When Boeing Co. will attempt to launch its Starliner space taxi to the International Space Station remains unclear after its engineers detected a problem with valves on a propulsion system on the vehicle.

The launch of the aerospace giant’s Starliner capsule was scheduled for Aug. 3, but officials decided to postpone the flight because some valves on a Starliner propulsion system weren’t properly configured, according to the National Aeronautics and Space Administration.

Officials had said they could try to fly the vehicle to the space station on Aug. 4, but later nixed that possibility, saying engineers needed time to assess what happened with the valves.

Boeing has faced problems with the Starliner before. A botched effort in late 2019 dented the record of a company that has been at the forefront of U.S. space exploration, including the Apollo missions to the moon. The Starliner is the latest of an array of new rockets, capsules and other vehicles aimed at furthering U.S. ambitions in a new space race to the moon, Mars and beyond.

The Starliner would give the U.S. more options to reach low earth orbit and the space station. U.S. astronauts had to hitch rides on Russian Soyuz spacecraft to get there following the retirement of the Space Shuttle in 2011. NASA opted to outsource a replacement through its Commercial Crew Program and picked Boeing and Space Exploration Technologies Corp., the formal name for Elon Musk’s SpaceX, to provide space taxi services.