Complex sentence: Fourthly, the extraordinary fuel consumption of the two ramjets made the helicopter uneconomical to operate, flight time and flight range were quite limited making the helicopter unattractive for commercial or military use.
Split into simpler sentences: Fourthly, the extraordinary fuel consumption of the two ramjets made the helicopter uneconomical to operate. Therefore flight time and flight range were quite limited which made the helicopter unattractive for commercial or military use.