NASA’s latest attempt to fuel its massive moon rocket for a countdown test was thwarted on Thursday by a dangerous hydrogen leak, the latest in a string of nagging equipment problems.

The launch team had just started loading fuel into the rocket’s center stage when the leak surfaced. This was NASA’s third attempt at a dress rehearsal, a necessary step before a test flight to the moon.

This time, the launch team managed to load some super-cold liquid hydrogen and oxygen into the center stage of the 30-story Space Launch System rocket, but it fell far short of the total amount. Liquid hydrogen is extremely dangerous, with officials noting that systems were checked for leaks prior to testing.

Technicians deliberately left the lower upper stage empty after discovering a bad valve last week. The helium valve inside the upper stage cannot be replaced until the rocket is back in its hangar at the Kennedy Space Center.

Two previous countdown attempts were marred by dogged fans and a large manually operated valve that workers mistakenly left closed on the block last week.

Officials said via Twitter that they were evaluating their next steps.

NASA was targeting June for the launch debut of the 322-foot (98-meter) SLS rocket. The empty Orion capsule on top will be sent on a four to six week mission around the moon and back.

The astronauts will make the second test flight around the moon, planned for 2024. This would be followed as early as 2025 with the first lunar astronaut landing since 1972. NASA plans to announce the crews for these two missions this summer.