How NASA Reinvented The Wheel

This metal is the closest thing to magic that can be found in nature. I just don't understand. It can adjust the arrangement of its atoms to return to a predefined shape, but it also converts between mechanical and thermal energy. And it can stretch up to 30 times more than ordinary metal and still return to its original size. I can feel it in my hands receding. Because of these unique properties, it is used in everything from medical devices to toys to bulletproof bicycle tires. And it's allowing NASA to reinvent the

wheel

for space exploration. - These are the bones of the tire. - The tire bones are sneaky. - Basically this is the slinky applied to the rim. - You just wrapped a slinky around an edge. - Yes.

There is nothing simpler than that, right? Here's a bike that has slinkies inside a polymer, if you look inside. - This tire does not require air pressure to operate. The structure and shock absorption are provided by that stealthy metal. - That's about a hundred psi or what it would feel like on a normal road bike. - Yes. Which means you should be able to drill it without loss of performance. So we'll run it over a bed of nails, but first we'll test a traditional tire just to make sure these nails are sharp. (tires exploding) (upbeat music) - Another flat, another flat. - This one, more or less expected.

So now I'm going to put these airless tires to the test by driving over the same bed of nails. Here we go. (tires bursting) I heard a lot of bangs. I must have hit some nails. I don't feel anything different. (upbeat music) It's still going well. I'm going to speed up a little. - It's definitely a nail. - I think the nail broke, why does it seem... - That's what it seems. - Yes, the nail is in the tire. - Now let's try to shoot a bullet at the tire and see what happens. 3, 2, 1. (bullets firing) (upbeat music) - There it is. - There is. - Oh! - Check it out. (Derek laughing) - Wow.

It's a really clean and direct shot. - Yes, you can barely see the mark on the tire. Looks like this one really hit the... - Alloy? - Yes, it seems that way to me. - Yes, that's what it feels like. - You can see that we cut part of the bullet before we even get to the cardboard. - How is the trip going? - If there are not problems. Bulletproof bicycle. This bulletproof bicycle tire actually comes from NASA's research into making

wheel

s for space missions. (upbeat music) It's really hard to make good wheels for other planets.

I mean, in many of the places we want to send rovers, there is no or very low atmospheric pressure. - We cannot use rubber tires due to the extreme conditions on the Moon and Mars, there is no confining pressure outside of it. It can basically explode. - Furthermore, with extremely low temperatures, the rubber becomes brittle. - If this were a flagpole, the temperature facing the sun would be 250 degrees Fahrenheit above zero. In the shade it is 250 degrees below zero. Let's put some rubber on the moon. - 90ºC is the glass transition temperature. It is when the polymer goes from being a flexible element to a rigid element. - This is what happens when you immerse rubber in liquid nitrogen. (rubber exploding) (bright music) (Jim chuckling) This is why you can't send rubber to the moon.

That is why almost all the wheels used to explore other planets have been made of hard metal. - Actually, this is a spare part for the Curiosity rover. They are made of aluminum. A single billet that is machined so you don't have to worry about fasteners or welding or anything like that, which could be a point of failure. - But since it is so expensive to launch matter into space, the wheels have to be as light as possible. It's a little light, but still heavy. - To meet those mass limitations, they thinned the skin to a thickness of 0.7 millimeters. - Thinner than a credit card. - Yes, these structural members here, which we also call claws, are there to give strength to the wheel, but also to help grip obstacles and the ground.

The problem is that because this rubber is so big and heavy and the terrain is so aggressive and nasty, they are actually seeing much higher peak loads concentrated in the areas between these grips than was predicted. This is the actual condition of the wheels on Mars right now. And as you can see, we have big holes and cracks where that skin was. Now the wheel still works, it hasn't immobilized the rover. You will still complete your mission, but it affects where you can go and how efficient you are. - When a force is applied to a material it is known as tension.

And what you're really doing is pulling all the atoms inside the object and as a result their spacing changes a little bit and therefore the material deforms. For example, if you pull on an object, it will lengthen a little. And the change in length per unit is called strain. Now, for most materials subjected to low stresses, the deformation is directly proportional to the applied stress. And the more you stress it, the more it stretches and the material is elastic. If you remove the tension, the object returns to its original size. So no atom has moved and no bonds have been broken or formed.

You just made them flex when you apply that tension. But if the applied stress exceeds the elastic limit of the material, then the stress is so great that the atoms cannot maintain their positions relative to each other. Defects called edge dislocations can pass through the material. In reality, the atoms are rearranging themselves and therefore the deformation is not reversible. It is plastic deformation. Therefore, the object will not return to its original shape when the stress is removed. If enough stress is applied, the material can fracture completely. In the worst case, this causes holes, such as in the wheels of the Mars rover, which reduce its performance and could ultimately jeopardize the mission.

Common metals can elastically resist deformation of only about 0.3 to 0.8%. More than that, they suffer plastic deformation so they will not return to their original shape. Ultimately, they might even fracture. Alright. - Yes, and you twisted it too. - He folded it and stretched it. And that's why every component of a spacecraft is designed to never stretch more than that 0.3 to 0.8%. But that is an important limitation. There is a different type of wheel that NASA has tested in space, which are those on the Apollo Lunar Roving Vehicle or LVR. - That particular structure they built is something we call a pantograph.

All it is is a set of cables that have been woven over, under, over and under. - And this on the surface here to tear off also to strengthen? - It is mainly to ensure that the tire does not sink into the ground. So they did some studies with these strips to determine how much coverage they needed. And then they discovered that about 50% was enough to keep the tire floating on the surface and still maintain that flexibility. - The Lunar Roving Vehicle's wheels worked well for short-distance trips on the Moon. I mean the furthest this vehicle went was 36 kilometers, but still, these wheels had to be designed to minimize plastic deformation of the steel mesh. - And then they put this internal structure in there.

We call it a cap. So when they hit a stop, and it deforms, when it hits, it stops the deformation to keep it just below that proportional limit where they would induce plasticity. - This wheel was good enough for the short Apollo missions, but for longer trips a stop will not be enough to prevent plastic deformation from building up over time. Steel mesh wheels have been tested in the dirt, but their performance degrades over time. - This was the Mars steel spring tire that we built and drove on that same test bench. And there's no fracture but you see a lot of permanent deformation there. - What we need is a material that is strong and durable like steel, but can withstand much more stress without permanently deforming.

And that's where this comes into play. In 1961, the Naval Ordnance Laboratory was conducting experiments with different alloys including nickel and titanium. A sample that had been repeatedly worked, heated, and cooled was shown to one of the associate technical directors, who happened to be a pipe smoker. So he decided to see what the sample would do if he applied a little heat to it with his lighter. And when he did that, he discovered that the material changed shape. This surprised everyone and led to more research into the material. Which became known as nitinol, due to its nickel and titanium components, and because of the Naval Ordinance laboratory where it was discovered.

So why did nitinol change shape? Well, it's actually because the alloy can undergo a phase change in the solid state. In heated nitinol, the atoms are arranged in a cubic lattice, and this phase is known as austenite. But when cooled, the atoms take on a shape known as twinned martensite. It is a more disordered lower symmetry arrangement of atoms. And in this phase, tension can be applied to the material and deformed. But unlike an ordinary metal, this deformation does not cause the bonds between atoms to break or cause edge dislocations that move throughout the material. Now in this case, the crystal structure is changing once again to an unpaired martensite form.

And now, when you heat it again, the material changes from martensite to austenite. Which means that all the atoms return to their original locations and therefore the material returns to its original shape. - We can basically establish this shape as the main known memory shape. That's why we call it shape memory. I can stretch this. If you cool it you might stretch it even more, but as soon as you warm it up again, it will remember its original shape. - And that is why nitinol is considered a shape memory alloy. The shape is fixed at high temperature when the material is in the austenite phase.

Then, as the material cools, it undergoes a phase transition to twinned martensite. If stress is now applied to the material in this phase, it can be extensively deformed changing the crystalline structure to unpaired martensite. When the stress is released, most of that deformation remains. But when the sample is heated, the atoms return to the austenite phase, returning the material to its original shape. (Derek laughing) It's like you're barely in the water. - No. - And just... - It's as fast as you can conduct heat to it or remove heat from it. - Whoa Whoa. I mean, that's great.

This is the property of nitinol that most people are familiar with and that makes it useful for many applications. So that's a period. - They cool them slightly just below until they obtain martensite and then crush or elongate it. As you can see, it becomes very thin. And then they put in a catheter and that catheter goes through the body to the place where they want to deploy the stent. And then when you unfold it, it bounces. Increasing that outside diameter and opening up that artery. Nitinol is absolutely perfect for that. - Shape memory alloys can generate significant forces when heated, meaning they can also be used as actuators. - You'll see a lot of force and tension build up in the cable, which we can see here with how much you pull. - Six pounds, seven, you can really see it contracting there. 13, 15, 16, 17, 20 pounds.

Oh, he's picking it up. That's about 90 newtons of force. Scientists have even used shape memory alloys to fracture a rock. Shape memory alloys are being investigated for use in aviation. I made a video before about vortex generators. What are these small fins that protrude from the wing of an airplane to cause turbulence in the air flow? This is important during takeoff and landing to keep the flow attached to the wings so you don't stall. - But when you're cruising and you don't need those vortices to generate, you want them to be saved because they're a drag penalty.

As the plane climbs from takeoff to cruise, we go from a temperature on the ground to something close to -50, -60 C at cruise. The alloy is designed between them so that we can take advantage of the ambient temperature change that occurs in the environment. When we cool it, without any controller or operator, it remains flat autonomously. - The temperature at which the material goes from austenite to martensite can be adjusted between -150 and -350 degrees Celsius. This is done by changing the proportion of the elements and using different heat treatments. - And then, as that would warm up again upon landing, it goes back up. - This principle has been extended to operate the main flaps of an airplane.

Now the heating and cooling are not passive, but are controlled by a heating element. - We've done demos where you have a 737 airplane and there are no hydraulic actuators in the wing box. All we have is a shuttle mechanism powered by two nitinol tubes and we have powered those air arms and flap elements on the wing box of a 737 in flight, with a 60 degree downward flap angle, a 30 findegrees up by simply heating and cooling two nitinol tubes, replaces the entire hydraulic system. - The shape memory effect is the most important thing people know about materials like nitinol, but they have another unique property that makes them ideal for making durable wheels. - And you're just going to take it and wrap it a couple of times around your hand like this, and you're just going to pull that wire and feel like 6 to 8% tension on a piece of metal. - Oh, that's really strange. - That's a tension of 6 to 8%, something you can't do with other cables, right? - But the strange thing is that it feels a little crunchy. - Because you're feeling all the reorientation. - Oh, how strange. - Very good, right? - Yes, very cool. (nitinol ping) Can you hear that? - Yes. - How weird is that? - That ping is 20. - Shape memory alloys can stretch up to 8% of their length and still recover their original size.

This property is known as superelasticity or pseudoelasticity, but these are misnomers because the material does not actually operate in its elastic regime. What really happens is that this nitinol is in the austenite phase. Its transition temperature is lower than room temperature. But applying stress, even without temperature changes, can force the crystal structure to change from austenite to unpaired martensite. And this rearrangement allows the nitinol to deform by that 8% and still return to its original configuration once the stress is removed and the atoms return to the austenite phase. (nitinol ping) That sound you're hearing is the material undergoing a stress-induced phase change in the solid state.

If you want to think about it in a stress and strain curve. Now, this transformation occurs entirely above the martensite transition temperature. So the material starts in the austenite phase, and then the applied stress is what induces the phase change from austenite to unpaired martensite. And when that stress is removed, the atoms return to the austenite phase, so the material returns to its original size and shape. - If this were a normal tube I would bend it here and it would be plasticized. If it were a brass pipe, which you know has a plastic buckling mode, it would be like this and it would actually buckle a wall.

I would never take my hands and fold them like that and they would completely snap back into shape. - In the curve, nitinol transforms from austenite to martensite and vice versa. - When we go from the phase with the greatest symmetry, austenite, to the daughter phase with the least symmetry, which one is it? Exothermic or endothermic? - I feel like that should be exothermic. - Good job, scientist. (Derek and Santo laugh) If you put your hand around this tube, you would actually feel the thermal energy, the enthalpy of that transformation evolving as heat. Are you ready? - Yes.

Oh, yes, that's very attractive. - Oh oh oh. Actually, that's like getting burned. Like I couldn't keep my hands on him. - No, keep your hand on it, it won't burn. - Damn, that's hot. - When the stress is removed and the material returns to being austenite, this phase change is endothermic. Absorbs heat. Court. (Derek laughing) Right? It's like you can use it as a refrigerator. - Then it is exactly correct. So another area where these materials are applied is in a field called elastocalorics, where we use this transformation to do things equivalent to heat pumping. - As heat pumping.

I want to photograph this with our thermal camera. We have a FLIR with us. How is that? - This dissipation potential can act a bit like the dissipation in the shock absorber, right? Therefore, the tire itself could realize some of that dissipation potential on its own. - It almost acts as a shock absorber, right? To get rid of that loss of energy. Then your tire really has the potential to become a complete suspension system. - Hmm. - Which obviously greatly simplifies the construction of vehicles for space. The original tire, when I put a load on it, okay, you can see I'm just transferring a load from the tread to this little section of the tire, okay?

By tying this stop element here, when I go over a tread, you can see now that I'm transferring load 360 degrees around the tire, right? By doing this, I have significantly increased my carrying capacity without adding more mass. - So, to make a tire with a shape memory alloy, they intertwine nitinol springs forming a mesh. It is quite a tedious and time-consuming process. - Then you're going to take it like that. - Yes. - Are you going to grab both ends? - No. - And I'll accept it. - No you are not. - Take it. - Yes. - And screw it on. - My God.

Are you kidding me? Is this what you do every day? - 684 Times. - 684 times - - Per tire. - But will these wheels work on rovers on the Moon and Mars? Will they test the wheels extensively on a spinning carousel on different types of terrain, from sand to small rocks to larger rocks? - So the terrain resistance equipment basically consists of a circular carousel that is independently powered. The wheel tire assembly is also driven independently. Then we can create a slip force condition, so we can drive with zero slip. (Rover wheel whirring) And it's about how slow a rover would travel on Mars.

The average speed is about 6.7 centimeters per second. That's a nominal speed, they don't go too fast. - Okay, I'm going to walk on the regular simulated moon. It looks like a beach and feels like a beach. This side is intended to simulate the surface of the Moon, and this side is intended to be the surface of Mars. It is very sunken sand. The wheel rolls, rolls, it's a rock. Am I pushing towards him or do I want to put him on top? - I would say get on it and put all your body weight on it. - That's basically my entire weight on it.

Shape memory alloy is strong enough to support the weight of a vehicle or a vehicle and its crew, but it is also incredibly flexible. So it can deform up to 8% without suffering permanent damage. And that's what is needed for long space missions. - So that's a pretty good amount of warp, right? - That's a lot of warp. - And it still does not exceed 8%. - It's so sticky. Just walking back to the car after the beach. Complicated for a rover, right? But these tires won't just be for space. They are also studying terrestrial applications. - Most airplanes, the tires on those airplanes, have to be pressurized to a really high pressurization, 300-400 psi.

It's not the conventional 30-60 psi that applies to car or truck tires, right? We have problems where, with that enormous pressurization, they can explode. The other build is maintenance, right? So if I'm a tire and I depend on that pneumatic system for the performance of the system, I always have to be checking the air pressure to make sure that I have the correct inflation pressure so that I'm not burning too much fuel, or I'm not in a place where You could burst a tire due to the loads. Opting for a structural system that does not depend on air and is designed specifically for the application.

All those things disappear. - They have tested one in a Jeep. Since it doesn't rely on pressurized air for support, you simply can't get a flat tire. Additionally, it can never be underinflated, which significantly improves fuel economy. With a metal that works like magic, airless tires can be made that will take us off the road, on the road, through the air and through other worlds. (fire hiss) (logo falling) NASA's nitinol tires are designed to last the entire life of a rover mission, even in rugged Mars terrain. But here on earth, few products last a lifetime. From bicycle tires to phones to toothbrushes, virtually everything wears out.

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