NASA called off plans to load its new Space Launch System moon rocket with more than 750,000 gallons of liquid oxygen and hydrogen fuel Sunday because of trouble with launch pad support equipment need to prevent buildups of hazardous gas. The team plans to try again Monday.

The giant 322-foot-tall SLS rocket was hauled to launch pad 39B at the Kennedy Space Center on March 18 for a dress-rehearsal countdown to clear the way for its maiden blastoff early this summer to send an unpiloted Orion crew capsule on a flight beyond the moon and back.

The countdown started Friday afternoon and reached the T-minus 6-hour point early Sunday when engineers had planned to begin pumping 196,000 gallons liquid oxygen and 537,000 gallons of liquid hydrogen into the rocket’s core stage. Another 22,000 gallons of cryogenic oxygen and hydrogen fuel was to be loaded into its upper stage.

NASA’s Space Launch System moon rocket atop pad 39B at the Kennedy Space Center early Sunday as engineers were preparing to load it with fuel for a dress-rehearsal countdown. The test was delayed by trouble with ground equipment.

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Once fully loaded, the team planned two terminal countdown practice runs, one down to about T-minus 33 seconds and another down to less than 10 seconds to test recycle procedures and to make sure the system will be ready for the real thing.

But the countdown was held up, first to give engineers time to get back on schedule after delays caused by stormy weekend weather and then by trouble with fans used to pressurize the rocket’s mobile launch platform. The fans ensure positive pressure inside the structure to prevent the buildup of any free hydrogen during the fueling operation.

After several hours of troubleshooting at the pad and in the launch control center, mission managers decided to call off the test for the day to give engineers time to correct the problem.

“Fans are needed to provide positive pressure to the enclosed areas within the mobile launcher (to) keep out hazardous gases,” NASA said in a blog post. “Technicians are unable to safely proceed with loading the propellants into the rocket’s core stage and (upper) stage without this capability.”

Former shuttle program manager Wayne Hale said he was not surprised problems cropped up during NASA’s first attempt to get through a complex countdown with the largest, most powerful rocket the agency has ever built.

“Just the sort of ground system issue that I expected,” he tweeted. “Short term delay. First time through these sort of issues are expected.”

NASA’s Exploration Ground Systems group tweeted: “This is the first time we are performing the integrated cryo operations at the pad and teams are taking lessons as they progress through countdown procedures. The events happening now represent a great training and learning opportunity for the team prior to launch.”

Equipped with two extended solid-fuel boosters and a core stage powered by four modified space shuttle main engines, the SLS rocket will tip the scales at 5.75 million pounds at liftoff, generating ground-shaking 8.8 million pounds of thrust, 1.3 million pounds more push than NASAs legendary Saturn 5 rocket.

Development of the “mega rocket” is years behind schedule and billion over budget, but NASA hopes to kick start its Artemis moon program by launching the first SLS early this summer, followed by a piloted flight around the moon in 2024. The first in a series of landings by NASA astronauts is planned for the 2025 timeframe or shortly thereafter.