▁considered ▁as ▁part ▁of ▁the ▁Apol lo ▁Sk ip ▁Gu id ance ▁concept , ▁but ▁this ▁was ▁not ▁used ▁on ▁any ▁cre wed ▁fl ights . ▁The ▁concept ▁continues ▁to ▁appear ▁on ▁more ▁modern ▁vehicles ▁like ▁the ▁Or ion ▁space craft , ▁using ▁on board ▁computers . ▁ ▁Fl ight ▁mechan ics ▁▁ ▁Using ▁simplified ▁equations ▁of ▁motion ▁and ▁assuming ▁that ▁during ▁the ▁atmos pher ic ▁flight ▁both ▁drag ▁and ▁lift ▁forces ▁will ▁be ▁much ▁larger ▁than ▁the ▁gravity ▁force ▁acting ▁on ▁the ▁vehicle , ▁the ▁following ▁analyt ical ▁relations ▁for ▁a ▁skip ▁re entry ▁flight ▁can ▁be ▁derived : ▁ ▁Where ▁g amma ▁is ▁the ▁flight ▁path ▁angle ▁relative ▁to ▁the ▁local ▁horizontal , ▁the ▁sub script ▁E ▁indicates ▁the ▁conditions ▁at ▁the ▁start ▁of ▁the ▁entry ▁and ▁the ▁sub script ▁F ▁indicates ▁the ▁conditions ▁at ▁the ▁end ▁of ▁the ▁entry ▁flight . ▁ ▁The ▁velocity ▁V ▁before ▁and ▁after ▁the ▁entry ▁can ▁be ▁derived ▁to ▁relate ▁as ▁follows : ▁ ▁Where ▁L / D ▁equals ▁the ▁lift ▁to ▁drag ▁ratio ▁of ▁the ▁vehicle . ▁ ▁Ex isting ▁or ▁in ▁development ▁ ▁Russian ▁Hyp erson ic ▁G lide ▁Ve h icle ▁Av ang ard ▁developed ▁and ▁deployed . ▁ ▁Chinese ▁Hyp erson ic ▁G lide ▁Ve h icle ▁D F - Z F ▁in ▁development . ▁ ▁US ▁Hyp erson ic ▁G lide ▁Ve h icle ▁HT V - 2 ▁in ▁development . ▁ ▁Indian ▁Hyp erson ic ▁G lide ▁Ve h icle ▁H GV - 2 0 2 F ▁in ▁development