On episode 296, we are joined by rocket scientists who discuss a new revolutionary rocket engine design tested at NASA’s Marshall Spaceflight Center last year. This episode was recorded on June 7, 2023.

Transcript

Gary Jordan (Host): Houston, we have a podcast! Welcome to the official podcast of the NASA Johnson Space Center, Episode 296, “Advanced Rocket Engines.” I’m Gary Jordan, 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 and more. Last year in 2022, a revolutionary rocket engine design was tested at NASA’s Marshall Space Flight Center in Huntsville, Alabama. It’s called a Rotating Detonation Rocket Engine, or RDRE. It’s a kind of rocket design that’s only been theorized until modern advancements allowed for this once-only-imagined technology to become a reality. The rotating detonation design is fundamentally different from how a traditional rocket engine works, and the traditional rocket design is one that is very widely used. So when I mentioned that this rocket engine design is revolutionary, I mean it. We’re talking about a potentially disruptive innovation, but we still have a ways to go and we’ll get into that. Now, to explain this kind of design, how it works and what it means, we’re going to have to bring in some rocket scientists. On this episode, we have Tom Teasley propulsion systems development and test engineer based out of NASA’s Marshall Space Flight Center in Huntsville, Alabama. And Steve Heister, Raisbeck distinguished professor at Purdue University, and co-owner of a small business in space, LLC, located in the Purdue Research Park in West Lafayette, Indiana. All right. Let’s get into it. Enjoy.

[Music]

Host: Steve and Tom, thank you so much for coming on Houston We Have a Podcast today.

Steve Heister: Thanks very much, pleased to be here.

Tom Teasley: Yep, likewise, pleased to be here as well. Thank you.

Host: Awesome. This is a really cool topic, advanced rocket engines. I need you guys to help me to talk about this, because this is a very interesting concept and it’s revolutionary in the sense — next to even something as complicated as a traditional rocket engine. And so, we’re just sort of going to lay out the foundation. You know, there are a lot of ways that you can get involved with spaceflight. And there’s, for me, I like to joke that mine is relatively easy compared to yours. Thinking about rocket engines, you guys are actually rocket scientists. And so, I wonder- I want to start with just a sense of why you guys are into this? You know, it’s a very complicated thing, but maybe, it’s exciting and maybe there was something in your childhood that sparked to get you to where you are right now working on these advanced rocket engines. Steve, why don’t we start with you. Tell us a little bit about yourself, your background and what led you to working on this?

Steve Heister: Oh, that’s a great question. I’m a professor at Purdue University. Been here a long time, since 1990. I had the advantage of being a 10-year-old boy when a man named Neil Armstrong set foot on the Moon. I remember that night as if it were yesterday. And that definitely set my path toward aerospace. And more specifically, as I got into college into propulsion, I was excited about working on the business end of the rocket. And that’s kind of how I got to where I am today.

Host: Awesome. Yeah. You, there’s nothing, you know, if you’re thinking about spaceflight, there’s not much that’s more exciting about just that smoke and fire, right? It’s just a very exciting moment. You, like, whenever you think about human spaceflight moments and Neil Armstrong, of course, you think about those steps on the Moon and those words from the Moon. But just the powerful launch of the Saturn V had to really capture your imagination.

Steve Heister: It sure did.

Host: [Laughter] That’s great to hear. Tom, what about you? What led you to where you are?

Tom Teasley: Yeah, so, I’m a liquid propulsion systems development and test engineer here at Marshall Space Flight Center in Huntsville, Alabama. More recently in the last couple years, we’ve been developing the Rotating Detonation Rocket Engine. And I guess for me, the reason why I got started on this track, so to speak, is predominantly due to my father. So my father was an airline pilot, and he would fly all around the country and he would bring me back home pictures of being at 35,000 feet. And that kind of spurred my interest in looking up at the stars. And so, I went on to my undergraduate, got a degree in physics. And from there, went forward to get a master’s in aerospace. And it’s kind of been all downhill from there. So…

Host: [Laughter] All downhill. All right. Awesome. You guys are both, you know, you took this path to get you to where you are, you sitting here talking about rocket engines. And we’re going to talk about, Tom, you alluded to what we’re going to be talking about today. This new kind of rocket engine called “rotating detonation.” And I think to help us to understand what that is, what would at least help me is understanding how we typically think about the, quote-unquote “traditional rocket engine.” And so, Tom, if you’ll kind of help to kick us off on just, if we think about what a traditional rocket engine is and how it works and what are the key components of it, that will help us to differentiate it when we start getting into this new design, this RDRE design. So, Tom, kick us off. What’s, if you had to describe a traditional rocket engine to someone who was really just getting started and really wanted to understand conceptually what it is, how would you start?

Tom Teasley: Sure. So, it, they’re, they’re pretty much, for all intents and purposes the same in how you would, you would classify their components. You would have an injector or an injection system. A chamber, or a barrel section that allows for the propellants to combust and then a nozzle to expand those combustion products to get usable thrust from the engine. Really, the key differences here is that the combustion process itself, that the RDRE or Rotating Detonation Rocket Engine uses, is completely different from a traditional liquid rocket engine. So that also inherently changes the geometry. Some of the design parameters completely change. And those are a lot of the challenges that we’ve been working through. And actually, I don’t know, Steve, you might be able to describe the main differences between deflagration and detonation a little bit better than myself, if you don’t mind.

Steve Heister: Yeah. Well, we’re both interested in the combustion part, and that’s the exciting part where the miracle happens. If you think about a conventional rocket engine, you know, we’re squirting in a fuel and oxidizer and mixing them intimately. And if you think about lighting that mixture and observing what happens, that flame would just start to move within the combustion chamber at speeds of the order of a few meters per second. And in a traditional rocket engine, we designed the combustion chamber to perform well with those flame speeds. In the rotating detonation engine, the flame front is actually provoked to combust by a detonation wave that’s moving over a thousand meters per second. So it’s, you know, in terms of power density, the amount of energy release we can get within a certain volume, it’s literally an order of magnitude higher than today’s devices. And that, I think is what’s exciting to both Tom and myself, is, it’s hell fire, right? You cannot eat propellant faster than with a rotating detonation wave.

Tom Teasley: Yeah. I couldn’t agree more. And I mean, that brings a lot of design challenges to the floor as well. So something that we’re trying to push through.

Host: Yeah. And so, maybe to help us to understand those design challenges, going back to this conventional or traditional rocket engine, you know, like the way you’re describing this combustion is the speed. And, you know, for me, who’s not a rocket scientist, I would think, OK, that this already sounds really nice. Now you have these flames that are, you know, supersonic, they’re superfast. And that’s a good thing, right? So, why haven’t we’ve been doing this? You know, we have the traditional rocket engine, so what’s really the reason that the conventional one, at sub or below supersonic speeds, why is that, you know, why has that been the case and we haven’t just jumped right to an RDRE?

Tom Teasley: Well, this is actually something the early NASA engineers discovered on the Apollo program when they’re in development of the F-1 engine. They encountered something called thermoacoustic instabilities, and they actually plagued the F-1 so much so that, I think at one point there was an entire field full of scrap engines because these instabilities, potentially detonations, were essentially burning through the wall of the chamber because the heat release was so high and the material compatibility was not high enough. And it isn’t until today because of the advent of things like additive manufacturing and high-conductance alloys such as GRCop-42, that we can really design a combustor that can handle such an extreme combustion phenomenon. So it’s not that we exactly couldn’t have, it’s just the traditional liquid rocket engine is tried and true and much easier to design to. And now, we’re trying to work through those challenges to try and get better performance and reach out further into the stars, I guess.

Host: Yeah. That’s what I’m hearing, right? It’s about control. I think that’s really the key thing, is it sounds like just given the technology and the capabilities at the time we weren’t ready to control this, to control a rotating detonation kind of design of engine. And so, like, you know, we needed– the most important thing is that the rocket engine works, right? And so, that’s it. What I’m hearing is that’s really the reason, is we just, we just didn’t really have the capabilities at the time.

Steve Heister: Yeah, I think that’s true. And Tom’s characterization, you know, reaching back to early engines, the Apollo program, our field has struggled with combustion instability since the very earliest days of liquid rocket engines. And now, as we start to pursue these rotating detonation devices, one might think of it as embracing the instability. Let’s make it unstable, but let’s try to control the way it’s unstable. We have difficulties. We don’t know how many detonation waves are going to show up when we start up this combustor. The delicate timing of the injection process and relative to a passage of a wave, when the wave passes by, it temporarily stops the flow of propellants. And that’s somewhat of a necessary condition to make these engines work. And we don’t really understand, you know, how the injectors respond in such a dynamic environment. We just kind of make it work. Sometimes it works, sometimes it doesn’t.

Host: And so that’s– maybe getting into RDRE just a little bit more is, that’s sort of what you’re trying to test. It sounds like what Tom alluded to was, you know, we have things like new kinds of materials, and we have capabilities for things like additive manufacturing, and these allow us to create an engine that won’t explode when you try to contain these supersonic flames. The detonation kind of capability within a rocket engine. But it sounds like even with the modern technology, you guys are still trying to figure out how this thing works. Is it, I guess maybe chaotic is the right way to describe it? Or maybe you guys have a better word?

Steve Heister: Chaos is one way of thinking about, or trying to control the chaos, or to understand it, and be able to produce a device that you can repeatably produce a certain number of waves and a certain performance, a certain thrust level. And in doing so, be able to last a long time, you know, for multiple missions.

Host: Tom, let’s go into, you referenced some of these things that help us to get us to the point where you guys were able to conduct a test of an RDRE engine. And you talked about just, I guess, if you think about the way that traditional or conventional rocket engines are made, you talked about new capabilities like additive manufacturing, like advanced materials. There are these things that help us to create a new engine. Can you help us, by just maybe laying the foundation for what we know about conventional rocket engine manufacturing and design, and how that works, and sort of help us to lead into these new capabilities that allow us to start playing with RDREs?

Tom Teasley: Sure. So, I guess a little bit of rocketry 101. So the way a liquid rocket engine typically works is there are coolant channels that are machined, traditionally machined usually, before the advent of additive manufacturing into the hot wall of the combustor. And then, typically a fuel is pumped through- either hydrogen, methane, which is typically used nowadays, or kerosene, which has been a good option in the past, as well. And that’s used to actively cool the wall of a combustor. Now, going back to rocketry 101, like I said, traditional liquid rocket engines, usually you have a barrel section, a throat section, and then an expansion section, right? So this means that the combustor in and of itself, regardless of what the scale is, can be fairly long. For a typical lander engine, we’re talking eight, 10, 12, some up to even 16 inches in length, right? So that’s maybe a foot, foot-and-a-half, in that range. With rotating detonation engines, we’re now seeing just a part, back to some of those challenges that we were mentioning before that the heat loads that the combustor is experiencing are much, much higher because you’re basically releasing all the heat within an extremely short and compact space. And so, you can imagine one of the main advantages there is, instead of having a combustor that’s 12, 16 inches in length, you’re now shrinking that down to a couple of inches. I’ve even seen some papers and some work going forward and testing combustors that are only an inch in length. That’s incredible. That means that you have outstanding cost savings in the production of the component, you have far less material that’s being used. You design trade space to be able to fit that propulsion system into the vehicle is now radically improved. But then, that also creates some substantial manufacturing challenges, right? How are you going to design an engine that can be– that can have everything like the instrumentation needed, the integrated cooling channels, the manifolding? Where is a port going to be fit to duct coolant through the walls? So those are challenges that we’re now facing with the technology, but at the same time, additive manufacturing is now enabling us to go and produce components that have complex integrated structures such as coolant channels that allow us to integrate things like advanced instrumentation and diagnostics. And so, whereas using traditional manufacturing methods, if you were to say, “oh, we want to make a combustor that’s only a couple inches in length.” That could be a real challenge, actually, to the point where it may even be impossible to do with those methods. But with additive, you can now go and produce components that are exceedingly short and then reap the benefits from it. And to that end, because of additive manufacturing, we’re now able to produce components with some more advanced materials. There’s been a lot of development over the last several decades in novel advanced materials, such as high-conductance GRCop alloys that allows the combustor to actually take in that extreme heat and put it back into the coolant as quickly as possible, whereas we were much more limited in the material selection back during, again, the Apollo era.

Host: Yeah. I think this is a really important theme to why, you know, RDRE work today is– that the advancements in these particular areas. In the, like, traditional manufacturing, you described it perfectly, Tom, you just couldn’t get the cut that you needed. It’s too small, it’s too delicate. You know, there are a lot of challenges to doing that successfully, almost perfectly, maybe is the right word to use. But additive manufacturing allows you to be more precise in that design. And then it sounds like this alloy, you said GRCop, this alloy, you know, one of the main things was, you keep referencing this heat and to think about this alloy. I think, maybe that’s the right characteristic when it comes to materials, right? You guys were looking for something that could deal with extreme heat. And I guess, maybe the materials at the time, either couldn’t do that, or just the materials that could do that were really tough to manufacture. From the materials perspective, what were the challenges with traditional engines when it came to materials?

Tom Teasley: So I believe for the most part, a lot of the materials that were traditionally used are referred to as super alloys. It’s actually a group of nickel-iron alloys. So one of the advantages there is that they were very high strength; however, when they come in contact with combustion products, if you exceed certain limits in terms of their thermophysical properties, they can then start to break down and actually become part of the fuel in the combustion process. And the wall would quite literally burn. They’re also relatively low in conductivity. I believe there are some combustors that were using wrought copper. But a lot of the time that copper was very low strength, and not typically capable of holding up to extremely high pressures, such that a lot of the missions required. Nowadays, since we’re able to produce these specific alloys, you can reap the benefits of the strength of super alloys, but at the same time, the conductivity of base materials such as copper.

Host: OK. There’s your factors. Now, I think another important thing we talked about– additive manufacturing, the materials that help us to achieve this design. I think another thing maybe to talk about is, when it comes to understanding the phenomenon, we have modern technology, modern software capabilities in computer modeling to better understand these designs. I think before you start making those cuts. And I wonder if that has helped you guys in terms of coming up with designs that eventually lead to actually cutting the materials, actually putting this through an additive manufacturing process? If this has really, you know, modern advances in computer modeling have helped you to achieve success here?

Steve Heister: Certainly, I think they have. Yeah. I think they have tremendously, actually. I think, Tom would be, as an experimentalist, would be the first to admit he can’t measure everything. And the flow fields in these devices are very transient and very three dimensional. So the high-resolution scientific computing allows us to see the entire combustion chamber at various instances in time. And to see this wave pass by an injection site and see what kind of pressures the injector sees and how the flow stops and how the flow recovers and starts to mix again. Any combustion that might occur prior to wave arrival, we call that parasitic deflagration. It’s something we don’t necessarily want. So all of those things come out of these high-resolution calculations, but they are very difficult because in general, we have to simulate almost the entire combustor. And rather than just simulating a single injection site, which is what we might do in a conventional engine, in a RDRE, because we’ve got this wave moving around, it’s challenging. You have to use basically more computer time to get the same type of answer.

Host: OK. Multiple layers, it sounds like, of understanding. Not just an isolating to one component, really, getting the biggest picture you possibly could.

Steve Heister: Yes. It all sings together.

Host: OK. Now, let’s go into RDRE. Let’s talk about this rotating detonation design, and maybe, continuing to compare to traditional rocket engines. Steve, why don’t you take this one, on how an RDRE works? As you know, and I guess to try to help to compare it to traditional rocket engines, and the complications of this design.

Steve Heister: Yeah. So we’ve touched on it. You know, one of the reasons the community has pursued this technology is it allows one to use what we would call a conventional feed system. You know, upstream of our combustion chamber, we have turbo pumps that are delivering the propellants to the chamber at very high pressures and flow rates. And we’d rather not have to go and develop new turbo machinery solely for a new type of combustor. And so, the RDRE naturally makes, as far as the pump is concerned, it’s delivering a constant flow rate. But when we start to look and focus in on injection sites in the combustion chamber, we see a very unsteady flow rate. We see a detonation wave passing by an injection site and temporarily stopping the flow, and then the flow’s recovering, and jets starting to issue back into the chamber and mixing starting to occur before another wave arrives and consumes that portion of fluid. So that’s the exciting part, I think for Tom and I, is trying to get that dance correct. To get the combustible mixture prepared just in time for that detonation front to come and consume it. And that’s really what is the real challenge. We don’t pretend to understand all of it. We try some things and sometimes they work, and in some conditions we don’t get a rotating detonation, we just get deflagration, we get constant pressure combustion, basically. So these are the challenges that one has to step up against when we’re thinking about a RDRE design.

Host: Yeah, it sounds like, I think you use the word “dance.” That sounds appropriate, because you have to balance the injection and it sounds like you have to match those. But Steve, if I’m correct, it sounds like matching those is a challenge because it’s hard to predict?

Steve Heister: Correct. Correct. It’s a little bit like, an automotive or a diesel engine, right? In timing the injection, when that cylinder head comes down to just the right location and getting the combustion event to happen exactly when you want. You know, the RDRE shares some similarities to automotive combustion.

Host: Tom, when you think about traditional engines and this rotating design, you know, in terms of injection sites and in terms of trying to characterize and explain how– what is, when you say rotating detonation, I’m really trying to understand what that means? What is rotating? What is detonating?

Tom Teasley: Right. So, so the propellant itself is what’s actually being combusted. And that combustion process drives a supersonic pressure wave. So you can think of it as, if you were to look into the engine from the throat, you could actually see waves traveling around either clockwise, counterclockwise, and sometimes in both directions. But what’s actually happening is, if you get down near the injection face, you can actually see that there is a wavefront that is pushing ahead of where the actual combustion process is occurring and it is traveling in a certain direction. It is basically slamming against streams of liquid propellant or gas-based propellant, and causing an additional mixing process. And then behind it, after that propellant has been compressed through that shock, it is then combusted, which then in turn drives that pressure wave ahead of it. So that’s quite literally what’s happening. And in terms of where the rotating detonation, so to speak, is occurring, it’s actually occurring very close to the injector face. A lot of folks have actually visualized this and seen evidence of where it’s occurring on the chamber wall itself. So, it’s very close to the injector face. But where exactly it sits, whether it’s quarter-inch, half-inch, in some cases an inch or two downstream, really depends on a number of factors such as what Dr. Heister mentioned injection timing, the wave speed itself, the propellant type, that sort of thing.

Host: OK. Yeah. Lots of different factors. And we’re really getting into what makes this kind of RDRE design very, very challenging. But I think what’s exciting, part of the reason we’re talking to you is not only to understand this design, but to really recognize that you guys got to a point where you could conduct a hot fire test. You actually, you know, you were thinking about this design is not necessarily new, it’s been theorized, but eventually, you were able to get to a point where you can print it and actually test it out. And so, Tom, take us through the hot fire test and specifically from the time of, you know, you guys were going through a bunch of different designs to really– kicking us off with a certain level of confidence and readiness that, “Hey, I think we’re ready to print this, and put this on a test stand, and give it a go.” What was that whole process like?

Tom Teasley: Yeah, so really, it started with industry and academia and Dr. Heister’s group put in a lot of the legwork over the last decade to really assess, “hey, is this technology ready to go up to large scale?” And so, a lot of their preliminary work that they conducted really laid the groundwork, so to speak, for this hot fire test program that we conducted summer of 2022. That hardware, we actually had a number of test goals. The primary test goal was really to take the engine technology from a somewhat small-scale, heat sink (meaning it does not have coolant channels integrated) gas configurations and up the technology to a point where, OK, thrust is now in the thousands of pounds. The propellants that we’re using are now cryogenic in phase, so what we would typically use on a flight-like configuration, I guess you could say. And pushing average chamber pressures that are something a little bit closer to what a flight vehicle might utilize. And so, one of the major test goals was, “Hey, can we fire one of these engines and it survive a true mission architecture’s duration?” And our test goal was to exceed 100 seconds, and we actually did that multiple times with rotating detonations that we observed in the combustor. We not only did that with one engine geometry, but we did it with two engines. The first engine scheme was actually more of a typical straight annular configuration that we designed specifically to measure what we call calorimetry, or to identify exactly what the heat loads at specific locations of the combustor were. And then the second geometry was a more, best effort optimized design to give it a much higher area ratio nozzle. And then, in addition to that, other goals were looking at much higher pressures in the chamber. So, we were successful at that as well.

Host: Very good to hear. I wanted to back up, Tom, just a little bit and go over to Steve for just a sec, because you mentioned that Steve did, over the past decade, maybe even more, working on leading us up to the hot fire test. And so, Steve, I wonder what that work entailed? What were you doing over the past decade to really prepare us for this moment?

Steve Heister: Yeah, thank you. So we got it, at Purdue. My group at Purdue got started almost a decade ago in this area. Colleagues at the Air Force had encouraged me to look into this because I was, a lot of work at the Air Force is done, is mainly aimed at air breathing engine technologies, jet engines, ramjets, and those sorts of things. So, we got started on it. And as Tom points out, generally the way combustor development goes, you start with what we call heat sink hardware, which is just very heavy pieces of copper and very short run duration. So, we start this thing up and we run it for literally just a second before the copper overheats and starts to burn. And we started to get some good measurements there. And then, a few years ago, Tom approached us and said, “hey, we should collaborate and try and do something more ambitious with the tools that are available at Marshall, real propellants, cooled hardware, long duration capabilities.” And those were the things that, really, Tom demonstrated in his test campaign last year. It was very exciting to be part of that and, you know, to be in the control room for a hundred-second duration test. As we discussed, it’s a lot of emotions there. A lot of banter going on and the test conductor telling people to be quiet because we’re getting ready to fire a big rocket combustor. But it was an exciting time.

Host: [Laughter] That’s right. Yeah. It sounded like up to that point, you had only had, you know, really short durations. And so, this sounded like it was going to give you a lot more data, a lot more understanding of what this was. And so, you know, all the work that got you to use these new capabilities to get a better understanding of the engine, I mean, it must have been, yeah. You sound like you guys had just a ton of energy in that room because, you know, all the work. You said you’ve been working on this for a very, very long time, and now you have this capability, this new understanding. You must have been, yeah. You must have been ecstatic.

Tom Teasley: Yeah. And if I can add that very first hot fire that we conducted, it, I get chills just thinking about it, but there were a lot of challenges with getting the combustor to ignite. I mean, as you can imagine, these things are so short, there isn’t any what we call stay time. And so, there was actually a friendly running wager around the test engineers. And the idea was that because this engine is so short, how much propellant is going to be spewed onto the pad that wasn’t combusted? And getting to see the combustor ignite for the first time, and you see this full envelope of a plume come out, and just clear Mach diamonds and shock formation, it– just reliving the moment. Sitting here, I can tell you, it all happened so quickly, it was such a blur. But I do remember distinctly, our test conductor essentially saying, all right, valves are open, ready, fire, and then there’s a pause. And then you see the engine come to life and you are getting performance metrics back. We see full chamber pressure, and you can hear an audible gasp in the room from the 20 to 25 people that were standing back and watching it. It really was a surreal moment.

Host: It was like, I guess you guys, that was what you were going for, but up to that point, you hadn’t really experienced it. And so, maybe it was the sound, maybe it was the visuals, maybe it was the flowing data that’s just like, it sounded like you– that’s what you were building, that’s what you were designing. But it, just, you know, it just sounded surprising in a way.

Tom Teasley: Yeah. It was quite shocking. And even better was when we were doing the high-pressure cases. When we eventually went to higher pressures, the performance metrics we were getting back in the control room were absolutely staggering. So staggering that one of my student interns at the time, basically nudged my shoulder and said, “this can’t be right, this, this doesn’t make any sense.” And that, that’s another thing I’ll say. I mean, the technology’s phenomenal. It really is, and it’s something that NASA is seriously considering and investing in for potential lunar or Martian lander missions. And it gives excellent opportunities to interns, such as Pathways, NASA NSTGRO (NASA Space Technology Graduate Research Opportunities) fellows. I even had five interns working that project last summer, and I’ll be having another four coming back this summer. So not only is the technology great, but it gives opportunities for that next generation.

Host: Yeah. Not a bad opportunity.

Steve Heister: Yeah, the lander mission is a very interesting one because, the shortness, the fact that you can shorten the thrust chamber and the nozzle has huge implications for a lander because you have long landing gear that have to extend beyond the bottom of the engine, of course. And so, by shortening the engine, you also shorten the landing gear, which reduces the weight, not only of the combustion system, but also of the, the vehicle itself. And that’s exciting, you know, one of the prime potential applications for the technology for NASA.

Host: Oh, yes. I mean, if we’re talking about, really the pros of the RDRE, I think that’s maybe something we haven’t made super clear. It sounds like the nozzle design is important, and having it be smaller. But I think really another key component here that maybe we haven’t said out loud is the performance of the thrust. It’s, I think, one of the biggest pluses here is what this design offers is essentially more bang for your buck; getting more performance, and more thrust out of less fuel.

Tom Teasley: Yeah, certainly. So actually on this front, so we’ve mentioned it a number of times now, but just to recap really quickly, because that detonation phenomenon is actually enabling rapid completion of combustion, and you’re able to shorten the length of your combustor by such a drastic margin, it really frees up not only the design space, but also the initial funding that is required to develop an engine. So by that, what I mean is– and most of the time a lot of companies will approach us and say, “hey, we’re, we’re developing this engine, but we’re getting really bad combustion efficiency.” Something that we quantify as a metric called characteristic exhaust velocity. So C-star efficiency can be very, very low, well below, 100% that of the theoretical limits. But what we saw in testing was that we’re hitting very, very close to 100% every time. So that, what that enables us to do is say, “Hey, you can now save all of your development funding required, go and develop an RDRE, and right off the bat, instead of getting 70%, 80%, like what you would traditionally see, and get very near that 100% mark.” So that’s an enormous advantage of the technology. That in addition to theoretical increases potentially in I_sp (Specific Impulse), but that’s something that we’re still parsing through.

Steve Heister: I_sp is our gas mileage, for experts in the area. That’s the thing we are trying to advance.

Host: I see. Yeah, I think, I think one of the important things here, you know, going back to the hot fire test– you guys were talking about the emotions experienced during the test, but then ultimately, you’re receiving this incredible data and you’re able to analyze that. And so, Steve, I know you co-authored a paper, can you give us an understanding of, just based on this hot fire test, based on all the work up to this point, and your work on RDREs, what this revealed to you in terms of the capability and just sort of summarizing, you know, as mostly for me as simply as possible, just what you learned from this hot fire test?

Steve Heister: Well, innumerable things, I think we’ve touched on a number of them. The hardware that we manufactured and tested, you know, Tom’s initial hardware was somewhat based on what we had done. Although our combustor was manufactured with conventional means, subtractive, you know, cutting hardware and drilling holes, whereas Tom’s was additively manufactured from laser built up from powder. So just the fact that you could do that. The fact that you could run long-durations with cooled hardware was certainly exciting to us. And the data that Tom and the crew gained in that area is invaluable. We need that data to design a flight-weight configuration, and to be able to understand what the heat loads are. What will our limits be? So, you know, that was another major advancement there. Unique geometries, Tom alluded to this tapered chamber. Checking out how that differed from performance that we had seen with a conventional straight chamber was another crucial, crucial outcome from the test campaign and from the work.

Host: I think, kind of touching on why we’re doing this work. I think, Tom, you sort of alluded to it, but, you know, in terms of what this is? Why we are doing this initiative? This is something under an interesting NASA umbrella of work, of exploring new technologies. And so, can you help to describe and help to characterize how NASA is actually supporting and enabling, you know, working with Steve and Purdue, and just how this works, how we’re able to explore new technologies?

Tom Teasley: Yeah, certainly. So, most of the time, if I had to boil it down, it really comes down to either the academic partner, the industry partner, government agency approaching us and saying, “Hey, we have this problem, what tools do you have to help us solve that problem?” And also, a lot of the time, yes, in this case it was development of a novel propulsion system, the rotating detonation engine, but there are also other technologies that you discover in the process, right? For example, in the additive manufacturing process, we actually discovered that there are specific ways that you can design coolant channels such that you lose very little pressure losses. And for an engine, pressure budget is just as important as fuel economy, right? You only have so much that you can expend. You can’t have all of your pressure being lost in the coolant channel geometry. You have to have some reserves so that you can push propellant into the combustor. You have to have an accounting of that pressure in the combustor itself as well, which completely drives what your fuel economy would end up being. So it enables us to go and develop new things like strategies for designing channels, and even other technologies like finding, oh, this injection system is far more effective than this other one. Or even, designing novel nozzle geometries, for example. But at the end of the day, it all comes down to what does the industry need. How can we help industry, and academia, and how can we better serve our U.S. partners?

Host: That’s perfect. And, and sort of catapulting from there, Tom, back to you, is really just thinking about what the next steps are. Now we’ve done this hot fire test, Steve went over some of the things that we learned from the hot fire test and this design, and I think, what I’m hearing from the level of excitement from you guys and just from understanding what this engine is, is there’s a lot of promise. So, what are the next steps that we’re taking in terms of RDREs and continuing to learn more about them?

Tom Teasley: Yeah. So, that initial hot fire test campaign that we conducted was really more of a, “Hey, can we do this?” sort of thing. It answered the fundamental question of: What are the initial heat loads we’re seeing? What are the other performance metrics we’re seeing? Is it viable to go forward for a, towards a more flight-like geometry? And after that work was shown to be such a staggering success in my view, the space technology mission director out of NASA Headquarters saw fit to fund our follow-on activities to do just that, to really parse out the limits of the technology. And so, I think that the highest thrust we were able to demonstrate was a little over 4,000 pounds under that campaign. Well, we’re going to be going bigger. We’re going to be pushing towards the 10,000-pound class that seems to be around the sweet spot of what most industry in the U.S. is looking for. Doing CLPS (Commercial Lunar Payload Services) lander missions, for example. Other lander missions to the Moon, to Mars. And so our next iteration engine is actually going to be a dual regenerative cycle, 10,000-pound rotating detonation rocket engine. And the reason why we have to go towards dual regenerative is because well, we no longer can take water with us for a flight configuration, right? We don’t have access to enormous amounts of water to help cool the hardware. So, we have to demonstrate that we can successfully cool the hardware with just the propellants that we’re taking with us into space, right? And because the heat loads are being shown to be so high, we have to use both fuel and oxidizer in this new combustor. And hopefully, we’ll be able to overcome a lot of the challenges that you might experience there as well. And we’re actually planning on testing a lot of those hardware this coming summer. So stay tuned.

Host: [Laughter] Very exciting stuff. What you’re talking about is leading up to– It sounds like what you’re doing is a — and when you talk about the 10,000-pound class, you’re not only talking about something that’s bigger, maybe more powerful, but what you said very clearly is that this is something that industry can use, that this is the need. And so, Steve, I’ll go over to you for a second. You’ve been working on this for a long time. I think there’s this interest in RDREs. When you think about the exciting potential and what this test has sort of kicked off in terms of continuing to scale up and continuing to refine this technology that eventually could, dare I say, disrupt the industry in terms of the engines that different companies, different space agencies even, seek in terms of the technologies needed to accomplish their missions. How does that kind of sit with you whenever you think about what you’ve been working on could possibly be used, you know, across the industry?

Steve Heister: Well, it’s certainly exciting. You know, and Tom’s campaign that he just described will be another step toward a real flight-type configuration. One challenge that we haven’t talked about is the fact that we’ve done very little, almost no work as a community verifying how well these systems work in the vacuum of space. The first thing we do is a ground test at sea level with ambient pressure. We need to understand better how they’re working in the vacuum of space. There are large facilities, just a few of them around the country, where you can run an ejector system to pull down the ambient pressure to simulate an altitude of maybe 100,000 feet, and to see how your system performs at that simulated altitude condition. Another way of getting at that might be to do a flight test, to do a sounding rocket and, using a long-duration burn, you’ll be able to see how the system performs over a range of altitudes. So those are things that I know Tom and I have been musing about, and hoping that we can get to in order to prove out the the vacuum performance, because a lot of the missions that NASA might be interested in will be space missions where this system has to be able to ignite and start up in the vacuum of space. The nozzle has to perform well in under vacuum conditions. So those are still things that lie ahead for us. But it is exciting, and I think the industry is, certainly, the interest in the industry globally has increased exponentially over the past few years. And certainly with Tom’s test campaign last summer and subsequent press release, we’re getting a lot of inquiries from industry as a result.

Host: That’s how I discovered you guys, was just the level of attention that seems to be brought over to this. And you’re right, you have a very leveled, very measured sort of response when it comes to that. You know, I think maybe I’m jumping ahead because I just, I’m really excited and I’m sure you guys are too, but I think it’s really important what you said, Steve, that this, it was so exciting to get to where we were with the hot fire test and understanding, but it’s really just a step and there’s much more that’s needed to give us a certain level of confidence and to really get us to a point. But the level of interest, I think is really exciting and the potential is really exciting. I think what’s also exciting is just, you know, just NASA’s participation in this. And Tom, I’ll toss over to you to sort of help us wrap up here, is just understanding this technology is, I think what’s exciting is the way that we’re exploring this, is not necessarily for it to be a proprietary thing, for this to be a disruptive thing. That’s something that we can share, that we can explore, that we can talk with industry about, understand their needs. And so, I think it’s something that, it’s exciting in a way that this is something that NASA’s investing in, in order to share, and to share the results to share the journey that we went on. And that’s something that you and your team work on.

Tom Teasley: Yeah. And let me just say, we are truly in a new golden age of spaceflight activities right now. The number of companies and individuals and institutions investing in capability and new technologies and new spaceflight activities is just staggering. I think we’ve had, this year alone, we had more people in space than any year previously, which is just incredible. And this technology is actually going to enable that a lot more for U.S. industry going forward. Now, let me be clear. I don’t think that this technology is going to completely replace every application of propulsion. But it is going to drastically open up the options there for spaceflight and exploration. And NASA very much wants to be centerfold in that, and really enable industry to be able to do that, because we are looking to go back to the Moon and eventually onto Mars, and we’re going to need advanced propulsion technologies just like this in order to get us there.

Host: I think, Tom, that’s exactly where I wanted to sort of wrap up for today, is just that level of excitement and just really, just understanding what this means for the future, and thanks to you guys and your teams that really helped us to get us here. And so, to Steve, to Tom, thank you so much for dedicating some time to speak with me about this new technology and what it means and help us to characterize and understand more about it. This is just a really exciting thing and I’m so happy that I got to hear this, the excitement from you guys on, on the actual hot fire test and help and help to break it down. This has just been such an enlightening and exciting conversation for me, personally. So I appreciate both of you for coming on and I appreciate your time. Thanks so much.

Steve Heister: Yeah. Thank you.

Tom Teasley: Yeah, thank you. It’s been a pleasure.

[Music]

Host: Hey, thanks for sticking around. Hope you learned something today. Really exciting to be talking with Tom and Steve. They had so much energy and so much knowledge and I definitely pulled a lot from it, walked away a lot smarter on RDREs. I hope you did as well. You can check out NASA.gov for the latest and make sure you check out NASA’s Marshall Space Flight Center for all of the great things they have going on there. We, of course talk a lot about a number of different topics on this podcast. You can check out our full collection at NASA.gov/podcasts, as well as the other podcasts we have across the agency. If you want to talk to us or maybe give us a suggestion about a topic that we should cover, we’re on social media on the NASA Johnson Space Center pages of Facebook, Twitter, and Instagram. You can use the hashtag #AskNASA on any one of those platforms to submit an idea. Just make sure to mention it’s for us at Houston We Have a Podcast. This episode was recorded on June 7th, 2023. Thanks to Will Flato, Justin Herring, Heidi Lavelle, Abby Graf, Belinda Pulido, Jaden Jennings, Pat Ryan, Marina Geurges, and Ray Osorio. And of course, thanks again to Tom Teasley and Steve Heister 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.