WASHINGTON, June 26 (Reuters) - The Boeing-Lockheed joint venture United Launch Alliance's new Vulcan rocket is expected to launch its second mission in September to clinch a long-sought Pentagon certification, after booting the rocket's delayed payload by Sierra Space, ULA's CEO said on Wednesday.

Vulcan, ULA's next-generation rocket that will compete with rockets from Elon Musk's SpaceX, needs to fly a second mission to prove to the Pentagon it can launch national security payloads. However, its original payload - the first Dream Chaser cargo spaceplane from Sierra Space - has been caught up in development.

ULA CEO Tory Bruno told reporters that a dummy payload will instead fly on the mission, while Sierra Space is open to fly on a future Vulcan flight. (Reporting by Joey Roulette; Editing by Franklin Paul and Emelia Sithole-Matarise)