▁that , ▁since ▁each ▁stage ▁may ▁have ▁a ▁lower ▁mass ▁ratio ▁than ▁an ▁S ST O ▁launch ▁system , ▁such ▁a ▁system ▁may ▁be ▁built ▁further ▁away ▁from ▁limits ▁of ▁its ▁struct ural ▁materials . ▁It ▁is ▁argued ▁that ▁a ▁two - stage ▁design ▁should ▁require ▁less ▁maintenance , ▁less ▁testing , ▁experience ▁fewer ▁fail ures ▁and ▁have ▁a ▁longer ▁working ▁life . ▁In ▁addition ▁the ▁two - stage ▁approach ▁allows ▁the ▁lower ▁stage ▁to ▁be ▁optimized ▁for ▁operation ▁in ▁the ▁Earth ' s ▁lower ▁atmosphere , ▁where ▁pressure ▁and ▁drag ▁are ▁high , ▁while ▁the ▁upper ▁stage ▁can ▁be ▁optimized ▁for ▁operation ▁in ▁the ▁near - v acu um ▁conditions ▁of ▁the ▁later ▁part ▁of ▁the ▁launch . ▁This ▁allows ▁an ▁increase ▁in ▁the ▁payload ▁mass ▁fraction ▁of ▁a ▁two - stage ▁vehicle ▁over ▁single - stage ▁or ▁stage - and - a - half ▁vehicles , ▁which ▁have ▁to ▁perform ▁in ▁both ▁environments ▁using ▁the ▁same ▁hardware . ▁ ▁Crit ics ▁argue ▁that ▁the ▁increased ▁complexity ▁of ▁design ing ▁two ▁separate ▁stages ▁that ▁must ▁interact , ▁the ▁log istics ▁involved ▁in ▁returning ▁the ▁first ▁stage ▁to ▁the ▁launch ▁site , ▁and ▁the ▁difficulties ▁of ▁conduct ing ▁increment al ▁testing ▁on ▁a ▁second ▁stage ▁will ▁out we igh ▁these ▁benefits . ▁In ▁the ▁case ▁of ▁air plane - like ▁lower ▁stages ▁they ▁also ▁argue ▁how ▁difficult ▁and ▁expensive ▁high ▁speed ▁aircraft ▁( like ▁the ▁SR - 7 1 ) ▁are ▁to ▁develop ▁and ▁operate , ▁and ▁question ▁performance ▁claims . ▁Many ▁'