Skylab B was a proposed second US space station similar to Skylab that was planned to be launched by NASA for different purposes, mostly involving the Apollo–Soyuz Test Project, but was canceled due to lack of funding. Two Skylab modules were built in 1970 by McDonnell Douglas for the Skylab program, originally the Apollo Applications Program. The first was launched in 1973 and the other put in storage, while NASA considered how to use the remaining assets from Apollo.
One considered option was to use Saturn V SA-515 to launch the backup Skylab station into orbit sometime between January 1975 and April 1976. That way, it could expand the Apollo-Soyuz mission by 56–90 days.
Further proposals were made for an International Skylab, launched using Saturn V SA-514. This station would have been serviced by Apollo, Soyuz and later by the Space Shuttle.
Some uses considered for the second Skylab module included putting it into a rotation mode where it could generate artificial gravity and a plan to celebrate the 1976 United States Bicentennial with the launch of two Soviet Soyuz missions to the back-up Skylab.[citation needed ]