SLS Core Stage Green Run WDR countdown ends early – Next steps considered

by Philip Sloss

NASA partially completed the Green Run Wet Dress Rehearsal (WDR) test of the Space Launch System (SLS) Core Stage on the second attempt in the B-2 Test Stand at the Stennis Space Center. The test was again conducted in secrecy on 20 December, and the test team was able to fill the rocket stage with its liquid oxygen (LOX) and liquid hydrogen (LH2) propellants for the first time ever; however, a crucial, 10-minute long terminal countdown was aborted inside of the 5 minute mark when a valve failed to close within pre-set time limits.

In a statement, NASA said that despite the early end to the test the issue doesn’t appear to be with the vehicle; after draining the stage, the agency’s SLS Program and Core Stage prime contractor Boeing are now reviewing data from the test and inspecting the stage after its first propellant loading cycle while they deliberate what to do next. The 8-minute long Hot-Fire test was expected about two weeks after a fully completed WDR, but with the countdown demonstration incomplete, SLS engineers and management will have to decide if and when to proceed to the stage firing.

WDR terminal countdown stopped inside T-5 minutes

The second attempt at the WDR resumed early on 20 December after an issue with a few heaters in the stage controlled by ground computers postponed tanking two days prior. In a first for the SLS Program, the Artemis 1 Core Stage flight article, which is performing double duty as a propulsion test article, was filled with liquid oxygen and liquid hydrogen.

The WDR is the seventh of eight test cases in the Green Run design verification campaign which has seen Core Stage-1 in the B-2 Test Stand at the agency’s southern Mississippi space center for all of 2020.

Modified loading procedures to lower the temperature of the liquid oxygen entering the vehicle were able to satisfy the vehicle temperature limits; the first WDR tanking attempt on 7 December was scrubbed after it was learned that the initial procedures did not cool the propellant enough.

Tanking a real Core Stage helped verify and calibrate procedures being developed to load the stage for future launch countdowns. The WDR was not planned as just a fill and drain test, but was to culminate with a full terminal countdown demonstration test.

Running through the terminal countdown from the start at T-10 minutes down to T-33 seconds was planned to similarly verify and calibrate the automated pre-ignition, pre-launch countdown procedures along with all the criteria defined to ensure the vehicle is safely operating within design parameters. The plan to cutoff at T-33 seconds would have demonstrated that all the propulsion, pressurization, and management systems in the complicated stage reached a point where they were ready for engine ignition within a few seconds.

In the WDR test, sources said the terminal countdown started at T-10 minutes and counting and ran down to T-4 minutes and 40 seconds where an unplanned hold occurred. The terminal countdown is governed by test commit criteria that defines limits for thousands of parameters such as temperatures and pressures, along with criteria for the vehicle configuration at specific points in time as well as criteria for how actions must be performed.

Credit: NASA/SSC.

(Photo Caption: Liquid oxygen barges are docked to the B Test Stand in this December 4 image. Barges for both liquid oxygen and liquid hydrogen are used to fill storage tanks in several test stands at Stennis; for the Core Stage seen in the right-hand B-2 position of the test stand in the image, the propellant flows from the barges through transfer lines directly into the vehicle’s propellant tanks.)

The criteria for how long it should take for a liquid hydrogen replenish valve to close was violated at that point in the countdown when the valve was commanded to the close position as a part of the process to pressurize the liquid hydrogen tank for engine firing. After holding at the T-4:40 point for a few minutes, teams decided the terminal countdown test couldn’t continue.

Vehicle safing and recycle sequences were then executed.

Although the countdown ran for over half of its intended duration, the early cutoff left several major milestones untested. With the countdown aborted at that point, the stage’s propellant tanks weren’t fully pressurized, the hydraulic Core Stage Auxiliary Power Units (CAPUs) were never started, the final RS-25 engine purge sequence was never run, and the vehicle power transfer didn’t occur.

Upcoming decision on what’s next

Following the latest WDR test, the Green Run test team is running through post-test data reduction and analysis, along with thermal protection system inspections. As the propellant tanks were fully loaded, partially pressurized, and then drained, the spray-on foam insulation that covers most of the outside of the stage also went through its first cryogenic tanking cycle.

On the NASA Stennis facility side, the different commodities that the stage consumes during a tanking and countdown are being replenished. In addition to the liquid oxygen and liquid hydrogen propellant barges that need to be refilled, the stage uses gaseous nitrogen and helium, which will also be resupplied for the next test attempt.

The SLS Program will be looking at all the data collected to decide if they are ready to move on to prepare for the final Hot-Fire Test. The test ended short, but NASA indicated that the tanking test part of the test went well.

“First looks at the data indicate the stage performed well during the propellant loading and replenishment process,” a post-test NASA statement said. Troubleshooting the cause of the early cutoff of the countdown will likely look at possible factors such as the performance of the liquid hydrogen replenish valve, the timing prediction from the analytical models, and whether the limits in the test commit criteria might need to be revised.

In the brief statement, NASA said the problem may be with the predictions and the limits rather than sluggish hardware. “The core stage and the B-2 test stand are in excellent condition, and [the problem] does not appear to be an issue with the hardware,” it said.

The SLS Program stated that while the intention with the WDR was to count all the way to T-33 seconds, there might be cases where the test ends early but is still complete enough to proceed to the Hot-Fire test. “If we have an earlier cutoff than 33 seconds, it really depends on why we had to cutoff early,” NASA SLS Stages Manager Julie Bassler said in a 10 December media teleconference.

“That would be part of the discussions we would have with all the technical authorities [and] with our program office to ensure we’re ready to go, but as long as we can get the majority of our verification and validation data out of the Wet Dress Rehearsal, we’ll be good to go into the Hot-Fire.”

As the Solid Rocket Boosters separate, Core Stage-1 continues to power the Artemis 1 mission towards Earth orbit. (Credit: Mack Crawford for NSF/L2)

Those discussion are now underway. If the decision is made to press ahead to the Hot-Fire, it is expected to take about two weeks to complete the data analysis, inspect the vehicle, and recycle the facility to be ready to start the test.

Core Stage work is continuing through the end-of-year holiday fortnight for the second year running; however, there are two or three federal holidays that may also factor into the schedule.

A test readiness review for the Hot-Fire will be held a day or two prior to the test to make sure everyone is ready to proceed. The initial timeline for the Hot-Fire test will be very similar to the WDR, with the test beginning 48 hours prior to the planned engine ignition.

Lead image credit: NASA/SSC.

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