NASA's second attempt to fuel its Artemis 1 moon mission megarocket hit another snag on Monday (April 4) due to a stuck valve on ground equipment.

A jammed vent valve high up on the mobile launcher structure supporting the Artemis 1's Space Launch System rocket at Launch Pad 39B of NASA's Kennedy Space Center forced NASA to scrub the Artemis 1 test after fueling began on Monday, agency officials said.

"Due [to] the vent valve issue, the launch director has called off the test for the day," Jeremy Parsons, NASA's deputy director for ground systems, wrote in a Twitter update after the scrub. "The team is preparing to offload LOX (liquid oxygen) and will begin discussing how quickly the vehicle can be turned around for the next attempt."

The stuck vent valve was on the 160-foot (49 meters) level of the mobile launcher, which serves as both a gantry and launch platform for the SLS, according to Parsons.

Monday's fueling attempt was NASA's second try to fill the core stage of Artemis 1's 322-foot-tall (98 m) SLS rocket with 700,000 gallons (2.6 million liters) of super-chilled liquid oxygen and liquid hydrogen propellant in what the agency calls a "wet dress rehearsal." The test, which began April 1, features a full launch countdown rehearsal, including the fueling process.

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NASA tried to fuel the Artemis 1 moon rocket on Sunday (April 3) but stopped before propellant loading began due to a problem with pressurization on the mobile launcher that keeps hazardous gases out of enclosed areas where technicians work. On Monday, technicians had loaded about 50% of the liquid oxygen needed for the fueling test before standing down for the day, Parsons wrote on Twitter.

Monday's test initially aimed to simulate a launch countdown that would end at 2:40 p.m. EDT (11840 GMT), but delays related to the rocket's nitrogen gas supplier stalled that work. Once that issue was solved, NASA was aiming for a simulated launch time of 6:02 p.m. EDT (2202 GMT) before the stuck valve prompted the scrub.

It is unclear if NASA will be able to recycle for a third fueling attempt on Tuesday (April 5) or have to stand down to replenish its propellant supplies and allow its pad crews and launch controllers time to rest. Meanwhile, a private mission to the International Space Station is waiting in the wings for its time to fly.

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SpaceX is aiming to launch four private astronauts to the International Space Station on the Ax-1 mission for the Houston company Axiom Space. A SpaceX Falcon 9 rocket will launch the mission from Pad 39A, which is located near Artemis 1's Pad 39B.

SpaceX and Axiom Space originally planned to launch the Ax-1 mission on April 3, but pushed it back to April 6 to allow NASA time for the Artemis 1 wet dress rehearsal. After NASA's Artemis 1 fueling delays on Sunday, SpaceX pushed the launch back again, this time to Friday (April 8).

Whether the Ax-1 mission stays on April 8 or is delayed again depends on NASA's plans for the Artemis 1 fueling test. In yet another launch traffic wrinkle, SpaceX is also preparing to launch four more astronauts to the space station for NASA on April 20 as part its Crew-4 mission. That flight will launch three NASA astronauts and one European Space Agency astronaut to the orbiting lab.

But Crew-4 must wait for the Ax-1 mission to launch (because both launch from Pad 39A), which in turn is awaiting NASA's completion of the Artemis 1 wet dress rehearsal. As it is, Crew-4 is currently scheduled to launch on April 20 and has already seen its own schedule delays.

Email Tariq Malik at [email protected] or follow him @tariqjmalik. Follow us @Spacedotcom, Facebook and Instagram.