One of the signs accompanying Apollo 15. It reads
The Apollo series of missions represents the deepest penetration into space by
a human crew to date. It also represents the end of the expendable vehicle as
the primary vehicle for manned access to space by the United States. Instead,
reusable spacecraft, exemplified at present by the Space Shuttle, and in the
future by the National Aero-Space Plane (NASP), expand versatility, keep costs
within reasonable boundaries, and provide a substantial ability to rescue
satellites and astronauts from orbit.