sentences:  The first rocket plane was the Lippisch Ente, and later examples include the Messerschmitt Me 163 rocket-powered interceptor.The American series of research aircraft starting with the Bell X-1 in 1946 up to the North American X-15 spent more time flying unpowered than under power.In the 1960s research was also done on unpowered lifting bodies and on the X-20 Dyna-Soar project, but although the X20 was cancelled, this research eventually led to the Space Shuttle.