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United Launch Alliance teams at Cape Canaveral hoisted a Centaur upper stage on top of an Atlas 5 rocket Friday at launch pad 41, completing the initial build-up of the launch vehicle slated to carry Boeing’s Starliner crew capsule into space in December on an unpiloted test flight to the International Space Station.

The dual-engine Centaur upper stage was connected to the top of the Atlas 5’s first stage Friday inside the Vertical Integration Facility, located just south of Cape Canaveral’s Complex 41 launch pad.

The addition of the Centaur stage completed a busy week of rocket assembly operations at the VIF. On Monday, Nov. 4, ULA teams raised the Atlas 5’s first stage, then added two strap-on solid rocket boosters Nov. 6 and Nov. 7.

The Centaur stage was outfitted with the Atlas 5’s interstage adapter and the Starliner spacecraft’s launch vehicle adapter inside a test cell at ULA’s Delta Operations Center at Cape Canaveral. The off-site integration tasks allow ULA to lift the Centaur upper stage and its associated adapters in one piece, rather than raising the components over multiple days.

The Starliner spacecraft will fly on a unique configuration of ULA’s workhorse Atlas 5 rocket. The Atlas 5 will launch without a payload shroud, and the Centaur upper stage is powered by two RL10 engines supplied by Aerojet Rocketdyne. All Atlas 5 missions to date have flown with the single-engine variant of the Centaur stage.

Dual-engine Centaur stages have flown on earlier versions of the Atlas rocket family, and have launched with the retired Titan rockets. Of the 251 Centaur upper stages flown to date, 166 have launched in the dual-engine configuration.

The Starliner spacecraft will be installed atop the Atlas 5 rocket as soon as this weekend to begin a month-long series of checkouts and tests before the mission’s target launch date of Dec. 17.

Boeing’s Starliner Orbital Flight Test to the International Space Station will demonstrate the capsule’s readiness fo carry astronauts on the next Starliner test flight, scheduled for some time in the first half of 2020.

Read our earlier story for details on the modifications to the Atlas 5 rocket to support Starliner crew launches.

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Follow Stephen Clark on Twitter: @StephenClark1.