4 D ▁thr uster ▁generated ▁ ▁of ▁thrust , ▁and ▁used ▁mon om eth yl h yd raz ine ▁( MM H ) ▁as ▁fuel ▁and ▁nit ro gen ▁t etro x ide ▁( N TO ) ▁as ▁ox id izer . ▁Each ▁quad ▁assembly ▁measured ▁ ▁and ▁had ▁its ▁own ▁fuel ▁t anks , ▁ox id izer ▁t anks , ▁hel ium ▁press ur ant ▁tank , ▁and ▁associated ▁val ves ▁and ▁regul ators . ▁ ▁Each ▁cluster ▁of ▁thrust ers ▁had ▁its ▁own ▁independent ▁primary ▁fuel ▁( MM H ) ▁tank ▁containing ▁, ▁secondary ▁fuel ▁tank ▁containing ▁, ▁primary ▁ox id izer ▁tank ▁containing ▁, ▁and ▁secondary ▁ox id izer ▁tank ▁containing ▁. ▁The ▁fuel ▁and ▁ox id izer ▁t anks ▁were ▁press ur ised ▁by ▁a ▁single ▁liquid ▁hel ium ▁tank ▁containing ▁. ▁Back ▁flow ▁was ▁prevent ed ▁by ▁a ▁series ▁of ▁check ▁val ves , ▁and ▁back ▁flow ▁and ▁u ll age ▁requirements ▁were ▁resolved ▁by ▁containing ▁the ▁fuel ▁and ▁ox id izer ▁in ▁T ef lon ▁bl add ers ▁which ▁separated ▁the ▁prop ell ants ▁from ▁the ▁hel ium ▁press ur ant . ▁ ▁All ▁of ▁the ▁elements ▁were ▁dup licated , ▁resulting ▁in ▁four ▁completely ▁independent ▁R CS ▁clusters . ▁Only ▁two ▁adjacent ▁function ing ▁units ▁were ▁needed ▁to ▁allow ▁complete ▁attitude ▁control . ▁ ▁The ▁lun ar ▁module ▁used ▁a ▁similar ▁four - quad ▁arrangement ▁of ▁the ▁identical ▁thr uster ▁engines ▁for ▁its ▁R CS . ▁ ▁Elect rical ▁power ▁system ▁ ▁Elect rical ▁power ▁was