SLS Green Run test-firing to verify Core Stage design, analysis before first launch

by Philip Sloss

The Green Run test campaign for NASA’s first Space Launch System (SLS) Core Stage will culminate in two critical tests that will demonstrate the real-world performance of the large, complicated vehicle. The last two of the eight test cases of the campaign at the Stennis Space Center in Mississippi will finally load the stage with cryogenic propellant for the first time and then fire its four RS-25 engines, running the stage propulsion systems through a full mission cycle.

At the center of the two tests is a ten-minute long terminal countdown leading up to the eight minutes the stage is planned to fire in the stand. NASA and Core Stage prime contractor Boeing have several experiments planned during the terminal count and test firing to help validate and calibrate their analytical models on how the stage design behaves.

Ground test program a first and last opportunity

Boeing is conducting the Green Run campaign with the stage bolted into the B-2 Test Stand at Stennis. The stage arrived at the test site in mid-January; work to prepare the stage for the hot-fire test at the end of the test series was disrupted in mid-March by the COVID-19 pandemic.

Overall, the Core Stage is undergoing an eight test case series of diagnostics at Stennis.  Test cases one through three were complete prior to August.

After a two-month long complete work stand down, power up testing finally began in late-June. Test case four procedures, testing the pneumatic components of the Main Propulsion System and verifying that those helium-based systems are free of leaks, were completed late on August 5. After reviewing data from the last test and checking that everyone is ready to continue, test case five is expected to begin in mid-August.

The tests are giving Boeing and the SLS Program, headquartered at the Marshall Space Flight Center in Alabama, their first data to measure the performance of the different subsystems when they work together as a whole rocket stage.  The test cases increase in complexity and build to the critical seventh and eighth test cases, the Wet Dress Rehearsal (WDR) and the hot-fire test.

Credit: Philip Sloss for NSF.

(Photo Caption: A view of the B Test Stand in February from the opposite side. One of the barges that supply the liquid oxygen and liquid hydrogen propellants to the test stands is docked in the foreground, lower right.)

The WDR is the first critical test because it is the first time a working Core Stage article has ever been subjected to its designed cryogenic conditions. The stage’s liquid oxygen (LOX) and liquid hydrogen (LH2) tanks are filled via a maze of Main Propulsion System plumbing in its engine section.

“The next big unknown for the program is when we put the cryogenic liquids in the oxygen tank and the hydrogen tank and we look at the plumbing and all the systems and make sure that they remain tight and that they perform as expected,” John Shannon, Boeing’s Vice President and Program Manager for SLS, said. “Through our qualification tests, we have high confidence that they will, but until you see it in an integrated fashion you don’t really know. So we have prepared the team with contingency procedures if they see anything unexpected to be able to handle that expeditiously.”

If that tanking and terminal countdown test goes well, the hot-fire could occur some time in October.  Following post-firing refurbishment, the stage will be transported to the Kennedy Space Center in Florida for Artemis 1 launch preparations.

The Green Run test campaign represents both a first and last chance for the SLS Program; the goal is to demonstrate that the rocket stage behaves as designed. The Core Stage is a new rocket design around key Space Shuttle heritage hardware.

The campaign is the first opportunity for the SLS Program to see the real interactions between the complicated stage systems in a test environment before committing the foundation of the vehicle to a first launch.

“I think some folks view Green Run as workmanship screening; I don’t view it as that. As a matter of fact, that’s not what it is,” NASA SLS Program Manager John Honeycutt said. “This Green Run and this Green Run only is for us to learn everything that we can about the Core Stage while we’ve got it here on the ground with us.”

“We need to learn everything we can about this stage before we start flying, and it’s going to make us much better off in the long run.”

Given the opportunity to fire the stage and run through a full mission cycle on the ground, the major, fueled Green Run tests have several design verification objectives. “It’s not a development test series, where you bring a new stage [or] a new test article out there and run it on all the corners of the boxes and see what it’ll do and see what it won’t do and then go make changes,” Marc Neely, Core Stage Green Run Test Operations and Execution Manager for NASA, said last year.

The different systems will be as designed, leading to the the “Green Run” labeling used for acceptance testing new hardware; however, the upcoming countdowns and hot-fire in the test stand allow Boeing and NASA the opportunity to run experiments that will help demonstrate design requirements and operating margins.

The stage is filled with ground test instrumentation that will be watched and recorded throughout the WDR and hot-fire; those additional channels of sensor data augment both operational and development flight instrumentation data sets that will also be captured simultaneously. Ground test specific cameras will also be set up inside the intertank and engine section to visually document the behavior of the equipment during the two fueled tests.

“I look at the team’s product as providing the data necessary to clear the test article, in this case the stage, for shipment to KSC for flight,” Neely said. “Our product is that data, that information.”

The first-ever test campaign for the SLS Core Stage is also a last chance; at the same time that NASA decided last year to keep one Green Run for the first working article, it also decided that this first working stage article would be the only one to ever visit Stennis, cancelling plans for any subsequent Core Stage acceptance tests in the B-2 Stand.

Credit: NASA/SSC.

(Photo Caption: The B Test Stand on July 14, with Core Stage-1 installed in the B-2 position on the left. The stage’s four RS-25 engines will fire in the refurbished flame bucket below it on the left. The right-hand B-1 position of the stand supports single-engine RS-68 testing for Aerojet Rocketdyne.)

The B-2 position of the B Test Stand at Stennis was rebuilt and refurbished to support SLS Core Stage test firings. “Not to be sly, but if you want to [describe how Stennis is] involved, we’re actually a filling station, Bryon Maynard, Project Engineer for NASA’s B2 Green Run Project, said. “Literally, we’re the filling station. We’re going to fill it up, but after we fill it up it is Marshall and Boeing all the way.”

The test stand also provides the sound suppression water to protect the stage from the acoustics of an eight-minute long static firing and a water deluge to protect the flame bucket while the stage is firing its engines into it. In addition to critical stage operating parameters during the engine firing, the water supplies are also required to continue running the test to full-duration.

Boeing provides the ground control computer system to orchestrate the test, called the Stage Controller. As the test team monitors from the test control center, the Stage Controller will command vehicle purges, load the propellants from barges docked at the test stand into the Core Stage, manage heaters in the vehicle, manage the terminal countdown sequence, safe all the elements after engine shutdown, and capture many continuous data streams from start to finish of the test results.

Green Run terminal countdown

Beyond demonstrating a first cryogenic propellant loading and unloading cycle and that the vehicle is leak-tight, the major objective of the Wet Dress Rehearsal, test case seven, is to run through a terminal countdown for the first time. The terminal sequence requires the ground control and test stand infrastructure to support preparing the Core Stage to fire its engines under launch conditions, pressurizing the fully-loaded fuel tanks, and timing thermal conditioning of the engines so they reach their start box temperatures and pressures at the right time.

The Stage Controller is a separate ground processing computer system implemented from the one that NASA’s Exploration Ground Systems (EGS) at the Kennedy Space Center in Florida is developing for Orion and SLS launch processing there, but the teams have worked to synchronize with each other. “We started this process working closely with the EGS team, and in fact it’s a new vehicle so they needed a lot of input from us to shape their countdown sequence,” Cipoletti noted.

“There’s been a propellant working group that started about 2013, and they meet weekly. That’s between Boeing and NASA Marshall [Space Flight Center] and NASA EGS here at the Cape and TOSC (Test Operations and Support Contractor, Jacobs). So they’ve been meeting for a while to create it.  So we’ve emulated the [launch] countdown; it’s nearly identical.

“I’m not going to say it’s exactly the same because … we have the Stage Controller, so it’s got a little bit of differences. The B-2 Stand isn’t exactly the same as KSC and naturally when they do their launch they have to worry about ICPS (Interim Cryogenic Propulsion Stage), Orion, Boosters, which fortunately we don’t,” he added.  “But the basic Core Stage sequence is exactly why we partnered with EGS, so we will be a really good proof of pudding for them if you will,” he added.

Credit: NASA/Tyler Martin.

(Photo Caption: Core Stage avionics are mounted to a semicircular ring in the Avionics & Software System Integration Lab at Marshall Space Flight Center. The avionics like the flight computers and the flight software for SLS are new and the Green Run tests are verifying that the computer systems are ready to fly for the first time on Artemis 1.)

As with the division of labor during a launch countdown, the ground-side terminal countdown sequencer is in charge; it orchestrates commands to the stage and to the stand while monitoring thousands of parameters of data being reported from the hardware and software of both the vehicle and ground systems. The Core Stage flight computers running the Green Run Application Software take over primary control of the vehicle for the final half minute of the countdown and throughout the firing.

The flight computers take over enforcing the large set of vehicle criteria needed to continue the countdown through ignition, but the Stage Controller also continues to monitor those and remains in charge of the health of critical test stand systems. The vehicle computers, the ground control computers, or test team members can call for an abort if necessary.

The terminal sequence of events begins at T-10 minutes, with the Stage Controller in command until T-30 seconds when it hands off to the Core Stage flight computers to fire the stage. There are three general phases of the last part of the countdown.

First is the initial four minutes from T-10 minutes to T-6 minutes where an extended hold capability still exists. The middle period runs from T-6 minutes to T-90 seconds, where the countdown can be paused for a short time if there is a quick resolution to a problem. The last 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.

“Terminal count starts, and this is an agreed upon thing with EGS as well, at T-10 minutes. Things are pretty quiet up to 10 minutes, and we have in Wet Dress a couple of verification objectives there,” Cipoletti explained. “The vehicle configuration between T-10 minutes and T-6 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.”

Once inside T-6 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-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 during the wet dress we will execute a verification objective to show that we can stay in that state for three minutes.”

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