0 0 % ▁efficient , ▁in ▁practice ▁there ▁would ▁be ▁thermal ▁losses ▁from ▁within ▁the ▁drive ▁system ▁and ▁resid ual ▁heat ▁in ▁the ▁exhaust ). ▁However , ▁in ▁most ▁cases ▁this ▁uses ▁an ▁imp ract ical ▁quantity ▁of ▁prop ell ant , ▁but ▁is ▁a ▁useful ▁theoretical ▁consideration . ▁ ▁Some ▁drives ▁( such ▁as ▁V AS IM R ▁or ▁elect rod eless ▁pl asma ▁thr uster ) ▁actually ▁can ▁significantly ▁vary ▁their ▁exhaust ▁velocity . ▁This ▁can ▁help ▁reduce ▁prop ell ant ▁usage ▁and ▁improve ▁acceleration ▁at ▁different ▁stages ▁of ▁the ▁flight . ▁However ▁the ▁best ▁ener get ic ▁performance ▁and ▁acceleration ▁is ▁still ▁obtained ▁when ▁the ▁exhaust ▁velocity ▁is ▁close ▁to ▁the ▁vehicle ▁speed . ▁Pro posed ▁ion ▁and ▁pl asma ▁drives ▁usually ▁have ▁exhaust ▁veloc ities ▁enorm ously ▁higher ▁than ▁that ▁ideal ▁( in ▁the ▁case ▁of ▁V AS IM R ▁the ▁lowest ▁quoted ▁speed ▁is ▁around ▁ 1 5   km / s ▁compared ▁to ▁a ▁mission ▁delta - v ▁from ▁high ▁Earth ▁orbit ▁to ▁Mars ▁of ▁about ▁ 4   km / s ). ▁ ▁For ▁a ▁mission , ▁for ▁example , ▁when ▁launch ing ▁from ▁or ▁landing ▁on ▁a ▁planet , ▁the ▁effects ▁of ▁gravit ational ▁att raction ▁and ▁any ▁atmos pher ic ▁drag ▁must ▁be ▁overcome ▁by ▁using ▁fuel . ▁It ▁is ▁typical ▁to ▁combine ▁the ▁effects ▁of ▁these ▁and ▁other ▁effects ▁into ▁an ▁effective ▁mission ▁delta - v . ▁For ▁example , ▁a ▁launch ▁mission ▁to ▁low ▁Earth ▁orbit ▁requires ▁about ▁ 9 . 3