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Wikipedia dicit:

Vulcan is a next generation heavy-lift launch vehicle under development by the United Launch Alliance (ULA) to meet the demands of the United States Air Force’s National Security Space Launch (NSSL) competition and launch program.

Vulcan is ULA’s first launch vehicle design, adapting and evolving various technologies previously developed for the Atlas V and Delta IV rockets of the USAF’s EELV program. The first stage propellant tanks share the diameter of the Delta IV Common Booster Core, but will contain liquid methane and liquid oxygen propellants instead of the Delta IV’s liquid hydrogen and liquid oxygen. Vulcan’s upper stage is the Centaur V, an upgraded variant of the Common Centaur/Centaur III currently used on the Atlas V. A lengthened version of the Centaur V will be used on the Vulcan Centaur Heavy. Current plans call for the Centaur V to be eventually upgraded with Integrated Vehicle Fluids technology to become the Advanced Cryogenic Evolved Stage (ACES). Vulcan is intended to undergo the human-rating certification process to allow the launch of crew.

The Vulcan booster will have a 5.4 m (18 ft) outer diameter to support the methane fuel burned by the Blue Origin BE-4 engines. The BE-4 was selected to power Vulcan’s first stage in September 2018 after a competition with the Aerojet Rocketdyne AR1. Zero to six Graphite-Epoxy Motor-63XL (GEM-63XL) solid rocket boosters (SRB)s can be attached to the first stage in pairs, providing additional thrust during the first part of the flight and allowing the six-SRB Vulcan Centaur Heavy to launch a higher mass payload than the most capable Atlas V 551 or Delta IV Heavy. Vulcan will have a 5.4 m diameter fairing available in two lengths. The longer fairing is 21 m long, with a volume of 317 m3.

Video Credit: ULA