Boeing postponed its Starliner capsule's second flight to the International Space Station on Tuesday, days after an earlier launch was delayed when the orbital outpost experienced difficulties caused by its new Russian module.
"We're confirming today's Starliner Orbital Flight Test-2 launch is scrubbed," Boeing said on Twitter.
During pre-flight preparations, Boeing engineers discovered that valves in the spacecraft's propulsion system were in "unexpected" positions, the company said.
“We’re disappointed with today’s outcome and the need to reschedule our Starliner launch,” said John Vollmer, vice president and program manager of Boeing’s Commercial Crew Program.
“Human spaceflight is a complex, precise and unforgiving endeavour, and Boeing and Nasa teams will take the time they need to ensure the safety and integrity of the spacecraft and the achievement of our mission objectives.”
The highly-anticipated test flight is a significant milestone for Boeing’s troubled human spaceflight programme.
A previous uncrewed Starliner orbital flight in 2019 ended with a nearly catastrophic failure of the spacecraft to dock with the space station.
The US aviation company, which has had a hand in almost every Nasa mission since the start of space exploration, is competing with Elon Musk’s SpaceX in Nasa's Commercial Crew Programme.
SpaceX has now delivered 10 astronauts to the ISS, with Boeing yet to fly its capsule with a human crew on board.
Tuesday's postponement was the second in a matter of days. The capsule was sitting on top of a United Launch Alliance Atlas rocket ready to blast off on Thursday last week when the launch was halted.
Cosmonauts on the space station told mission control in Moscow they were having difficulties with the newly arrived Nauka module hours after it had docked with the ISS.
"Nasa and Boeing have decided to stand down from Friday's launch attempt of the agency's Orbital Flight Test-2 mission," Nasa said in a statement posted online, and added that launch teams were assessing the next available launch opportunity.
"The move allows the International Space Station team time to continue checkouts of the newly arrived Roscosmos Nauka module to ensure the station will be ready for Starliner's arrival," the US space agency added.