Complex sentence: By 1959, the Boeing X- 20 Dyna-Soar space-glider program became the USAF's preferred means for launching military manned spacecraft into orbit; however, this program was canceled in the early 1960s before an operational vehicle could be built.
Split into simpler sentences: By 1959, the Boeing X- 20 Dyna-Soar space-glider program became the USAF's preferred means for launching military manned spacecraft into orbit. However, this program was canceled in the early 1960s before an operational vehicle could be built.