NASA will resume its efforts to conduct a major fuel test of the Space Launch System rocket on Tuesday.
However, the space agency has decided to change this test due to a problem with a non-return valve on the upper stage of the rocket leading to a pressurized helium bottle. The valve turned out to be stuck last week and needs to be replaced.
With the valve in this position, NASA does not feel it would be safe to load the top step with cryogenic oxygen and hydrogen during the “wet dress” test as originally planned. Therefore, Thursday’s test will only burn the core stage – the largest and least proven part of the rocket – during tank operations. As part of this test, the firing system will be brought into a terminal countdown before being interrupted at T-10 seconds.
NASA plans to collect a lot of data from this test, and that 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 failed. It can then be replaced, which should be a relatively simple operation, said John Blevins, SLS chief engineer.
A way forward
“We are very comfortable with the way forward,” said Tom Whitmeyer, Deputy Associate Administrator for Joint Exploration Systems Development at NASA’s headquarters in Washington. “We think it’s a great way forward.”
Officials at Monday’s teleconference seemed convinced they could get a lot of good data from Thursday’s test. For example, launch director Charlie Blackwell-Thompson said, during the terminal countdown from T-10 minutes to T-10 seconds, there are nearly 25 “critical events” in the rocket’s test targets. Only two of them are specific to the upper phase, she said.
“There is a significant amount of testing and data and risk buying you get in relation to the core stage, to the ground systems and in relation to the boosters,” she said.
The top phase, known as the Interim Cryogenic Propulsion Stage, was manufactured by the United Launch Alliance and delivered to the Kennedy Space Center about four years ago. Chief SLS engineer John Blevins, however, said he does not believe the valve problem was due to any durability issues. The check valve in question, he said, is estimated to work for 20 years or longer.
“Two days” becomes “two weeks”
The wet dress test was originally scheduled to last two days as it began Friday, April 1st. But in part due to a problem with fans at the Mobile Launch Tower, the first attempt to provide fuel for the rocket was to be scrubbed on April 4th. Another attempt last week saw NASA fill the core stage about halfway with liquid oxygen before the agency discovered that a manually adjusted “vent valve” at the core stage was mistakenly left in the wrong position. Then NASA discovered the problem with the non-return valve on the top step.
Now, teams of NASA employees and contractors will be called to their stations again Tuesday night to prepare the vehicle and ground systems for loading fuel for the third time. The actual refueling of the vehicle is scheduled to begin Thursday morning, with the terminal countdown reaching at 6 p.m. 14:40 ET (18:40 UTC). Of course, the exact timeline does not require any further delays, which seems unlikely for a two-day test extended to two weeks.
Asked to assess the next steps after this test in terms of preparing the SLS rocket and Orion spacecraft for an unmanned demonstration flight later this summer, NASA officials did not want to look too far beyond the conclusion of this nuclear phase refueling. They declined to say whether the rocket could be subjected to another wet dress test for the entire vehicle to ensure flight readiness for the upper stage and its ground systems.
“I do not think we are ready to really say, one way or another, how we think the next step is going to look like,” Whitmeyer said. “I think we really need to do the test on Thursday and then look at the data.”