Boeing's first crewed Starliner flight launches amid competition with Space X

Boeing’s new Starliner astronaut capsule has launched from Florida in a much-delayed first test flight carrying a crew, a milestone in the aerospace giant’s ambitions to step up its competition with Elon Musk’s SpaceX.

The CST-100 Starliner, with two astronauts aboard, lifted off from the Cape Canaveral Space Force Station in Florida, strapped to an Atlas V rocket furnished and flown by the Boeing-Lockheed Martin joint venture United Launch Alliance (ULA).

The gumdrop-shaped capsule and its crew headed for a rendezvous with the International Space Station (ISS), following years of technical problems, delays and a 2022 test mission to the orbital laboratory without astronauts aboard.

On Wednesday, the Atlas V’s engines thundered to life in flaming clouds of exhaust and coolant-water vapour as the spacecraft roared off its launch pad into sunny skies from Florida’s Atlantic Coast.

The rocket’s upper stage separated from its lower section about four minutes into flight, followed by Starliner’s separation from the second stage.

Now on its own, the spacecraft fired its onboard thrusters to start pushing itself into orbit, mission managers said, a process that will kick off its 24-hour catch-up journey with the ISS, the orbiting research outpost 400km above earth.

Starliner will need to execute precise manoeuvres to dock with the ISS as planned on Thursday, demonstrate it can stay docked for about eight days, then safely return the two astronauts - Barry “Butch” Wilmore and Sunita “Suni” Williams - to earth.

Boeing intends for Starliner to compete with SpaceX’s Crew Dragon capsule, which since 2020 has been NASA’s only vehicle for sending ISS crew members to orbit from US soil.

Boeing wrote on social media that Starliner “has reached a safe, stable orbit on its Crew Flight Test” and is headed to the space station.

“Starliner ascends to the heavens,” NASA chief Bill Nelson wrote on social media, congratulating Boeing, ULA and the US space agency.

Nelson called the launch “a milestone achievement for the future of space flight. Butch and Suni - safe travels through the stars. See you back home.”

Last-minute issues had nixed the Starliner’s first two crewed launch attempts.

A May 6 countdown was halted two hours before lift-off over three issues that required weeks of extra scrutiny.

Another try last Saturday was halted less than four minutes before lift-off because of a glitch with a launchpad computer.

The inaugural crew for the seven-seat Starliner includes two veteran NASA astronauts:

Wilmore, 61, a retired US navy captain and fighter pilot, and Williams, 58, a former navy helicopter test pilot with experience flying more than 30 different aircraft.

They have spent a combined 500 days in space over the course of two ISS missions each.

Wilmore is the designated commander for the flight, with Williams in the pilot seat.

They are due to spend about a week at the ISS before returning to earth.

Boeing, with its commercial aeroplane operations, rocked by a series of crises involving its 737 MAX jetliners, needs a win in space for its Starliner venture, already several years behind schedule and more than $US1.5 billion ($A2.3 billion) over budget.

The longtime NASA contractor has built modules for the decades-old ISS and rockets designed to loft astronauts toward the moon.

But Boeing had never before built its own operational spacecraft, a feat complicated by years of software issues, technical glitches and management shakeups on the Starliner program.