5 ▁meg aton ▁war head ▁intended ▁to ▁destroy ▁enemy ' s ▁war head s ▁with ▁a ▁burst ▁of ▁x - ray s ▁outside ▁the ▁atmosphere . ▁A ▁second ▁shorter - range ▁miss ile ▁called ▁S print ▁with ▁very ▁high ▁acceleration ▁was ▁added ▁to ▁handle ▁war head s ▁that ▁ev aded ▁longer - r anged ▁Spart an . ▁S print ▁was ▁a ▁very ▁fast ▁miss ile ▁( some ▁sources ▁claimed ▁it ▁acceler ated ▁to ▁ 8 , 0 0 0   m ph ▁( 1 3   0 0 0   km / h ) ▁within ▁ 4 ▁seconds ▁of ▁flight — an ▁average ▁acceleration ▁of ▁ 9 0 ▁g ) ▁and ▁had ▁a ▁smaller ▁W 6 6 ▁en hanced ▁radiation ▁war head ▁in ▁the ▁ 1 – 3 ▁kil ot on ▁range ▁for ▁in - at mos phere ▁inter ception s . ▁ ▁The ▁experimental ▁success ▁of ▁N ike ▁X ▁persu aded ▁the ▁Ly nd on ▁B . ▁Johnson ▁administration ▁to ▁propose ▁a ▁thin ▁AB M ▁defense , ▁that ▁could ▁provide ▁almost ▁complete ▁coverage ▁of ▁the ▁United ▁States . ▁In ▁a ▁September ▁ 1 9 6 7 ▁speech , ▁Def ense ▁Secretary ▁Robert ▁Mc N am ara ▁referred ▁to ▁it ▁as ▁" S entin el ". ▁Mc N am ara , ▁a ▁private ▁AB M ▁oppon ent ▁because ▁of ▁cost ▁and ▁feas ibility ▁( see ▁cost - exchange ▁ratio ), ▁claimed ▁that ▁S entin el ▁would ▁be ▁directed ▁not ▁against ▁the ▁Soviet ▁Union ' s ▁miss iles ▁( since ▁the ▁USS R ▁had