SLS Core Stage nearing home stretch for Green Run tanking test

by Philip Sloss

Final preparations are in work to ready the first Space Launch System (SLS) Core Stage flight article for its first-ever propellant loading in the B-2 Test Stand at NASA’s Stennis Space Center in Southern Mississippi. Prime contractor Boeing is leading the Green Run test campaign for the central component of the space agency’s new launch vehicle as the test team approaches the critical Wet Dress Rehearsal (WDR) now tentatively targeted for early November.

The WDR is the seventh of eight Green Run test cases and the precursor to a full-duration Hot-Fire test of the stage once the tanking test is completed. Final closeouts of the stage to configure it for its first live countdown will take up much of the remaining time before the full Green Run team meets for a Test Readiness Review (TRR); the meeting is planned two days before the test to verify that the vehicle, the test stand, and the test team are ready to proceed.

In the WDR, the Core Stage will be prepared for ignition exactly as it will for the Hot-Fire and for a launch, activating all its systems to run together and meet all their firing criteria before the countdown is stopped several seconds before engine start.

Post-hurricane status, road to WDR

A couple of days after the Green Run test team completed the series of simulation practice runs that made up most of test case six in early October, Hurricane Delta became the fifth tropical weather system to disrupt the work schedule at Stennis.

Coupled with a two-month standdown from March to May due to the COVID-19 pandemic, natural causes are expected to stretch the Green Run campaign out to a full year; the stage arrived on NASA’s Pegasus barge at Stennis on January 12, 2020, and the current plan has Pegasus departing with the SLS Core in mid-January, 2021, assuming things go well with the last two Green Run tests.The Mississippi space center returned to normal hurricane status on October 10 and back on-site, final preparations for the big Wet Dress Rehearsal and Hot-Fire tests resumed. “The next thing that we had to go do was load the new version of Stage Controller software, so that’s behind us and now what we’re doing is doing the last set of safing checks that basically validate that the new load of software is working just like the old one did,” Mark Nappi, Boeing Green Run Test Manager, said on October 15.

The safing checks were originally run earlier in the test campaign as test case three, but were executed again now that the ground control system’s software is updated for the big, fueled tests. “It is almost identical to test three, you’re just going to go through and check all that stuff again and make sure it’s still working right with the final check of the big red button, which is our cutoff button,” he explained.

“We’ve already run these tests once before but we’re doing it as a confidence-builder since the Phase 2 Stage Controller software was loaded. That will start tomorrow (October 16) and that’ll work through Sunday (October 18) and then in parallel with that we’ll be removing engine section platforms which is really our critical path on getting to the Wet Dress [Rehearsal] date.”

Credit: NASA/Joel Kowsky.

(Photo Caption: Boeing Green Run Test Manager Mark Nappi (left) is one of the NASA and Boeing SLS officials briefing agency administrator Jim Bridenstine (middle) while standing under the Core Stage in the B-2 Test Stand at Stennis on February 10. Mr. Bridenstine is situated under the boattail’s “manhole,” which provides one of the access points into the engine section/boattail compartment. The nozzles of the four RS-25 engines are covered with yellow bagging in part to keep the engine components dry.)

Internal work platforms are deployed in the dry volumes of the Core Stage, the forward skirt, intertank, and engine section for pre-launch processing and then must be removed before flight or flight-like operations such as the final two Green Run tests. The liquid oxygen (LOX) and liquid hydrogen (LH2) propellant tanks are the biggest elements and have some machinery and sensory equipment, but the dry volumes contain almost all of the moving parts of the rocket and the platforms provide the reach and support for workers to get to them when necessary.

The engine section is the most complicated and densely-packed area on the vehicle, with the aft dome of the LH2 tank filling the top and the RS-25 engine powerheads sticking up through the boattail into the bottom. In between and around those prominent parts are foam-covered propellant lines and valves, large, high-pressure bottles of helium gas, hydraulic power units and actuators, and dozens of computer devices all connected with miles of wiring runs.

The engine section has a two-level set of ground platforms that provides access when the Core Stage is oriented vertically; due to the cramped quarters inside the aft compartment the platforms must be brought inside mostly one piece at a time and assembled in place. Similarly, for disassembly and removal the platforms cannot be taken apart in sections, but piece-by-piece in order to minimize the chances of damaging the rocket machinery.

“The Engine Section platform set is two-hundred separate pieces small enough to fit thru the engine section access door and then can be assembled around all the engine section hardware to form a continuous area for employees to work off of,” Nappi explained. “It is two separate levels, one at the lower boat tail area and a second story right below the LH2 tank. It has to be disassembled piece by piece and removed from the engine section thru the access door while inspections are performed of the area that was uncovered.”

“Every time you take a platform out you’re looking for anything that may have been covered up so you can fix it and properly document it and move on,” Nappi added. “So that’s going to take a good ten, twelve days or so and that is our critical path.”

After completion of test case six, NASA and Boeing announced a forecast for the final two big Green Run tests on October 13. The WDR tanking test planned to culminate in the final countdown on October 30 and the Hot-Fire test about two weeks later. “What we’ll do is we’ll set up a couple of days beforehand. The vehicle will be all powered up and the team will be on console and then we’ll start the loading of the 537,000 gallons of liquid hydrogen and the 196,000 gallons of liquid oxygen,” John Shannon, Boeing’s Vice President and Program Manager for SLS, said.

“We’ll spend about two weeks of looking at data to make sure that that all the systems behaved as expected, we’ll go out and inspect the vehicle, make sure there are no surprises and that’ll set us up well for a hot-fire that right now we’re anticipating will be in the middle of November,” Shannon added.

Shannon gave a tentative date of November 14 for the Hot-Fire at the time; however, the tentative dates for both tests have subsequently slipped a few days. NASA Human Exploration and Operations Mission Directorate Associate Administrator Kathy Lueders indicated on October 20 that the Wet Dress Rehearsal had slipped into early November, which is likely to delay the Hot-Fire out a similar number of days.

WDR test and tanking preparations

On the eve of starting the WDR with the standard “call to stations” for the test team on console in the control center, NASA and Boeing SLS management will hold the Test Readiness Review (TRR). “The test readiness reviews are normally conducted the day before the test starts, so we start Test Case Seven on the 28th is what we’re planning right now and so the TRR will be on the 26th,” Nappi said prior to the delay.

“So that’s where we’ll start test case seven and then it’s a four-day test. Two days into it we start tanking, so it’s a pretty short-duration type of operation.”

Final preparations in the area, also called “pre-operations,” will begin the week before the pre-test review. The new date for the TRR is still under review.

“[Pre-operations] is when Stennis will start getting ready with bringing the barges in and configuring the stand,” Nappi noted. “So there’s some work that we’ll do on the vehicle and there’s some work that Stennis will do leading up to the official start and call to stations for test case seven.”

The large, 212-foot long Core Stage will require all six of the space center’s LOX and LH2 barges to be docked at the B Test Complex. The cryogenic fuel boils off over time; once docked, their propellant loads will need to be topped off by tanker trucks brought to the test complex.

At the conclusion of the TRR, the SLS management team would give a go to start the test a day or two later. One of the first activities is to power up the Stage Controller, which then commands power up of the Core Stage computer system.

“Once we’re ready to power up, we have to 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,” Nappi explained. “In parallel with that Stennis operations will get the stand ready and that’s about a day and a half of operations for that to occur.”

The countdown clock won’t start until later in the test, but Nappi noted the WDR is a launch countdown type of operation. “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,” he said.

“We don’t put planned holds into this count but we have plenty of time to get things down so we stay on schedule.”

Credit: Aerojet Rocketdyne.

(Photo Caption: Green Run infographic of the propellant storage that will be used to fill the Core Stage during the Wet Dress Rehearsal and Hot-Fire tests. Stennis Space Center uses barges docked to its A and B Test Complexes to support testing “hydrolox” rocket engines and rocket stages. The SLS Core Stage Green Run will use most of the barge propellant capacity and all six positions at the B Test Complex will be occupied for the two fueled Green Run tests.)

The remaining schedule leading up to the pre-test final review meeting includes some time for solving “non-conformances” (NCs) to the work plan, but with only a handful of days left to the test there’s not much time to account for climatological factors and nothing for another hurricane. “Between now and wet dress it’s a pretty sequential schedule,” Nappi said. “When we build the bars of work we account for some NCs that may happen. We don’t account for big weather events.”

“If we were shut out of the stand for days because of bad weather obviously that’s not accounted for,” he added. “We put a little of that in the plan that we create.”

The 2020 Atlantic hurricane season is not over yet; if another tropical weather system were forecast to get too close to Stennis, forward work would be put on hold. Nappi explained that it takes about three days of preparations to protect the Core Stage and the test stand against tropical storm force winds and weather conditions.

“What that involves is that you have to close out all of volumes, so you have to get the equipment out of each, the forward skirt, the intertank, and engine section, put the doors on and then you have to secure the outside of the vehicle,” he said. “So any scaffolding you have out has to come down, any tooling that you have up has to come down, and then we have these things that are in between the platforms and the vehicle called debris catchers that have to come out in case the winds got so strong that they could impact the vehicle and cause damage.”

The schedule impact each time the Green Run team had to stop and protect the hardware was different from one storm to the next, in part depending on the timing of the approach and what test work was being performed. “It usually is about seven days of an impact when a storm comes through unless we can figure out a way to perform critical path work at the same time, in parallel with the securing and post-storm reconfiguration,” Nappi noted.

“You have that three days of prep when you’re not doing anything else other than prepping for the storm, and then of course you’re down for the storm, and then you come back and you’ve got another three days that you’ve got to do to reconfigure the vehicle so that you can go back to work. So for example when Sally came through it was a seven-day impact, but when Delta came through it was only a two-day impact to the critical path because we managed to get a lot of critical path work done as we were securing and doing post-storm reconfiguration and so it really is dependent on where we are in the flow and what kind of work do we have going on at the time.”

WDR tanking and terminal countdown objectives

The current plan is for tanking day to be two days after the call to stations begins the WDR test. A pre-tanking management meeting will be held as a final decision point about twelve hours before the planned T-0 time. If a go is given to proceed, the primary test team will be on console in the test control center and they will begin final tanking preps.

Loading the Core Stage with propellant is a major objective of the test; it will be the first time in the SLS Program that a full-scale article is filled with its flight load of LOX and LH2. The tanking procedures have been analyzed with different math models for flow rates, temperatures, pressures, and other system behavior, and this will be the first opportunity to measure the predicted system response against an actual one by a flight article.

“The first fun thing is going to be to thermally condition the tank, where you put a little bit of cryos in and you start to chill the tank down before you go into what you call the fast fill time-frame and then it’s going to be a lot of looking at stable replenish and make sure we can keep pressure on the tank,” Shannon said back in January when the stage completed production at the Michoud Assembly Facility and was transported to Stennis.

“Primarily we’re looking at the [hazardous] gas [detection] system to make sure that we don’t have any leaks once we get down to cryogenic temperatures but it is called ‘wet dress,’ the dress is dress rehearsal, it’s the rehearsal for how we’re going to do things on hot-fire day so we will count and we will cut off and we will drain.”

Credit: NASA/SSC.

(Photo Caption: The top of the -Z side of Core Stage-1 in the B-2 Test Stand at Stennis in late January, showing fire-extinguisher water lines and platforms around the forward skirt and liquid oxgyen tank. The first working stage article will go on to Kennedy Space Center to help power the Artemis 1 mission to the Moon.)

The countdown and the pre-ignition cutoff is the other major objective of the WDR test; the test team will be running a live, fueled Core Stage through a terminal countdown for the first time. The terminal sequence was simulated during test case six but a full-up, final countdown requires the ground control and test stand infrastructure to support preparing the Core Stage to fire its engines under launch conditions.

Beginning at T-10 minutes on the countdown clock, the automated final countdown sequence arranges activation of the final remaining unpowered systems around completing the thermal conditioning of the engines so they reach their start box temperatures and pressures at the right time. Replenishment of LOX and LH2 boiloff in the fully-loaded fuel tanks will be terminated and the tanks will be pressurized for engine start. The vehicle’s Core Stage Auxiliary Power Units (CAPU) will be started and the hydraulic systems will be tested by running the four RS-25 engines through a canned sequence of movements.

One of the design verification objectives unique to the WDR is a two-hour hold with the stage configured to proceed into the terminal countdown sequence; this will demonstrate the capability of the vehicle to wait through a hold as long as the two-hour long launch windows that SLS lunar missions will have.

An additional design verification objective for the WDR will be performed during the final minutes of the countdown to demonstrate the ability of the vehicle in a “ready to start” configuration to hold for a short period of time without needing to cutoff and recycle the clock back to the top of the automated sequence.

The countdown will be taken all the way down to a few seconds before the hand-off of control from the Stage Controller to the Core Stage flight computers. After the countdown is stopped, safing will begin on the ready-to-fire vehicle and both the stage and ground systems will be recycled back to the T-10 minute configuration.

Following recycle and safing, the propellants will drained out of the two tanks. The test will conclude after residual propellant boils off, the tanks are inerted, and the vehicle and ground control systems are powered down.

Lead image credit: NASA/SSC.



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