SLS on pad
NASA said April 9 it will delay a fueling test of the SLS until April 14 and modify it with "minimal propellant operations" on the upper stage because of a faulty valve. Credit: NASA/Joel Kowsky

WASHINGTON — After discovering a problem with a valve on the Space Launch System’s upper stage, NASA is delaying a countdown rehearsal and fueling test and modifying it to limit fueling of the upper stage.

NASA said in an April 9 blog post that it was delaying the wet dress rehearsal (WDR) for the SLS by another three days. NASA previously planned to restart the test with a “call to stations” for personnel April 9, leading up to the tanking test and practice countdown April 11. NASA said the call to stations is now set for April 12 and tanking on April 14.

The latest delay is linked to a faulty helium check valve in the Interim Cryogenic Propulsion Stage (ICPS), the upper stage of the SLS. The valve prevents helium, used to purge propellant lines and drain propellant, from escaping the rocket.

NASA said April 7 that engineers found a problem maintain helium purge pressure in the ICPS after changing out a regulator in the mobile launch platform. At that time, the agency said it was able to restore normal pressure but was still studying the source of the problem, now linked to the faulty valve in the ICPS itself.

Because that issue, NASA now plans to limit the amount of liquid oxygen and liquid hydrogen propellant loaded into the ICPS during the WDR. NASA said the countdown rehearsal will be modified with “minimal propellant operations” on the ICPS, but didn’t elaborate on how much propellant would be transferred into the upper stage.

“Wet dress rehearsal is an opportunity to refine the countdown procedures and validate critical models and software interfaces,” NASA said in the post. “The modified test will enable engineers to achieve the test objectives critical to launch success.”

The ICPS is based on the second stage of the Delta 4 launch vehicle and uses a single RL10 engine. This particular stage was one of the first components of the overall vehicle that was completed. It was delivered and placed into storage several years ago while the core stage and solid rocket boosters were still being manufactured.

The valve issue is the latest glitch in the WDR process that has delayed the full test of the vehicle by more than a week. NASA scrubbed the first countdown rehearsal April 3 after delays caused by severe weather the night before, followed by malfunctions of fans in the mobile launch platform designed to prevent the buildup of hazardous gases. A second attempt the next day resulted in filling the core stage’s liquid oxygen tank halfway, but several other problems, including a vent valve that failed to open, led controllers to halt the test before the start of liquid hydrogen loading.

The delays in the in the WDR and the need to replace the helium valve in the ICPS now put into doubt the ability of SLS to be ready for a launch window that runs from June 6 to 16. NASA said in its April 9 blog post that it is “confident in the ability to replace the valve” once the SLS returns to the Vehicle Assembly Building after the WDR, but didn’t estimate how long that would take and what additional testing might be required.

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...