How NASA Reinvented the Wheel with Shape Memory Metal

TL;DR

NASA developed airless tires made from nitinol, a shape memory alloy that can stretch 8% and return to its original shape, solving problems with wheels on Mars and the Moon while enabling bulletproof, puncture-proof tires for Earth applications.

Key Takeaways

1Nitinol (nickel-titanium alloy) is a shape memory metal that can deform extensively and return to its original shape through phase transitions in its crystal structure
2Traditional rubber tires fail on Mars and Moon due to extreme temperatures; aluminum wheels damage easily on rough terrain, requiring a more durable alternative
3Nitinol wheels distribute load 360 degrees around the tire instead of concentrating it, increasing load capacity without adding mass—critical for space missions
4The metal exhibits both shape memory (deforms when cooled, returns when heated) and superelasticity (deforms under stress up to 8% strain and springs back instantly)
5Airless nitinol tires are puncture-proof, eliminate flat tires, never become underinflated, and improve fuel economy on Earth while enabling long-duration Mars rover missions
6NASA's weaving process creates a mesh of nitinol springs that absorbs energy through stress-induced phase changes, acting as a built-in suspension system
7Shape memory alloys have diverse applications: medical stents, aircraft vortex generators, wing flap actuators, and terrestrial vehicle tires

Notable Quotes

“This metal is about as close to magic as it is possible to find in nature. It can adjust its arrangement of atoms to return to some predefined shape, but it also converts between mechanical and thermal energy.”
— Derek (host)

“We can basically set this shape as the parent known memory shape. That's why we call it shape memory. I can stretch this out. If I cooled it down I could stretch it out even more, but as soon as I heat it back up, it'll remember that original parent shape.”
— Santo

“All we have is a shuttle mechanism that's driven by two tubes in nitinol and we've driven those air arms and flap elements on the wing box of a 737 in flight, 60 degrees flap angle down, 30 degrees flap angle up just by heating and cooling two tubes of nitinol, replaces all the hydraulics.”
— Santo (on aircraft application)

“By tying this bump stop element to here, when I go through a footprint, you can see now I'm transferring load 360 degrees around the tire, right? By doing that, I have now increased my load carrying capacity significantly without adding any more mass.”
— Engineer discussing nitinol tire design

Chapters

1. The Problem: Why Traditional Wheels Fail in Space

Rubber tires are unsuitable for Mars and Moon due to extreme temperatures (-250°F) that make rubber brittle; aluminum wheels, though lightweight, suffer from metal fatigue and cracking under rough terrain, causing damage to rovers like Curiosity.

2. Introduction to Nitinol and Shape Memory Alloys

Nitinol, discovered in 1961 at Naval Ordnance Laboratory, is a nickel-titanium alloy that undergoes solid-state phase transitions between austenite (high-temperature cubic lattice) and martensite (low-temperature twinned arrangement), enabling it to remember its original shape.

3. The Shape Memory Effect and Medical Applications

When heated above its transition temperature, nitinol returns to its programmed shape. This property is used in medical stents that compress for catheter delivery, then expand to open arteries. The alloy can generate significant forces when heated, making it useful as an actuator.

4. Superelasticity and Stress-Induced Phase Changes

Nitinol can stretch up to 8% and spring back instantly through stress-induced phase changes, even at room temperature. This differs from normal elastic deformation; the material undergoes a crystal structure transformation (austenite to detwinned martensite) that is reversible once stress is removed.

5. Thermal Properties and Energy Conversion

Nitinol transforms exothermically when stressed (releasing heat) and endothermically when stress is removed (absorbing heat). This enables applications in elastocalorics and heat pumping, and allows the tire to dissipate energy like a damper, reducing vibration.

6. Engineering the Nitinol Tire

NASA weaves nitinol springs into a mesh tire that distributes load 360 degrees around the wheel rather than concentrating it, increasing load capacity without added mass. A single tire requires 684 manual weaving operations, each consisting of wrapping and screwing springs together.

7. Testing on Simulated Terrain

Extensive testing on a terrain endurance rig with moon sand and Mars rocky sand confirms the tire can deform up to 8% without permanent damage. The airless design prevents punctures from nails and bullets, and the flexible structure supports full body weight on rocks.

8. Terrestrial Applications and Benefits

Nitinol tires eliminate flat tires, prevent underinflation, and improve fuel economy for Earth vehicles. High-pressure aircraft tires benefit from reduced explosion risk and no maintenance checks. One test on a Jeep demonstrated the tire's durability and reliability for everyday driving.

9. Aviation Applications of Shape Memory Alloys

Nitinol is used in autonomous vortex generators that flatten passively during cruise (no temperature control needed) and deploy during takeoff/landing. Main wing flaps can be operated by heating and cooling nitinol tubes, replacing traditional hydraulic actuators—tested successfully on a 737.

10. Broader Impact and Future Potential

Shape memory alloys are being explored across industries for their durability, reliability, and ability to simplify mechanical systems. The technology transforms everyday products from razors to spacecraft, enabling lighter, more efficient, and longer-lasting designs.

Key People & Entities

Derek: Host and presenter of the video; explains scientific concepts and tests the nitinol technology
Jim: Co-demonstrator who participates in tire testing and puncture/bullet tests
Santo: NASA engineer and shape memory alloy expert; explains nitinol properties and aircraft applications
Emily: Participant in demonstrating the acoustic properties of nitinol under stress
NASA: Space agency developing and testing nitinol wheel technology for Mars rovers and lunar missions
Naval Ordnance Laboratory: Research facility where nitinol was discovered in 1961 through experiments with nickel-titanium alloys
Curiosity Rover: Mars rover that uses aluminum wheels, which have suffered damage and cracking on Martian terrain
Apollo Lunar Roving Vehicle (LRV): Historic moon vehicle that used a pantograph tire structure (mesh of wires) for short-distance lunar exploration

Glossary

Nitinol: A shape memory alloy composed of nickel and titanium, discovered at Naval Ordnance Laboratory. It can undergo phase transitions between austenite and martensite, enabling it to return to a programmed shape or deform up to 8% elastically.
Austenite: The high-temperature crystalline phase of nitinol with a cubic lattice arrangement. Atoms return to this phase when heated, causing the material to return to its original shape.
Martensite: The low-temperature crystalline phase of nitinol with a lower-symmetry twinned arrangement. In this phase, the material can be deformed but returns to austenite when heated.
Shape Memory Effect: The ability of nitinol to return to its original shape when heated above its transition temperature, even after significant deformation at lower temperatures.
Superelasticity (Pseudo-elasticity): The ability of nitinol to stretch up to 8% and spring back instantly without permanent deformation, caused by stress-induced phase changes from austenite to detwinned martensite.
Phase Transition: A change in the crystal structure of a material from one lattice arrangement to another, such as from austenite to martensite in nitinol, triggered by temperature or stress.
Yield Strength: The maximum stress a material can withstand while remaining elastic. Beyond this point, permanent (plastic) deformation occurs.
Elastocaloric Effect: The use of stress-induced phase transitions in materials like nitinol to generate temperature changes, applicable to heat pumping and refrigeration.
Grousers: Structural ribs or treads on rover wheels designed to provide strength and grip on soil and obstacles.
Pneumatic Tire: A tire that relies on pressurized air inside a rubber chamber for support and shock absorption, as opposed to airless designs.

Explore