NASA's deep space exploration rocket is set to fire all four rockets of its behemoth core stage for the first time in a "once-in-a-generation" test.
The test marks a crucial point for the years-delayed US government project, which is facing mounting pressure from emerging private sector technology.
The Boeing-built Space Launch System (SLS) is known as a 'megarocket' and is being developed with the aim of putting Americans back on the moon within a few years.
The hot fire test for SLS is expected to begin at 5pm local time on Saturday at NASA's Stennis Space Centre in Mississippi, capping a nearly year-long campaign to validate the rocket's design.
It is seen as a vital step before a debut unmanned launch later this year under NASA's Artemis program, the Trump administration's push to land humans on the moon again by 2024.
Saturday's test will see the rocket's four Aerojet Rocketdyne RS-25 engines ignite for roughly eight minutes, generating 1.6 million pounds of thrust and consuming 700,000 gallons of propellants on a test stand to simulate internal conditions of liftoff.
"This is a once-in-a-generation kind of test," Jim Maser, Aerojet Rocketdyne's Senior Vice President of Space, told Reuters. "This will be the first time four RS-25s fire together at the same time."
The expendable super heavy-lift SLS is three years behind schedule and nearly $US3 billion ($A3.9 billion) over budget.
Critics have long argued for NASA to transition from the rocket's shuttle-era core technologies, which have launch costs of $1 billion or more per mission, to newer commercial alternatives promising lower costs.
By comparison, it costs as little as $90 million to fly the massive but less powerful Falcon Heavy from Elon Musk's SpaceX, and some $350 million per launch for United Launch Alliance's legacy Delta IV Heavy.
While newer, more reusable rockets from both companies promise heavier lift than Falcon Heavy or Delta IV Heavy potentially at lower cost, SLS backers argue it would take two or more launches on those rockets to launch what SLS could carry in a single mission.
NASA and Boeing have faced hiccups in the project that included five tropical storms and a hurricane that swept over Stennis, as well as a three-month closure after some engineers tested positive for the coronavirus in March.
Australian Associated Press