Lesso ns Lea rned 

1) The delive ry sched ule of the grin ding wheel s for inte gration into th e flight asse mbly was such that we  did not have  eno ugh tim e to te st the  beh avior of the diam ond resin wheels in a simul ated Ma rs  environ ment prior to deliv ery. Obvio usly this i s not the way we would have li ked to  procee d but  we  had no choice a s the delivery d eadlin e was fast ap proachi ng as we finished  our last  round of testing.   Later, we were abl e to test the final  wheel d esign unde r Mars enviro nmental temperature an d  pressure. Du ring the se tests, we di scovered that  the we ar rate  of the diamond/re sin d ecreased  dram atically i n a simulate d Mars e nviron ment. U nder Earth co nditions, the wear rate wa s abo ut 0.510.64 mm per basalt grin d event. Unde r Mars enviro nmental cond itions, we sa w wea r rates of about  0.1-0.12 7 mm per basalt grind event. This i s a five-fold d ecrease in th e we ar rate o r a five-fold   increa se in to ol life or abo ut 25 basalt grind eve nts. System level requi rements impose d a tool life of  three grinding s with a  margin of o ne (six grindi ngs). In  retros pect, if we k new that this wear effec t was  present, we woul d have d ecreased the concentratio n of diam ond in the resi n mix. This wo uld have  the effect  of reducing the tool life  through increased wear (something  we could live  with) and  increa sed the  grindin g performance of the tool beca use with fe wer ston es in the matrix there wo uld  be more force per st one, thus in creasing the grindi ng prope rties. 

After learni ng of this diam ond be havio r unde r Ma rs conditions, a pape r written by Hitchi ner et al.   (1983) from Oxford Unive rsity provid ed compl ement ary inform ation. Hitchin er compa res the effects of  polishing dia mond on a n iron scaife in Earth atmo sphere and  at a vacuum of 0.1 Torr. Hi s  conclusions describe a five -fold decrease in the wear rate under vacu um. Th erefore Mars a cts more   like a vacuum  than Earth atmosp here wit h rega rds to diamon d we ar rates. 

2) The Z-axis le ad screw hard stop should have bee n designed so th at it acted a s a rotation al hard stop.  With sm all po sts p rojecting upwa rd on t he lead screw and a smal l post projecti ng do wnward  from the  ground struct ure, this rota tional hard st op could hav e been imple mented. It would have gi ven the  instru ment m ore repeatabl e positional i nformatio n and it wo uld n ot have de pended as heav ily on th e  temperature and thu s current limit at  which the sy stem is activated . 

3) The only Platinum Re sistive Therm ocouple (PRT ) that was availa ble to us wa s mounted o n the Z-axi s  motor with th e thoug ht tha t it would m ore accu rately provide th e true mo rning tempe rature o f the RAT  as well as limit the n umber of wi res that mu st flex while  the RAT was in motion. However,  in use, it  seem s that t he PRT would be p ut to better use if it were mo unted to the gri nd motor that h as by fa r  the hig hest duty cycle  of a ll the moto rs. This woul d have allo wed direct readin g of the grind motor’ s  temperature durin g ope ration as op posed to a the rmal mod el looki ng for o verheatin g, whi ch is  currently used. 

4) During th e assembly of the  RAT e ngineering and flight model s, the 37 pin mi cro D connectors were  backfilled wit h an epoxy, model # PR-1590, made by  PRC Desoto Intern ational.  Th roughout the  process of ve rifying our mi xing pro cedure, testing our Teflon mold, and ap plying this pro cedure to the   engin eering and flight mod els we lea rned that t he co mpoun d was extremely se nsitive to atm ospheric  humidity.  If the humi dity levels were g reater th an 30%, the comp ound fo rmed bubbl es th at coul d not   be elimin ated even when i mmersed i n a vacu um ch amber, as part of our procedure de signed as a  precautio nary step to prevent bubble fo rmation.  Regardl ess of the amou nt of applied vacu um, the  bubbl es continued to fo rm in the p resence of high humidity levels.  Ultimately we ke pt ou r clean room   at or belo w 30% humidity, yielding successful mold s with no bu bbles. 

Conclu sion 

Using data o btained from grindi ng test s perform ed at 0ºC and dyn amomete r tests pe rform ed at the same  temperature, it is po ssible to close the loop bet ween the amo unt of  force an d torque requi red by the RAT  motors to grind a rock, an d the corresp ondin g curre nt. However, the torq ue va ries g reatly with the type of  rock th at is being grinded. In ord er to determine the clo sed loop rel ationship, a wo rst-ca se rock was  grind ed (h ard basalt with flat surfa ce topology). It was found that  the worst -case torq ue re quired by the  289