The rocket was set to lift off on a mission to propel a crew capsule into orbit around the moon, bringing the US a big step closer to putting astronauts back on the lunar surface.

The next launch attempt will not take place until Friday at the earliest and could be off until next month.
The next launch attempt will not take place until Friday at the earliest and could be off until next month. (AA)

A fuel leak and then an engine problem during final liftoff preparations have led NASA to call off the launch of its mighty new moon rocket on a shakedown flight with three test dummies aboard.

As precious minutes ticked away on Monday, NASA repeatedly stopped and started the fuelling of the Space Launch System rocket with nearly 1 million gallons of super-cold hydrogen and oxygen because of a leak of highly explosive hydrogen. 

The leak happened in the same place that saw seepage during a dress rehearsal back in the spring.

Then, NASA ran into new trouble when it was unable to properly chill one of the rocket's four main engines, officials said. Engineers continued working to pinpoint the source of the problem after the launch postponement was announced.

The next launch attempt will not take place until Friday at the earliest and could be off until next month.

“This is a very complicated machine, a very complicated system, and all those things have to work, and you don’t want to light the candle until it’s ready to go," said NASA Administrator Bill Nelson.

The rocket was set to lift off on a mission to propel a crew capsule into orbit around the moon, bringing the US a big step closer to putting astronauts back on the lunar surface for the first time since the Apollo programme ended 50 years ago.

READ MORE: NASA opens sample taken from the Moon 50 years ago

Most powerful rocket ever built

The 98 metre spaceship is the most powerful rocket ever built by NASA, out-muscling even the Saturn V that the Apollo astronauts rode.

As for when NASA might make another liftoff attempt, launch commentator Derrol Nail said engineers were still analysing the engine problem and “we must wait to see what shakes out from their test data.”

No astronauts were inside the rocket's Orion capsule. Instead, the test dummies, fitted with sensors to measure vibration, cosmic radiation and other conditions, were strapped in for the six-week mission, scheduled to end with the capsule's splashdown in the Pacific in October.

Even though no one was on board, thousands of people jammed the coast to see the rocket soar. Vice President Kamala Harris was expected among the VIPs.

The launch, when it happens, will be the first flight in NASA’s 21st-century moon-exploration programme, named Artemis after Apollo’s mythological twin sister.

Assuming the test goes well, astronauts will climb aboard for the second flight and fly around the moon and back as soon as 2024. A two-person lunar landing could follow by the end of 2025.

READ MORE: NASA's rocket rolled out to launch pad ahead of Moon mission

Source: AP