s ▁a ft ▁heat ▁shield . ▁It ▁remained ▁attached ▁to ▁the ▁command ▁module ▁throughout ▁most ▁of ▁the ▁mission , ▁until ▁being ▁j ett ison ed ▁just ▁prior ▁to ▁re - entry ▁into ▁the ▁Earth ' s ▁atmosphere . ▁At ▁j ett ison , ▁the ▁C M ▁um bil ical ▁connections ▁were ▁cut ▁using ▁a ▁py rote chn ic - activ ated ▁gu ill ot ine ▁assembly . ▁Following ▁j ett ison , ▁the ▁SM ▁a ft ▁translation ▁thrust ers ▁automatically ▁fired ▁continu ously ▁to ▁distance ▁it ▁from ▁the ▁C M , ▁until ▁either ▁the ▁R CS ▁fuel ▁or ▁the ▁fuel ▁cell ▁power ▁was ▁de ple ted . ▁The ▁roll ▁thrust ers ▁were ▁also ▁fired ▁for ▁five ▁seconds ▁to ▁make ▁sure ▁it ▁followed ▁a ▁different ▁traject ory ▁from ▁the ▁C M ▁and ▁faster ▁break - up ▁on ▁re - entry . ▁ ▁Service ▁prop ulsion ▁system ▁The ▁S PS ▁engine ▁was ▁used ▁to ▁place ▁the ▁Apol lo ▁space craft ▁into ▁and ▁out ▁of ▁lun ar ▁orbit , ▁and ▁for ▁mid - course ▁corre ctions ▁between ▁the ▁Earth ▁and ▁Moon . ▁It ▁also ▁served ▁as ▁a ▁retr or ocket ▁to ▁perform ▁the ▁de or bit ▁burn ▁for ▁Earth ▁orb ital ▁Apol lo ▁fl ights . ▁The ▁engine ▁selected ▁was ▁the ▁A J 1 0 - 1 3 7 , ▁which ▁used ▁A ero z ine ▁ 5 0 ▁as ▁fuel ▁and ▁nit ro gen ▁t etro x ide ▁( N 2 O 4 ) ▁as ▁ox id izer ▁to ▁produce ▁ ▁of ▁thrust . ▁The