Rock Abrasion Tool 

Thomas M. Myrick*, Kiel Da vis* and Jack Wilso n* 

Abstract 

The Rock Ab rasion Tool  (RAT) is de signed  to expo se th e interior of Martia n rocks for ide ntification a nd  analysi s. The RAT, contained within  a cylinde r 128-mm lon g and 85-mm i n diameter, has a ma ss of 687  gram s. Using three sm all DC brushed motors, the RAT grinds a 5-mm d eep, 4 5-mm diamet er hole into a  targeted rock. Durin g grin ding, the gri nding wheel, shap ed a s a paddl e wit h a syntheti c diam ondimpre gnate d resin, enga ges a target ro ck whe n the Z-axis is exten ded. Un realized duri ng testing, under  Mars conditions th e grinding wheel wear rate decreases five fol d when  compared to operation under  Earth conditi ons. In total,  four Rock Abra sion Tools were ma nufactu red, t wo flight u nits, and t wo  engin eering test units. Afte r deliveri ng th e flight units, the motor current mea sured by the RAT’ s ele ctronic  interface did not provide a  true readi ng. Follow-on te sting pe rmitted correlatin g the erro neous data with   proper data. The flight units we re installed on two separate spacecrafts for deli very to Mars i n 2004. 

Introduc tion 

The Ro ck Ab rasion To ol (RAT) i s a criti cal sub system of the Athena scien ce payload. De signed to mimic  the effect of a geol ogist’s hamme r, the RAT i s used to  expo se a ro ck’s i nterior for i dentification an d  analysi s. Martian rocks ca nnot be thoro ughly examin ed from the outside be cause of dust an d oxidation.   Consequently , from a surfa ce view, mo st Martian ro cks app ear visu ally similar. 

To effectively grind rocks on Mars, sev eral key ch aracteristics a re req uired, primarily a de sirable cutting   material, en ough po wer and force rea ction while working, and knowledge of th e target mate rial. The first  two re quirem ents, cutting  material a nd power and force can  be re searched an d tested; howeve r,  information pertaining to the Martia n rock composition was illu sive prior to the succe ssful use of the RAT.  During th e design phase, the behavio r of Martian  rocks with rega rds to cutting or grin ding was not known  and h ad to b e estim ated leadin g to co mplicatio ns in deriving ou r force a nd torque margins. Throug hout  the iterative RAT d esign process vari ous discove ries provided critical knowledge on the development  of  drilling and grinding units for use on non-Earth planetary bo dies. Particul arly, important findings were  made with regards to the g rindin g wheel developme nt, the diamond  abra sive mat erial. 

RAT Ov erview 

The RAT is designed to grind a 5 -mm deep and  45-mm dia meter hole i nto rocks so  that scien ce  instru ment s can view the rock’s interi or. The grinding depth i s selectable  within  the rang e of t he RAT’s Zaxis a nd the t opog raphy of the rock surface. The typical comman d depth  is 5 mm. A small rotating b rush  (Figu re 3, lab el 7) can b e used to clean a rock’s surface  with out grinding. T hrough thi s method, a rock  can b e studi ed with a suite of instrum ents before and after the su rface h as been swept clea n or gri nded. 

The RAT (Figure 3 ), 128 -mm long, is co ntained within  a circle 85 -mm in diamete r, and has a m ass of 685  g. It is made primarily from  a low density, high modul us alloy of aluminum  and beryllium  called Albemet ,  which has a coeffici ent of thermal exp ansion simila r to steel. Othe r stru ctural material s incl ude alumin um  (7075 and 6061),  steel (Vascomax C-250,  4140, 302, 3 03, 4 40C, A2 86, Nitronic 6 0) and titaniu m  (6AL4V ), Vespel, Nickel a nd 18-ka rat yellow gold. All of the be aring s and gears are lu bricated with a   grease plate  appli cation  of Braycote 600 EF mi croni c and are sealed with Teflon V seals to  prevent d ust  from ente ring the mech anism. The RAT, using three small DC b rushed motors with mag netic en coders,  consumes between 8 a nd 11 watts fo r a typical three -hour g rinding session.  

                                                  * Honeybe e Robotics, Ne w York, NY 

Procee dings of the 37th Aeros pace Mec hanisms Sy mpos ium, Joh nson S pace Ce nter, May 19-2 1, 200 4  277