SLS Core Stage tanking test is first Green Run moment of truth for NASA

by Philip Sloss

NASA stood down from a second attempt to complete a critical propellant loading and countdown demonstration test of its first Space Launch System (SLS) Core Stage December 18 in the B-2 Test Stand at NASA’s Stennis Space Center in southern Mississippi. Prime contractor Boeing is conducting the Wet Dress Rehearsal (WDR) test which is a full rehearsal of the countdown for the final test, an eight-minute long Hot-Fire of the Core Stage.

Issues with activating ground-controlled heaters stopped the second attempt before propellant loading could begin. The first attempt to load the rocket stage with propellant on December 7 was terminated early when the real-world behavior of the vehicle and Stennis test facility equipment deviated from the agency’s analytical modeling; liquid oxygen (LOX) wasn’t cold enough when it reached the vehicle, which prevented conducting a full test.

The plan is for the countdown dress rehearsal to be cut off at T-33 seconds and also includes test objectives to demonstrate hold capabilities for long launch windows and for last-minute troubleshooting. NASA will not broadcast this test, but if it goes well, the flight-duration test firing of the stage could take place sometime in January after the holidays.
Test is full dress rehearsal for test-firing

The Wet Dress Rehearsal is a prerequisite for the Hot-Fire test, so the engines won’t fire until NASA and Core Stage prime contractor Boeing has demonstrated that the stage is ready to fire them. Boeing is conducting the test, and the test team will follow the same countdown timelines planned for the Hot-Fire and for the Core Stage during a launch countdown.

“With the exception of a couple of different objectives and then the detanking, we’re going to do our best to run the same sequence that we would run during the Hot-Fire, which is the same sequence that we would run on the launch pad,” Jonathan Looser, NASA SLS Core Stage Propulsion Lead, said in an October interview. “There’s not much difference other than those couple of test objectives in terms of demonstrating holds and then draining the tanks.”

In this full rehearsal for the Hot-Fire test, the propellant tanks will be fueled, the test team will countdown towards T-0, and the computers will run the final pre-launch sequence. With the stage and its four Aerojet Rocketdyne RS-25 engines ready to fire, this WDR countdown will be stopped with a little over 30 seconds left on the clock; the ground and vehicle computers will then safe the vehicle, and the test team will practice recycling the live stage back to its configuration at the start of the terminal countdown.

NASA will not broadcast this pivotal test, but the countdown is targeting T-0 for 4 pm Central Standard Time (2200 UTC). Next year, a similar Wet Dress Rehearsal will be conducted at Kennedy Space Center on the integrated Artemis 1 SLS vehicle at the launch pad prior to its first launch.

Credit: NASA/SSC.

(Photo Caption: The B Test Stand at Stennis Space Center on July 14, with the first SLS Core Stage installed in the B-2 position on the left. The B-1 position on the right is used for single-engine acceptance firings of Aerojet Rocketdyne RS-68A engines.)

Fully loading a Core Stage with propellant for the first time and then finishing the test with a first-ever live countdown for the stage is a major milestone in the SLS Program. The Green Run campaign for the stage is the culmination of nine years of design, development, production, and testing for the new piece of the agency’s new launch vehicle.

If the management team gives a go to tank the vehicle, propellant loading could begin by 8 am Central (1400 UTC).

The test will be the first opportunity to examine how closely the new rocket stage behaves under launch conditions to predictions from analytical models; it is also an opportunity to learn something before firing the stage during the next and last Green Run test.

“To summarize, it’s really demonstrating the performance of the integrated Core Stage prior to getting into a hot-fire test,” Looser explained. “We want to demonstrate the ability to chilldown the propellant lines and fill the propellant tanks, [and] be able to demonstrate that we can replenish and stay at a hundred percent fill level during test.”

“And then ultimately demonstrate the propellant feed system. We have various heaters across the vehicle, [an objective is] being able to demonstrate the operation of those in a cryogenic environment. The propellant conditioning and the pressurization and pneumatic systems, demonstrating the operation of those.”

“And also our thermal protection system (TPS), being able to demonstrate the properties and the ability of the TPS to perform its function during the test,” he added. “So really just demonstrating overall the capability of the integrated Core Stage under cryogenic conditions.”

“This will be the first time that we will introduce cryogenics to the system and demonstrate the vehicle’s performance, and of course, we’ll take that data and go back and correlate our math models that we used to design both the vehicle and the loading conditions and make sure that everything is right before we go into Hot-Fire test.”

The Core Stage and test team at Stennis already received a full dose of 2020 as they reach the two, final, pivotal tests. After the stage arrived in January, the test campaign was suspended in mid-March for two-months by the onset of the COVID-19 pandemic.

After a deliberate, phased return to work, vehicle testing began in late June, and the WDR was scheduled for late September with the Hot-Fire in mid-October. Then in August, the hurricanes started arriving in coastal Louisiana, interrupting or delaying Green Run test work several times from August to October.

The WDR was rescheduled for late October and the Hot-Fire in mid-November when late-breaking technical issues stopped preparations for the WDR. In the middle of troubleshooting, the last Louisiana hurricane of the season saved the worst for last for New Orleans, with Zeta plowing through the city and the regional NASA facilities there, including Stennis on October 28.

Credit: NASA/SSC.

(Photo Caption: A view of the first Core Stage flight article in the B-2 Test Stand on February 9 less than a month into the Green Run campaign. At the end of the campaign, this stage will be barged to Kennedy Space Center, where it will fly with the rest of the SLS and Orion hardware for the Artemis 1 launch.)

After inventing a method and the tools to repair a sticky liquid hydrogen (LH2) prevalve buried deep inside the engine section and then pulling off the repair, Boeing was able to finish preparations and is ready to fuel the stage and conduct the first live terminal countdown.

The first attempt to conduct the WDR was scrubbed on December 7 when liquid oxygen (LOX) temperatures violated both anti-geyser and fill and drain limits during the initial chilldown phase of loading.

The LOX temperature didn’t exactly follow forecasts by analytical models for the first-time ever propellant loading. With the temperature a few degrees higher than required when the cryogenic propellant entered the vehicle in the first attempt, the test team worked on modifying loading procedures to lower the propellant temperature so that it is within limits when it reaches the vehicle.

After reviewing the new procedures the second WDR attempt started over from the beginning of the test on December 16, 48 hours before T-0, with powering up ground and vehicle systems.

“Once we’re ready, we power up the Stage Controller, and then we move into vehicle power up and then we do a short check of all of our systems and black boxes to make sure everything is working properly,” Mark Nappi, Boeing Green Run Test Manager, explained in October.

“In parallel with that [was] about a day and a half of operations [budgeted] for that to occur, and we’ve got a good amount of time in there in case anything goes wrong because we want to get to our T-0 when we plan our T-0, so it’s similar to launch countdown time where on launch countdowns we used to put planned holds in but we don’t put planned holds into this count, but we have plenty of time to get things down, so we stay on schedule.”

A long work day will cover all the propellant loading and terminal countdown operations. “[When] you get to the twelve-hour mark things start to pick up, and you start with the tanking meeting, where we will review the weather, we’ll review any problems that we have, and we’ll make a go/no-go [decision] with program management on whether or not we’re in good shape to go tank,” Nappi said.

“And then we move into the preps for tanking so that we can start filling the vehicle with hydrogen and oxygen. [At] about nine hours we’ll have finished our tanking preps, we’ll start with turning the [hydraulic circulation] pumps on, get heater activation, and then about eight hours we’ll start loading liquid oxygen.”

“That’ll take us all the way down to about the T-6 hour mark where we get into fast fill, and we go down to the T-4 hour mark, and we’re complete with liquid oxygen,” Nappi added. “That’s a very key milestone to get the oxygen loaded.”

“Similarly, we start working hydrogen loading preps,” Nappi continued. “We start the chilldown at about the six and a half hour mark, and then we begin loading liquid hydrogen into the system until we get to a topping event at about four hours. And at that point, we’re sort of in a sit and wait mode, we’re all fueled, and we’re just topping off the system as we continue the count and get down to the ten-minute mark.”

First WDR attempt retires some risk of hydrogen leaks

Although the full WDR test was scrubbed on the first attempt, the issue with liquid oxygen loading did not prevent a similar, first-ever test of the liquid hydrogen loading sequence.

“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,” NASA SLS Stages Manager Julie Bassler said in a December 10 media teleconference. “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.”

During chilldown and initial slow-fill operations the LH2 propulsion system was wetted up to the low-level engine cutoff sensors. “That was a big milestone for the team and everything looked really great there, we saw no leak on this first load,” Bassler said.

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 the successful LH2 chilldown and initial filling operations in the first tanking attempt increased confidence in the stage’s MPS design.

The first-ever Core Stage propellant loading operations will also demonstrate the integrity of the cryogenic system. Previous programs like the Space Shuttle, which SLS heavily borrows from, had sporadic issues with liquid hydrogen leaks during its thirty years of launch operations.

Credit: NASA/Jared Lyons, Jude Guidry, Eric Bordelon, Steven Seipel.

(Photo Caption: The four LH2 prevalves on Core Stage-1 can be seen sticking out of the LH2 tank aft manifold in the upper right part of this image taken in September, 2019, at Michoud Assembly Facility during operations to mate the engine section (left) to the bottom of the hydrogen tank (right). A suspect clutch mechanism in one of the valves was recently repaired from inside the engine section with the stage installed vertically in the B-2 Test Stand at Stennis.)

“We’re continuously monitoring for leaks both internal to the volume and external to the vehicle and around the stand, particularly with hydrogen,” Looser noted. A hazardous gas detection system is deployed to look for higher than allowed gas concentrations in those areas.

“In general, once you’ve reached stable replenish in the tanks, everything has steadied out at cryogenic temperatures, and you haven’t seen a leak, you feel a lot better about the hardware at that point,” Looser said. “We would experience our Shuttle leaks as you first hit the vehicle with the first bit of cryogenic liquid as you start [chilldowns]; a lot of times we would see leaks right off the bat, but there’s been other times where it’s been late in the count where we’ve seen leaks appear. But most of the time, it’s typically early on where you’ll see those.”

“There are some components that don’t see those cryogenic temperatures until later in the count, like the GUCP (Ground Umbilical Carrier Plate) from Shuttle or as we start configuring for pre-press and you start opening and closing different valves that maybe haven’t cycled yet throughout the countdown,” Looser added. “As those see cryogenic temperatures in different parts of that valve for the first time, you have the opportunity to have leaks late like that.”

First live terminal countdown test

Once the vehicle is fully loaded with propellant, the other crucial part of the Wet Dress Rehearsal is the countdown demonstration test. The WDR terminal countdown sequence that will be run for the first time with a live vehicle under launch conditions is a demonstration that the stage is ready for its first ignition and a mission-duration, 500-second duty cycle in the subsequent Hot-Fire test.

Beginning at T-10 minutes, the countdown test will demonstrate the propellant tank pressurization sequence and timing, activation of the vehicle hydraulic system, the final RS-25 engine purge, and completion of the conditioning of the engines into their start boxes.  As the countdown nears the point of engine ignition, then the stage will transfer to internal power, and subsequent operation of all the Core Stage in-flight systems will be on the vehicle’s own batteries.

The terminal countdown begins with the Stage Controller ground sequencer in control and includes multiple points where the countdown can be paused to evaluate a last-minute issue. The WDR includes objectives specific to the test and will demonstrate required vehicle hold capabilities for countdown operations. At a high-level there are three phases to the terminal countdown sequence.

The first phase is the initial four minutes from T-10 minutes to T-6 minutes where an extended hold capability still exists, the second period runs from T-6 minutes to T-90 seconds where the countdown can be paused for a short time to troubleshoot a last-minute problem, and the third and final part is the final 90 seconds of the countdown where a hold is no longer possible.

If any of the critical parameters for proceeding with the test strays outside the rules inside of T-90 seconds, the countdown is aborted, and the vehicle is recycled back to its configuration at T-10 minutes. During the Wet Dress Rehearsal, the ability of the vehicle design to support unplanned holds during the terminal countdown will be demonstrated along with a deliberate abort and recycle sequence.

“Terminal count starts, and this is an agreed upon thing with EGS (Exploration Ground Systems) as well at minus ten minutes, things are pretty quiet up to ten minutes, and we have in Wet Dress a couple of verification objectives there,” John Cipoletti, Boeing SLS Green Run deputy test director, explained in an interview early this year. “The vehicle configuration between minus ten minutes and minus six minutes, we have to demonstrate that we can stay in that vehicle configuration for two hours so we’re going to demonstrate that the vehicle can execute a two-hour hold at that point. That lines up with the goals of being able to support a two-hour launch window.”

The two-hour hold demonstration is planned for the end of the test day and the ability to demonstrate the full two hours will depend on whether there’s enough LOX remaining from modifying the loading procedures from the first attempt to keep the vehicle’s propellant tank fully replenished.

Once inside T-minus six minutes, several of the final steps to prepare a rocket stage to fire begin. Once those steps are performed, they put systems in a configuration they can stay in for only a limited amount of time. Cipoletti noted that during the WDR, they will demonstrate that shorter unplanned hold-time capability.

“The next critical point is at T-minus 6 minutes; that’s when we start to pressurize the tanks for flight,” he said. “At that point, your options become more limited; the vehicle can only stay in that condition for three minutes max, and also during the wet dress, we will execute a verification objective to show that we can stay in that state for three minutes.”

These demonstration hold periods are not planned to be repeated in the Hot-Fire test; the plan for the countdown to the test-firing is to proceed through the terminal sequence without stopping. The WDR will run-through the whole sequence first with the extra test stoppages.

Credit: NASA/SSC.

(Photo Caption: On the left, the upper four-fifths of NASA’s first SLS Core Stage in the B Test Stand’s number two position on July 14. The liquid oxygen tank on top and the liquid hydrogen tank below will be filled with propellant and pressurized for engine ignition for the first time in the Wet Dress Rehearsal.)

At T-minus four minutes, the hydraulic systems that help operate and steer the engines will be activated by spin-starting the Core Stage Auxiliary Power Units (CAPU). The CAPUs are highly-modified Shuttle APUs; in Shuttle, the APU turbines were driven by burning a supply of hydrazine fuel internal to each unit. In the Core Stage, the hydrazine fuel elements were removed, and the turbines are now driven by exhaust pressure from the RS-25 engines while they are running; prior to engine start, the CAPUs are driven by a helium spin-start ground system.

The next major milestone occurs around T-minus three minutes when the four engines will start purge sequence four, the final purge sequence before ignition.  The purges ensure that the engines are free of contaminants, which is another criterion that must be satisfied to start them.

With the four hydraulic systems up and running, a slew or gimbal test of the engines will be performed at about T-minus two minutes, thirty seconds. The stage’s hydraulic thrust vector control (TVC) actuators will be commanded to move all four engines simultaneously in a programmed pattern to verify the TVC system is ready to position the engines as commanded while they are firing.

Then at T-90 seconds, the Core Stage transfers from ground power to its own internal power supply. During its mission, the stage is powered by four batteries located in the intertank, which are conditioned to be at full charge at that point in the count.

The power transfer at T-90 seconds is the last hold point in the countdown; once the stage is running on its own batteries, there is no time to stop and troubleshoot. “At that point, we have no capability to hold, the only option that the test team has is they could recycle back to ten minutes and try and resolve the problem, but there’s no ability to hold and continue forward once we’re on internal power,” Cipoletti said.

“And that’s true for launch day — once we’re inside of ninety seconds, we’re either going to go or recycle.”

During the WDR, the countdown will continue down until a few seconds before the handoff of primary vehicle control to the Core Stage at T-30 seconds. This will allow a first look at how the stage behaves as an integrated whole leading up to the point where the four engines would be started.

The tanks will be filled and pressurized for flight, the hydraulics will be activated and running, the vehicle systems will be running on its own battery power, and the engines should be ready to start. The plan is to cut off the countdown at T-33 seconds and begin the safing and recycle sequence.

After the vehicle is safed and recycled back to the T-10 minute configuration in the WDR then the test objective to demonstrate the two-hour hold capability would be run into the evening past sunset. The full WDR test will continue through detanking overnight, another day to allow boiloff of residual propellant, and inerting of the tanks. Finally, the Core Stage and Stage Controller systems will be powered down to conclude the test.

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