The joy and excitement of the team at JPL was worth watching when they completed the significant milestone of landing a 900-kilogram rover which was the size of a mini cooper on Mars. The Mars rover is a marvelous creation of engineering. However, this post will deal with only one part of the rover, its tires. While creating the wheels of the rover, engineers faced a lot of difficulties. The rough Martian terrain caused substantial damage to the wheels of the previously sent rovers. It was easy to detect and predict this but not as simple to create something new. Even on the flat ground, the rover was able to attain a speed of only 0.15 km/h. Despite moving at such a slow pace, the force was still enough to tear holes into the aluminum wheels of the rover. The design criteria being followed by the engineers was very rigorous which made the job even more difficult.
The wheels of the rover were required to be stiff enough to support the weight of the rover which was nearly one tonne. The wheels were needed to be as light as possible to reduce the launch costs and should be able to maintain traction and navigate the unpredictable martian terrain. Currently, the record holder for the longest journey on another planet is held by the Opportunity rover. It traveled 45 km on the Mars. Previously the record was owned by a Russian rover, Lunokhod 2 which went 39 km on the moon. All the Mars rovers used solid aluminum wheels. All the previous rovers used the same aluminum wheels, however, Curiosity rover is a little different. Each wheel of the Curiosity rover is almost half a meter in diameter and 400 millimeters wide. It is created from a solid block of aluminum. It is 0.75 mm thick which is the same thickness as a credit card. When a vehicle that weighs more than 1 tonne runs over sharp rocks, it will be getting a lot of damage to its aluminum tires. The load-bearing threads which are almost 6.4 millimeters thick are also damaged. If they continue to break, the wheels will become useless in no time. This was perhaps the biggest concern of the scientists.
The Mars rover cost 2.5 billion dollars to develop and has a nuclear reactor for power. The top priority of the engineers was to somehow extend the life of the project so finding an appropriate solution to this problem was a high priority on NASA’s list for plans. Engineers needed tires which can bend and conform according to the terrain, Just like our rubber tires do on earth. However, rubber tires cannot be used on Mars since the temperature there can be as low as -130 degree Celcius. In this low temperature, the rubber can transform from an elastic material which is capable of absorbing stress to a brittle-glass like material which makes it useless to be used on the Mars. Rubber also degrades from the UV radiation when it is exposed. Moreover, it is also essential to save weight, and rubber tires along with rims are very heavy.
The Lunar Rover used flexible steel mesh wheels with a stiffer inner frame to prevent over deflection and thin strips of metal attached to avoid the wheel from sinking into the lunar soil. This option was studied for the application on Mars, but there was a considerable difference between the mass of the Lunar Rover and Curiosity rover. Then the higher gravity of Mars also made the wheel unsuitable for the application. The wheels were not able to hold the weight of the vehicle without deforming permanently. Goodyear that created the original Lunar wheel, was working with NASA and in 2010, they developed spring tires which were light, capable of bending and conforming to the terrain without permanently deforming. They were given the R&D 100 award for this spring tire as well.
This spring tire has its applications not only on the Mars but earth as well. It uses a material called Nitinol. It’s a nickel-titanium alloy which has been named a shape memory alloy. It can be bent and deformed and then by applying a little heat, it can retain its original shape. The alloy is capable of remembering its original shape. A material when goes under stress, either of two things can happen. If the pressure is below its yield point, the material will deform elastically like a rubber band and then return to its original shape when the stress is removed.
On the other hand, when the stress crosses its yield point, the material will deform permanently and will not return to its original shape again. This permanent deformation occurs when the bonds are not broken and the material can return to its original preferred crystal lattice orientation. But when the sufficient force is applied small defects in the crystal lattice are able to move. This is what happens to wheels of the rover.
When the wheels are driven over pointed rocks and gravel, the stress exceeds the yield stress and the wheel face permanent damage and crack. Deformation occurs slightly differently for Nitinol since it has some very unique properties based on its internal crystal structure. When Nitinol is below a certain temperature, it has a specific crystal structure called martensite. This crystal structure is arranged in such a way that it can accommodate deformation very easily. Martensitic nitinol forms grains of twinned atoms where the direction aligns. When stress is applied these twin grains deform and align to absorb the stress. Some grains can grow and deform at the expense of other grains, this phenomenon is called detwinning. This phenomenon is permanent without providing the energy which is needed to revert backward. However, Nitinol can get that energy from heat. When heated, the Nitinol forms austenite, an ordered and regular crystal structure which resets the crystal structure. When the Nitinol cools again, it remembers its original shape.
What makes this material even more useful is that this transformation in temperature can be used in various applications. Generally, increasing the titanium content of the metal will increase the transformation temperature. This property has found many useful applications across many industries. One of its applications is in the stunt development for heart where the material is made to remember its shape at 37 degree Celsius body temperature, as a result when veins or arteries shrink, it automatically expands and takes its original shape. When the material will be used to create tires for the Mars rover, it will undergo martensite to austenite transformation through stress and strain. When the stress passes a threshold level, it causes the crystal lattice to transform to austenite. When the stress is released, it returns to martensite and retains its original shape. This is called super elasticity and it is the property which allows these new Mars rover wheels to deform to the rim and then recover to its shape again. This property combined with an interlocking coil design allows the tire to tolerate strains in a way no other tire could. No other tire is able to survive the Martian environment but there are strong chances that these tires could appear in NASA’s design for the future Mars rovers. It is interesting to think that something as simple as a wheel will never be reinvented. However, there are always ideas and problems which needs thinking and innovation to bring about a change.