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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 192, Yajaira Sierra-Sastre and Vivake Asnani from NASA’s Glenn Research Center detail the history of tires used on the Moon and Mars and describe an innovative new tire called the Mars Spring Tire that may be used on future rovers. This episode was recorded on March 22, 2021.
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Transcript
Gary Jordan (Host): Houston, we have a podcast. Welcome to the official podcast of the NASA Johnson Space Center, Episode 192, “Drive Like a Martian.” I’m Gary Jordan and I’ll be your host today. On this podcast we bring in the experts, scientists, engineers and astronauts all to let you know what’s going on in the world of human spaceflight. You don’t hear very often that it’s time to “reinvent the wheel,” but when that wheel is spinning on another planet, it might be a good idea. Tires from Mars are going to have to deal with rocks, sand, slopes, craters, and cold. So, the idea of a wheel becomes a bit more complex as you start to realize what has to go into the design to deal with all of that. So, today, we dive into the history of space tires, and discuss work being done for an innovative new tire for Mars called the Spring Tire. Joining us from Glenn Research Center is the Mars Spring Tire Project Manager Dr. Yajaira Sierra-Sastre, and the Lead Tire Engineer Vivake Asnani. So, let’s jump right in to learn what it takes to drive like a Martian. Enjoy.
[ Music]
Host: Yajaira and Vivake, thanks for coming on Houston We Have a Podcast today.
Vivake Asnani: Hey, excited to be here.
Yajaira Sierra-Sastre: Hello, Gary, thank you for having us.
Host: Hey — great to have both of you guys. So, this is a very interesting topic, Martian Tires, and we’re going to get into a lot of the — we’re going to start with the history, talking to you guys a head of time. And we’re going to actually reveal what is a tire, what is that especially in the space environment, and then get to some of the super cool technology that you guys are working on. Yajaira, I want to start with you. What is your background? How’d you get involved in tires, particularly with your materials science background?
Yajaira Sierra-Sastre: Yeah, that’s a very interesting question. And I don’t have a really traditional path to NASA, but I can tell you that I am a materials chemist by training. I have a Ph.D in materials chemistry. And back in grad school I focused on the synthesis and structural evolution of nanoscale materials from molecular thin films to metallic and semiconductor nanostructures. And prior to joining NASA Glenn I worked for two startup companies specializing in nanomaterials for different types of industries and applications from textiles to biomedical applications. And most recently I also served as a civil servant. I worked as a project manager for the U.S. Bureau for Engraving and Printing, that’s our U.S. money factory. And there I led material science projects to keep our U.S. bank notes secure. Nothing related to space, but space has always been my passion, so while looking for opportunities to transition my career to the space sector, back in 2013, I was part of a Mars analog mission funded by NASA — the Hawaii Space Exploration Analog and Simulation. And was their contact crew member for four months in — a habitat in Mauna Loa volcano in Hawaii. But, besides that analog and that simulation, I also established collaborations with Johnson Space Center for an exercise work study. So, we were looking at Mars garments and intravehicular clothing for astronauts — for ISS. So, these experiences plus other you know, other types of opportunities provided to us over the years like, taking planetary geology courses and trying to get, gain more field work experience related to Mars exploration, I believe that my path to now serve as NASA Glenn. So, when the opportunity came my way to manage this project I — just said, yes. And in the context of these Mars Spring Tires Project, I am the project manager for — the project, for — and as — you will learn through these conversations; we are part of that — the Mars Sample Return mission, part of the Sample Retrieval Lander Project, we are delivering, will be building and delivering the new tires and wheel and tire assemblies that will be integrated with a Sample Fetch Rover. So, my goal as a project manager, and my role and responsibilities to make sure our team executes and successfully deliver these — tire, wheel and tire assemblies to ESA (European Space Agency).
Host: I can’t wait to get into this topic. I’m curious though, it seems like — you say, you have kind of a roundabout path to get into NASA. I wonder what — maybe, and earlier in your life, there was something that maybe, that maybe triggered some spark that was, that you were thinking, “Hey, NASA is somewhere that I want to go. I want to work in the space sector.” Do you have a moment like that in your — earlier life?
Yajaira Sierra-Sastre: Oh, yes, definitely. I think since my earlier years, like since I was very young, and my first passion and my first love was astronomy and planets and stars. I am originally from Puerto Rico, so growing up in that beautiful island and just staring at the night sky really influenced me, instill in me the passion for science.
Host: Wonderful. Wonderful. I’m glad to have you on today, Yajaira, and — Vivake, you are bringing to the table a mechanical engineering background, tell us a little bit about yourself.
Vivake Asnani: Yeah, actually — you know, going into school I wanted to work on acoustics, so I started in electrical engineering, and then in graduate school did mechanical engineering. My path really wasn’t directed towards NASA — they recruited me. And when I got to NASA, we were just going back to the Moon, right. So, we hadn’t really thought about Moon rovers in a while, there wasn’t going on that, in that area. And I — found myself in this meeting where we were trying to figure out how to land a nuclear reactor on the Moon and get it away from the lander. So, we’re starting to think about mobility on the moon, and — kind of looking at each other not really knowing how to handle this problem. And that just sent me down a rabbit hole, and since then, I’ve been really interested in mobility on other planets.
Host: That is awesome. And this is like the ultimate — mobility, right. Now we’re going to be talking about driving like a Martian, there’s — you know, a history here when it comes to — this type of mobility, Vivake, that you’re talking about on other planets. Let’s — before we go into the history though, set a foundation for what we know about tires on other planets, and how they work. Give us a little bit of, you know, the what is, when it comes to tires.
Vivake Asnani: Yeah. If I could start with Earth tires. We didn’t always have them, we used to roll on rigid wheels, like wagon wheel type of things. And, you know, the pneumatic tire was a breakthrough, it was invented like 130 years ago, and its purpose was to create flexibility. And it helps in a couple ways. First, if you’re not on a road, and you need to gain traction; the larger contact area really helps you. But something that’s not really appreciated is, it also saves you a lot of energy. So, a vehicle on wagon wheels is going to — going to go up and down quite a bit as it goes over rocks, but the tire, it actually conforms to the surface, and saves the vehicle all that energy that it takes to go up and down. So, that’s — on Earth, you know, we always use tires now. Looking to other planets though, you look to Mars or the Moon where it can get very cold, we can’t use the rubber. And we also don’t want to use the air, because that’s a single point of failure, and — as NASA missions go, we don’t like to have single points of failure.
Host: That’s right, you know, you — get that punctured tire, right, that — when you’re driving down the highway on Earth, you just pull over to the side of the road, and you — call AAA or some insurance company, they bring up, pump up your tire, change it out for you, whatever, maybe even tow it. You know — don’t have that service on another planet as much. So, you know, let’s think about some of the tires we know from the past, right. So, this problem, right, you don’t want a single point of failure, what are some of the tires that — or some examples of tires in the past that sort of shaped this narrative of what a space tire would look like?
Vivake Asnani: Yeah, so, this problem, obviously was new to the people that were working on the Apollo program and the precursors, as well as, you know, in the Soviet Union, they were seeking to land a rover on the Moon. And you know, the real issue was you couldn’t take the regular Earth tire and get the same kind of performance, and you have the risk of the puncture. The Soviet Union sent a rover called — Lunokhod, or moonwalker and they just made the decision, we’re going to use rigid wheels, we’re not going to handle the problem with flexibility. But, you know, with that comes performance limitations, and it was unmanned, so not as much risk. But the Americans, we decided we were going to go for a flexible tire. Yajaira might be able to explain better, but Goodyear actually produced a pneumatic tire to start before we even got into the woven wire tires that we’ll talk about today. Yajaira, you want to tell us more about that?
Yajaira Sierra-Sastre: Yeah, so, I wanted to add to the history of tires before — I get to — that topic, that, as a material scientist, one of the things that I consider very interesting in evolution of the tire is that, that evolution has been driven by the need for better material. So, Vivake was talking about, you know, these rigid wheels that were used, right. And initially, tires were made of leather bands, and then replaced with steel, and eventually as Vivake mentioned, rover, right. The rover material was tuned in such a way to make it more a pliable material, and the material of choice for tires. So, all those materials advancements have — had a really, really impact and have driven these type of evolutions of tires. And besides the Russian Lunokhod tires — back in the Apollo era, there were different types of tire — wheel and tire families, and tires that were developed for different types of purposes. So, we had the LRV, the Lunar Roving Vehicle, that which we can explain in more detail. And also, another type of vehicle called the Modularized Equipment Transporter, MET, which its function, is — was basically carrying equipment and samples collected by the astronauts on Apollo 14. So, depending on the functionality, the purpose, the mission that these vehicles were going to serve and meet, there were different types of requirements for the type of tires that were needed.
Vivake Asnani: Yeah, the — MET was really interesting; it was basically a wheelbarrow that the astronauts used to tow around equipment. If you look back at all the Apollo missions, each mission they tried to get, you know, produce more equipment to carry more rocks. And so, the MET carried a bunch of tools for geology and carried rocks. There was an emergency memo that went to Goodyear that said, “we need a tire, we need a tire for this, you know, instrument carrying machine.” And so, they did what they knew, they produced a pneumatic tire, super interesting though, because it’s going to the Moon, there’s not atmosphere. They put this tire in a vacuum chamber on Earth and in the vacuum chamber, they introduced just a few air molecules, which allowed it to open up to the right size that it would be on the Moon. Then, took it out of the vacuum chamber, and it got crushed under Earth gravity, and they put it into the spacecraft. And when they landed on the Moon, and experienced that vacuum atmosphere again, it expanded to the correct size. So, it was a really interesting trick they played to get a tire on this wheelbarrow on the Moon.
Host: So —
Vivake Asnani: Yeah.
Host: — you went through a couple — Vivake, you went through this — you went through the Russian, is it Lunokhod, am I saying that right?
Vivake Asnani: Lunokhod.
Host: Lunokhod, OK. So, you said that was a rigid tire, and then, you’re — the one you’re just talking about, the Goodyear tire, this is a pneumatic tire. So, can you describe those a little bit, just for, you know, for — well, this is — — unfortunately, this is not like a visual medium, right, so, I’m like trying to imagine it. When I hear rigid, I just think of like, this like — you know, basically a wheel of steel maybe. And then, Goodyear, I think about — I don’t know, like one of those — you know the old cars like 1950s cars that like, they’re just kind of like a flat tube around the car. Am I imagining this right?
Vivake Asnani: You are, yeah.
Host: OK, good.
Vivake Asnani: Because the Russian Lunokhad vehicle had — it had eight rigid wheels. Each of them looked like a bicycle wheel with spokes and on the perimeter was this rigid mesh structure. So, just a screen type of construct. Really basic — it was really intended just to be light in weight and roll. And the Goodyear tire, you’re right, it was just like an old timey smooth tire. It wasn’t there for traction; it was there to be pulled. And so, it just could be smooth without any tread — and the main thing is, they wanted it to be light in weight, so, you know, with everything we launch we just go for low mass. So, it wasn’t until the next vehicle, the Apollo Lunar Roving Vehicle, that carried astronauts, that traction became a really big deal. Traction, and they wanted to travel fast. So, the trick to traveling fast on the Moon where there’s really low gravity and the vehicle doesn’t weigh much, when you hit those rocks, you want to absorb them, you don’t want a hard impact, because literally, it will send all four wheels off the surface and you’ll lose control of the vehicle. That’s what drove the innovation. So, if you want to hear more about the tires that were produced for the Lunar Roving Vehicle, that’s very interesting.
Host: Yeah, so, that’s — OK, so, they have to — I’m trying to paint the picture in my head. So, are they really digging into the regolith, and just kind of pushing it a little bit more so, than just kind of scrapping off the top, maybe like a Goodyear tire would, as the — am I getting that right?
Vivake Asnani: The big difference was the Lunar Roving Vehicle tires needed to wrap themselves around obstacles at high speed.
Host: Oh, OK.
Vivake Asnani: And so, this led to like an innovation of making a tire out of wire that could reorganize every time it hit an obstacle and that quality is called an envelopment. And instead of the obstacle creating an impact for the vehicle, the tire absorbs it, and then you can continue driving with, you know, contact in the surface. So, that was an invention that started at Goodyear, actually, they made the earliest versions. And then, as it became more of a like a spaceflight piece of hardware — the General Motors Defense Resource Laboratory, they further developed that. They wove it like a shirt out of what’s basically guitar strings. So, if you play a guitar like the — E — the A string is about the same thickness as the wire they used. And they wove it like a shirt and shaped it into a tire and then tuned it to behave kind of like a tire does on Earth. So, it was kind of a mix between engineering and art. It’s something worth googling, it’s a beautiful tire.
Host: Very interesting, now — Yajaira, I’m thinking about the materials used on these — Martian rovers, when you’re — are we just, you know, are we using guitar string metal? Like — what are the materials consideration when you start designing a lunar rover?
Yajaira Sierra-Sastre: So, one of the considerations, right, as I mentioned previously is, what type of application, what type of functionality we would like to see in these tires? What type of vehicle — we — these tires are needed for? So, for instance, the Russian Lunokhod tire allowed that vehicle to travel long distances. I — learned that the travel distances exceeding ten kilometers, so comparing to the distances traveled by Mars rovers, these Lunar Rovers — cover, covered greater distances. The Lunar Roving Vehicle for instance, could sustain maximum speed of 16 kilometers per hour. It was a 4 by 4-wheel vehicle. And in terms of materials — do you have more insight about the different types of materials that were used for these tires?
Vivake Asnani: Are you asking about Lunokhod or the Lunar Roving Vehicle?
Yajaira Sierra-Sastre: Yeah, — both, like — Lunar — what are the differences between Lunar Roving Vehicle and Russian Lunokhod tires?
Vivake Asnani: I don’t know. You know, we didn’t have a ton of insight into the Soviet design for Lunokhod. It seemed to be some sort of steel, and the materials that were used on the wire tires for the Apollo Lunar Roving Vehicle were essentially music wire.
Host: Oh, interesting — very —
Yajaira Sierra-Sastre: Yeah, now, that’s interesting, yeah, that’s real interesting —
Host: Yeah, I was making a joke about the guitar string, but it sounds like, yeah, it sounded like that’s what it was. That’s very, very interesting.
Vivake Asnani: Yeah, that’s what it was.
Host: How about that. All right, now, sort of leading into Mars, right, because I think that’s part of the, part of what we’re going to be talking about today is — the innovations you guys have been working on. I want to explore a little bit of history of the tires that have been on Mars, so — some of the — there’s, you know, been — rovers, you know, no humans yet, but rovers on the surface of — Mars — that have driven. So, anything you can share about those technologies.
Yajaira Sierra-Sastre: So, we all know that in terms of wheels on Mars, all the rover wheels are rigid wheels. For instance, Curiosity wheels are made of aluminum. And — but we have seen each of, you know, with those Curiosity wheels on Mars — engineers were — surprised by the magnitude of damage — that they saw in these wheels. And Mars is hard. Mars is hard, as we can share with you today, it’s — the type of embedded pointy rocks, bedrock that were encountered in that traverse path by Curiosity really have an impact in these type of wheels. But, in general, we know as a matter of fact, you know — the tires, the wheels that we have sent to Mars are rigid. So, that has driven some of these — innovation and focus work on trying to look at better wheels and tire designs that, to address some of the challenges associated with struggles in very difficult terrain on Mars.
Host: Yeah, I think I’ve seen those pictures, right. So, Curiosity did, you know, awesome things when it — just after it landed, we got to see some super high-resolution photos of the rover, right. It traveled across the Martian surface doing a bunch of science, and sometimes it took some selfies. And over time, I think we started to see, right, some of those tires. it was taking selfies of the tires. And we could see holes in them, right. So, that’s what you’re talking about, rigid tires, it’s made out of that aluminum, but those sharp — those sharp rocks are — doing some damage to the rigid tires.
Yajaira Sierra-Sastre: Yeah, that’s correct.
Vivake Asnani: Yeah.
Yajaira Sierra-Sastre: I mean, you can imagine, right, the impact that it will have — in mission operations, in how, you know, these traverse paths will need to be carefully, you know, planned. In order to extend the life of these rigid wheels, and therefore, you know, extend the life of — the Curiosity rover to meet its mission objective.
Vivake Asnani: Yeah, and Curiosity, I mean, it’s just an awesome rover. And it’s — as an engineer you look at it, you’re — you get a pit in your stomach when you see something going wrong like that.
Host: Yeah.
Vivake Asnani: We — You know, JPL (Jet Propulsion Laboratory) — they took a hard look at the reasons for this, right, the — there’s — as Yajaira said, embedded rocks that are very sharp, they’re not going to move — when you encounter them. And if you have a six-wheel rover, and a heavy one like Curiosity, something like 850 kilograms or perhaps more — when one wheel encounters one of those embedded rocks, the other five try to drive it through. So, it’s, you know, five wheels ganging up on one, it doesn’t have a choice, and it gets punctured by this embedded rock. It’s not something that was seen in, on Earth for testing. And so, anymore, our risk posture for testing is quite careful — we test almost everything.
Host: So, let’s dive into that Vivake, thinking about why Mars is hard, right. So, you talked about, you know, when you’re talking about JPL rovers exploring the surface of Mars, one of those reasons that Mars is hard is because of these rocks. You know, when you think about driving on another planet, I think that’s the one that may be most obvious when it comes to tires, right, is that puncture, right. You don’t want to puncture the tire, so, these rocks are one of those things that makes Mars so difficult.
Vivake Asnani: Rocks are one of those things. And you know, Mars is surprisingly rocky, and when you get into the dry lake beds like, you know, Gale Crater where Curiosity — went — and you get to Jezero where Perseverance is right now, you can just look around and see all these embedded — cemented in rocks — would make sense, right, it was in an ancient lake bed. And — that, that’s — a massive challenge if you’re trying to fight against those rocks, and what tires that are flexible to do — is to stop the fight, to accommodate and conform to the surface so, that there’s not as much stress, and there’s no puncture. Mars is hard for other reasons too. If you look just about anywhere, any region you can see ripples. And this is essentially the wind picking up certain particle sizes, really small, and blowing them into sort of a rippley pattern that your tires encounter. And if you get all four or all six, depending on your vehicle, tires on these ripples, you can sit there and spin in place. We saw that with the MER (Mars Exploration Rovers) rovers, both Spirit and Opportunity. And — you know, tires help with this as well, flexible tires they spread out the footprint and keep you on top of the surface, you dig in less. But, you know, bringing something flexible to a planet like Mars is hard, because it gets cold, it gets really cold at night, like minus 130 degrees Celsius — in Fahrenheit it’s about minus 200 degrees Fahrenheit and your material needs to stay flexible. So, I mean, that’s the primary engineering challenge on the material level. Yajaira being a material scientist could probably explain that challenge.
Host: Yeah, Yajaira, it sounds like something that — when you’re thinking about a tire, right, what’s going to work on this surface? Something that is going to easily deal with these punctures, right, possible punctures, the rocks that are in place. Something that has enough grip, so you don’t get stuck like — Vivake was saying, and something that can deal with extreme temperatures on another planet.
Yajaira Sierra-Sastre: Yes, that’s correct, and that’s where our team, the Mars Spring Tire team has been focusing on for many years on understanding that interplay between the microscopic, you know, properties of a tire. You know, what type of spring geometry, and how we — weave those springs to make a tire. How that has — or relates to — materials properties, to the properties of those materials that made up those springs? So, we are talking about, you know, like how we can trim the compliance, flexibility of this tire — by design, by spring geometry selections, as well as materials, the material of choice, you know, here. So, in GRC (Glenn Research Center) Vivake mentioned earlier, a lot of their work related or focus on flexible tires and compliant tires made use of steel, steel a conventional metal, was a metal that can be irreversibly deformed. So, after that metal is deformed over and over — several cycles it can be irreversible, irreversibly deformed. And that’s where, you know, these materials innovations come to play, where GRC material scientists, along with tire engineers were able to identify materials such as, shape memory alloy materials, materials that have super elastic properties, and we can discuss what that means. And materials that can — withstand the very extreme temperature conditions that we find on Mars, and keep that material — with, you know, reversible, meaning a material that can undergo reversible reformation within that wide range of temperatures on Mars. A material that will remember, will have a memory and will remember its original shape. So, it’s — that combination of materials, use of shape memory alloys to make and manufacture these springs that we use to then build the tire. And the macro, you know, level design of that tire, which makes these Mars Spring — tires — suitable, you know, solution for — these upcoming Mars Sample Return missions, and — upcoming and future rover missions.
Host: Interesting. So, when you’re describing that material design, you use the terms reversible and irreversible, right, you have something I guess that forms to the shape of whatever you’re driving on, but doesn’t snap back to its original form, the shape that I guess it’s supposed to maintain. It makes me think of a memory foam mattress, right. You get on and it makes that imprint of yourself, right —
Yajaira Sierra-Sastre: Yeah.
Host: — and then you get up, and then, boom, it slowly just snaps back to that nice flatbed that you’re wanting. That’s what I’m imagining, but I guess in a tire form, is this — am I on the right track here?
Yajaira Sierra-Sastre: Yes, you are, you are. And what we are talking here is — on the materials level, right.
Host: OK. OK.
Yajaira Sierra-Sastre: What we — when we talk about reversible deformation, let’s imagine that material at that, at the atomic scale. So, pretty much a shape-memory alloy is a material that when it’s — when stress applied to the material, what we see is how these atoms, you know, rearrange in such a way that the material itself can — can withstand very large strain. But that material and those — you know, atomic bonds reverse to its original you know, shape or structure or crystalline structure at the atomic level. So, these are processes that are happening at the atomic level, and that’s why I get very excited as a material chemist as you can imagine for the uses and application of these type of Mars materials. A material that is stimuli responsive, and the public may be more familiar with the use of shape-memory alloys as actuators. Let’s say shape-memory alloys that can move parts in systems, mechanical parts in systems by adding certain types of stimuli, specializing on electrically, an electrical stimuli or heat, for example. But in our case, the shape-memory alloy application that we are developing for these Mars missions is a type of shape-memory alloy that doesn’t require the several electrical activation or heat activation, it’s a material that has been defined chemically and processed in a way that we can just leverage the ability of the material to withstand very large, you know, like deformations of the material without irreversibly deforming or breaking the material. The material doesn’t break, we can continue deforming the material reversibly over, you know, certain – light and length of time, and the material will — remember that original shape or atomic structure that it has.
Host: I see. OK, so I’m imaging a tire going over one, like a big pointy rock almost like a pyramid, right. So, what I’m, what I imagine when it comes to this shape-memory alloy is the tire rolls over that pyramid, takes the shape of the pyramid, and as it rolls off the pyramid, it goes back to its original shape, that’s the idea here.
Yajaira Sierra-Sastre: That’s correct, that’s correct. And that good result or good outcome — comes from the design of these Spring tires along with tuning the material’s properties at the chemical atomic level, so that even at the atomic level, that material can, yes, can go back to that original structure or shape.
Host: So, diving into this, right, we’re, now we’re getting into the Spring Tire itself, tell me about, a little bit of the history, was the Spring Tire — did it come up — because of these problems that you were trying to solve with driving on the Martian surface? Maybe shape-memory alloys came up first? Give us a little bit of background on this project.
Vivake Asnani: Sure. Yeah, so this came up very early in my career. I mentioned we started thinking about how to drive on the Moon and we saw that it had been done before, right. So, the first thing that we did was try to find the original engineers, the people who developed the Moon Tire, the wire mesh Moon Tire. And — you know — we’d found several experts that helped us by giving us documentation, but then we hit the jackpot. We found the actual lead rover designer named Ferenc Pavlics. He worked at General Motors Defense Research Laboratory. So, we got a chance to meet him in Santa Barbara. And at first it was kind of like, what we found, what we received from others. We had PowerPoint slides, and we were looking at their story and things they built. And then they did this big reveal where they came into the room and showed us one of the original tires.
Host: Get out. [Laughter]
Vivake Asnani: And — yeah, we were like, OK — you could have led with that.
Host: Right? [Laughter]
Vivake Asnani: So — I think it probably was one slated for Apollo 18 that never flew.
Host: OK.
Vivake Asnani: And they explained to us that, you know, living through that time, there was a conflict between funding the Apollo program and the cost of the Vietnam War. So, the, you know, the congress of that time they — abruptly shut down Apollo, a lot of the hardware for Apollo 18 existed but wasn’t used. So, they put one of these tires in their closet and kept it there until we came knocking on the door. So, you know, engineers from Johnson Space Center, Jet Propulsion Laboratory and myself, we were just, you know, tripping over ourselves to learn about this. Eventually, we decided to bring this to the experts, so we took this hardware to Goodyear, which is close to where we — do our engineering in Cleveland at the Glenn Research Center, Goodyear is the neighboring city in Akron. And we set up an agreement that we’re going to — you know, we’re all going to get together and try to learn what this is — reverse engineer it, remanufacture it and get all the value of it out of this original invention. So, we set up shop in the Goodyear Blimp hangar, and made essentially a manufacturing shop. We built all of the machines that were needed to remake this tire. And in the end, we started to understand it. We made it, tested it, understood its limitations and then we wanted to see, can we use it for the next Moon Missions, which were much heavier vehicles. So, we started trying to make heavier versions of this, so we increased the wire thickness and such. And we found out that the structure locks up, it’s no longer flexible when you try to make it support heavier loads. So, at that point we were stuck, and we went back to the agency, we went back to NASA Headquarters and we said — “We need to make a change.” And so, we kicked off this program called Generalizing Moon Tire Technology. It had the goal to try to make it a useful technology for the heavier Moon vehicles and for Earth vehicles and Earth gravity. And so, you know, that was a really, really fun/frustrating thing to work on, because we tried all kinds of design changes. And just hit walls, we couldn’t figure out how to make something as good as the Earth tire, which has air and rubber, but without those ingredients, we’d just be working with the wire. We tried making tries that looked like slinkies, we tried changing the tire to look like Earth tires, we had like radial construction. In the end, we stumbled upon the Spring Tire. We were trying to make these wire tires, and we’re having trouble keeping the wire in place. So, we were using springs as spacers to keep the wires separated from one another. And over time we kept adding springs and removing wires, and eventually the whole thing was just a Spring Tire. And we created that Spring Tire sort of like a fireplace spring if you can imagine. Where, each spring is intercoiled with its neighbor, kind of like DNA, and then shaped into the structure of a tire. So, it was kind of an accident, we can’t say we had a really clear design logic, but in the end made for a really useful tire.
Host: Yeah, you know, it’s almost like, it sounds like a trial and error journey, right. Let’s try this, no, that didn’t work. Let’s try this, no, that didn’t work. And then it seems like springs were sort of leading you in the right direction. At what point did shape-memory alloys come into the story?
Vivake Asnani: Later on. Actually, I want to, if you don’t mind, I want to give a quick shoutout to Jim Benzing who —
Host: Yeah.
Vivake Asnani: — I think put us on. He was a Goodyear engineer who put us on the right path, because — we tried a lot of, I would say academic solutions, and he was like, “OK, guys, let’s get in there and try stuff,” and that’s how we got — we arrived at a design solution. Shade-memory alloys came many years later when we shifted our attention from the Moon to Mars. And you know, the initial step was to try the original Spring Tire on a Mars, what we call a life test. And essentially, that’s driving, in this case, it was like a half of the Mars 2020, or the Perseverance rover around a test track and just seeing if your tires will survive. Well, the original Spring Tires survived, but it would get dented, it would get plastically deformed, and that just created the problem to solve and the material innovation was what was brought in, the shape-memory alloys in order to solve that problem.
Host: I see, and that’s where Yajaira comes into the mix, right. Yajaira, so how did that, how did you start incorporating this shape-memory alloy with the Spring Tire design?
Yajaira Sierra-Sastre: Well, I should — say that in our team, that to add to that story, right.
Host: Yeah.
Yajaira Sierra-Sastre: So, we have in our Mars Spring Tire Team, we have material scientists, we have tire engineers and back in the days when as Vivake said, when these steel tires started giving issues associated with plastic deformation, it was a moment of serendipity between Dr. Santo Padula and Colin Creager, Engineer Colin Creager, where they started chatting about, you know how to fix that problem. And Dr. Santo Padula, like our SMA expert, subject matter expert in our project, he offered the use of SMA tires as a solution. And since then — the, that moment, you know, that moment between that, that, is what started, you know, the, this new engineering development, you know, incorporating these shape-memory alloys as the material of choice for — for these tires.
Host: So, Vivake, taking this concept, right, the Spring Tires as we know it today, tell me about some the testing that you guys have been doing to prepare this tire for its next step on another, on the surface of another planet.
Vivake Asnani: Sure. Yeah, so, you can kind of put testing in two categories, performance and survival. So, we obviously, we focused on survival as sort of the primary thing. We — the same engineer who was coinventor with myself on the Spring Tire, Jim Benzing — we hired him after he retired from Goodyear. And he’s a career machine designer, he’s building our life test. And if you can picture this, this is a chamber that holds Mars’s temperature and a carousel of Mars’ terrain revolves within that chamber, really harsh rocks. We spied on Mars so to speak, we used the Mars Reconnaissance Orbiter’s, high-rise stereo imaging tool to look at the surface where we’re going, see the terrain that we might encounter in this next mission, and construct the life test to represent that terrain. So, we have the right rocks — and we’re in the right temperature environment and to account for the gravity difference, we have an interesting mechanism within this chamber that, you know, balances like a seesaw the right amount of — rover mass but also allows you to have that light weight that you would have on Mars. And we drive to four times the mission life to understand if we can survive and have, you know, reserve life, just to be sure. So, that’s the survival side of things. And on the traction side of things, we have a whole laboratory, we call it the Slope Lab. We actually developed it for the Constellation Program, but if you can picture that, inside is this adjustable tilt bed, which — it’s essentially two dump truck beds welded together, and we can angle it with the hydraulics up to like 40-something degrees. So, we can use that to evaluate slope traversal and we have another big sandbox and other machines that allow us to drive and evaluate different rover operations. So, you know, we are doing our best to try and cover our basis and go in with our eyes wide open.
Host: I’m trying to get a, paint a picture on that first test that you were talking about, Vivake. Is — I’m — I imagine like some chamber, right that’s simulating the Martian surface, and you said drive to four times the design life. So, how I imagine it is a tire just kind of going around like a carousel — for — I don’t know, in my mind, I think years. Is that what’s going on here, you’re just kind of driving this thing over and over, and over?
Vivake Asnani: You know, fortunately, it doesn’t take that long.
Host: OK.
Vivake Asnani: We can get ourselves a full life test under our belt in about a week.
Host: Oh, interesting.
Vivake Asnani: What’s — yeah — it’s not too bad. You know, if you compare that to driving on Mars, there’s a lot of downtime, you’re not driving at night, it’s too cold. You’re — with this particular rover it’s solar powered, so your energy limited as far as how you go each day. But, you know, it’s also autonomous, so, it’s not like the Apollo rovers where you could drive quite fast and quite far. It’s going to set the land speed record on Mars, but still quite slow. So, when you test on Earth we go faster and because we’re going continuously, we can get the whole mission done in a couple days, and, you know, get ourselves an even longer test in a week. Basically, we keep the wheel and the tire attached to an, one arm of what would be the rover. And the carousel moves while the tire rotates.
Host: Aha.
Vivake Asnani: So, we’re not driving around, it’s kind of fixed, and that’s what lets us be inside of a temperature chamber by keeping everything stationary and having the carousel move underneath the tire.
Host: OK, I get it now, yeah. And — I, that totally makes sense, right, because the — even the rovers they’re driving now, they drive a little bit, they pause for a long time, and then they drive a little big again, you know. So, it’s, I — see, you’re just basically getting that, all of that time done, the driving part done in a week. So, I — understand that.
Vivake Asnani: Yeah.
Host: OK. So, what’s next for the Spring Tire? You keep talking about missions, you know, getting ready for the missions. So — what are you — what are you preparing for? What’s the — Mars tire going to, or the Spring Tire going to do?
Yajaira Sierra-Sastre: Yes, excellent. So, the Mars Spring Tire is the subsystem that will be delivered to ESA — Sample Fetch Rover. So, have been very excited, right, about the successful landing of Perseverance. Now Perseverance Rover is on Mars, will be collecting samples, putting — Martian soil samples in — in this tube that a future Mars Sample Return mission will be collecting to return them back to Earth. So, we are part of that endeavor. We have been collaborating with ESA — in their — with their Sample Fetch Rover design, and — right now, with our part or role it’s to design and build the wheel and tire assemblies that will be integrated in these — in the Fetch Rover. And, I should mention that earlier in the conversation you were wondering about the time, you know, that these future rovers will have to traverse the Mars terrain.
Host: Mm-hmm.
Yajaira Sierra-Sastre: And we should say that for the Mars Sample Return mission and in contrast to Perseverance, which is a scientific missions, right, these sample Fetch Rover won’t have a lot of time, so that’s why new rover — development, new tire developments are needed to ensure, as we have mentioned, to ensure that these — 4 by 4 Sample Fetch Rover — Rovers will have sufficient traction, that these wheel and tire assemblies will be durable enough, that will — that these wheel and tire assemblies — will help this rover to save energy, so that the rover travels more terrain, and go and catch the samples. And the tube that the Perseverance will be — leaving in — default and specified locations — to then, you know, return them to — the Mars Ascend Vehicle that will bring them, you know, back to Earth. So, the, lots of work with design, lots of work, you know, co-engineering efforts to define those –wheel and tire assembly requirements. And Vivake — you can add more to that.
Vivake Asnani: Oh, I’d love to. This is one of my favorite subjects. So, the next mission — the next mission to Mars as Yajaira just said is retrieval mission, so it’s called Sample Retrieval Lander. It’s really unusual in two ways — number one is, we’re taking a rover and a rocket in the same lander, they have to coexist. So, usually, we talk a lot about mass, mass is a big deal when you engineer a space craft. Here, we’re also really volume constrained, so the rover folds up, and it has — there’s a neat little compartment inside the back-shell interface structure where it lives. And it doesn’t have the ability to be as big as Curiosity, right, or you know, the Perseverance Rover — it’s much smaller — but it still needs enough traction to get through the region where the big rover currently is in Jezero Crater. So, the tires were enabling — the tires allow the baby rover to have enough traction to keep up with the big rover. And so, in that sense tires are really important to this mission. And the other thing that Yajaira touched on is it’s time constrained — it’s a solar rover that’s doing the pickup. And — the solar rover can’t be around for any kind of global dust events that might occur up on Mars. So, those tend to happen in the fall, wintertime, so we’re trying to constrain the mission to spring and summer, which means you have to get your mission done in two seasons. And to make that happen, you really want to go as far as you can every day. And the thing I mentioned earlier is, tires save energy, and energy lets you go an additional distance. So, in two ways, you know, having these tires is enabling for us to pick up samples from the surface of Mars.
Host: See, this is so fascinating, there’s a lot of application here, Vivake. As you’re talking about this upcoming mission, right, some of the things that are going through my head is, you know, there’s a lot of application here, there’s a lot of, there’s been a lot of testing for this. Is this scalable, right. So, we’re talking about returning humans to the Moon in the very near future. That means a — not necessarily a continuous presence, but a sustained presence of humans on the Moon. And what I’m thinking is maybe humans can benefit from a technology scaled up to a lunar rover that’s, that — uses a lot of this technology. Is there a future for scaling up the — Spring Tire Technology? I know you’re talking about Mars, right, for a lot of this, but is there a potential for it to be — on use on the Moon?
Vivake Asnani: I do think so. The Spring Tire was originally developed for the Moon. I can say with the materials we have today, it could support an equatorial mission. So, you know, it’s not going to get as cold as if you were to say, go to — a crater — at one of the poles, which is hidden from the sun, and gets — you know, colder than our current material can tolerate. So, today, you know, the Spring Tire is scalable for you know, larger vehicles with heavier loads. And it’s really great at keeping a rover on the ground when you drive fast on the Moon, and that’s what you want. If you’re going to bring people to the Moon, you want to efficiently move them around. And so, a fast-moving rover on the Moon would really benefit from a set of spring tires. We have more work to do if we’re going to go to the poles. We need to do further material innovation to start getting down to polar temperatures.
Host: Now, Yajaira, you talked — you know, Vivake just mentioned material innovation, right. There was a shape-memory alloy that went into the Spring Tire design, and a lot of innovation to make that possible for the upcoming Mars mission. But I wonder — you know, there seems like there was a lot of work in here, and there’s a lot of benefits of shape-memory alloys, potentially of Spring Tires. Is there anything that can be brought back down to Earth for the benefit of us here on, that are stuck on this planet?
Yajaira Sierra-Sastre: Yeah, excellent, excellent question. So, yes, the answer is yes. Shape-memory alloys — have been around for decades, have been around for decades. And some predict that their demand will reach the $20 billion dollars by 2025. So, there are lots of earth-bound applications of shade-memory alloys that are — currently in place, and that will be further expanded and developed. In the area of biomedical devices, for instance, we know shade-memory alloys are currently used as the material for a stent — the stents. Another applications, you know, would be for engines, or you know, anything that — makes a mover, right. Or a shade-memory alloy as an actuator to move parts. So, those are — applications that are currently in place. What I can tell you, there has been so much interest from the industry and private sector on these GRCs, shade-memory alloy development. So, currently there is a company that commercializing shade-memory alloy Spring Tires for — bikes, for — bicycle applications. And we also see it, I mean, we also see potential for, you know, medium fit of the rove tires, for vehicles for military, for any type of application that, where you want to remove and avoid that single point of failure, right, of a flat tire. So, really exciting times, really good to see how our engineers, our scientists, you know, they — are now — at this point, they are enjoying, right, the, all the — good outcome and fruit of all these years of effort. Seeing these companies now, you know, looking for applications. The list is quite interesting, because when we look back at the history of tires, and the history of rover made tires, everything started to, with, you know, like the design of a rover tire for bicycles. And now we are seeing like kind of the same thing but, of course, I mean, the applications go beyond these type of you know, bicycles and – vehicles, we see potential in many other areas as well.
Host: Wow. Yajaira and Vivake, what a fascinating conversation that we just had today. Tires on different planets, and just wonderful stories that you were able to share throughout that whole process. This has been a really, really good conversation. So, to both of you, I appreciate you going over the Spring Tire and the history of tires when it comes to driving on different planetary bodies. And I thank you both for coming on Houston We Have a Podcast today, it was a pleasure.
Vivake Asnani: Thank you so much, it was great.
Yajaira Sierra-Sastre: Thank you so much.
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Host: Hey, thanks for sticking round. I hope you enjoyed our conversation today with Yajaira and Vivake as much as I did, I definitely learned a lot about tires, not only the history, but a lot of the cool technology that’s under work right now for missions to Mars, and perhaps the Moon. You can check out more our podcasts at NASA.gov/podcasts, there’s also a couple of other shows there from the different centers across NASA that you can find. If you want to listen to Mars specific episodes, we actually have a collection of those specifically. You can search it, just Houston We Have a Podcast, Mars episodes, it’ll be the first thing that comes up. The URL for — everyone though is — is NASA.gov/Johnson/HWHAP, as in Houston We Have a Podcast/mars-episodes. We, Houston We Have a Podcast, are on the Johnson Space Center pages of Facebook, Twitter, and Instagram. So, if you want to talk to us or ask us a question, you can use the hashtag #AskNASA, and just make sure to mention it’s for us at Houston We Have a Podcast. This episode was recorded on March 22, 2021. Thanks to Alex Perryman, Pat Ryan, Norah Moran, Belinda Pulido, Jennifer Hernandez, and Jimi Russell at Glenn. Thanks again to Dr. Yajaira Sierra-Sastre and Vivake Asnani 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. We’ll be back next week.