher altitude, you’re no longer — Bernoulli does not apply.

Tom Temin And you can’t fly the barn door with enough thrust.

Sarah Popkin Exactly. I love that phrase. So you need to think differently about how you design your spacecraft. That’s, your air particles are more like billiard balls. Or I like to call them like bouncy balls. They’re basically so far apart from each other that they only interact with the surfaces on the spacecraft. And we are, an area of research that I’m interested in is called gas surface interactions. And it’s essentially looking at, okay, your bouncy ball hits the surface. Is it going to bounce back like kind of when you throw a football at the ground and you don’t know which way it’s going to go, and when it comes back at you it incurs the most drag? Or is it going to be very smooth and it’s going to bounce right off, and then you have very low drag? And then there’s of course, how skinny can you be? Basically the skinnier you are in VLEO, the lower drag you can achieve.

Tom Temin Well, what’s the programmatic approach to getting this research and development done and prototypes built? Is it through grants to companies trying to accomplish this physics and chemistry?

Sarah Popkin So, we have contracts with some industry performers, generally smaller businesses. And what was sort of realized in the beginning is that it’s this is such a new type of technology, that one company that’s maybe very good at thrusters may not be very good at inlets or vice versa, or a spacecraft designer may be accustomed to a different type of challenge area than, let’s say, trying to ionize air or trying to collect air. So there are unique entities participating in order that have unique areas of expertise, and we are funding them through. It’s not a grant, it’s their contracts. And or I guess I’m not sure what the right phrase is for the contracts, but they are working on.

Tom Temin Grant, I mean. In effect, it’s a grant.

Sarah Popkin Well, so I came from AFOSR. So, a grant means a very specific thing. That means fundamental research, and this is not fundamental research. This is applied research.

Tom Temin More in the OTA area.

Sarah Popkin Right. Exactly. More in the OTA area. Exactly, sorry. That’s the right term. And so we are allowing them in this first phase to develop their technology. They are aware of each other within, you know, proprietary boundaries, etc. and then ultimately going forward, if we go forward, then they would be integrated together on a single VLEO satellite.

Tom Temin And by the way, you mentioned inlets and ducts and so forth in the shape of the satellite. It sounds like there is a little bit of aerodynamics involved here, even though, I mean, most satellites are ungainly looking things, as if they’re never going to encounter wind in any way, but VLEOs, you can shape them so that they’re more efficient as they move through whatever little bit of error there is.

Sarah Popkin Yes, it is. And that’s actually where my aerodynamic background really starts to come in, is I know how I’m accustomed to aerodynamically shaped things. And I look at a satellite, a traditional satellite. And you’re right, it is a gangly, boxy thing. In fact, solar panels are potentially problematic, right? Solar panels are a traditional way for collecting electricity, but they may not be the best solution because you’re not 100% of the time facing the sun. And if you were, you might basically turn your spacecraft into some slick aerodynamic shape and then turn it into that barn door just because you’re trying to collect solar energy. So that’s a VLEO video workshop coming up at the end of this month in September, where we’re going to try and encourage the community to look at other solutions for power for satellite design. How do you make it low drag, what materials are the best to use, etc.?

Tom Temin So I imagine NASA must be looking at this also, as well as the defense agencies and intelligence agencies that have VLEOs.

Sarah Popkin Yes, NASA is interested, although I think that most of the interest comes from the Department of Defense at this point. But NASA technology there is definitely relevant NASA technology that’s been invested in that we’re looking to leverage. And so we’re talking with NASA to say, hey, how far did you get with that technology? Maybe it doesn’t apply to what you were going to do with, but we think it could be really awesome for what we’re doing.

Tom Temin And by the way, this idea of air-breathing and ionizing and thrusting and so forth. Could that work in the regular atmosphere at some point in the future? Everybody’s chasing so-called green jet engines and so forth. Is there any or will the whole process fail if you get too far down toward Earth?

Sarah Popkin So one of the realizations I came to when I sort of looked into the VLEO world is electric propulsion. Traditional space electric propulsion provides Miller Newtons of thrust per kilowatt of electricity.

Tom Temin That doesn’t sound like very much.

Sarah Popkin It’s not very much. It was a very, I will tell you, it was a very sobering moment, realizing how little thrust can come from a whole kilowatt of electricity. So if you try and let’s say you’re flying in sort of normal atmosphere and you want to provide tens or hundreds of Newtons of thrust, you just need an incredible amount of electric power. So it just, it breaks down really quickly.

Tom Temin Is there any timeline for this or what? What progress have you made so far to date?

Sarah Popkin For Otter, specifically?

Tom Temin Yeah, for Otter.

Sarah Popkin So, the timeline. So in our program solicitation that we put out, it was a planned launch in the 2027 timeframe. So we are still a few years out from that, but definitely aggressively developing the technology demonstration through test in order to meet those timelines and burn down risk as quickly as we can, as DARPA does.

Tom Temin And by the way, I asked if it could work in lower atmospheres. And the economics, if you will, don’t quite hold up. It sounds like it would not work up higher in space as you get thinner and thinner, because there’s not enough molecules.

Sarah Popkin Your strategy would become different. So the question is, can you harvest air in a way so that you could use it at a different time from when you actually harvest it? So you can use it in two ways. You can envision two ways of using it. You could basically collect it, use it immediately, or you could collect it in a way where you’re going to use it at a different time when you’re at a higher altitude. But yes, you are definitely limited as to how high you can go.

Tom Temin And ultimately, it’s still fuel. And so, no satellite will no VLEO can stay there forever under this process, but much longer than they can now.

Sarah Popkin Absolutely. By being able to harvest your own air, your lifetime limitations start to fall back in the category of regular spacecraft limitations. Except for that space rust, I will say that space rust is a real problem.

Tom Temin Well, you need Earl Scheib up there, I guess, to touch it up from time to time. Dr. Sarah Popkin is a program manager in the tactical technology office at DARPA. Thanks so much for joining me.

Sarah Popkin Oh, you’re very welcome. Thank you again for having me.

Tom Temin And we’ll post this interview along with a link to more information at the federalnewsnetwork.com/federaldrive. Hear the Federal Drive on demand. Subscribe wherever you get your podcasts.

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