NASA Glenn reinventing the wheel to aid future Mars rovers
GLENN RESEARCH CENTER, Ohio — One of the most important high-tech components of future Mars rovers may be, of all things, its wheels.
Engineers at NASA’s Glenn Research Center in Cleveland, Ohio, are reinventing the wheel in a way that may allow rovers and other future mobile vehicles to traverse the treacherous surface of Mars with greater and longer success. They have developed a woven mesh tire made of a shape-memory alloy, or SMA, that “remembers” its ideal shape and immediately springs back into form after absorbing the bumps and beatings of traversing rocky terrain.
In a recent test, the newly developed tire was subjected to 6 miles (10 kilometers) of difficult driving over a punishing simulated Martian surface. At the end of the test, the tire was as good as new. By comparison, the rigid aluminum wheels of the Curiosity rover, which has driven not quite twice that distance during its entire mission so far, are currently riddled with holes and dents from the rougher-than-expected punishment of traveling over the Martian landscape.
The coupling of the mesh tire structure design with the use of shape memory alloy came about through a chance meeting of two NASA Glenn engineers, Colin Creager and Santo Padula.
Creager, a mechanical engineer, works at NASA Glenn’s Simulated Lunar Operations Lab, or SLOPE, where many rover technologies and designs are tested. Creager was already working with wire mesh design tires that were made of spring steel. The spring steel mesh design was an improvement over previous designs. However, after taking some beatings in simulated terrain tests, the spring steel material would eventually become dented and misshapen due to repeated stresses and strains on the material.
Padula, a materials scientist at NASA Glenn, then bumped into Creager. Padula had previously known Creager when he was a student but had no idea he was back at NASA Glenn and working in the SLOPE lab. When Creager described the work he was doing, Padula was immediately curious.
“‘Aren’t you having problems with this plasticizing?’, I said,” Padula told Spaceflight Insider. “Colin just smiled at me and said, ‘Yeah, that’s the biggest problem we’re having.’ So I said [that] we have the solution, and it’s called shape memory.”
The next day Padula came over to SLOPE to show Creager the shape memory material. Upon seeing it, Creager immediately wanted Padula to meet an engineer he knew at Goodyear.
“When we originally started doing tire developments here about eight years ago, we set up a collaboration with Goodyear where we rebuilt the original Apollo Moon rover tires because there weren’t any available for us to test,” Creager told Spaceflight Insider. “That collaboration was two-phase, to re-build the lunar rover tires to give us a testing baseline to determine what we needed to improve upon. And then the second phase was to determine how to design a tire that can carry ten times as much load. At the time, it was for a big mobile habitat for the Moon; it was at the time of the Constellation program. That was the basis of our relationship and why we wanted to talk with Goodyear, because they had been heavily involved with our developing this tire.”
They moved from the Lunar Rover tires, which were made for lighter loads and in the lighter one-sixth gravity of the Moon, to tires that were designed to operate on Mars with heavier loads over rockier terrain and to possibly climb steeper inclines. They developed a tire that consisted of weaving together many strands of steel springs to form a mesh. The new tire outperformed previous tires, but the spring steel’s plasticity after repeated strains was proving to be its limitation.
Discarding the spring steel material and, instead, making the mesh out of spring memory alloy would change all that. The improvement in performance, Padula explained, is brought about by unique characteristics at the material’s atomic level.
“There is a principle in solid mechanics that we call deformation or deformation field,” Padula said. “Every deformation that we deal with on any type of body in solid mechanics is [composed] of three elements: translation, rotation, and stretch, or what is usually called strain. At the material element level, we’re talking about how the material itself is deforming in terms of how the [atomic] bonds are accommodating deformation.”
The problem, Padula explained, is that most designs for rover tires are trying to limit the strain component. Most conventional materials, such as the spring steel they have been using, has an upper limit of strain that is 0.3 percent.
“That’s what we can do in elastic deformation,” Padula said. “In that elastic deformation, what we get is basically the limit of what we can do in stretching the atomic bond. Once we go beyond that, we get plasticity. Permanent deformation. If it gets a lot of plasticity, it will go to the next mode of deformation, which is [a] fracture.”
Padula introduced the use of the shape memory alloy nickel-titanium (NiTi). Springs made of NiTi were woven together into a mesh in the same design as the spring steel tires. However, the shape memory alloy behaves very differently.
“Instead of doing the elastic bond stretch at the atomic level that we do with other materials, what’s happening with shape memory alloys is that we have opened up a unique well in the energy landscape,” Padula said. “That well exists in a handful of different material alloy systems. Nickel-titanium is the predominant one of choice because this particular one, of all the ones we know of currently, has the highest energy density. It gives us the most capability.”
Padula explained that instead of the elastic bond stretch that occurs in other materials, NiTi actually undergoes a solid-state phase transformation. Movements or bends in the material take the crystal atomic lattice and reorient it, or transition it, from one crystal lattice to an entirely different crystal lattice. An atomic rearrangement occurs in order to accommodate energy instead of an elastic bond stretch.
“These are actually storing energy based on a reversible solid-state phase transformation,” Padula said. “It is what we call an inelastic deformation. Plasticity is an inelastic deformation, but it is irreversible. This is a new type. It’s the first time we’ve ever had an inelastic deformation that is reversible. This crystal rearrangement, or solid state phase transformation, can store 30 times the energy that we can store in an elastic bond stretch.”
This means that while the spring steel can withstand 0.3 percent of strain before it dents and the crystals permanently rearrange, the NiTi shape memory alloy can withstand 10 percent strain – 30 times better elasticity.
The next step in testing the new tire design is to test it under simulated Martian temperature conditions. That test is currently under preparation in a cryogenic test chamber at NASA’s Jet Propulsion Laboratory (JPL).
Creager said that the technology is too late in its development to be included on NASA’s upcoming Mars 2020 rover. However, the new tire design may be ready for use on the as yet unnamed Mars-sample-return mission in 2024.
Michael Cole is a life-long space flight enthusiast and author of some 36 educational books on space flight and astronomy for Enslow Publishers. He lives in Findlay, Ohio, not far from Neil Armstrong’s birthplace of Wapakoneta. His interest in space, and his background in journalism and public relations suit him for his focus on research and development activities at NASA Glenn Research Center, and its Plum Brook Station testing facility, both in northeastern Ohio. Cole reached out to SpaceFlight Insider and asked to join SFI as the first member of the organization’s “Team Glenn.”