COLORADO SPRINGS, Colo. – United Launch Alliance's next-generation rocket, the future Vulcan Aces, will be able to refuel in space, opening the door to in-orbital assembly and other complex space operations.

Most aerospace companies focus on reusing the first-stage rocket, ULA president and chief executive Tory Bruno said in a Wednesday interview at the Space Foundation's annual National Space Symposium. SpaceX, for example, recently landed a first-stage Falcon 9 rocket on a barge in the Atlantic Ocean after it successfully launched a payload into space.

The concept of a reusable first-stage rocket, if it proves reliable, could transform the space launch market, potentially driving down the cost of space launch to unprecedented levels, analysts contend.

But ULA is taking a different approach. Bruno wants to reuse the upper stage, which — unlike the first stage, which falls to the ground before it reaches space — is orbital.

"We had the idea, well, why do you have to bring it back to Earth just to reuse it?" Bruno said "Why don't we just leave it in space?"

Atlas V's upper stage, dubbed Centaur, can operate for about seven or eight hours in space, which allows ULA to directly inject a spacecraft into geosynchronous orbit. The next-generation Vulcan's upper-stage rocket, dubbed Aces, will be able to operate for seven or eight days using its initial loaded propellant, Bruno said.

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This will allow the Aces to perform multiple missions in space, such as rendezvous and proximity, and even refueling operations to further extend its life, Bruno stressed. ULA plans to build up a fleet of upper-stage Aces in space that can refuel each other using excess fuel, Bruno said.

"So that means that you can do multiple missions, and you can do rendezvous and now it's also practical to refuel it in space and just use it forever," Bruno said.

This capability opens the door to a variety of complex space missions, such as "distributed lift," or taking spacecraft up in pieces and assembling them in space, he said.

"It will really change the way we go to space," Bruno said. "It's going make it practical to build giant structures and infrastructure in space."