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

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

As precious minutes ticked away, NASA repeatedly stopped and started the fueling 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.

“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.