NASA started April 3 hoping to get through a practice countdown of the SLS, but a technical problem with its mobile launcher forced a scrub hours later. Credit: NASA/Joel Kowsky

Updated 6:30 p.m. with comments from briefing.

DENVER — NASA called off the first attempt to fuel its Space Launch System rocket and go through a practice countdown April 3, citing a problem with the rocket’s mobile launcher, but hopes to try again April 4.

NASA announced shortly before 12 p.m. Eastern that it was scrubbing its wet dress rehearsal (WDR) for the SLS before crews started loading any liquid hydrogen and liquid oxygen propellant into the vehicle. The agency said fans in the mobile launch platform, required to create positive pressure in enclosed areas of the platform to avoid a buildup of hazardous gases, were not working.

Charlie Blackwell-Thompson, NASA Artemis launch director, said in a call with reporters late in the day that one fan malfunctioned when a breaker tripped. Technicians were sent in to look at it but could not resolve it, so controllers switched to a redundant fan. “It wasn’t long after that we got the call that that fan, too, had experienced an issue,” she said, adding it was a different problem. “We made the decision to stand down to get into a configuration to go troubleshoot that.”

She said it should take four to five hours to resolve the problems. That would allow a second attempt at loading propellants onto the vehicle early April 4, following a schedule similar to what had been planned for April 3. The malfunctioning fans “is the only real issue” teams are examining, she added.

Even before the fan problem NASA was running behind schedule with the test, where the core and upper stages of the SLS are filled with propellants and go through a countdown that stops at approximately T-10 seconds. Severe thunderstorms April 2 delayed WDR activities at Launch Complex 39B, including several lightning strikes on protective towers at the pad.

There were four lightning strikes, including one particularly powerful one that hit a catenary between two lightning protection towers, said Mike Sarafin, NASA Artemis mission manager, on the call. All four were cleared before the the decision early April 3 to proceed with the WDR.

Blackwell-Thompson said the fan problems don’t appear to be linked to the lightning strikes. The fans had been running continuously at a lower speed since the launch platform arrived at the pad, and moved into a different mode for hazardous operations for several hours before the problem.

If NASA doesn’t complete the WDR on April 4, future plans will depend on how far into the test they get. Sarafin said if they get past a “certain point” in loading propellants, NASA would have to stand down for a couple days to replenish stocks of liquid hydrogen. If the test is scrubbed before that point, NASA may seek to try again April 5, but after that they run into “a number of range conflicts” like the launch of a Falcon 9 on the Ax-1 crewed mission to the International Space Station from neighboring Launch Complex 39A.

“We’ve got Monday [April 4] in the bag for sure as as attempt. Tuesday is a definite possibility but requires some additional coordination,” he said. “After that, we’ll have to play it day by day.”

NASA started WDR activities April 3 about one hour behind schedule, according to tweets posted by Jeremy Parsons, deputy manager of the Exploration Ground Systems program at KSC. Tweets and blog posts were the primary means NASA used to provide updates about the test after claiming that export control restrictions prevented it from providing audio from the launch control center or any other commentary.

The WDR is the final major test before the inaugural launch of the SLS on the uncrewed Artemis-1 mission. NASA officials said at a March 29 briefing that, if the test goes well, the agency could be ready in about a week to set a launch date for the mission, likely no earlier than June.

The WDR, while intended to simulate a countdown, offers greater flexibility to correct problems than what crews would have during an actual countdown. “We are fortunate to have this flexibility during today’s test, but would operate differently on launch day,” Parsons tweeted shortly before the test was scrubbed for the day.

Jeff Foust writes about space policy, commercial space, and related topics for SpaceNews. He earned a Ph.D. in planetary sciences from the Massachusetts Institute of Technology and a bachelor’s degree with honors in geophysics and planetary science...