Following last month’s ravages of Hurricanes Marco and Laura and the ongoing devastation being wreaked worldwide by the COVID-19 coronavirus pandemic, teams at NASA’s Stennis Space Center in Bay St. Louis, Miss., continue to press ahead towards a full-flight-duration test-firing of the four RS-25 engines on the first Space Launch System (SLS) core stage as early as next month.
As this article was being prepared, NASA was only days away from completing the fifth of eight “Green Run” functional tests, before three final “operational” tests begin as soon as 19 September to simulate countdown operations, fuel the core stage with liquid oxygen and hydrogen propellants and run the engines for over eight minutes at their full thrust of over 1.6 million pounds (720,000 kg).
Standing 212 feet (64.6 meters) tall, the core stage is the central component of the SLS and, together with a pair of five-segment Solid Rocket Boosters (SRBs), will provide a combined 8.8 million pounds (3.9 million kg) of thrust to lift Artemis-1—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 at some point late next year.
The boosters were delivered to the Kennedy Space Center (KSC) in Florida for processing back in June, whilst the core stage has been at Stennis since January as its integrated systems are wrung through a series of eight critical tests known as the “Green Run”.
But as we all know, 2020 has posed far more than just technical obstacles. The core stage 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 in the form of the “Modal Test”, in which mechanized “shakers” imparted dynamic forces onto the core to identify bending modes as part of efforts to verify vehicle models needed to operate the SLS Guidance, Navigation and Control (GNC) systems.
However, the stead 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 needed to perform mission-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 recently 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 included several simulations of potential problems. The fourth (“Propulsion”) test, which was wrapped up early last month, checked the core 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 brough 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,” NASA’s Tracy 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, with an initial expectation that Test 5—the final “functional” test, dedicated to evaluating the core stage’s Thrust Vector Control (TVC) and hydraulics—could wrap up as early as 7 September.
“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.”
Although Stennis teams were originally working towards completing Test 5 as early as this week, Ms. McMahan cautioned that NASA was not working toward any firm 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. That said, she added that Test 4 was completed earlier than intended.
At the current pace of activity, Ms. McMahan anticipates that the team will finish up Test 5 “early next week”, which will complete the final “functional” test of the performance of integrated core stage systems. The final three steps of the Green Run—simulating the countdown sequence, performing a fully-fueled Wet Dress Rehearsal (WDR) and hot-firing the four RS-25 engines for a full-flight-duration of 8.5 minutes—are considered a test of core stage operations.
“The countdown simulation is scheduled to start around 19 September,” Ms. McMahan told us. “WDR and the hot-fire are still scheduled for October. There are certain activities with the stand [and] software that happen between tests to prepare for them.” She previously advised us that the WDR process is expected to take “much longer”, due to the fact that engineers from NASA and prime contractor Boeing will be physically loading propellant—537,000 gallons (2.4 million liters) of liquid hydrogen and 196,000 gallons (891,000 liters) of liquid oxygen—into the core.
“By the time NASA and Boeing are ready to do the hot-fire, they will have methodically checked out and operated every system on the stage from top to bottom,” she added, “because they are all needed for the hot-fire and ultimately for the launch of Artemis-1.”