325 composite (flex fatigue, compression and tensile modu lus) improve at cryogenic temperatures. This has  enabled JPL and Michelin to demonstrate driving the wheel more than 5000 km under load while being  fully immersed in liquid nitrogen. With a mass of only 15 kg, this wheel has a load carrying multiplier (load capability/wheel mass), which is three times that ac hieved by the original Apollo Lunar Rover wheel. 

Figure 9. Michelin Lunar Wheel Impacting a 10-cm Tall Rock 

Lessons Learned 

The Tri-ATHLETE robot is the second generation of ATHLETE vehicles. The first vehicle was completed  in 7 months from concept to fully functional robot. The vehicle was then tested for three years and its shortcomings became evident. In the design of the second generation ATHLETE vehicle, the development team tried to solve all of the problems with  the first vehicle and add substantial functionality,  which in turn added significant complexity, cost, and schedule over the development of the first vehicle. From the onset of the re-design effort the team was very aware of “the second  system effect” but was  unable to avoid it [8]. Continually trying to solve every problem led to a significant slip in the schedule and  a significant increase in the scope of the project. What started as t he re-design of only two joints to  increase the load carrying capability of the vehicle,  quickly escalated to a complete re-design with  significantly more complex end-of-year milestones. In hindsight it would have been easier to break the redesign of the vehicle in to smaller incremental deve lopments so that new innovations could be tested and  refined instead of one large re-design effort.  Tri-ATHLETE is a research project with a team of fo ur mechanical design engineers. This is a relatively  small project, but large enough to necessitate consis tent guidelines and require ments across the team.  This was implemented using clear load cases, stru ctural margin philosophy, and design procedures. The  load case was clearly defined as a two-leg iron cross, which is the vehicle supporting itself on two opposing limbs completely outstretched. This sets the load capabilities of each of the pitch joint actuators  and associate structure. Similar pose derived requirem ents size the necessary capabilities for the yaw  and roll joints. The mass of components was reduced to the extent possible while still meeting the  functional and structural requirements. A guideline of  $1000/kg was used to evaluate the merit of lightweighting features, meaning special machining operat ions would be employed simply to remove mass  from custom designed parts if the incremental cost of  that particular machining operation per unit of mass  removed was below $1000/kg. Even with a small team, these clear guidelines established at the beginning of the project kept the entire team de signing to the same requirements and reduced the  amount of time spent debating the proper balance of capability, complexity, and cost.    In contrast with the implementation of rigorous stru ctural guidelines, much looser rules were used when  designing the cable routing. Schedule allowed for the te sting of the cable spool clock springs that were  used inside the joints because of rapid prototyped cable trays, but it did not allow for an entire joint to be  NASA/CP-2010-216272