NASA said that even without fueling the SLS upper stage in the modified version of a countdown rehearsal, they will still gain most of the information they're seeking from other systems. Credit: NASA/Ben Smegelsky

WASHINGTON — NASA officials defended their decision to proceed with a modified version of a countdown rehearsal for the Space Launch System that does not involve fueling the rocket’s upper stage, saying they’ll wait until after the test to determine the next steps toward launch.

During an April 11 call with reporters, NASA SLS managers said they were ready to proceed with a third attempt to load the SLS at Launch Complex 39B with liquid hydrogen and liquid oxygen propellants and go through a countdown that stops just before ignition of the core stage’s four RS-25 engines. That fueling and terminal countdown is scheduled for the afternoon of April 14.

NASA announced April 9 it was modifying the plan for the third attempt after discovering a faulty helium check valve in the rocket’s upper stage, the Interim Cryogenic Propulsion Stage (ICPS). The agency announced it will perform only “minimal propellant operations” on that upper stage during the wet dress rehearsal, and will not completely fill the stage with liquid hydrogen and liquid oxygen as originally planned.

“We believe that this is the best option moving forward,” said Charlie Blackwell-Thompson, NASA Artemis launch director, during the call. “We believe that we’ll be able to meet the majority of our test objectives and provide us with a reasonably good set of data prior to rollback” to the Vehicle Assembly Building (VAB).

She and other agency officials argued that they will still get valuable data even though the test falls short of the wet dress rehearsal originally planned, where the core and upper stages are both fully loaded. Blackwell-Thompson said there are about 25 “critical events” in the terminal phase of the countdown, from T-10 minutes to the cutoff just under T-10 seconds. Only two, she said, were specific to ICPS.

NASA plans to wait until after the test is complete to determine what to do next in preparing the SLS for launch. “We’ll take a look at where we’re at and ask ourselves what’s the right next step,” said Tom Whitmeyer, deputy associate administrator for common exploration systems development at NASA Headquarters. “There’s a value of taking a step-by-step approach.”

There will be a small amount of propellant that will flow into the ICPS during the test, said John Blevins, NASA SLS chief engineer, enough to cool systems to cryogenic temperatures. That will stop just before “fast fill” of the propellant tanks would begin.

“That’s a really important piece of data to get,” said Blackwell-Thompson. “Historically, when you see leaks, especially with something small like hydrogen, it is usually when you get down to cryogenic temperatures.”

Blevins said it was too soon to say if NASA will need to do another wet dress rehearsal that includes tanking the ICPS, explaining that it will depend on data collected from other systems from the upcoming test. “If that missing data were the only missing data, and every model acted perfectly, I think we would look long and hard at whether we needed to or not,” he said, adding that it wasn’t possible to do a tanking test of the ICPS alone without also loading the core stage.

Once the SLS is rolled back to the VAB, he said replacing the faulty helium check valve should be straightforward. “Easy to get to, easy to change out.”

He declined to speculate on why the valve, which he described as a “high-reliability part,” malfunctioned. “It is somewhat of an unexpected outcome, an unexpected failure,” he said.

NASA has also declined to speculate on what these issues mean for the schedule of the Artemis 1 launch itself. That mission has launch windows of June 6 to 16, June 29 to July 17 and July 26 to Aug. 9, defined by orbital mechanics of the Earth and moon and other mission constraints. Whitmeyer said there are no issues with the SLS or Orion spacecraft that would prevent a launch “well into the fall” if necessary.

That margin may be needed since agency officials acknowledged there may be more problems yet to be found as NASA attempts another countdown test. “I can say that these will probably not be the last challenges we’ll encounter,” said Mike Sarafin, Artemis mission manager, “but I’m confident we have the right team in place and the ability to rally around those problems.”

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...