Technicians also discovered that a valve in the upper stage was defective, and on the third attempt, last Thursday, the test was modified so that only the fuel tanks were filled in the boost stage. But then a hydrogen leak was detected in the so-called secret tail service shaft that connects to the bottom of the rocket, and the test was cleared. The oxygen tank was not half full, and the hydrogen supply had just begun.

Last week, NASA officials said they hoped to fix the hydrogen leak while the rocket was still on the launch pad. But on Monday they said they had changed their minds. Next week, the rocket will be returned to the Vehicle Assembly Building, where technicians can easily access parts of the rocket. This will allow the defective upper stage valve to be replaced.

Meanwhile, an outside supplier that supplies nitrogen gas – used to purge hazardous gases – is upgrading their systems. During two of the training trials, disturbances in the nitrogen supply delayed the countdown.

“The massive lunar rocket is doing really well,” Tom Whitmaier, NASA’s associate deputy administrator for Joint Exploration Systems Development, said during a press conference Monday. “I think we got really smart with this missile. But we have a lot of work to do.”

The Space Launch System, a key component of NASA’s Artemis program to send astronauts to the Moon, is years behind schedule and has a $1 billion budget. With recent problems, Whitmer said, getting the missile ready for launch within a two-week period in early June will be a challenge. There are additional two-week opportunities starting in late June and late July.