Boeing's new Starliner capsule was launched Thursday on a do-over uncrewed test flight bound for the International Space Station, aiming to deliver the company a much-needed success after two years of delays and costly engineering setbacks.

The gumdrop-shaped CST-100 Starliner blasted off shortly before 7 p.m. ET from the Cape Canaveral Space Force Station in Florida, carried atop an Atlas V rocket furnished by the Boeing-Lockheed Martin joint venture United Launch Alliance (ULA).

About 30 minutes after liftoff, the Starliner reached its intended preliminary orbit, after separating from the upper-stage Atlas V rocket and flying on its own power toward a trajectory for later rendezvous with the space station.

It was at that point in Starliner's previous test flight in late 2019 that a software glitch effectively foiled the spacecraft's ability to reach the space station.

The capsule's flight to orbit on Thursday was not without a hitch. Two onboard thrusters, out of a set of 12, failed during Starliner's 45-second "orbital insertion" manoeuvre, NASA and Boeing officials told a post-launch news conference.

However, a backup thruster kicked in, and the manoeuvre was completed, they said, adding that the malfunction, while yet to be explained, should not prevent the spacecraft from reaching its destination or returning safely to Earth.

"The system is designed to be redundant, and it performed like it was supposed to," said Mark Nappi, Boeing's Starliner program manager. "We have a safe vehicle, and we're on our way to the International Space Station."

The capsule was due to arrive at the space station about 24 hours after launch and dock with the research outpost in orbit some 400 kilometres above Earth on Friday evening.

The Boeing craft is to spend four to five days attached to the space station before undocking and flying back to Earth, with a parachute landing cushioned by airbags on the desert floor of White Sands, N.M.

A successful mission will move the long-delayed Starliner a major step closer to providing NASA with a second reliable means of ferrying astronauts to and from the International Space Station (ISS).

Since resuming crewed flights to orbit from American soil in 2020, nine years after the space shuttle program ended, the U.S. space agency has had to rely solely on the Falcon 9 rockets and Crew Dragon capsules flown by Elon Musk's company SpaceX.

Previously the only other option for reaching the orbital laboratory was by hitching rides aboard Russian Soyuz spacecraft.

"Having a backup is important to the country," NASA chief Bill Nelson told Reuters hours before liftoff.

Thursday's launch also comes at a pivotal time for Boeing as the Chicago-based company scrambles to climb out of successive crises in its jetliner business and its space-defence unit. The Starliner program alone has forced Boeing to take $595 million US in charges since the failure of its first uncrewed test flight to orbit in 2019.

The Starliner will not be flying to orbit empty. The capsule was carrying a research mannequin to collect data on crew cabin conditions during the journey, plus 500 pounds of cargo for delivery to the space station's crew — three NASA astronauts, a European Space Agency astronaut from Italy and three Russian cosmonauts.

Two of the U.S. astronauts will be tasked with boarding the capsule during Starliner's stay to take measurements of its interior environment and unload the supplies.

Thursday's launch marks a repeat of a 2019 test mission that failed to achieve a successful rendezvous with the space station because of a flight-software malfunction. Subsequent problems with Starliner's propulsion system, supplied by Aerojet Rocketdyne, led Boeing to scrub an attempt to launch the capsule last summer.

The spacecraft remained grounded for nine more months while the two companies sparred over what caused its fuel valves to stick shut and which firm was responsible for fixing them, as reported by Reuters last week.

Boeing says it has since resolved the glitch with a temporary workaround and plans to redesign the propulsion system's fuel valves system after this week's flight.

Starliner was developed with a $4.5 billion US fixed-price NASA contract to provide the U.S. space agency a second avenue to low-Earth orbit, along with SpaceX.

If the second uncrewed trip to orbit succeeds, Starliner could fly its first team of astronauts in the fall, though NASA officials caution that time frame could get pushed back.

NASA astronauts Butch Wilmore and Mike Fincke had been designated to fly Starliner's maiden crewed mission. But NASA officials, reluctant to tie down two astronauts to a flight whose launch date is uncertain, said Wednesday the mission could end up carrying at least two of any of the four astronauts now training to test-fly Starliner.

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