“So as the fluid got warmer, we were actually getting more pressure just in the [hydraulic] system anyway, so the system was more ready to support the gimbaling operations,” Jackson said. The same circular gimbal profile was repeated on the running engines near the end of the firing, less than a minute before Main Engine Cut-Off (MECO).
“It was the same intent with both tests, but the TVC system was warmed up and in better shape to do the gimbaling at the end,” Jackson added.
The static-firing deliberately ended with a test of the low-level cutoff system. The final test, while the stage was firing, demonstrated the performance of the cutoff sensors and the flight software logic when the multiple cutoff sensors begin to read a tank going dry. It also provided additional data on the performance of the propellant feed system and accuracy of the modeling.
“[Propellant feed system performance] was for the most part right down the middle, and you saw an indication of that by the fact that we shut down so close to where the planned cutoff time was,” Jackson noted. “The feed systems performed remarkably well.”
“Since [Green Run] was really our first time being exposed to cryogens, I think there was concern that we would be chasing leaks [or] see something that would actually cause us to stop testing, and we really didn’t. [For] the entire test procedure, we had excellent feed system performance with practically no leaks at all, nothing that caused any slow down in the testing.”
“Pneumatics was similar,” Jackson added. The MPS valves are actuated pneumatically with gaseous helium, which the stage also provides to the engines for critical purges while they are firing and as a backup shutdown capability.
“There wasn’t a whole lot of news there because the pneumatic system performed excellent as well,” he said. “We saw that we had a good amount of margin in that system. The system performed exactly as we expected.”
“Again, we were not chasing any leaks, we didn’t have any significant issues with any of the components in the pneumatic system. All of that performed excellent. We showed the system was sized appropriately.”
Carrying Green Run knowledge base to launch preps and countdown
With three terminal countdowns, two firings, and what amounts to a full-duration flight completed in the test stand at Stennis, the Core Stage team is working with the Exploration Ground Systems (EGS) launch team at KSC to support the vehicle’s upcoming real first flight with the launch of Artemis 1.
“The things we’ve learned through Green Run and transferring that knowledge base over to future operations at KSC,” Looser said, “we’re evaluating whether some of those things are going to be a potential issue that we need to have the same type of contingency procedures or processes that we developed at Stennis [in place here] or whether we don’t think those are going to be an issue and we don’t have to do that.”
“We’re have great communication with our counterparts at KSC and are working through all of those potential changes or issues that we saw at Stennis and whether or not we need to do anything different than our original planning for KSC.” Boeing was in charge of executing the Green Run campaign along with the SLS Program, but the EGS launch team followed all the Green Run countdowns and test-firings from the Launch Control Center at KSC, with a real-time data stream fed into their firing room consoles.
Credit: Chris Gebhardt and Nathan Barker for NSF.
(Photo Caption: Core Stage-1, the Artemis 1 flight article, is moved into the Vehicle Assembly Building at the Kennedy Space Center on April 29 after post Green Run delivery from Stennis. The stage is being prepared to begin launch preparations.)
Some of the lessons learned were with issues where the behavior was slightly different than predicted and/or where workarounds could be employed. “One of the things that we learned was just how critical valve timing performance is, and I’m talking milliseconds,” Jackson noted.
“[In] the second Wet Dress [Rehearsal], we got all the way into terminal count, and there was a valve timing issue. As you marry computer logic and timing with the actual valve performance that you see, especially on the ground valves or even in the flight valves, milliseconds really matter.”
“We had a couple of valves that closed just a little bit after we had expected them to close, we’re talking 30 milliseconds after, but in the computer that means you failed that test. And that’s what happened that kicked us out of that terminal count sequence,” he explained. “We were able to fix that real quick, turn it around. The design was all good, it really was just that sometimes you need the vehicle to talk back to you a little bit to tell you ‘you’re real close, but you got this number just a little bit off.'”
Another issue that required fine-tuning was seen with the engine “bleed flow” during the first tankings. “Shuttle had [hydrogen recirculation] pumps that would keep the propellants on board and drive them through the engine,” Jackson explained. We are doing overboard bleed, where we go through and use the same paths, but from the engine we just route it overboard.”
“[With] hydrogen, the fluid is very prone to staying gas rather than liquid, and so in the second Wet Dress we saw what we believe was just a bubble that was stuck in the LH2 feed system.” The issue was with only one of the four feedline legs from the LH2 tank to each RS-25 engine inlet.
Behold: @NASA_SLS’s incredible journey. 😍
After a 900-mile journey, the final component of the #Artemis I rocket has arrived at @NASAKennedy. The core stage was taken to the Vehicle Assembly Building to be assembled with the rest of the rocket elements: https://t.co/5aMQmZ8tj8 pic.twitter.com/4uiHVRQuHM
— NASA Artemis (@NASAArtemis) April 30, 2021
“The simple fix [was] pressurizing that hydrogen tank for just a minute to force liquid to go through that line,” Jackson noted. “Once the liquid went through that line, it pulled the heat out, and then our bleed flow was established. So it was definitely fine-tuning because three of the four legs were right on, but that fourth one just needed a little, what we call a ‘kick start.'”
“Once that [bubble] was out, it started flowing just fine with no issues,” he added.
Continuing to investigate prevalve clutch failures
One open issue discovered during the Green Run test campaign that is still being investigated is the separate failures of two prevalve clutch mechanisms. The first issue was identified with the LH2 prevalve for engine four in October 2020; the second one was found with the liquid oxygen (LOX) prevalve for engine one in late February 2021.
There are eight prevalves in the Core Stage engine section that control the flow of propellant from the tanks to each of the RS-25 engine inlets, four for LOX and four for LH2. “We are still in the investigation phase of the prevalve issue that we saw at Green Run,” Looser said.
“The team is actively working through some engineering tests and additional analysis and inspections of hardware that we have on hand as well as some Shuttle prevalve clutches and trying to understand the exact root cause of the issues that we saw.”
Credit: NASA/Frank Michaux.
(Photo Caption: Scaffolding and environmental enclosures are set up around Core Stage-1 in the VAB on May 5. Post Green Run thermal protection system (TPS) repairs are being performed in different spots around the circumference of the rocket’s outer mold-line. After the stage is stacked vertically on the Mobile Launcher with the two SLS Boosters, those areas will become inaccessible.)
The prevalves are newly manufactured units from a Space Shuttle design modified for SLS use. “The prevalve is what we refer to as a heritage design reuse, so it’s not a direct component reuse like some of the TVC components are where we have physical hardware that was removed from a Shuttle,” Looser explained.
“[It is] the design that’s reused [with] some slight modifications to that hardware mostly to accommodate for SLS environments, housing, thicknesses, things like that.”
“When you go back to build things that haven’t been built in a long time, there are always different materials that need to change out, different application processes, there’s all sorts of things that can go in there,” Jackson added. “The research [that] is going on is to understand the production and any material changes that were made over time because you’re 30 years later than when the Shuttle built them. So that’s what we’re working through.”
While the investigation continues, there is no decision on what actions might need to be taken, if any. “In the event that the team and the program does determine that something needs to be done to the hardware on the vehicle, [we are] working through the logistics with our friends at KSC on what it would take to access and change out any potential hardware if the team determines that is necessary,” Looser noted.
Boeing went through the process of performing repairs at Stennis to the clutch mechanisms with the overall prevalve assemblies themselves remaining installed. Tooling to make the repairs inside the engine section is already designed and fabricated, and has now been used twice.
“I think you can see, with what we did at Green Run when the decision was made to change out a clutch, we were able to execute that very successfully and relatively quickly in terms of getting hardware on hand, accessing it in the vehicle, and making that change once it was determined that it was necessary,” Looser said. “I don’t anticipate it being any different with the team being able to react very quickly and access the hardware and successfully change out hardware again if the team determines that’s something that’s necessary.”
(Lead image credit: Brady Kenniston for NSF)