▁to ▁communicate ▁through ▁the ▁A TS - 6 ▁satellite ▁in ▁ge ost ation ary ▁orbit , ▁an ▁experimental ▁prec ursor ▁to ▁the ▁current ▁T DR SS ▁system . ▁ ▁On ▁the ▁Sk yl ab ▁and ▁Apol lo - So y uz ▁miss ions , ▁some ▁additional ▁dry ▁weight ▁was ▁saved ▁by ▁removing ▁the ▁otherwise ▁empty ▁fuel ▁and ▁ox id izer ▁storage ▁t anks ▁( le aving ▁the ▁partially ▁filled ▁sum p ▁t anks ), ▁along ▁with ▁one ▁of ▁the ▁two ▁hel ium ▁press ur ant ▁t anks . ▁This ▁permitted ▁the ▁addition ▁of ▁some ▁extra ▁R CS ▁prop ell ant ▁to ▁allow ▁for ▁use ▁as ▁a ▁backup ▁for ▁the ▁de or bit ▁burn ▁in ▁case ▁of ▁possible ▁S PS ▁failure . ▁ ▁Since ▁the ▁space craft ▁for ▁the ▁Sk yl ab ▁miss ions ▁would ▁not ▁be ▁occupied ▁for ▁most ▁of ▁the ▁mission , ▁there ▁was ▁lower ▁demand ▁on ▁the ▁power ▁system , ▁so ▁one ▁of ▁the ▁three ▁fuel ▁cells ▁was ▁deleted ▁from ▁these ▁SM s . ▁ ▁The ▁command ▁module ▁could ▁be ▁modified ▁to ▁carry ▁extra ▁astr onaut s ▁as ▁passengers ▁by ▁adding ▁jump ▁seat ▁c ouch es ▁in ▁the ▁a ft ▁equipment ▁bay . ▁C M - 1 1 9 ▁was ▁fitted ▁with ▁two ▁jump ▁seats ▁as ▁a ▁Sk yl ab ▁Res cue ▁vehicle , ▁which ▁was ▁never ▁used . ▁ ▁Major ▁differences ▁between ▁Block ▁I ▁and ▁Block ▁II ▁ ▁Command ▁module ▁▁ ▁The ▁Block ▁II ▁used ▁a ▁one - pie ce , ▁quick - release , ▁out ward ▁opening ▁h atch