utt le ▁Land ing ▁Fac ility , ▁touch ing ▁down ▁on ▁Kennedy ▁Space ▁Center ' s ▁Run way ▁ 1 5 ▁at ▁ 0 2 : 3 4 ▁ED T ▁after ▁comple ting ▁its ▁final ▁mission , ▁ST S - 1 3 4 . ▁ ▁External ▁tank ▁fuel ing ▁test ▁Atl ant is ' s ▁external ▁tank ▁for ▁the ▁ST S - 1 3 5 ▁mission ▁was ▁put ▁through ▁a ▁tank ing ▁test ▁on ▁ 1 5 ▁June ▁ 2 0 1 1 ▁to ▁check ▁the ▁health ▁of ▁the ▁tank ' s ▁string ers . ▁It ▁was ▁slightly ▁delayed ▁due ▁to ▁a ▁light ning ▁storm ▁which ▁passed ▁over ▁the ▁Kennedy ▁Space ▁Center . ▁During ▁the ▁test , ▁techn icians ▁detected ▁a ▁hydro gen ▁fuel ▁val ve ▁leak ▁in ▁Atl ant is ' s ▁main ▁engine ▁No . ▁ 3 , ▁as ▁it ▁recorded ▁temper atures ▁below ▁normal ▁levels . ▁The ▁le aking ▁hydro gen ▁val ve ▁was ▁replaced ▁on ▁ 2 1 ▁June . ▁ ▁On ▁ 1 8 ▁June , ▁engine ers ▁also ▁comm enced ▁X - ray ▁ins pe ctions ▁to ▁verify ▁the ▁performance ▁of ▁the ▁radius ▁block ▁dou bl ers ▁that ▁were ▁installed ▁over ▁the ▁top ▁of ▁the ▁string ers . ▁The ▁string ers ▁form ▁the ▁back bone ▁of ▁E T - 1 3 8 ' s ▁central ▁" inter t ank " ▁compart ment ▁that ▁separ ates ▁the ▁upper ▁liquid ▁o xygen ▁tank ▁from ▁the ▁larger ▁liquid ▁hydro gen ▁tank ▁below . ▁The ▁installation ▁of ▁the ▁dou bl ers