▁risk ▁of ▁failure . ▁ ▁Ex amples ▁ ▁Ang ara ▁ 1 . 2 ▁ ▁Atlas ▁V ▁( 4 0 1 ▁and ▁ 5 0 1 ) ▁ ▁Star ship ▁( pro posed ) ▁ ▁New ▁Gl enn ▁( pro posed ) ▁ ▁Cos mos - 3 M ▁ ▁D elta ▁IV ▁( medium ) ▁ ▁Fal con ▁ 1 ▁ ▁Fal con ▁ 9 ▁ ▁Electron ▁ ▁Sat urn ▁IB ▁ ▁Sat urn ▁V - SL 1 ▁( Sk yl ab ▁launch ) ▁ ▁Ts y kl on - 2 ▁ ▁V ul can ▁( pro posed ) ▁ ▁Z en it - 2 ▁▁ ▁It ▁is ▁not ▁always ▁clear ▁when ▁a ▁vehicle ▁is ▁a ▁T ST O . ▁Many ▁designs ▁which ▁use ▁boost er ▁rock ets ▁at ▁lif to ff ▁are ▁referred ▁to ▁as ▁single - stage - to - or bit ▁if ▁there ▁is ▁no ▁additional ▁upper ▁stage . ▁Some ▁have ▁also ▁co ined ▁the ▁expression ▁ 1 . 5 ST O ▁for ▁' one - and - a - half - stage - to - or bit ', ▁e . g . ▁for ▁the ▁Atlas ▁miss ile ▁( the ▁half ▁stage ▁refers ▁to ▁the ▁boost er ▁engines ▁which ▁were ▁j ett ison ed ▁early ▁in ▁the ▁launch ). ▁ ▁Re usable ▁launch ▁systems ▁With ▁reference ▁to ▁a ▁re usable ▁launch ▁system ▁this ▁approach ▁is ▁often ▁proposed ▁as ▁an ▁alternative ▁to ▁single - stage - to - or bit ▁( or ▁S ST O ). ▁Its ▁supp or ters ▁argue