The Saturn IB was an uprated version of the Saturn I, which featured a much more powerful second stage, the S-IVB. Unlike the earlier Saturn I, the IB had enough throw weight to launch the Apollo Command/Service Module or Lunar Module into Earth orbit, which made it invaluable for testing the Apollo spacecraft while the larger Saturn V needed to send them to the moon was still being developed. The Saturn IB was later used for manned Skylab flights, and the Apollo-Soyuz Test Project.