NASA’s Space Launch System (SLS) program, Core Stage prime contractor Boeing, and RS-25 prime contractor Aerojet Rocketdyne have completed the biggest test in the 10-year old program, firing the Artemis 1 flight stage for 500 seconds in the B-2 Test Stand at Stennis Space Center on March 18. The test was the highest hurdle for the program to clear before the agency’s new launch vehicle lifts off on its inaugural launch to send an Orion spacecraft to the Moon.
The Green Run design verification campaign at Stennis has taken longer than hoped, but after over a year in the stand at Stennis, the Hot-Fire test provides one of the last development data drops that need to be analyzed before first flight. While one team starts going through the multi-terabyte data set, the test team at Stennis will begin vehicle inspections and start to get the stage ready to ship to Kennedy Space Center in Florida for launch.
Full-duration test ran all the way to LOX depletion
Boeing is conducting the Green Run campaign on its first Core Stage build for NASA; after loading the stage with its cryogenic liquid hydrogen (LH2) and liquid oxygen (LOX) propellants during the morning of March 18, the ten-minute long terminal countdown was started at 3:27 pm Central time (20:27 UTC) with T-0 at 3:37:13 pm Central (20:37:13 UTC). Thirty seconds before T-0, the stage’s three flight computers took over command and control of the final countdown, starting its Automated Launch Sequencer (ALS).
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For this special ground test firing, the three flight computers on the Core are running Green Run Application Software (GRAS), a variation of the Flight Computer Application Software (FCAS) they will run for Artemis 1 and beyond. Just as planned for launch, engine start commands were issued beginning at T-6.6 seconds to the four RS-25 engines individually at 120-millisecond intervals.
The engines start to 100 percent of rated power level as they did in their Space Shuttle Main Engine (SSME) days. Now, with SLS, at T-0 they throttled up to 109 percent of their original, 1970s rating. The engines fired for about 499 seconds, with a final test objective to let the RS-25s run until they uncovered the low-level sensors in the LOX tank which triggered shutdown.
“From our initial look at all the data, we achieved all the test objectives even the secondary objectives,” John Shannon, Boeing’s Vice President and Program Manager for SLS, said in a briefing a few hours after the test. “We did see a LOX low-level cutoff and the system behaved exactly as it should and so we’re very excited about the data that we’ve gotten.”
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“It’s terabytes and we’ll be working through that over the next couple of weeks as we do very detailed inspections of the hardware, but the initial look is that everything worked perfectly,” he added.
“We also can’t forget to say that it wasn’t just the vehicle that worked perfectly today it was the B-2 Test Stand and the work that the Stennis Space Center did to pull off an amazing test like this was unbelievable and all of their systems worked exactly like they should.”
Credit: NASA/Robert Markowitz.
(Photo Caption: The B Test Stand with Core Stage-1 installed in the B-2 position at the Stennis Space Center dances with lightning on the night of March 17. The weather was a challenge for the test team to complete preparations for propellant loading the next morning, but the pre-tanking management meeting was able to be held on schedule.)
A repeat of the test was necessary when the first attempt on January 16 stopped 67 seconds into the planned eight-minute plus long firing, when the first hydraulic thrust vector control (TVC) engine gimbaling test caused pressures and reservoir levels to drop farther than analytical models and subsystem-level bench testing had predicted.
A strong weather front passed through the area on March 17 the night before the second test, bringing lightning, heavy rain, and high winds to Stennis and threatening to slow final test preparations.
“We actually had a weather challenge last night,” Julie Bassler, NASA SLS Stages Element manager, said in the post-test briefing. “We were watching the bad storms come through but we had a Stennis team that was on the ready and they came out around 2 am and got on the facility and finished up our pre-test work that we needed to do out on the stand.”
“At 5:30 when the test team came in we had an all-clear sign and we were ready to go into test. At 6 am we did our tanking ‘go/no-go’ and we had no anomalies, no issues were being worked, and that was pretty much the way the whole day went,” she added.
This was the second full terminal countdown that the test team conducted, tanking the stage, conditioning the propellant to reach engine “start box” requirements, and then initiating the automated final, 10-minute pre-ignition sequence.
The test team has used data and experience gained from previous Wet Dress Rehearsal and Hot-Fire tests to fine-tune procedures and protocols for both the countdown and the hot-fire test sequence. Data from the system response in the first Hot-Fire test was also used re-calibrate analytical modeling and NASA said an early look at data from the second test indicates the updated model predictions were close to actual performance.
“We had no major issues and we went all the way up to the T-10 minutes and we did our hold and checked everything and did our final go/no-go from the test team and went right into the hot-fire,” Bassler said. “All the data that we’re seeing now [indicates] everything looks like we were right down the middle of our modeling and our analysis.”
The 500 second engine firing was punctuated by three stressing engine gimbaling profiles. The first TVC test began at T+60 seconds and ran for about 30 seconds, with all four engines were being simultaneously driven in a small circle at high rates.
Within that first gimbal test with the engines moving in circles they were throttled down from 109 to 95 percent at T+65 seconds, which put additional demand on the hydraulic systems that also control engine valve positions. At T+87 seconds, the engines were throttled back up to 109 percent while still in motion.
Credit: Brady Kenniston for NSF
(Photo Caption: The flight article for Artemis 1, Core Stage-1, fires in the B-2 position of the B Test Stand at Stennis Space Center on March 18. The second Hot-Fire ran a full 500 seconds in duration, accomplishing all planned test objectives.)
A two-minute suite of sinusoidal frequency response test (FRT) engine movements began at T+2 minutes, 30 seconds, followed by a second circular motion TVC test that ended a few seconds before shutdown.
“The gimbaling that we did and the engine throttling all matched exactly what we predicted,” Bassler said. “The team will go through and look at that data but we saw nothing anomalous there.”
“It looked really smooth and the data that we do have we’re going to have to go through that more thoroughly but it is looking like we have a very strong TVC system and we hit all the ends of the boundaries. We were testing the limits to make sure that we could fly through any kind of environment and since this is a generational vehicle it’s something we wanted to test for all future Core Stages and Artemis missions.”
The engines were also throttled down to 85 percent before shutdown. “We collected a lot of data today,” Shannon noted. “Being able to see the hydraulic system work with very little propellant in the tanks with some aggressive gimbaling was a stress test for the vehicle and it just gives us great confidence the vehicle can handle exactly what it was designed for.”
“The vehicle really performed like a champ today.”
Test-firing also stressed thermal protection system
The full-duration test was the big hurdle for SLS to clear before the program’s first launch. The Core Stage is the most complicated piece of the new launch vehicle and, in contrast to the Shuttle-era engines and Constellation-era solids, it was the new development in the program.
Core Stage-1, which is the flight article for the Artemis 1 launch, served double-duty as the propulsion test article for Green Run and effectively just performed a full flight cycle while held in place on the stand in the March 18 test. Completion of the eighth and final test case of the Green Run campaign came almost a year to the day from when Green Run work was suspended and Stennis Space Center closed for two months due to COVID-19.
The high-fidelity test ran the integrated stage and its high-energy systems at flight pressures and temperatures, and in some cases stressed the stage above and beyond what would be seen in flight. With the vehicle computers and SLS flight software commanding the firing engines through the TVC stress test cases, the thermal protection system (TPS) on the aft end of the vehicle in particular experienced radiant heat loads beyond flight levels.
The base heat-shield of the Core Stage, which is a part of the boat tail structure, will see heat loads from the RS-25 engines and also the two Solid Rocket Boosters (SRB) flanking the stage and firing side-by-side with the engines; however, the vehicle will be flying at high-speed and accelerate out of the atmosphere within a few minutes.
Credit: NASA/SSC.
(Photo Caption: The four RS-25 engines are throttled at their 95 percent power level during the Core Stage Green Run Hot-Fire test on March 18. The multi-layer insulation blankets around the engine powerheads look different from the first test in January because their plastic rain covers were removed.)
In the stand, the heat loads are worse; the stage sees the same radiant heat from the engines for the entire eight-minute firing and an extra layer of reflective tape was applied to all down-facing surfaces around the aft end of the stage for Green Run. The tape provided additional protection on top of the cork that will provide the insulation for launch, but NASA and Boeing still expected a full-duration firing to eventually burn away some of the tape, cork, and adhesive in between.
NASA and Boeing made some changes to the extra, test-specific TPS after the first test-firing in January. The blankets that protect the engine powerheads were configured with a plastic water barrier during the first Hot-Fire test; that “rain cover” over the multi-layer insulation was removed the second time around.
Additional layers of cork and tape were also added in some areas on the boat tail. “After the first test we saw some base heating data that made us look at the amount of cork insulation that we had on the boat tail area of the engine section,” NASA SLS Program Manager John Honeycutt said.
“The team went off and made a decision, number one to go ahead and remove those rain covers that were on the engine blankets. We saw those burn off in the previous test.”
“Number two was to add some [additional] cork insulation and reflective tape and we did that I would guess at something on the order of probably four or five inches of cork in layers applied with adhesives. So what you saw there was some of the tape burning off as well as once one of the layers of cork got ablated we’d see the adhesive burn.”
“We don’t expect to see that during flight just due to the environments are different and the aft end of the vehicle won’t experience that same radiant heat load,” Honeycutt added.
“While it looked kind of interesting, two points,” Shannon also noted. “One is we have temperature sensors under that cork and none of those sensors got above 100 degrees so we were in great shape, the cork did its job.”
“And as John [Honeycutt] said, after two minutes of flight we’re out of the sensible atmosphere and you wouldn’t have any burning like that.”
Lead image credit: Brady Kenniston for NSF.
