Boeing's company’s Starliner capsule was due to rocket away at midday with a pair of test pilots to the International Space Station for a weeklong stay.
The test drive should have happened years ago. But problems kept piling up, most recently a leak that went unnoticed until the first launch attempt with a crew in early May.
NASA wants a backup to SpaceX, which has been flying astronauts for four years.
United Launch Alliance's Atlas V rocket is providing the lift from Cape Canaveral Space Force Station.
