4 ▁when ▁it ▁was ▁clear ▁the ▁treat y ▁was ▁going ▁to ▁be ▁followed ▁by ▁both ▁countries . ▁ ▁Work ▁on ▁M X ▁also ▁continued , ▁and ▁by ▁the ▁mid - 1 9 7 0 s ▁the ▁preferred ▁M X ▁bas ing ▁mode ▁was ▁the ▁Multiple ▁Prote ct ive ▁Sh elter ▁( MP S ) ▁concept . ▁In ▁this ▁system ▁each ▁M X ▁miss ile ▁would ▁be ▁installed ▁in ▁a ▁network ▁of ▁hard ened ▁sh el ters , ▁ 2 3 ▁in ▁most ▁plans , ▁and ▁moved ▁between ▁them ▁at ▁random . ▁The ▁trans ports ▁would ▁be ▁cam ou fl aged ▁and ▁dup licated ▁so ▁the ▁S ovi ets ▁would ▁not ▁know ▁where ▁the ▁miss iles ▁were . ▁A ▁counter force ▁attack ▁would ▁thus ▁require ▁the ▁S ovi ets ▁to ▁exp end ▁ 2 3 ▁war head s ▁for ▁every ▁M X ▁miss ile ▁to ▁ensure ▁destruction . ▁With ▁a ▁total ▁of ▁ 4 , 5 0 0 ▁sh el ters ▁and ▁ 2 0 0 ▁miss iles ▁moving ▁between ▁them , ▁the ▁system ▁could ▁so ak ▁up ▁a ▁significant ▁portion ▁of ▁the ▁Soviet ' s ▁ 5 , 9 2 8 ▁IC B M ▁war head s ▁and ▁surv ive . ▁In ▁this ▁bas ing ▁mode , ▁a ▁system ▁like ▁Saf egu ard ▁was ▁of ▁little ▁use . ▁If ▁the ▁S ovi ets ▁needed ▁to ▁fire ▁ 4 , 5 0 0 ▁war head s ▁to ▁ensure ▁the ▁destruction ▁of ▁the ▁M X ▁force , ▁exp ending ▁another ▁ 1 0 0 ▁to