Complex sentence: Since 1946, NACA (NASA's predecesor), and since 1958, NASA have conducted the X-Plane Program, originally a family of experimental aircraft not intended for production beyond a limited number and built solely for flight research.
Split into simpler sentences: Since 1946, NACA (NASA's predecesor), and since 1958, NASA have conducted the X-Plane Program. The program was originally intended to create a family of experimental aircraft not set for production beyond the limited number of each design built solely for flight research.