CAPE CANAVERAL, Fla. (AP) — NASA called off the launch of its mighty new moon rocket on its debut flight with three test dummies aboard Monday after a last-minute cascade of problems culminating in unexplained trouble related to an engine.

The next launch attempt will not take place until Friday at the earliest and could be delayed until mid-September or later.

The mission will be the first flight in NASA’s Artemis project, a quest to put astronauts back on the moon for the first time since the Apollo program ended 50 years ago.

As precious minutes ticked away Monday morning, NASA repeatedly stopped and started the fueling of the Space Launch System rocket because of a leak of highly explosive hydrogen, eventually succeeding in reducing the seepage. The leak happened in the same place that saw seepage during a dress rehearsal in the spring.

The fueling already was running nearly an hour late because of thunderstorms off Florida’s Kennedy Space Center.

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

Mission manager Mike Sarafin said the fault did not appear to be with the engine itself but with the plumbing leading to it.