Take three for Boeing Starliner crewed launch attempt

Boeing will be hoping the third time's a charm on Wednesday as they try once more to launch astronauts aboard a Starliner capsule bound for the International Space Station.

Liftoff is targeted for 10:52 am (1452 GMT) from the Cape Canaveral Space Force Station in Florida, for a roughly one-week stay at the orbital laboratory.

The last attempt, on Saturday, was dramatically aborted with less than four minutes left of the countdown as the ground launch computer went into an automatic hold.

The problem was later traced to a faulty power supply source connected to the computer, with the malfunctioning unit since replaced.

And a buzzy valve on the United Launch Alliance Atlas V rocket scuppered a previous attempt on May 6, a few hours before launch.

In both cases, astronauts Butch Wilmore and Suni Williams were strapped in and ready to go, only to be forced to return to strict quarantine in their quarters.

The Starliner program has already been beset by years of safety scares and delays, and a successful mission would offer Boeing a much-needed reprieve from the intense safety concerns surrounding its passenger jets.

NASA meanwhile is looking to certify Boeing as a second commercial operator to ferry crews to the ISS -- something Elon Musk's SpaceX has already been doing for the US space agency for four years.

- Embarrassing setbacks -

Both companies received multibillion-dollar contracts in 2014 to develop their crew capsules, following the end of the Space Shuttle program that left the US temporarily reliant on Russian rockets for rides.

Boeing, with its 100-year history, was heavily favored, but its program fell badly behind.

Setbacks ranged from a software bug that put the spaceship on a bad trajectory on its first uncrewed test, to the discovery that the cabin was filled with flammable electrical tape after the second.

While teams worked to replace the faulty rocket valve that postponed the previous launch attempt, a small helium leak located in one of Starliner's thrusters came to light.

Rather than replace the seal, which would require taking the spaceship apart in its factory, NASA and Boeing officials declared it safe enough to fly as is.

When they do fly, Wilmore and Williams will be charged with putting Starliner through the wringer, including taking manual control of the spacecraft on its way to the ISS.

During their stay on the research platform, the crew will carry out more tests, including simulating whether the ship can be used as a safe haven in the event of problems on the orbital outpost.

After undocking, Starliner will re-enter the atmosphere and carry out a parachute and airbag-assisted landing in the western United States.