sentences:  Since the USSR lacked a production-ready turbojet engine, development efforts were directed toward mixed-power aircraft utilizing a conventional piston engine-driven propeller for the majority of propulsion with a small rocket or jet engine for bursts of speed.The Su-5 (initially I-107) and the conceptually similar Mikoyan-Gurevich I-250 were designed in 1944.The aircraft first flew on 6 April 1945 and underwent limited flight testing.