3 and the chuck), enables the bit boxes to be passive devices while minimizing the impact to the Drill’s  volume and mass. The success of this execution is  evident by the presence of only two additional  features on the Drill for the sole purpose of bit exchange: the retaining features on the bit and the  alignment posts on the contact sensor assembly for docking  the Drill to the bit box.    It’s a dirty and dangerous job…

The collection of extraterrestrial samples from a mo bile robot on rough terrain poses the unique challenge  of ensuring reliability and robustness in a highly indeterminate environment. One example is the sustained and maintenance-free operation in a dirty environment. Wherever possible, the moving components were placed internally within the sealed Drill housing, other wise they were designed to be  tolerant of dirt. The specific implementation of t he latter will be discussed in the subsequent sections.    Since many highly valued science targets on Mars such as outcroppings are located on rough and steep terrain, the MSL rover has been designed to acquire samples from sloped surface up to 20 degrees. The compressed development schedule required the generat ion of a load case that could be defined and  articulated relatively quickly. Without any ensured fr iction between the rover wheels and the surface of  Mars, a worst-case load scenario was generated: a complete  loss of friction at the rover wheels on a 20degree slope while the tip of the bit was locked to the surface. One component of the Drill could not be designed to show positive margin for this load case: the bit. To meet this requirement the bit diameter would have to grow considerably, increasing the volu me of sample collected. Larger cavities would be  needed to process the additional sample causing the tu rret to exceed its volume allocation. Since the  failure of one mechanical component cannot be counted on to protect the others downstream, all the nonreplaceable elements of the Drill have been designed and will be qualified to the worst-case loads. Throughout the remainder of the paper, it will be show n how this single require ment drove the overall  volume, structural design and required capability of many of the mechanisms of the Drill.    Live long (reemphasis) – even if you can’t drill

Penetrating the surface from a mobile  robot requires that a single mechanism fault in the Drill cannot  result in the anchoring of the entire rover to the Martia n surface; and the bit being “stuck” in a rock is not a  mechanism fault – it is part of the process. This requirement flowed into two capabilities: (1) generate a  large force to extract a stuck bit and (2) release the bit subjected to the worst-case load scenario. These functions are entirely independent throughout the system  (including being powered by separate electronic  drivers) such that a loss of one function will not preclude the operation of the other. Defining a “stuck” bit is somewhat of a slippery slope – pun intended. Lacking a more accurate  definition of stuck, it was the  intent of the Drill design to provide a sufficient retraction force to dead-pull the bit without percussion from a rock while under the worst-case load scenario. Howe ver, development testing showed that the force to  extract a bit while subjected to 40% of the worst-case load scenario was higher than the maximum capability of the Drill. Mass and volume constraints of the turret precluded additional capability. However,  a promising result observed in the force time hist ory plots showed a stick-slip phenomenon which implies  that the load required may be significantly reduced by hammering the end of the bit – a capability that the  Drill has. This conservative test case was based on the assumption that the flight system would do  nothing to alleviate the loads on the bit. It is emphasiz ed that the flight system must carefully assess risk  when sampling on steep and uncertain terrain.   Divide and conquer 

Through a series of unfortunate event s, the brunt of the design and implementation of the Drill started  mid-summer of 2007 when the completion of the conceptual design kicked-off an intense effort to meet a launch date of fall 2009 (prior to the slip to 2011). As a result, a large emphasis of the Drill design architecture was placed on modularity allowing variou s mechanisms of the Drill to reach design maturity  relatively independently. This approach demanded heavy coordination of interfaces; however, by keeping the interfaces simple and avoiding an overly inte rtwined design, the process was streamlined by only  requiring the consensus of two mechanism engineer s with the cognizant engineer providing review,  arbitration and concurrence.    NASA/CP-2010-216272