Vulcan will initially fly with a Centaur upper stage. The move allows ULA to begin transitioning its Atlas V launch manifest to Vulcan without disruption. Eventually, the Centaur will be replaced with ACES, the Advanced Cryogenic Evolved Stage. ACES, which runs on liquid hydrogen and liquid oxygen, uses pressure-stabilized "balloon tanks" to lower weight and increase propellant capacity.

"We call them balloon tanks because they are so thin and so lightweight that on Earth, they cannot even support their own mass," Bruno said. "They would collapse without propellant or pressure in them to hold their shape."

ACES could be powered by anywhere from one to four engines, Bruno said. ULA is currently looking at Aerojet Rocketdyne’s RL-10, Blue Origin’s BE-3, or an XCOR-built engine.

A key part of ACES is the Integrated Vehicle Fluids System, which captures gaseous hydrogen and oxygen that would normally be vented into space. ULA said this will greatly increase the stage’s on-orbit lifetime. "The thing that limits the performance of upper stage systems in launch vehicles is time—time in space," Bruno said.

The captured propellant will be used re-pressurize the fuel tanks, generate electrical power and provide thrust for an attitude control system. ULA is touting the capability for its "Distributed Lift" system, in which an upper stage lofts cargo and propellant to low-Earth orbit, where it awaits the launch of a crewed spacecraft.