2 2 , ▁Atl ant is ▁with ▁the ▁newer ▁and ▁less ▁ris ky ▁E T - 1 3 8 ▁would ▁be ▁po ised ▁to ▁rescue ▁Ende av ours ▁crew . ▁ ▁In ▁early ▁December ▁ 2 0 1 0 , ▁ground ▁techn icians ▁installed ▁the ▁main ▁engines ▁on ▁Atl ant is . ▁The ▁Sh utt le ▁received ▁the ▁center ▁engine ▁on ▁ 7 ▁December ▁ 2 0 1 0 , ▁followed ▁by ▁the ▁lower - right ▁engine ▁and ▁the ▁lower - left ▁on ▁ 8 ▁and ▁ 9 ▁December ▁ 2 0 1 0 ▁respectively ▁inside ▁Or b iter ▁Process ing ▁Fac ility ▁( OP F - 1 ). ▁The ▁event ▁marked ▁the ▁last ▁set ▁of ▁main ▁engines ▁ever ▁to ▁be ▁installed ▁on ▁a ▁Space ▁Sh utt le . ▁ ▁Stack ing ▁operations ▁of ▁the ▁Sol id ▁Ro cket ▁Bo ost ers ▁( SR B s ) ▁for ▁the ▁mission ▁comm enced ▁in ▁the ▁evening ▁hours ▁of ▁ 2 9 ▁March ▁ 2 0 1 1 . ▁Techn icians ▁inside ▁the ▁V AB , ▁lifted ▁the ▁left - aft ▁segment ▁from ▁the ▁handling ▁cr ate ▁and ▁carefully ▁man eu ver ed ▁into ▁High ▁Bay ▁No . ▁ 1 ▁and ▁finally ▁onto ▁the ▁mobile ▁launch ▁platform . ▁The ▁boost er ▁stack ing ▁was ▁completed ▁in ▁mid ▁April . ▁The ▁completed ▁boost ers ▁had ▁a ▁mixture ▁of ▁ref urb ished ▁and ▁un flow n ▁elements ▁( 1 1 ▁sections ▁on ▁each ▁boost er ). ▁For ▁example , ▁the ▁forward ▁d ome ▁for ▁the ▁right -