NASA will try again on Saturday to launch its new moon rocket on a test flight, after engine trouble halted the first countdown this week.

Managers say they are changing fuelling procedures to deal with the issue.

The 98-metre rocket remains on its pad at Kennedy Space Centre, with an empty crew capsule on top. It’s the most powerful rocket NASA has built.

The Space Launch System rocket, or SLS, will attempt to send the capsule around the moon and back.

No one will be aboard, just three test dummies. If successful, it will be the first capsule to fly to the moon since NASA’s Apollo program 50 years ago.

During Monday’s abandoned launch attempt, one of the four main engines in the rocket’s core stage could not be chilled sufficiently before planned ignition moments before liftoff. The three others came up just a little short.

The chilling operation would be conducted half an hour earlier for the try on Saturday (4.17am Sunday AEST), once fuelling was under way at the pad, officials said.

Tweet from @NASA

John Honeycutt, NASA’s program manager for the rocket, said the timing of this engine chill-down was earlier during successful testing last year, and so moving it to sooner might do the trick.

Mr Honeycutt also questioned the integrity of one engine sensor, saying it might have provided inaccurate data.

To change that sensor, he noted, would mean hauling the rocket back into the hangar, which would mean weeks of delay.

The $US4.1 billion ($A5.9 billion) test flight is the opening shot in NASA’s Artemis moon-exploration program, named after the twin sister of Apollo in Greek mythology.

Astronauts could strap in as soon as 2024 for a lap around the moon and actually attempt a lunar landing in 2025.

-AAP