The regression testing will verify that the parameter changes are input to the flight software correctly, that only the specific values are changed and not something else, and that no other unintended side effects occur when the file is loaded by the flight software.
(Photo Caption: The four Aerojet Rocketdyne RS-25 engines in the SLS Core Stage start during the Hot-Fire test on January 16. The engines operated normally during the test-firing, the first time the former Space Shuttle Main Engines had been throttled up to 109% power. A bad reading in one sensor value on Engine 4 during startup was quickly disqualified by its engine controller and was unrelated to the later abort of the test.)
In contrast, a code change would require more work and take longer to complete. “Without having to do a code change, we can turnaround the creation of [an] I-load file, the specific parameter change testing, in a couple of general regression tests to show proof that it did no harm.”
“The last thing we would want to do is make a code change for something like this because then you get into more extensive process and regression testing that instead of three or four days can easily go to maybe two to four weeks depending on what the change is.”
Vehicle in good shape post-firing
NASA reported that the Core Stage itself was in good shape after the short test-firing. The extra thermal protection system (TPS) layer for the Green Run Hot-Fire also appears to be in good condition.
A layer of reflective foil was applied to the bottom of the stage’s boattail and any other down-facing hardware in the area to protect against the higher heating expected in an eight-minute long static-firing. Within a few minutes of flight, the stage would be out of the appreciable atmosphere; instead, during a continuous static-firing at sea-level, the vehicle is subjected to extra heating and acoustic effects.
The Core Stage CAPUs also exhaust hydrogen gas while they are powered by the running engines. During the engine start and shutdown sequences, groups of hydrogen burn-off igniters are fired make sure that hydrogen gas from the engines or the CAPUs doesn’t build up to dangerous levels.
“Before we light off the engines, we [use] the hydrogen burn off igniters, the sparklers if you will,” Terry Prickett, NASA’s Deputy Chief Engineer for the SLS Core Stage, said. “We actually have dedicated sparklers that are shooting up into this area to ignite the CAPU exhaust so we don’t get a build up of the hydrogen gas and have a big pop event [that would] overpressurize that area.”
Prickett added that once the engines are up and running, they would then consume the CAPU’s hydrogen gas exhaust. “We would expect it to get entrained in the aspiration flow that’s coming down around the vehicle down through the flame bucket,” he said.
(Photo Caption: Residual hydrogen gas is consumed by hydrogen burn-off igniters after the engines shut down in the January 16 Hot-Fire test. The outside igniters are deployed around the bottom of the stage and its engines and are fired to allow a controlled burn of free hydrogen gas in the area rather than letting it build up and ignite all at once during engine startup and shutdown in the test stand at Stennis.)
In the test stand at Stennis, the hydrogen burn-off igniters are run not only before and during engine startup but also during and after the engines and CAPUs shutdown.
The other TPS system around the engines are the engine mounted heat shield blankets that protect the engine section and the engine powerheads while the engines are running during a Hot-Fire test and also during launch when the Solid Rocket Boosters are burning on either side of the Core Stage.
The blankets are multi-layer insulation designed to handle the severe launch environment, but they also have an outer layer to protect against moisture instead of combustion. “The outermost layer is a thin, waterproofing barrier and the inner layers are insulation material,” Prickett said.
“The outer layer was expected to be consumed during the Hot-Fire, and like I say, it has no effect on the insulating capability of the blanket, it’s just there for a moisture barrier — a weather cover, if you will — while we’re sitting out there on the stand for months at a time. We don’t want rain and moisture getting into those blankets.”
“As far as how it looks afterwards, the moisture barriers are torn up and burned, there’s some burning that we saw on those, but the insulating part of the blankets looked intact and no damage basically at all on those,” he added.
Since the firing only lasted for about a minute, NASA and Boeing are still discussing whether they needed to do any work on the blankets before the second Hot-Fire. A new, fresh set of blankets will be installed for launch.
Stage and engines have reserves for another test-firing
NASA had planned to conduct more propellant loadings of the first flight Core Stage as a part of the Green Run campaign at Stennis and first-time launch integration activities at the Kennedy Space Center in Florida. In addition to filling the Core Stage with propellant for the Wet Dress Rehearsal and Hot-Fire tests for Green Run, a full SLS vehicle WDR is planned at KSC to demonstrate and verify the integrated launch vehicle and ground system capabilities.
At a minimum, propellant loading for a launch countdown was expected to be the fourth cryogenic cycle on the Core Stage. The Core can be loaded and unloaded (with one load and one unload counting as a single cryo cycle) a total of 23 times per the design.
“Before Green Run testing began, SLS had allocated nine cryogenic cycles for testing at NASA’s Stennis Space Center in Bay St. Louis, Mississippi, and has used two of those during the Hot-Fire and Wet Dress Rehearsal, with seven cryogenic cycles remaining for additional testing. For the Artemis I launch, NASA is preserving 13 of the remaining 20 cryogenic loading cycles,” another NASA blog post said.
“There are some differences in the way we count [cryogenic loading] cycles on this program versus what we did on External Tank,” Prickett noted. “All these cycles go into a big spectrum that the tank is going to see, so we book-keep them as major cycles and minor cycles and it’s a combination of cryo and pressure.”
“There’s a little bit of difference here in the fact that on Shuttle we loaded against pressure, so we pressurized the tanks and then we loaded them. On [SLS], we do not do that.”
Likewise, the Stage Green Run testing was initially planned to be more expansive, and Aerojet Rocketdyne prepared the four RS-25 engines assigned to this first SLS Core Stage as well as the next engine set for the second Core for the possibility of additional pre-launch testing. Early plans to conduct two Green Run campaigns on the first two Core Stages and to perform two Hot-Fire tests in each one were scaled back over time.
“We had a ‘six-six, three-three’ requirement,” Aerojet Rocketdyne’s Doug Bradley said in an interview in 2020; at the time, he was RS-25 Deputy Program Director for the company. “That means, the first two flights– they had to be good for six tests or [firings] without any real changes in how we had to inspect them. The next two [flights] would be three.”
(Photo Caption: The four RS-25 engines are seen following shutdown after the January 16 test. The white heat shield thermal protection blankets above the engine nozzles have a thin, outer water-barrier cover that was expected to be burned over the course of a full, eight minute engine firing; however, they appear to have survived one minute of firing with less damage.)
“At one time, there were going to be two Core Stage tests on the engines. And so we put an abort in for both of those; that gets you up to four,” he explained. “Then we said ‘put in an on-the-pad abort for the flight’ and then the flight, so we came up with six. Then we said, ‘after you do the first two [flights], you’re not going to be doing two [ground tests] any more,’ so we said three.”
Following the first Hot-Fire, during the refurbishment and turnaround process to get the engines ready for the second Hot-Fire, an issue with the readings from one of four Main Combustion Chamber pressure sensors on Engine 4 was also repaired. The measurement from that sensor was “noisy” during engine startup, and that sensor was disqualified by the engine controller 1.5 seconds after the start command.
The noisy data was not due to a bad sensor but was somewhere within the wire harness that carries the data from the sensor to the engine controller. The issue was not serious enough to either shutdown the engine or to stop the ground test from continuing and was unrelated to the issue that stopped the test over 60 seconds later.
While not an issue for the Hot-Fire test, such a noisy start on one sensor on a single engine would have resulted in an on-pad abort per flight safety rules that mandate full redundancy of the system at liftoff.
January Hot-Fire short, but many firsts demonstrated
Although there wasn’t much of a middle to the January 16 Hot-Fire, the test accomplished many SLS Program firsts: including the first time a Core Stage completed a launch countdown, the first four-engine start sequence, first steady-state run-time at 109%, and first safe engine shutdown.
“Even before engine start we got a lot of firsts,” Looser said. “If you remember in the second Wet Dress Rehearsal, we stopped at around the five minute mark, so [January 16] was the first time to go up to flight pressure on both of those tanks as we enter the terminal count. We demonstrated the engine start box, starting all four CAPUs, transitioning the engines into their final purge sequence, and then getting the engine ready command.”
“We [also demonstrated] a three-minute launch ready hold as we were working an issue just prior to engine start; so we demonstrated that launch-ready hold capability,” he added.
Many of the firsts in the January 16 test were under control of the brand new SLS flight software; after vehicle power went to internal batteries at T-90 seconds, the vehicle was virtually isolated from the test stand, and the flight computers took over control of the vehicle from Boeing’s Stage Controller ground computers at T-30 seconds and completed the Core Stage’s part of the final launch countdown.
“That was the first time we’ve been able to go through that T-4 minutes 40 seconds [mark] all the way to T0 and into plus count with the real vehicle,” Mitchell also added. “Of course we performed that testing thousands of times in the various labs.”
Once the engines came up before T0 and the engine tap-off gas was powering the CAPUs — another first for the program — the stage was running off its own resources under its own control.
“What’s interesting is that when you look at and understand all the interactions that need to happen between the Stage Controller and the test stand and the Stage Controller and stage working through the Green Run Application Software to configure the stage so that it can become a self-contained entity and for GRAS to take over and execute the Hot-Fire and how seamlessly that worked, there’s hundreds of activities that happened during those last four minutes to get to T0, and I couldn’t be more pleased with how that whole integrated system performed,” noted Mitchell.
“It provides us a lot of assurance, certainly for a second Hot-Fire, but also gives us a lot of confidence when we go to do integration at KSC.”
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