After an uncertain and tragic year for us all, NASA’s efforts to prepare the core stage of the first Space Launch System (SLS) rocket for its forthcoming maiden mission are progressing with apparent pace through the final milestones of “Green Run” testing. The space agency yesterday opened media accreditation for a full-flight-duration “Hot Fire Test” of the core’s four shuttle-heritage RS-25 engines in early November.
The 212-foot-tall (64.6-meter) core has resided in the B-2 Test Stand at NASA’s Stennis Space Center in Bay St. Louis, Miss., since January 2020, where it is being put through a comprehensive series of eight tests to validate its integrated performance. The fifth and final “functional” test wrapped up earlier this month, with the first of three “operational” tests set to begin next week.
As previously reported by AmericaSpace, Stennis teams envisaged starting the sixth test as early as 19 September, but temporarily halted some activity in response to Tropical Storm Sally. The storm arose as a tropical depression two weeks ago and, after making landfall in Miami, gradually strengthened into a tropical storm, then intensified to hurricane strength: firstly to Category 1 and by the evening of the 14th to Category 2. Despite a period of temporary weakening from increased windshear and upwelling, Sally intensified with sustained winds of 80 mph (130 km/h). It finally made landfall in Gulf Shores, Ala., then gradually lost power and degenerated into a remnant low by the 17th.
“We did have to stop work after Test 5 and prepare for the storm and restart,” NASA’s Tracy McMahan told AmericaSpace. “The storm hit East of Stennis, so luckily no impacts during or after the storm. We always have to prepare like it will hit Stennis.”
Together with a pair of five-segment Solid Rocket Boosters (SRBs), the core will provide SLS with a combined 8.8 million pounds (3.9 million kg) of thrust to lift the Artemis-1 mission—the first voyage by a human-capable vehicle to lunar distance since Apollo 17 in December 1972—on its three-to-six-week voyage around the Moon late in 2021. The booster segments were delivered by prime contractor Northrop Grumman Corp. to the Kennedy Space Center (KSC) in Florida for processing back in June.
Meanwhile, the core arrived at Stennis from NASA’s Michoud Assembly Facility (MAF) in New Orleans, La., aboard the Pegasus barge in the second week of January and was hoisted into the B-2 Test Stand, its home for several months of Green Run activity. Within days, it completed its first test: the “Modal Test”, in which mechanized “shakers” imparted dynamic forces to identify bending modes as part of efforts to verify vehicle models to operate the Guidance, Navigation and Control (GNC) systems.
However, the worldwide march of COVID-19 pushed Stennis into a “Level Four” posture on the scale of NASA’s response framework in March, with only personnel for essential activities relating to the safety and security of the center permitted on site.
“When Stennis closed in March, the team was initiating activities to start the Test 2, the avionics test,” SLS Stages Manager Julie Bassler previously told AmericaSpace. “There were activities to secure and safe the hardware. When crews returned to work in late May, systems had to be reactivated and checked out for both the test stand and test control center. Teams were also working under constraints to ensure their safety and follow federal and CDC guidelines. With that in mind, once the test started, we maintained the same duration for the test as originally planned. There are periods for vehicle power-down, data analysis and re-activation between each test and to date, the tests are taking the amount of time that was planned.”
Work resumed in a reduced capacity in May and in late June the second Green Run test—the “Avionics Test”—was successfully completed. The avionics, including the flight control computers and electronics, as well as a multitude of sensors which gather flight data and monitor the health of the core stage in flight, were powered-up and checked out. The third test, dubbed “Fail-Safes” and completed in early July, checked the rocket’s safety systems and simulated potential problems. The fourth (“Propulsion”) test, which wrapped up at the beginning of August, checked the core stage for leaks and evaluated command-and-control operations for the Main Propulsion System (MPS) elements which directly interface with the RS-25 engines.
However, the traumatic human consequences of the coronavirus pandemic was not the only direct impactor which hit the Green Run schedule. Last month also brought a pair of exceptionally powerful natural forces in the menacing form of Hurricanes Marco and Laura, which devastated the Caribbean Sea and threatened the Gulf Coast in mid-to-late August. This prompted NASA to pause Green Run testing on 24 August and secure both the core stage and B-2 Test Stand until the storms passed.
This was a pity, because the fifth Green Run test had originally been targeted to begin at around that same time. “We had started the facility preparation work for the Green Run Test 5,” Ms. McMahan told AmericaSpace last month. “Due to the prediction of two hurricanes in the Gulf of Mexico with potential impacts to Stennis Space Center, NASA made the prudent decision to put the valuable Artemis-1 core stage flight hardware and B-2 test stand in a safe configuration.”
Fortuitously, Marco quickly lost power and dissipated by 26 August and Laura herself did likewise by the 29th. As a result, Stennis teams resumed Green Run testing on the 31st and Test 5—the final “functional” test, dedicated to evaluating the core stage’s Thrust Vector Control (TVC) and hydraulics—concluded earlier this month. “The Thrust Vector Control System is the part of the engine that steers, or gimbals, the engine during launch, so they’ll actually move the engine just like they move it for launch,” NASA’s Tracy McMahan recently told AmericaSpace. “It has huge components, called actuators, that steer the engines and they will also check out systems that provide hydraulics to the engine valves.”
But Ms. McMahan cautions that NASA is not working toward any definitive dates. “For each of these tests, it is the first time they are conducting the test on a brand-new stage,” she previously told us. “So they don’t really have a hard-stop date. It all depends on what they find during the test, if they have to fix anything or make any adjustments.”
With the completion of Test 5, the final “functional” test of integrated core-stage systems is now behind the team. The final three steps of the Green Run—simulating the countdown sequence in Test 6, performing a fully-fueled Wet Dress Rehearsal (WDR) in Test 7 and hot-firing the four RS-25 engines for a full-flight-duration of 8.5 minutes in Test 8—are considered a test of core stage operations. With Test 6 slated to begin early next week and the Hot Fire Test expected in early November, the exact No Earlier Than (NET) dates for the completion of each test have yet to be announced.
“That will be assessed and determined after the countdown simulation, when NASA will conduct a test readiness review,” Ms. McMahan told us. “Right now, we are targeting October for the WDR and early November for the hot-fire.”
However, it must be borne in mind that the entire Green Run campaign is a series of highly complex “firsts” for the core stage. “The time between each test depends on what we find in the prior test,” she added. “The WDR is a big deal. It will be the first time the tanks have been filled with cryogenic propellants. The WDR will exercise every part of the stage that needs to work for the hot-fire, except actually firing the engines.”
During the WDR, the core stage tanks will be filled, then drained, with 733,000 gallons (3.3 million liters) of liquid oxygen and hydrogen, the former cooled to -182 degrees Celsius (-297 degrees Fahrenheit) and the latter to -252 degrees Celsius (-423 degrees Fahrenheit). There are two reasons for this fill-and-drain procedure. “It will allow the whole system to be checked out and allow the tanks [and] propellant lines to be inspected afterwards and provide time for data analysis of all those systems,” said Ms. McMahan, “and it will allow the team to simulate a scrub on the launch pad, should that happen in the future on a mission.”
Assuming that the Hot Fire Test occurs as planned in early November, NASA currently plans to refurbish the core stage and ship it to the Kennedy Space Center (KSC) in early January. Key areas for refurbishment, unsurprisingly, are the engines themselves. “The engine team will inspect the engine and determine what needs o be done after the test,” she told us. “Another area where they expect some refurbishment is in the foal thermal protection system.” Most of this work will be conducted at Stennis, prior to shipment.