28 Overall, as shown in Table 2, the coring penetration rates were lower than observed during the coring bit  development testing with the breadboard fixture. Th is was not surprising because of the inherent  hammer/rotation phase drift that prevented a rapidly-repeating strike pattern from developing. The breadboard fixture contained separate rotary an d percussion motors which were run open loop and  always drifted with respect to each other. This was naturally conducive to preventing a rapidly repeating  strike pattern. Indeed, on many rock samples cored by the integrated corer protot ype, deep strike marks  were seen in the rock that was suggestive of a repeating strike pattern. Performance could have been improved if a design feature had been introduced to shift the hammer/rotary phasing relationship. This  could have been in the form of separate drive and hammer motors, a cam device to oscillate the LSAS  Corer with respect to the robotic arm, or a number of other mechanical options. 

Table 2. Prototype Corer Penetration Rate 

Average Penetration Rate, mm/min   Corer Prototype Breadboard Testing  Sandstone 0.0727-0.229 15.812  Shale 0.370 6.789  Basalt 0.1017-0.1467 2.817   The low input power required to core, even in the most aggressive setups, was a pleasant surprise.  Approximately 90 watts were required to core effectiv ely with the breadboard fixture. During all of the  representative runs with the core r prototype, power consumption wa s below 50 watts. Even in the one  extreme case where the weight on bit was taken to a very high level (Run 4I), the maximum current draw  was still only 2.5 amps (60 watts). To ensure conservati vely that data could be co llected in any likely test  condition, the corer prototype was designed around a 120-watt motor. Further mass reduction could have  been possible from motor substitution alone.  One of the primary differences in power consumption between the breadboard fixture and the integrated corer prototype was initially thought to be caused by t he lower WOB used with the most recent testing.  However, the high WOB settings investigated during Run 4I suggested this was not the case. It is suspected that the keys to the high efficiency of the current corer were the use of ball bearing cam  followers for lifting the hammer mechanism (the origin al LSAS Corer Prototype used bushings), a hammer  mechanism that did not slow down the rotation spe ed of the coring bit at every strike, a freely moving  hammer mechanism, and careful tailoring of the hammer cam profile. In contrast, the hammer on the breadboard fixture did not rotate with the coring bit, causing loss of bit speed each time the hammer struck the shank of the bit. The hammer on the breadboard fixture also had excessive frictional losses.  Differences in the dynamics of the core bit fluting were noticeable during testing with the corer. In some  cases the fluting appeared to be very effective removing debris, with rock fines being ejected with such velocity that they actually became airborne. In ot her cases, the rock fines tended to gather around the  coring bit. One variable not previously explored is vibr ation of the overall test setup, helping to cause the  dispersal of the fines. Indeed, when a rather large rock sample was used, the fines did not disperse in a  similar manner to a nearly identical test setup that used a smaller rock that had fractured during the test.  It was suggested that vibration of the small rock fragments helped with  the fines dispersal. It appeared  that more realistic testing (e.g. testing in the field on large in-situ rocks) might have yielded results that  are more applicable.  Core Handling

With the exception of the one basalt sample (see Figure 12), all of the coring runs yielded core samples  that were either too short to be snapped by the eccentric collet breakage feature, or the rock samples were already fractured. In the latter case, despite t he core already being broken, the eccentric collet still  retained the captured pieces in the order they were produced. In all four cases of obtaining captured rock pieces via the eccentric collet, the pieces were reta ined and successfully inserted into the thin-walled  containment sleeve.  NASA/CP-2010-216272