It’s about as close to magic as it is possible to find in nature, but this metal is allowing NASA to reinvent the wheel for space exploration.It’s a metal slinky that can stretch up to 30 times more than an ordinary metal and still spring back to its original size.This metal is being used in everything from medical devices to toys, to bulletproof bike tires, and it can even withstand extreme temperatures and punctures from bullets.It’s no wonder NASA is using it to make wheels for space missions as it is lightweight and strong, making it a perfect solution for the extreme conditions of other planets. And it was designed to be a little bit more robust, and as you can see, it has these structural members called grousers.
Derek and Jim discussed the different types of wheels used in space exploration. The wheels on the Mars Rover have large rubber tires with grousers to provide strength and help grab onto obstacles and soil. These grousers are experiencing higher peak loads than predicted, resulting in holes and cracks in the skin of the wheel. Derek explains that when a force is applied to a material, it is known as a stress and causes the material to deform. For most materials, strain is directly proportional to the stress applied and the material is elastic. If the stress applied exceeds the yield strength of the material, it can undergo plastic deformation and not return to its original shape. The Apollo Lunar Roving Vehicle used a pantograph structure of wires to keep the tire from sinking into the ground. The wheels worked well for the short distance journeys, but for longer journeys a bump stop won’t be enough to prevent plastic deformation. Mesh steel wheels have been tried on Earth, but their performance degrades over time. The Mars steel spring tire has structural members called grousers to be more robust. This helps create lift at lower speeds, and so it’s really useful for takeoffs.
Shape memory alloys, like nitinol, are strong and durable like steel, but can endure much more strain without deforming permanently. This property of nitinol was discovered in 1961 at the Naval Ordnance Laboratory when a sample of nickel and titanium alloy was repeatedly worked, heated and cooled. When exposed to heat from a lighter, it changed shape, leading to more investigations into the material. This is because the alloy can undergo a phase change in the solid state, where atoms are arranged in a cubic lattice arrangement known as austenite. When cooled, the atoms ease into a form known as twinned martensite. This messier, lower symmetry arrangement of the atoms allows stress to be applied and the material to be deformed. When heated, the atoms return to the austenite phase, returning the material to its original shape. This is known as shape memory.
Shape memory alloys can generate significant forces when heated, and are used in applications such as stents, actuators and vortex generators on aircraft. Scientists have even used shape memory alloys to fracture a rock. Shape Memory Alloys, such as Nitinol, have a unique property which enables them to be used for a variety of applications. These alloys can be used to aid in the takeoff and landing of an aircraft by creating vortices which help to keep the airflow attached to the wings. This prevents the aircraft from stalling. During cruise, these vortices are not needed and can be stowed away to reduce drag. The alloy is designed to take advantage of the ambient temperature change which occurs when the aircraft climbs from takeoff to cruise. This transition between austenite and martensite can be tuned to be between -150 to -350 degrees Celsius by changing the ratio of the elements and using different heat treatments. This same principle has been extended to operate the main flaps on an aircraft. Heating and cooling is now controlled by a heating element. Demonstrations have been done on a 737 aircraft where no hydraulic actuators were used on the wing box. Instead, two tubes of nitinol were used to drive the air arms and flap elements on the wing box of a 737 in flight.
Another unique property of Shape Memory Alloys is their super elasticity or pseudo elasticity. This property allows the material to stretch up to 8% of its length and still be able to snap back to its original size and shape. This transformation occurs entirely above the martensite transition temperature. When a stress is applied, it induces a phase change from austenite to detwinned martensite. Once the stress is removed, the atoms spring back to the austenite phase, thus returning the material to its original size and shape. This is demonstrated when a nitinol tube is bend and it will return to its original shape. When we go from the higher symmetry phase, the austenite, to the lower symmetry daughter phase, it is exothermic. If you were to put your hand around this tube, you would feel the heat energy, the enthalpy of that transformation evolving as heat. When the stress is removed and the material goes back to being austenite, that phase change is endothermic. It absorbs heat. Another area where these materials are being applied is in a field called elastocalorics, where they use this transformation to do things equivalent to heat pumping.
To make a tire out of shape memory alloy, they weave nitinol springs together into a mesh. The dissipation potential can act like the dissipation in the shock absorber, so the tire itself can perform some of that dissipation potential on its own. It almost acts as a damper, to get rid of that energy loss. So then the tire has a potential of becoming a complete suspension system.
The terrain endurance rig consists of a circular carousel that is independently driven, and the wheel tire assembly is also independently driven. This is about how slow a Mars rover would be traveling, at an average speed of 6.7 centimeters per second. The shape memory alloy is strong enough to support the weight of a vehicle or vehicle and crew, but it is also incredibly flexible, able to deform up to 8% without being permanently damaged.
These tires won’t just be for space, they are also looking at terrestrial applications. Most aircraft tires have to be pressurized to very high levels (300-400 psi) compared to conventional car or truck tires (30-60 psi). This carries the risk of them exploding. Maintenance is also an issue, as they need to be constantly checked to ensure they are at the right inflation pressure to avoid burning too much fuel or having a tire pop due to the loads.
NASA has developed an alternative to air-pressurized tires, which is a metal that works like magic, allowing for airless tires that can take us off road, on road, into the air, and across other worlds. This has been tested on a Jeep, and the benefit is that it can never be under-inflated, thus significantly improving fuel economy.
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