▁() ▁is ▁fixed , ▁for ▁a ▁mission ▁delta - v , ▁there ▁is ▁a ▁particular ▁ ▁that ▁minim ises ▁the ▁overall ▁energy ▁used ▁by ▁the ▁ro cket . ▁This ▁comes ▁to ▁an ▁exhaust ▁velocity ▁of ▁about ▁ <0xE2> <0x85> <0x94> ▁of ▁the ▁mission ▁delta - v ▁( see ▁the ▁energy ▁computed ▁from ▁the ▁ro cket ▁equation ). ▁D riv es ▁with ▁a ▁specific ▁imp ulse ▁that ▁is ▁both ▁high ▁and ▁fixed ▁such ▁as ▁I on ▁thrust ers ▁have ▁exhaust ▁veloc ities ▁that ▁can ▁be ▁enorm ously ▁higher ▁than ▁this ▁ideal , ▁and ▁thus ▁end ▁up ▁powers ource ▁limited ▁and ▁give ▁very ▁low ▁thrust . ▁Where ▁the ▁vehicle ▁performance ▁is ▁power ▁limited , ▁e . g . ▁if ▁solar ▁power ▁or ▁nuclear ▁power ▁is ▁used , ▁then ▁in ▁the ▁case ▁of ▁a ▁large ▁ ▁the ▁maximum ▁acceleration ▁is ▁in vers ely ▁proportional ▁to ▁it . ▁Hence ▁the ▁time ▁to ▁reach ▁a ▁required ▁delta - v ▁is ▁proportional ▁to ▁. ▁Thus ▁the ▁latter ▁should ▁not ▁be ▁too ▁large . ▁ ▁On ▁the ▁other ▁hand , ▁if ▁the ▁exhaust ▁velocity ▁can ▁be ▁made ▁to ▁vary ▁so ▁that ▁at ▁each ▁instant ▁it ▁is ▁equal ▁and ▁opposite ▁to ▁the ▁vehicle ▁velocity ▁then ▁the ▁absolute ▁minimum ▁energy ▁usage ▁is ▁achieved . ▁When ▁this ▁is ▁achieved , ▁the ▁exhaust ▁stops ▁in ▁space ▁ ▁and ▁has ▁no ▁kin etic ▁energy ; ▁and ▁the ▁prop uls ive ▁efficiency ▁is ▁ 1 0 0 % ▁all ▁the ▁energy ▁ends ▁up ▁in ▁the ▁vehicle ▁( in ▁principle ▁such ▁a ▁drive ▁would ▁be ▁ 1