The Curiosity Mars rover has been a remarkable success except in one area – its wheels are falling apart faster than expected. To prevent this from happening to future rover missions, NASA's Glenn Research Laboratory is developing a new wire mesh tire made out of special memory alloy that is much tougher than previous designs, and may pave the way for larger, more robust rovers and vehicles.
A common criticism leveled at NASA is that it has a tendency to reinvent the wheel, and there was some justification for this charge. The space agency was notorious in its early years for completely redesigning every new series of satellites from a blank sheet rather than exploiting a proven design. And then there was the notorious story of developing a complex gas-charged pen that would write in zero gravity, rather than trying off-the-shelf mechanical pencils or ink sticks.
But one area where NASA has spent half a century literally trying to reinvent the wheel has been with, well, wheels. On Earth, wheels with pneumatic tires have proven very effective and efficient, but the Moon and Mars are different and not very friendly to the average steel-belted radial with tiger paw grip. Worse, the engineers were faced with the problem of how to design a wheel for someplace where no one knew what the surface was like.
From the 1960s on, NASA came up with all sorts of wheels, tires, and things that did what a wheel and tire do. There were weird, screw-like things for plowing through lunar dust as fine as talcum powder, vehicles that were one large caterpillar track, and others dominated by giant donut wheels like something off a sand and snow cruiser.
When the first manned Lunar Rover set out during the Apollo 15 mission in 1971, it had large, flexible wire mesh wheels with soft inner frames and titanium strips to handle the soft lunar soil. So successful was this design that Glenn engineers used it as the model for similar mesh wheels for future lunar rovers that would be the size of an RV, which resulted in the award-winning Spring tire.
But when the Mars rovers touched down beginning with Sojourner in the 1990s, they had solid aluminum wheels instead of mesh. These worked fine for Sojourner and the later Opportunity and Spirit rovers, but things changed when Curiosity set down in 2012. Curiosity is the size of a 4x4 and the surface of Mars turned out to be much rougher than expected. Within a year, the soft metal alloy of the unmanned explorer's wheels was showing obvious signs of wear as treads came loose and holes started to appear.
According to NASA, the Glenn engineers considered using an advanced form of the Spring tire for future Mars missions to provide better traction and durability, but the tight spring steel mesh couldn't handle the rough terrain of a simulated Mars. In going over sharp rocks and other obstacles, the mesh would deform under pressure and lose its shape.
Then a chance meeting between Engineer Colin Creager and Materials Scientist Santo Padula provided a solution. On hearing of the problem, Padula suggested using a shape memory alloy, which is a special alloy with crystals that deform and snap back at an atomic level. When new wheels were fabricated from nickel titanium alloy, they could deform right down to the axle and then return to their original shape.
Though still under development, NASA sees great promise for the new alloy mesh tires. They are not only more durable, but they conform to the terrain without sinking and they can carry heavier payloads at moderate to high speeds. The hope is that one day they will not only improve the lifespan of unmanned rovers, but will be used on manned vehicles as well. See them in action below.
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