▁cycle . ▁Development ▁years : ▁ 1 9 6 4 - 1 9 7 1 ▁. ▁For ▁use ▁as ▁main ▁engine ▁of ▁the ▁second ▁stage ▁of ▁R - 2 9 . ▁Sub mer ged ▁in ▁the ▁prop ell ant ▁tank . ▁ 4 D - 7 6 M : ▁A ▁U DM H / N 2 O 4 ▁burning ▁engine ▁in ▁the ▁gas ▁generator ▁cycle . ▁Development ▁years : ▁ 1 9 6 9 - 1 9 7 0 ▁. ▁Im pro ved ▁version ▁of ▁the ▁ 4 D - 7 6 . ▁ 4 D - 2 8 : ▁A ▁U DM H / N 2 O 4 ▁burning ▁engine ▁in ▁the ▁gas ▁generator ▁cycle . ▁Development ▁years : ▁ 1 9 6 8 - 1 9 7 0 ▁. ▁For ▁use ▁on ▁the ▁second ▁stage ▁of ▁the ▁ro cket ▁R SM - 2 7 K . ▁ 3 D - 2 0 : ▁A ▁U DM H / N 2 O 4 ▁burning ▁engine ▁in ▁the ▁st aged ▁comb ust ion ▁cycle . ▁Development ▁years : ▁ 1 9 7 1 - 1 9 7 3 ▁. ▁For ▁use ▁as ▁main ▁engine ▁R SM - 2 7 U . ▁ 3 D - 2 0 : ▁A ▁U DM H / N 2 O 4 ▁burning ▁engine ▁in ▁the ▁gas ▁generator ▁cycle . ▁Development ▁years : ▁ 1 9 7 1 - 1 9 7 3 ▁. ▁For ▁use ▁as ▁ste ering ▁engine ▁R - 2 7 U . ▁ 3