NASA to restart fueling test of SLS rocket, with key modifications

NASA will resume its efforts to complete a key fueling test of the Space Launch System rocket on Tuesday.

The space agency has decided to modify this test, however, due to a problem with a check valve on the rocket's upper stage that leads to a pressurized helium bottle. The valve was found to be stuck last week and will need to be replaced.

With the valve in this position, NASA does not feel it would be safe to load the upper stage with cryogenic oxygen and hydrogen during the "wet dress" test as originally planned. Therefore, Thursday's test will fuel only the core stage—the largest and least-proven part of the rocket—during tanking operations. As part of this test, the launch system will be brought into a terminal countdown before cutting off at T-10 seconds.

NASA plans to collect a trove of data from this test, and this information will inform the agency's plans going forward, officials said during a media teleconference with reporters on Monday. About 10 days after the test, NASA will roll the SLS rocket back into the Vehicle Assembly Building. There, technicians will remove the check valve, which is about 8 cm long, and inspect the part to understand why it malfunctioned. It can then be replaced, which should be a relatively simple operation, said John Blevins, the SLS chief engineer.

A path forward

"We’re very comfortable with the path forward," said Tom Whitmeyer, deputy associate administrator for common exploration systems development at the NASA Headquarters in Washington. "We think it’s a great path forward."

The officials on Monday's teleconference seemed confident that they could get a lot of good data from Thursday's test. For example, said launch director Charlie Blackwell-Thompson, during the terminal countdown from T-10 minutes to T-10 seconds, there are almost 25 "critical events" in the rocket's test objectives. Just two of those are specific to the upper stage, she said.

"There is a significant amount of testing and data and risk buy-down you get relative to the core stage, to the ground systems, and relative to the boosters," she said.

The upper stage, known as the Interim Cryogenic Propulsion Stage, was manufactured by United Launch Alliance and was delivered to Kennedy Space Center about four years ago. However, the chief SLS engineer, John Blevins, said he does not believe the valve issue was due to any shelf-life issues. The check valve in question, he said, is rated to function for 20 years or longer.

“Two days” becomes “two weeks”

The wet dress test was originally supposed to last two days when it began on Friday, April 1. But partly due to a problem with the fans on the Mobile Launch Tower, the first attempt at fueling the rocket had to be scrubbed on April 4. A second attempt last week saw NASA fill the core stage about halfway with liquid oxygen before the agency discovered that a core stage "vent valve," which is manually adjusted, was errantly left in the wrong position. Then NASA discovered the check valve issue on the upper stage.

Now, teams of NASA employees and contractors will be called to their stations again on Tuesday evening to prepare the vehicle and ground systems for propellant loading for the third time. The actual fueling of the vehicle is scheduled to commence on Thursday morning, with the terminal countdown reached at 2:40 pm ET (18:40 UTC). That precise timeline, of course, assumes no further delays, which seems unlikely for a two-day test that has expanded to two weeks.

Asked to assess next steps after this test in terms of readying the SLS rocket and Orion spacecraft for an uncrewed demonstration flight later this summer, NASA officials did not want to look too far beyond the conclusion of this core stage tanking. They declined to say whether the rocket might be subjected to a second wet dress test for the entire vehicle to ensure the flight readiness of the upper stage and its ground systems.

"I don't think we're ready to really state, one way or the other, what we think the next step is going to look like," Whitmeyer said. "I think we really do need to do the test Thursday and then look at the data."