The first-stage propellant tanks share the diameter of the Delta IV Common Booster Core but will contain liquid methane and liquid oxygen propellants rather than the Delta IV's liquid hydrogen and liquid oxygen. Blue Origin's BE-4 engine was selected to power Vulcan's first stage in September 2018 after a competition with the Aerojet Rocketdyne's AR1. ULA may use the Sensible Modular Autonomous Return Technology (SMART) which seeks to capture and re-use the BE-4 engines. ULA is working on the 'Sensible Modular Autonomous Return Technology' (SMART) reuse concept. The booster engines, avionics, and thrust structure would be detached as a module from the propellant tanks after booster engine cutoff, with the module descending through the atmosphere under an inflatable heat shield. After parachute deployment, the module would be captured by a helicopter in mid-air. ULA estimated that this would reduce the cost of the first stage propulsion by 90%, and 65% of the total first stage cost.