▁thrust ▁level ▁was ▁twice ▁what ▁was ▁needed ▁to ▁accomplish ▁the ▁lun ar ▁orbit ▁rende z vous ▁( L OR ) ▁mission ▁mode , ▁because ▁the ▁engine ▁was ▁originally ▁s ized ▁to ▁lift ▁the ▁C SM ▁off ▁of ▁the ▁lun ar ▁surface ▁in ▁the ▁direct ▁as cent ▁mode ▁assumed ▁in ▁original ▁planning ▁( see ▁Cho osing ▁a ▁mission ▁mode .) ▁A ▁contract ▁was ▁signed ▁in ▁April ▁ 1 9 6 2 ▁for ▁the ▁A ero jet - General ▁company ▁to ▁start ▁developing ▁the ▁engine , ▁before ▁the ▁L OR ▁mode ▁was ▁officially ▁chosen ▁in ▁July ▁of ▁that ▁year . ▁ ▁The ▁prop ell ants ▁were ▁pressure - f ed ▁to ▁the ▁engine ▁by ▁ ▁of ▁g ase ous ▁hel ium ▁at ▁, ▁carried ▁in ▁two ▁ ▁diameter ▁spher ical ▁t anks . ▁ ▁The ▁exhaust ▁no zz le ▁engine ▁bell ▁measured ▁ ▁long ▁and ▁ ▁wide ▁at ▁the ▁base . ▁It ▁was ▁mounted ▁on ▁two ▁g imb als ▁to ▁keep ▁the ▁thrust ▁vector ▁aligned ▁with ▁the ▁space craft ' s ▁center ▁of ▁mass ▁during ▁S PS ▁fir ings . ▁The ▁comb ust ion ▁chamber ▁and ▁press ur ant ▁t anks ▁were ▁hous ed ▁in ▁the ▁central ▁tunnel . ▁ ▁Re action ▁control ▁system ▁Four ▁clusters ▁of ▁four ▁reaction ▁control ▁system ▁( R CS ) ▁thrust ers ▁were ▁installed ▁around ▁the ▁upper ▁section ▁of ▁the ▁SM ▁every ▁ 9 0 ° . ▁The ▁six teen - th r uster ▁arrangement ▁provided ▁rotation ▁and ▁translation ▁control ▁in ▁all ▁three ▁space craft ▁axes . ▁Each ▁R -