NASA astronauts Eric Boe, from left, Barry "Butch" Wilmore and Suni Williams listen as United Launch Alliance engineer Tom Harper discusses aspects of an RL10 engine during a tour of Aerojet Rocketdyne's facility in West Palm Beach, Florida. The engine will be one of two used for the Centaur upper stage during a United Launch Alliance Atlas V mission to launch Boeing's CST-100 Starliner on a flight test carrying a crew. The engine was test-fired as part of acceptance testing to confirm the engine is ready for flight.