After the Space Shuttle retired, Pratt & Whitney Rocketdyne and Dynetics entered the "advanced booster competition" for NASA's next human rated vehicle, the Space Launch System (SLS), with a booster design known as "Pyrios", which would use two more advanced F-1B booster engines derived from the Rocketdyne F-1 LOX/RP-1 engine that powered the first stage of the Saturn V vehicle in the Apollo program. In 2012, it was determined that if the dual-engined Pyrios booster was selected for the SLS Block 2, the payload could be 150 metric tons (t) to Low Earth Orbit, 20 t more than the congressional minimum requirement of 130 t to LEO for SLS Block 2. In 2013, it was reported that in comparison to the F-1 engine, the F-1B engine was to have improved efficiency, be more cost effective and have fewer engine parts. Each F-1B was to produce of thrust at sea level, an increase over the of thrust of the initial F-1 engine.