SLS, Stennis working on colder LOX for next Green Run WDR attempt

by Philip Sloss

Space Launch System (SLS) Core Stage Green Run team members are working on new procedures to supply liquid oxygen (LOX) to the vehicle at lower temperatures in the next Wet Dress Rehearsal (WDR) attempt. The first attempt on December 7 at the Stennis Space Center in Mississippi was scrubbed because the temperature of the LOX was a few degrees higher than design requirements as it entered the vehicle; NASA did not want to continue loading with the temperature out of limits.

The problem is not with the Core Stage itself; engineers with the SLS Program, Core Stage prime contractor Boeing, and NASA Stennis are looking at ways to reduce the temperature of the cryogenic liquid before it reaches the stage from barges docked next to the B-2 Test Stand. In the meantime, the test team is recycling the stage and other equipment to be ready to start a new Wet Dress Rehearsal attempt no earlier than the week of December 14, with the tanking and countdown demonstration no earlier than the end of that week.

Working on LOX temperature conditioning to meet requirements

The first attempt to execute the Green Run WDR, a full dress rehearsal for the Core Stage of a launch day countdown, was scrubbed on December 7 when liquid oxygen temperatures during propellant loading didn’t meet the vehicle’s criteria. “On Monday (December 7), we began loading the liquid oxygen and the temperature in the system was not getting down to the levels we needed to meet our temperature requirements,” NASA SLS Stages Manager Julie Bassler said in a December 10 media teleconference.

“This was during the slow fill tank load operations.” Monday’s attempt was the first time that SLS had loaded cryogenic propellants into a Core Stage, and the temperature issue meant that proceeding with a full, end-to-end countdown would not be possible.
“The team decided to pause and look at this closer, so it took a couple of hours on Monday to see if we had any opportunity there to move forward,” Bassler said. “Given the condition of the vehicle, we wanted to ensure we got into a safe configuration, so we decided to close down on the liquid oxygen part of the tank filling.”

“This was the first time that LOX was flowed from the facility to the Core Stage, and it was the first test of our LOX propellant loading as an integrated system between the facility and the Core Stage vehicle.”

The temperatures measured where the oxidizer enters the vehicle at an umbilical connection were about four degrees Rankine or Fahrenheit too high. “The requirement is about 169.1 Rankine, that’s -290.57 degrees Fahrenheit, so we didn’t quite get there on Monday,” Bassler explained. “What we saw at the temperature for the interface to the Core Stage was 173 Rankine, which is -286.67 degrees Fahrenheit.”

A slow flow rate of propellant chills down the facility and vehicle lines to start the loading process, so they aren’t shocked by the sharp, several-hundred degree drop in their temperature. Normally after the chilldown phase, the loading transitions to a slow fill phase, which is at a higher flow rate, and then a fast fill phase to load the propellant tank to the top.

The concern with the warmer propellant in the early phases of loading is geysering. “The thing that the team is trying to protect in the [temperature] limits that got tripped are relative to the concern of developing a gas bubble in [a] feedline,” SLS Program Manager John Honeycutt said. “[A bubble] could end up collapsing and then you could have quite a bit of energy released when the liquid oxygen above that bubble could release and fall back down the feedline.”

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 space agency says that the problem is not with the Core Stage hardware or its design but with the procedures for loading LOX on the vehicle. “There’s not anything wrong with the rocket,” Honeycutt said. “This is an operational activity associated with how we load the liquid oxygen and I’ve got a high level of confidence that the team is going to be able to work the fix and it’s going to have to be on the ground side to do that.”

NASA used testing and analytical models to predict how this first-time ever propellant loading would go and defined the parameters used to perform the sequence of steps to fill the tank with LOX. The models predicted how the temperature of the propellant would be influenced by how fast it was pumped into the vehicle and how quickly the bulk temperature of the rocket would cool down as the cryogenic propellant passed through the lines from the barges to the engine section of the vehicle and finally into the propellant tank.

During the test on December 7, the change in temperature of the propellant didn’t exactly follow the predictions of the modeling. “We think it was just a part of the integrated modeling and capturing the exact temperature and the heat load that we’re receiving off of the Core Stage,” Bassler noted.

Both liquid oxygen and liquid hydrogen (LH2) are supplied by storage barges docked at the test stand. The more dense LOX is pumped from the barges docked at the dual-position B Test Stand.

The B-1 position is used for single-engine testing of Aerojet Rocketdyne RS-68A engines. That side of the facility has industrial run tanks that the barges are pumping propellant into; in the case of the B-2 position used for stage testing, the barges are pumping LOX into the propellant tanks of a rocket stage.

“The systems that we use for RS-68 on the B-1 side [are] the same [up] to a point [on the stand] and then it branches in two different directions,” Maury Vander, NASA Stennis Chief of Test Operations, said. “We’ve got the pumps on the barges that supply the LOX and they perform best when they’re running basically I’m going to say ‘wide open’ at a thousand gpm (gallons per minute) or so.”

“We’re operating a little bit below that and with that slower velocity, I think that may have had a little impact on the temperature.”

Now engineers are trying to balance Core Stage requirements between propellant temperature and flow rate. The slow introduction of the propellant into the vehicle during the chilldown and slow fill phases of loading allows the temperature of the hardware to more gradually equalize with the propellant. Still, insulation can only minimize the amount of heat from the outside being absorbed.

The longer the propellant is in the transfer lines, the more the temperature increases. “If you imagine this really cold LOX going through this facility line, there’s obviously going to be some heat input into it and we go through a process to condition the tank during chilldown and slow fill where we have a requirement to flow at a slower rate, so that gives the liquid oxygen a much better opportunity to pick up the heat,” Honeycutt explained.

Any significant physical modifications to the ground support equipment and test stand infrastructure would be more time-consuming, so the engineering and operations teams are looking for procedural changes they can make. “We may wind up having to flush those [ground transfer] lines more often just to make sure we can keep them conditioned as we go along, which may require us to use a little bit more LOX in that initial phase of chilldown,” Vander said.

“But what we think we can do is operationally by cycling valves and doing some things differently on the supply we think we can deliver the liquid oxygen at the temperature that the rocket requires without any facility modifications to the system.”

Analysis of the new procedures is also looking at whether the extra LOX consumption early in propellant loading will leave enough to perform a two-hour hold demonstration planned after the terminal countdown demonstration is completed. “If we have to reduce the time period in order to make sure we have enough LOX available, that two-hour hold is where we would get some of that margin back if this new procedure requires us to have more commodities available to do this earlier part of this procedure,” Bassler noted.

Credit: NASA.

(Photo Caption: The bottom end of a replica of the Core Stage LOX feed system used for anti-geyser testing at the Marshall Space Flight Center in 2014-2015, showing the semi-circular crossover line that runs between the two LOX feedlines that enter the engine section. The testing helped to develop the initial LOX loading procedures for the vehicle, but more was learned loading the full-up first-flight article in the stand at Stennis and NASA is updating their procedures to meet temperature requirements.)

Although the full WDR test was scrubbed on Monday, the issue with liquid oxygen loading did not prevent a similar, first-ever test of the liquid hydrogen loading sequence. “On Monday, the team successfully loaded a limited amount of liquid hydrogen,” Bassler said. “It was cooled to -423 degrees Fahrenheit.”

“We had no issues with this operation, all the data looked really good on it, and it really provided an opportunity to see how the tank and the Main Propulsion System reacted with the fuel loaded. It actually decreased one of the risks we had going in with this new design and development, to get this hydrogen tank partially loaded for the first time.”

“That was a big milestone for the team and everything looked really great there, we saw no leak on this first load,” she added.

The SLS Core Stage is a new design around the heritage Space Shuttle Main Engine (SSME) design, now called RS-25 and uprated to SLS operating requirements. The Space Shuttle Program had episodic issues with liquid hydrogen leaks across thirty years of launch operations, so Monday’s LH2 operations with this first Core Stage increased confidence in the stage’s MPS design.

“One of the things that were at the top of my worry list and has been there for a very long time, given the experience and the history that we’ve had with dealing with hydrogen leaks in the Shuttle program, was running into a problem where we were chasing a hydrogen leak,” Honeycutt said. “Coming out of this abbreviated test that we did on Monday, I have a lot more confidence in the system in and of itself because we wetted with liquid hydrogen all the potential areas of concern that I thought we might have.”

“The team did a really good job with the test program and the data that we saw coming out of the test showed that that system performed in an excellent fashion. Another one that was probably pretty high on my worry list that has gone down is the avionics system.”

“During the first six [Green Run] tests, we learned a lot about the subsystems and the ability for the control system to operate with the rocket and in the Wet Dress Rehearsal we did on Monday there were no issues there,” Honeycutt added. “So, from what I’ve seen, my confidence has gone up significantly.”

Next attempt tentatively end of next week.

Analysis of new procedures between the engineering and operations teams is continuing, but SLS is looking to make the next WDR attempt next week, the week of December 14. “Where we are today is we are reviewing some changes, we’re in the process of implementing those changes to ensure that we are able to reduce the temperature on the LOX side during this initial start of loading in the Core Stage,” Bassler said. “This is utilizing existing Stennis facility infrastructure, so it’s really just a procedural update that we’re making at this time.”

“The integrated NASA Stages, Boeing, and Stennis teams we are completing our analysis right now just to make sure we thoroughly understand how this new procedure will work, and then we’ll take the next couple of days, and we’ll define the final schedule for the next WDR attempt. Right now, it is looking like it will be next week, so we’re getting the team all ready and getting all of our procedures updated to support that.”

The Wet Dress Rehearsal test is the seventh of eight tests in the Green Run design verification campaign. A tentative forward schedule was looking at starting the WDR again from the beginning of the test with systems power up around December 16. If the analysis can be completed and the hardware and test team can be reset in time to start on the 16th, then the next attempt to perform the full tanking and countdown dress rehearsal would be two days later on December 18.

With completion of the WDR delayed until mid-December, the mission-duration, 500-second Hot-Fire test that follows it is now expected no earlier than the last week of December somewhere around New Year’s Day. “It’s about a two-week turnaround from Wet Dress to Hot-Fire, but that’s really contingent on reviewing the data, so as long as we get a clean Wet Dress and get all the data that we need to close our verification items out, it’ll be on a two-week time-frame,” Bassler said.

Credit: NASA/Glenn Benson.

(Photo Caption: The right-hand aft booster assembly for Artemis 1 is lowered by a crane onto the Mobile Launcher on November 24 at the Kennedy Space Center in Florida. Stacking of the boosters in the Vehicle Assembly Building on the Mobile Launcher is being coordinated with the Core Stage Green Run test schedule.)

The earliest that the stage could be refurbished at Stennis after the Hot-Fire and depart Stennis on the agency’s Pegasus barge to Kennedy Space Center (KSC) in Florida for Artemis 1 launch preparations is now February. “We’re getting to a point where we’ve got very little margin left in the schedule relative to our commitment to our delivery date,” Honeycutt said. “We’ve been working closely with the KSC folks [on] the operations that they need to execute in order to get the Core Stage and the rest of the SLS rocket ready to fly in November.”

The Exploration Ground Systems (EGS) integrated operations team started stacking the aft Solid Rocket Booster (SRB) assemblies on Mobile Launcher-1 in the Vehicle Assembly Building in late November. The SLS Boosters have a twelve-month stack life and that clock starts when the next segment is mated on top of one of the assemblies, so the SLS and EGS programs are continuing discussions about when to perform that mate and begin the segment-to-segment stacking.

The programs are also looking at whether the twelve-month stack life requirement can be extended. “Obviously, the clock doesn’t start until we start to stack the next segment on the aft segments,” Honeycutt said.

“We do believe that we could extend that stack life requirement and the team is actively working that but the key thing to note today is we’re meeting with EGS and their leadership team on a regular basis as to when we want to pull the trigger [on segment-to-segment] stacking.”

That next segment, the left-hand aft center segment, was moved from the Rotation Processing and Surge Facility (RPSF) to VAB High Bay 4 on December 7, where it would begin the final stacking preparations; however, the SLS and EGS programs decided to wait on finishing the WDR before stacking that segment and starting the clock.  The segment was subsequently moved back to the RPSF.

“I think once we get through the Wet Dress Rehearsal, we’ll be able to give the green light to start the stacking,” Honeycutt said.

Lead image credit: NASA/SSC.

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