Stennis, SLS core stage teams reflect on Green Run campaign

by Philip Sloss

He also noted that keeping inert purges running on the vehicle during hazardous operations with rocket propellant was another demanding aspect of the stage’s size, in particular for its gaseous nitrogen need. “That’s key. That keeps it inert so that you don’t have to worry about a fire breaking out if there was any spark or anything,” he said. “It was a significant amount of nitrogen; helium, as well.”

“[The Core Stage] pushed the site to its limits,” he added. “We did even have to upgrade some systems to have some margin because it pushed us to our extremes. Even the year or two before [it arrived] we were doing infrastructure upgrades site-wide.”

“For example, I remember when they added a large electrical water pump to the pump house. That was to make sure that it could handle both the deflector and the vibroacoustic water demands. So it was a major undertaking for the site to be able test something of this magnitude.”

Big tests

There were eight Green Run test cases during the campaign, building from powering up the flight and ground control systems through systematic functional checkouts to the two big tests that culminated the series. “They all built on each other,” McKibben said. “The second one was basically powering on the systems.”

“On some of the early ones, since this was the first time they would be powered up, some things got obviously streamlined after that. Any lessons learned would get incorporated, and we got a little faster each time we would power up the vehicle [to start the next test case].”

“The fourth [test case] was an important one because that was really where we did our final connections to the vehicle and the final leak checks,” he added. “Once we got through four, the facility and the test article were fully mated up. Then the fifth one was hydraulics and moving around of the engines, so that was key as well.”

The sixth test case was a team training simulation of the two big Wet Dress Rehearsal and Hot-Fire cases that culminated the campaign.

Credit: NASA/SSC.

(Photo Caption: The high-pressure industrial water plant that serves the A and B Test Complexes at Stennis. The circular storage reservoir holds water pumped from the center’s canal system. In this view, the A-2 Test Stand can be seen behind the water plant on the left and the dual-position B Test Stand can be seen on the far right. High-demand water flows were required to continuously supply water for the flame bucket and the acoustic aspirator ring in during the eight-minute Hot-Fire stage test in B-2 .)

For the test team, the countdown timeline for the two tests was similar. “The vehicle is powered up basically [at] T-2 days, and then from there on we’ve got generators on and a lot of the systems are powered, so we are actually manning the stations in the control room around the clock,” McKibben noted.

“Some of the other test cases even went three to four weeks long, where we had people around the clock in the control center. So some of the early test cases kind of helped us practice how to minimize the disturbance of a hand-off [between shifts]. For tanking on Wet Dress or on test day, most of the actual tanking duties would be done by first shift, but some of the barge checkouts and setups would be done by night shift and then handed off to day shift when [they] came in around six in the morning.”

In addition to the teams in the test control center, which was southwest of the test stand, other teams were positioned around the greater Stennis test area. “The gas house is [another] area that we had people stationed,” McKibben said.

“Due to the large volume [of nitrogen] that was necessary for this one, we ended up having to expand some of our nitrogen capabilities and even some of our helium. Each [test] complex actually ended up tying in and back-feeding the site to help us keep our gases up.”

“So we had people at the gas house and at E Complex, we had people at the water plant that were running the generators and big diesel engines for water pressure,” he added. “Also we had some barge personnel that were inside on the east pier. In case we had any issues, they were located fairly close inside the pier so they could go handle an issue pretty quick.”

McKibben also noted that a “red team” stayed close to the test stand itself until later in the countdown. “The red team ended up being our stand engineer and some of our people who were the most intimate with all of our systems,” he said.

“They stayed inside the test stand until a certain point in the countdown and then we would end up clearing them back. That way, if we had any issues that we couldn’t remotely do, they could go and troubleshoot.”

The red team cleared back to the original blockhouse for the B Test Stand, which was still being used for single-engine tests in the B-1 position of the stand. “We did try to minimize all personnel inside what we call the ‘QD,’ which is the quantity distance. And the QD varied during the day based on how much propellant was in the tanks,” McKibben noted.

Three terminal countdowns were executed during the Green Run between one Wet Dress Rehearsal test and two Hot-Fire tests. “Whether it was Wet Dress or Hot-Fire, it was very similar [in the control room],” McKibben noted.

During the second run of the Wet Dress Rehearsal on December 20, 2020, the Boeing Stage Controller ground control systems orchestrated the first full tanking cycle of the program. “We did have one additional objective for the Wet Dress where [Boeing and NASA] wanted to see it thermally-stable for a couple hours, so we had a two-hour hold in there. But that wasn’t there [for the Hot-Fire tests] which helped us kind of pull in the timeline a little bit.”

After the two-hour long wait, the first terminal countdown was only half completed before cutoff due to a valve closing more slowly than the defined test commit criteria. The next test was conducted on January 16, 2021, which was designated as both the third Wet Dress Rehearsal and first Hot-Fire test attempt.

Credit: Brady Kenniston for NSF.

(Photo Caption: The flight article for Artemis 1, Core Stage-1, completes a full 500-second Hot-Fire test in the B-2 position of the B Test Stand at Stennis Space Center on March 18. The second Hot-Fire accomplished all planned test objectives.)

The first full terminal countdown was completed in the mid-January test, with the Stage Controller handing over control of the stage firing sequence to the Core Stage flight computers late in the count, but the first Hot-Fire test was cut short after ignition when the behavior of the stage’s hydraulic systems went outside conservative test commit criteria limits.

After recycling the stage, the engines, and the infrastructure at Stennis, all the Hot-Fire test objectives were completed in the full-duration firing in mid-March. The three redundant computers, running a modified version of the flight software called the Green Run Application Software (GRAS), began the Autonomous Launch Sequence (ALS) at T-30 seconds and “flew” the stage through Main Engine Cutoff (MECO).

“One interesting thing, actually from our camera views, is at the umbilicals where they mate,” McKibben noted. “During different parts of the profile, you could actually see the rocket lift a few inches and then go back down some from the thrust. The umbilicals [were designed so they] could travel some and stay fully mated, and it was really interesting to see that in some of the video.”

Although the stage was virtually isolated from ground equipment and operating on its own power and supplies, the NASA Stennis team in the test control center was still busy monitoring systems and ready to begin safing the vehicle after shutdown. The Stage Controller was still monitoring and controlling ground systems throughout the firing, with the flame bucket and acoustic ring water flows being critical to complete the full test.

“Whether it’s full duration or whether it’s cut short, we’re very task-oriented. And we end up having to just move on to kind of the next task of safing the system, so at the moment you end up just continuing to work,” McKibben said. “The pause and reflection comes the next day or later that night because there’s still multiple shifts worth of work left to do to open the test stand back up.”

“For some people, it was six or seven years of working together, and at times some people had doubts on whether we’d get through a full duration on the stand. So it really was impressive, especially with it being such a demanding test for the entire site, pushing every boundary we had.”

“For us the next step will be seeing it launch, because that’s really the next hurdle and the people down at KSC are doing a fantastic job preparing the vehicle for that,” he added. “We’re all very excited to see it actually fly.”

Lead image credit: Brady Kenniston for NSF.

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