° ▁angles , ▁but ▁varied ▁according ▁to ▁required ▁size . ▁ ▁S ector ▁ 1 ▁( 5 0 ° ) ▁was ▁originally ▁un used , ▁so ▁it ▁was ▁filled ▁with ▁ball ast ▁to ▁maintain ▁the ▁SM ' s ▁center - of ▁gravity . ▁On ▁the ▁last ▁three ▁lun ar ▁landing ▁( I - J ▁class ) ▁miss ions , ▁it ▁carried ▁the ▁scientific ▁instrument ▁module ▁( SI M ) ▁which ▁contained ▁a ▁package ▁of ▁lun ar ▁orb ital ▁sens ors ▁and ▁a ▁subs at ellite . ▁ ▁S ector ▁ 2 ▁( 7 0 ° ) ▁contained ▁the ▁service ▁prop ulsion ▁system ▁( SP S ) ▁ox id izer ▁sum p ▁tank , ▁so ▁called ▁because ▁it ▁directly ▁fed ▁the ▁engine ▁and ▁was ▁kept ▁continu ously ▁filled ▁by ▁a ▁separate ▁storage ▁tank , ▁until ▁the ▁latter ▁was ▁empty . ▁The ▁sum p ▁tank ▁was ▁a ▁cyl inder ▁with ▁hem is pher ical ▁ends , ▁ ▁high , ▁ ▁in ▁diameter , ▁and ▁contained ▁ ▁of ▁ox id izer . ▁Its ▁total ▁volume ▁was ▁▁▁ ▁S ector ▁ 3 ▁( 6 0 ° ) ▁contained ▁the ▁S PS ▁ox id izer ▁storage ▁tank , ▁which ▁was ▁the ▁same ▁shape ▁as ▁the ▁sum p ▁tank ▁but ▁slightly ▁smaller ▁at ▁ ▁high ▁and ▁ ▁in ▁diameter , ▁and ▁held ▁ ▁of ▁ox id izer . ▁Its ▁total ▁volume ▁was ▁▁▁ ▁S ector ▁ 4 ▁( 5 0 ° ) ▁contained ▁the ▁elect rical ▁power ▁system ▁( E PS ) ▁fuel ▁cells ▁with ▁their ▁hydro gen ▁and