NASA's launch of its new moon rocket has been plagued by problems once more after it sprang a hazardous leak on Saturday.
The US space agency's launch team had begun fuelling the rocket when the leak occurred, calling into question the the likelihood of lift off for the test flight that must go well before astronauts can climb aboard.
The 98m Space Launch System rocket was set to lift off on Monday with three test dummies aboard on its first flight.
But the attempt was thwarted by a bad engine sensor and leaking fuel.
For the second time this week on Saturday, the launch team began loading nearly 1 million gallons of fuel into the 98m Space Launch System rocket, the most powerful ever built by NASA.
As the sun rose, an over-pressure alarm sounded and the tanking operation was briefly halted, but no damage occurred and the effort resumed, NASA's Launch Control reported.
But minutes later, hydrogen fuel began leaking from the engine section at the bottom of the rocket.
NASA halted the operation, while engineers scrambled to plug what was believed to be a gap around a seal.
The countdown clocks continued ticking toward an afternoon lift off, leaving NASA with just two hours to get the rocket off.
NASA wants to send the crew capsule atop the rocket around the moon, pushing it to the limit before astronauts get on the next flight.
If the five-week demo with test dummies succeeds, astronauts could fly around the moon in 2024 and land on it in 2025. People last walked on the moon 50 years ago.
Forecasters expected generally favourable weather at Kennedy Space Centre, especially toward the end of the two-hour afternoon launch window.
At the same time, the rocket's lead engineers expressed confidence in the tightened-up fuel lines and procedure changes.
On Monday, a sensor indicated one of the four engines was too warm, but engineers later verified it actually was cold enough.
The launch team planned to ignore the faulty sensor this time around and rely on other instruments to ensure each main engine was properly chilled.
Before igniting, the main engines need to be as frigid as the liquid hydrogen fuel flowing into them at -250C.
If not, the resulting damage could lead to an abrupt engine shutdown and aborted flight.
Mission managers accepted the additional risk posed by the engine issue as well as a separate problem: cracks in the rocket's insulating foam. But they acknowledged other problems could prompt yet another delay.
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The potential for more delays didn't stop thousands from jamming the coast to see the Space Launch System rocket soar on Saturday.
Local authorities expected massive crowds because of the long Labour Day holiday weekend in the US.
The $4.1 billion test flight is the first step in NASA's Artemis program of renewed lunar exploration, named after the twin sister of Apollo in Greek mythology.
Twelve astronauts walked on the moon during NASA’s Apollo program, the last time in 1972.
Artemis - years behind schedule and billions over budget - aims to establish a sustained human presence on the moon, with crews eventually spending weeks at a time there. It's considered a training ground for Mars.