A critical multi-day test of NASA’s Space Launch System was called off on Monday due to an issue with a cryogenic propellant pressure vent valve. The space agency seeks to resume the wet dress rehearsal in the near future, saying there’s nothing fundamentally wrong with the gigantic rocket.

Space is hard, as the saying goes, and that’s certainly true when it comes to preparing a never-flown rocket for a mission to the Moon and back. NASA is currently fitting its much-anticipated SLS rocket for launch at Kennedy Space Center in Florida, but the wet dress rehearsal failed to reach the finish line. The rocket was to be fully prepped—including tanks topped with super-cold propellant and the countdown started—but not launched.

“The mega Moon rocket is fine. We’re working to get it into a launch position,” Tom Whitmeyer, deputy associate administrator for Common Exploration Systems Development at NASA, told reporters yesterday during a media teleconference. “We’re just going to have to work our way through it,” he said, adding that the ground teams are “doing a really good job.”

This work is being done in preparation for the uncrewed Artemis 1 mission, the inaugural flight of SLS. The next-gen rocket is a critical component of the Artemis program, which seeks to land a man and woman on the Moon later this decade. NASA is currently targeting a June launch, but that will depend on the results of the yet-to-be completed wet dress rehearsal.

The space agency halted the test on Monday after ground teams were unable to proceed with the loading of cryogenic liquid hydrogen propellant. The problem was eventually traced to a manual vent valve that was left in the closed position, an unfortunate configuration that couldn’t be remedied remotely. In a statement, NASA said “the valve positioning has since been corrected.” The team did manage to load approximately 50% of the required cryogenic liquid oxygen propellant into the core stage, which was subsequently drained.

The misconfigured vent valve, located on the 160 level of the mobile launcher, was hardly the only problem faced by ground teams during the rehearsal, which got underway on Friday, April 1. Four lightning bolts struck the launch pad on Saturday, resulting in a slight delay, but the test came to full stop on Sunday when two fans, which are designed to ventilate the rocket’s 370-foot-tall (113-meter) mobile launcher, glitched out.

Despite this and another problem having to do with the third-party supplier of gaseous nitrogen, NASA resumed the wet dress on Monday. But again, new problems appeared, including a temperature limit issue for the cryogenic liquid oxygen, causing a delay of several hours. Resolved, the rehearsal continued, but the vent valve problem forced the launch director to call it a day at 5:00 p.m. EDT on Monday.

NASA is now preparing for the next wet dress attempt, but it’s stepping aside to allow for the launch of the Axiom Space Ax-1 mission, which is set to blast off from Kennedy Space Center on Friday morning. A date for the resumption of the launch rehearsal hasn’t been announced, but NASA officials said it’ll happen soon. The fully integrated rocket, with the Orion capsule up top, continues to stand on launch pad 39B.

Whitmeyer brushed off the less-than-ideal launch rehearsal, saying the ground teams learned “a couple things” from this “highly choreographed dance” that simply need to be cleaned up. “Sometimes you run into something that you weren’t really expecting,” he told reporters, comparing it to puzzle pieces that don’t quite fit. The “vehicle is doing pretty good,” said Whitmeyer, adding that similar issues were encountered during the SLS Green Run tests at NASA’s Stennis Space Center and during the development of the Space Shuttle.

At the press conference, Mike Sarafin, Artemis mission manager, said the teams have detected “no fundamental design flaws or issues” with the rocket and the problems experienced are best characterized as “nuisance” or “technical issues” that couldn’t be detected during prior testing.

“By putting it all together, you learn where the uncertainties are, and we’re working our way through that,” Sarafin said. “Sometimes you learn that a full system is slightly different than the subscale, but there are no major issues to overcome.” Most of the problems are small or procedural in nature, he said, such as slight adjustments to timing or limits, but “in terms of the rocket, the hardware is fine, the spacecraft is fine—we just gotta get through the test and the test objectives,” he said.

“It was a significant day for us. Our team accomplished quite a bit,” Charlie Blackwell-Thompson, Artemis launch director at NASA’s Kennedy Space Center, told reporters. Indeed, while it’s tempting to focus on the negatives, the team did manage to cross many items off their substantial checklist. These included the configuring of Launch Pad 39B and the mobile launcher, powering up Orion and the rocket in launch configuration, checkouts of the guidance, navigation, and control system, and the draining of propellant after the test, among others.

No date has been set for Artemis 1 or the resumption of the wet dress, but the good news is that the rehearsal won’t have to start from scratch. The clock is currently on hold, and the launch system remains in an ideal configuration, NASA officials said. The main priority moving forward will be to finally fill the core and second stage with cryogenic propellants and stop the countdown at T-10 seconds. When asked if SLS will still launch in June, Sarafin said: “We’re not giving up on it yet.”

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