text: As of January 2016, full-engine testing of the BE-4 was planned to begin prior to the end of 2016, while ULA was designing two versions of the Vulcan first stage, one using the BE-4 with a 5.4 m (18 ft) outer diameter to support the less-dense methane fuel and an AR1 design with the same 3.81 m (12.5 ft) diameter as Atlas V for the denser RP-1 (kerosene) fuel.