When Chris Voorhees thinks about wheels, he doesn't imagine the rubber hitting the road, but aluminum crawling across the surface of Mars.
He has already seen some of his handiwork making its way across the Red Planet. One of the first jobs Voorhees was handed as an intern was stamping out over 1,000 stainless steel cleats for the Sojourner rover on NASA's Mars Pathfinder mission. Fast-forward six years and tack on a 365-pound weight-gain and mobility specialists are dealing with a whole new animal -- the large twin "robot geologists" known as the Mars Exploration Rovers, Spirit and Opportunity.
"We started with the Sojourner wheels as a base to work from," Voorhees said. "Because of many different engineering demands on the wheels, the wheels for our new rovers didn't mature until late in the game."
Mobility engineers were tasked with making the wheels lightweight, so as not to add any more weight to an already hefty spacecraft; compact, so that when the rover is stowed in the lander they would fit; and capable, so the twin geologists can maneuver off of the lander safely and climb rocks up to ten inches high. Basic parameters were set, based on the weight of the rover and the contact area on the surface and then the challenge began to make the wheels deliver on all requirements.
A Design to Keep on Turnin'
The rocker-bogie suspension that was developed for Sojourner, the first vehicle to rove on another planet, will be used again in a modified design. This flexible mobility system allows the wheels to conform to obstacles like rocks, strengthening their grip and maximizing their ability to clear any "road blocks." At 26 centimeters in diameter (a little over ten inches), these aluminum wheels are twice the size of those on Sojourner and are missing the recognizable sharp cleats.
"A big challenge is to be able to get enough traction to get through soil and over rocks but also to be benign enough to get off of the lander without getting entangled in the deflated airbags," Voorhees said. The design is "basically like a paddlewheel that is machined onto the outside of the wheel, providing both safety and capability."
Each wheel has its own drive and steering actuators, which control movement and direction. The internal volume that each wheel can hold was increased to house both systems within the wheel's crown-shaped design. When steered, the wheel's unique shape bears the load continuously from inside to outside and prevents it from riding up on its outside edge.
Hubcaps to Minimize the Shock
Inadvertently adding to the rovers' panache are the spiral flectures. The futuristic-looking "hubcaps" were chosen over dozens of other flecture and spoke options and are designed to absorb shock and to protect the rest of the vehicle during driving. Next Intent, a company in San Luis Obispo, California that specializes in machining complex shapes, manufactured the wheels. The overall wheel design allowed them to machine each wheel from one piece (or billet) of aluminum. Being able to use just one piece of aluminum minimizes what's called scar mass, or useless leftover material where parts would join and makes the wheel stronger, Voorhees noted.
This up-close photo shows the spiral flectures which act as shock absorbers and the orange Solimide that fills the flectures, preventing rocks and debris from interfering with the driving and steering actuators.]]The outside of the wheels are anodized, or covered with a black coating, to provide additional strength. This smooth surface also minimizes the threat of the wheels getting caught up in the deflated airbags.
The "orange filling" between the spaces in the spiral flecture is an open-cell foam called Solimide. It was cut into crescent shapes and bonded to the wheel.
"The idea came from a concern that because the wheel has an open geometry design to the drive and steering actuators, it could pick up rocks and debris and cause a problem," Voorhees said. "We needed to fill the gaps but still be flexible - we couldn't use a solid for shock absorption. Solimide maintains its flexibility even at very low temperatures so it's ideal for conditions on Mars."
Test Tracks: A Race against Time
Planning such a complex mission is, as Voorhees said, a race against time. Designs are fluid and subject to intense testing and subsequent change. While nothing can substitute for being on Mars, the next best thing is to run trials in simulated martian environments at the JPL's test beds. An obstacle course dubbed the "rock gauntlet" challenged test wheels to scale everything from small rocks to concrete blocks. Engineers also conducted airbag interaction tests in which they drove the wheels into the deflated airbags again and again until they had enough information to proceed with wheel design changes. The mobility team and the assembly test and launch operations team gathered to conduct ramp tests with the flight rovers to make sure the rover brains were communicating effectively with its legs and wheels.
Preparing for the Rover's First "Steps"
Preparing a robot to perform to exact specifications on a harsh planet 460 million kilometers (286 million miles) away is no easy task. Still the excitement of sending a spacecraft to another planet has not waned. While engineers are anxious to see Mars through the eyes of a rover again, they know that the deployment process will be slow and precise once the rovers land on Mars in January, 2004. Once the lander petals open and the rover "wakes up," it may take up to five days for it to drive off the lander.
"It's hard to explain the minutiae - everything has to work exactly as you plan," Voorhees said. "After every command sequence we give the rover, we have to wait to make sure everything is working properly before we proceed. And due to the delay in sending and receiving signals from Earth to Mars and back, it's like taking 20 minutes just to talk to yourself!"
When ground controllers confirm that all systems are working as they should, they will tackle the decision of which direction to go. Nearby obstacles like rocks or deflated airbags will determine the safest route to leave the tetrahedron-shaped lander. As it emerges from the lander, its interplanetary cocoon, the rover will not be breaking any speed records to conduct its research. Top speed for the rovers is five centimeters (two inches) per second. However, as many scientists and engineers are quick to point out, the goal is not to travel as far and fast as possible, but to uncover the most interesting science wherever it presents itself. And as long as the wheels do their job, Voorhees and the mobility team can live without wheelies.
Mars Exploration Program
Last Updated: 2 March 2011