Interplanetary Chassis Technology
Consider what the Mars Exploration Rover (MER) design team was up against: two missions each launching a 384-lb vehicle toward Mars. Each vehicle spending months traveling aboard a “cruise stage” (a trip of some 303 million miles), before separating from this lifeboat and entering the Martian atmosphere. After discarding their aerodynamic shells, the parachute and retro rockets slowed their descent until the rovers dropped to the surface (from about 50 feet) encased in airbags resembling a pyramid made up of beach balls. The rovers bounced and rolled across the surface—about a kilometer was covered—before the airbags deflated, the landers protective “petals” folded down, and they drove off this platform. According to Peter Illsley, Mars Rover Structures lead engineer at the Jet Propulsion Laboratory (Pasadena, CA; www.jpl.nasa.gov), “This was a very quick-burn program.” Indeed. The design team had just three years—from the summer of 2000 to the summer of 2003, when the vehicles launched—to develop the rover, the cruise system, the entry, descent, and landing systems, and put them together into a cohesive whole.
Illsley, who refers to himself as, “just a mechanical engineer,” is responsible for the central structure, or chassis, of the Mars rovers. This piece goes by the name “Warm Electronics Box” (WEB) and carries all of the flight computers, motor controllers, as well as “all of the things we use to drive and power the various systems on the vehicle,” he says. The side and bottom are made of carbon fiber and aluminum. At the center of each panel is a 5056 aluminum honeycomb surrounded by eight plies of carbon fiber. Insulation is via an extremely light aerogel that fills the voids in the honeycomb core, while “Astroquartz” softening layers are used at the point where the titanium fittings are bonded to reduce peak stresses at the bond line. “The electronics module—which is isolated from the box via boron tubes between it and the chassis—has to share its heat,” says Illsley, “and aluminum is really good at conducting it.” Insulation alone, however, won’t keep the electronic systems and batteries warm. (Nighttime temperatures on Mars can fall to -157?F, and the batteries must be kept above -4?F when supplying power and 32?F when recharging.) The WEB uses the heat given off by the electronics, electrical heaters, and eight radioisotope heater units—each produces 1.0 Watt of heat from a 2.7g pellet of plutonium dioxide—to keep the temperatures within the required range. (Each pellet is encapsulated in a platinum-rhodium alloy surrounded by multiple layers of carbon-graphite, and is about the size of a C-cell battery.) “The rovers contain an extremely complex mix of materials,” deadpans Illsley.
That complexity extends to the suspension system as well. Invented by JPL and first used on the 1997 Mars Sojourner mission, the “rocker-bogie” design eliminates the need for springs, dampers, and axles. It also gives the craft the ability to climb sizable obstacles (up to twice the 10-in wheel diameter.) and withstand a tilt of 45? in any direction without tipping over; all while keeping the chassis relatively flat. The rocker portion of the design refers to the chassis-mounted differential that keeps the rover’s platform balanced and enables it to “rock” up or down based on the position of the rover’s six wheels. Meanwhile, train buffs will recognize “bogie” as the term for the multiple-wheel undercarriage found at the front of the engine that swivels to follow railroad tracks as they curve.
“Titanium is the primary material for the rocker-bogie system,” says Illsley, “and provides a good springing medium, which made it the logical choice.” The wheels are a one-piece aluminum design that is machined on its surface to create a tread to grip the Martian surface. Between the wheel motor/hub assembly and this surface sit long arcing arms that act as springs and keep the assembly’s weight to a minimum. “We rely on the springiness of the material in the wheel and the natural lever of the arm to do the job,” says Illsley. “After all, you can’t send a repair truck up to fix it if something goes wrong.”
Longevity takes on a different meaning when speaking about interplanetary vehicles. Whereas 10 years and 100,000 miles is a pretty standard measure of durability for passenger vehicles here on Earth, the Mars rovers were expected to work for just 90 “sols” (a Martian day lasting 24 hours, 39 minutes, 35 seconds) and travel 600 meters. “Much of that was based on the expected build up of dust on the solar arrays,” says Illsley, who notes the mission has been very lucky in that the Martian winds have kept the cells relatively clear. “The rovers landed on Mars in January of 2004,” he states, “and are still running today.” Even though they have a top speed of 5 cm/second, the rovers usually travel less than half that speed, and have covered approximately 11 km in their time on Mars. As the mission has progressed, the excursions have moved from “ground-in-the-loop” drives to semi-autonomous journeys where commands are uploaded at the start of the day, and the vehicles radio back to JPL at a prescribed time.
Next on Illsley’s plate is the successor to Spirit and Opportunity, due to launch in 2009. It builds on the basic architecture created for the Sojourner mission, and upsized to create the Mars rovers. Projected to be twice as long and three times as heavy as Spirit and Opportunity, the Mars Science Laboratory (MSL) will, says Illsley, “have the wheelbase of a Mini Cooper, but be rollin’ on 20s,” making this a large package with much larger wheels. It will not, however, be dropped in an airbag onto the surface, but will hang from a retro-rocket/parachute pack and lowered to the surface via a tether. A radioisotope power system will give it enough juice to run a full Martian year—687 Earth days.
As can be seen on this illustration of the Mars Science Laboratory, there is a separate rocker-bogie system for the left and right wheels, and each consists of a forward and aft bogie arm that connects to the middle and rear wheels, respectively, via struts. Drive motors are located within the wheel hubs and attach to the struts. An aft rocker arm attaches to the forward bogie near the main bogie attachment point through a hinge and actuator assembly, and extends forward before ending in a U-shaped bracket. This bracket holds a structural member that provides a mounting point for the rocker deployment actuator, and permits the forward rocker to rotate around the rover’s longitudinal axis. The forward rocker contains the attachment point for the front drive motor and wheel assemblies.