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# Rover Wheel Design

An artist's concept of the Mars Exploration Rovers. Image credit: NASA/JPL/Cornell

March 4, 2010

We received this question from a high school advanced functions and modeling class in North Carolina. They did research on why NASA's Mars rover Spirit has had trouble driving out of a sand trap.

Dear NASA,

We did a lab experiment to analyze how different sizes of wheels affect the ability of a vehicle to climb or roll over obstacles. We found that the wheels have to be large in relation to the wheelbase (the distance between the wheels. This keeps the rover from bottoming-out. Also, wheels with big treads and better traction would help. We also thought that an arm that extends out of the bottom of the rover could push the rover out of trouble.

We noticed that the Mars Rover has small wheels in relation to its wheelbase. Our results suggest that small wheels make it more likely for a vehicle to get stuck. We understand that many considerations go into every decision made in designing such a complex machine. What made the designers use small wheels?

Dear Advanced Functions and Modeling Class,

Thanks very much for your interest in NASA's Mars Exploration Rover Spirit.

Your wheel experiment sound like a great project but a major constraint in designs for spacecraft is to keep mass as small as possible. An extra ounce for wheels would mean one less ounce for science instruments. For the rovers, size was also a constraint -- with complicated origami-like positioning needed for folding the rovers into the landers that were inside air bags.  The engineers' choices about the wheels, the arms and other features on Spirit and Opportunity were based on judgments of what would be needed to accomplish the planned missions.

The key reason that Spirit has not been able to get away from the sand trap at Troy is that the rover no longer has six working wheels. (Both Spirit and Opportunity have operated so many times longer than originally intended.) The original design choices by engineers were successful in enabling very successful missions and extended missions for both rovers.

The students' finding that bigger wheels could be helpful is good work, and indeed the next rover to Mars, called Curiosity, will have bigger wheels than Spirit and Opportunity. Curiosity will also weigh more, carry a more massive set of science instruments and require a more expensive launch vehicle. The rover will have a robotic arm to use for scientific investigation. It will not have an arm for pushing itself, as the class suggests, since that is not judged enough value for replacing some instrument mass or other hardware mass in the fixed budget for mass lifted by the launch vehicle.

More research topics

Mars Missions