(CAPE CANAVERAL, FLA) — NASA’s new moon rocket was set to launch Saturday afternoon, however, for the second time this week the launch has been halted after another hazardous leak.

On Monday’s attempt, NASA discovered a bad engine sensor and leaking fuel. It forced the team to move the launch to Saturday.

Early Saturday morning the launch team began fueling it for liftoff on a test flight that must go well before astronauts climb aboard.

The launch team began loading nearly 1 million gallons of fuel into the 322-foot (98-meter) rocket, the most powerful ever built by NASA.

As time went by, an over-pressure alarm sounded and the tanking operation was briefly halted, but no damage occurred and the effort resumed. Minutes later, hydrogen fuel began leaking from the engine section at the bottom of the rocket. Engineers attempted to plug what was believed to be a gap around a seal in the supply line.

The countdown clocks continued ticking toward an afternoon liftoff; NASA had two hours Saturday to get the rocket off. Eventually the launch was halted after three to four hours of futile effort.

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.