With scientific advancements in space technologies, the need for more robust and efficient components is on the rise. To meet the demands of smoother and more stable rides in exoplanet missions such as Mars exploration, scientists are on the verge of reinventing our very concept of tires. Enter Superelastic Tires - a new research domain that inherits from the idea of Spring Tires, presented by NASA Glenn and Goodyear - inspired by the Apollo lunar tires.

Spring tires introduced a new concept of airless tires from what we see every day. These off breeds consist of several coiled steel wires woven into a flexible mesh providing the tires with the capabilities to support high loads while also conforming to the terrain, making it suitable for operations in unknown and complicated terrains we might expect from Martian or Lunar missions.

But what differs between Spring tires and the superelastic tires developed by NASA? To answer this, we’ll need to look into the revolutionizing concept of “Shape Memory Alloys.”

A Metal that remembers - NITINOL

NiTiNOL is an alloy of nickel and titanium, developed by William Beuhler in the laboratories of the US Navy in the 1960s. The exact ratio between nickel and titanium is 55% and 45%, respectively. Apart from their excellent superelasticity, durability, and corrosion-resistant properties, these alloys feature a very peculiar shape memory concept.

These alloys can be deformed when cold but return to their pre-deformed shape when heated to a specific cutoff temperature. This property is in virtue of the temperature-induced phase transformation that reverses deformation. When Nitinol is below a particular temperature, it has a crystal structure called martensite. Its crystal structure is arranged in such a way that it can accommodate deformation very easily. When stress is applied, the grains deform and align to absorb the pressure best.

Like the dislocation movement in other metals, this deformation is permanent in Nitinol, without external energy providing the energy needed to revert backward. Still, Nitinol can get that energy from heat. Upon heating, the Nitinol forms austenite, an ordered and regular crystal structure, which effectively resets the crystal structure, and when the Nitinol cools again, the Nitinol remembers its original.

It is this use of shape memory alloys which results in a tire that can withstand excessive deformation without permanent damage due to their abilities of undergoing high strain as load-bearing components, instead of typical elastic materials.

Nitinol en route

Using shape memory alloy as stiffening elements can also increase the load-carrying capacity of the tire. The Superelastic Tire fabricated from these allows traction superior to conventional pneumatic tires. It also eliminates both the possibility of puncture failures and running “under-inflated,” thereby improving automobile fuel efficiency and safety. Furthermore, the tire design does not require an inner frame that simplifies and lightens the tire/wheel assembly.

To reduce the amount of energy transferred to the vehicle during high deformation events, the Glenn Superelastic Tire can be made to soften causing increased deflection. Besides, the use of shape memory alloys in radial stiffeners instead of springs provides even more load-carrying potential and improved design flexibility. This type of compliant tire would allow for increased travel speeds in off-road applications paving way for a safer and better tomorrow.