A Saturn IB rocket with an Apollo spacecraft was positioned on the launch pad on December 5, 1973 as a contingency for Skylab 4 and remained there until the Skylab 4 crew returned to Earth.
The Saturn IB and Apollo used as the Skylab 4 contingency were returned to the Vehicle Assembly Building after the mission and later used for the Apollo-Soyuz Test Project.
Three crewed Skylab missions were planned as Skylab 2, Skylab 3, and Skylab 4, with the SL-R plan using the next vehicle in line as the rescue vehicle.
Three crewed Skylab missions were planned as Skylab 2, Skylab 3, and Skylab 4 with the next vehicle in line designated to serve as the rescue vehicle under the SL-R plan.
Skylab 4 launched on November 16, 1973 on an 84-day mission.
For Skylab 4 NASA had a Saturn IB rocket with an Apollo spacecraft positioned on the launch pad on December 5, 1973 until the Skylab 4 crew returned to Earth.