Final preparations began this weekend at NASA’s Stennis Space Center in Mississippi to load super-cold liquid hydrogen and liquid oxygen into the massive core stage of the Space Launch System heavy-lift rocket for the first time as soon as Monday.
The fueling test, called a wet dress rehearsal, is a major step before engineers from NASA and Boeing, which built the SLS core stage, fill the rocket with propellant a second time later this month for an eight-minute test-firing of the rocket’s four main engines.
The Space Launch System, a critical piece of NASA’s Artemis program to return astronauts to the moon, is running at least four years behind schedule. Its first test launch, known as Artemis 1, is scheduled for late 2021, at the earliest. That schedule hinges on a successful completion to the “Green Run” test campaign at Stennis, and the readiness of the Orion crew capsule that will launch on top of the SLS mega-rocket.
The Artemis 1 mission will fly without astronauts, but future SLS/Orion missions will carry up to four crew members to the moon.
NASA officials gave the “go” for the SLS wet dress rehearsal a test readiness review Friday, the agency said in a post on its website.
“The wet dress rehearsal is the first time the stage will be fully loaded with propellants and is planned to last approximately 48 hours,” NASA said.
Over the weekend, technicians powered up the the core stage’s avionics systems to begin preparing the rocket and test stand systems for the wet dress rehearsal. If final preps proceed as planned, NASA could load the core stage with liquid hydrogen and liquid oxygen as soon as Monday.
Covered in orange foam insulation, the huge Boeing-built rocket stage has been fastened in the B-2 test stand at NASA’s Stennis Space Center since January, when it arrived by barge from a manufacturing plant at NASA’s Michoud Assembly Facility in New Orleans.
Since January, work to ready the rocket stage for its first test-firing has been suspended several times, first by the coronavirus pandemic, and then by several close calls with hurricanes approaching the Gulf Coast.
The next step in the SLS test campaign at Stennis will be to load 733,000 gallons of super-cold liquid hydrogen and liquid oxygen into the core stage. Numerous sensors will measure how the core stage and its intricate plumbing respond to the loading of cryogenic propellants.
The liquid hydrogen is stored at minus 423 degrees (minus 253 degrees Celsius) in the case of liquid hydrogen, and liquid oxygen is kept at minus 298 degrees Fahrenheit (minus 183 degrees Celsius).
NASA said six barges filled liquid hydrogen and liquid oxygen will supply the SLS core stage during the wet dress rehearsal. The vessels can be positioned in a waterway near the B-2 test stand.
“Engineers will monitor the core stage’s giant propellant tanks and complex propulsion systems for potential leaks or other issues that stages have historically experienced the first time cryogenic propellants are loaded,” NASA said. “To prepare for Artemis launches, engineers also will put the stage through scenarios it might experience on the pad before liftoff.
“They plan to conduct two different holds in the countdown timeline while the stage is in a launch-ready state,” NASA said. “This provides an opportunity to observe how the stage would respond if the countdown was paused during the upcoming hot fire test or a future Artemis launch. At the end of the test, all the propellant will be drained following similar procedures that would be used during a launch scrub on the pad.”
So far, all the testing of the SLS core stage has been at ambient temperatures, not in cryogenic conditions.
“Filling everything with cryo is huge because that’s the first time you have your tanks moving the way they’re going to move when they’re filled with cryo fluid,” said Alex Cagnola, a propulsion engineer at NASA’s Marshall Space Flight Center.
“You’re testing all your joints. You’re testing how everything basically acts under a cryogenic environment,” Cagnola said in an interview earlier this year.
“Things change when you get down at cryogenic temperatures, and as we saw after our experience with the shuttle, you can end up having systems perform differently,” said Shannon, a former NASA flight director and space shuttle program manager.
Teams will check for leaks, and ensure valves and other hardware still work at cryogenic temperatures. It takes about six hours to pump liquid hydrogen and liquid oxygen into the core stage.
“The next big unknown as a 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 through our qualification test,” Shannon told reporters earlier this year. “We have high confidence that they will, but until you see it in an integrated fashion, you don’t really know.”
Engineers will drain the core stage of its cryogenic propellants after the wet dress rehearsal, then analyze the test results and perform inspections of the rocket before proceeding with another fueling operation. That will culminate in ignition of the rocket’s four RS-25 engines.
All four engines on the SLS core stage have flown on multiple space shuttle missions, and the high-performance powerplants will provide up to 2 million pounds of thrust at full throttle. The reusable engines on the single-use core stage will be discarded after each SLS flight.
The RS-25 engines will fire for more than eight minutes, just as long as they would burn during a real launch. Programmed commands will adjust the engines’ power settings during the test, mimicking their thrust profile during launch.
Assuming a successful hotfire test, teams at Stennis will prepare the core stage for shipment to NASA’s Kennedy Space Center early nexts for final processing to ready the rocket for its first test flight.
The fueling test is the penultimate test in the Green Run.
The hotfire test is a final exam in the development of the core stage, the tallest rocket stage ever built. The SLS core is derived from the space shuttle external tank, and its four RS-25 engines — built by Aerojet Rocketdyne — are leftovers from the shuttle program.
The 212-foot-long (64.6-meter), 27.6-foot-wide (8.4-meter) SLS core stage has the same diameter as the shuttle’s fuel tank. It weighs about 188,000 pounds (85 metric tons) empty, and will weigh around 2.3 million pounds (more than 1,000 metric tons) fully fueled.
After the core stage arrived at Stennis in January, ground teams lifted the rocket by crane and lowered it onto the B-2 test stand, a facility once used for test-firings of the powerful first stage of NASA’s Apollo-era Saturn 5 moon rocket.
The first major test on the core stage was a modal test on the core stage to measure the resonant frequency of the rocket.
In March, NASA halted operations on the test stand for two months with the onset of the coronavirus pandemic. NASA and Boeing engineers resumed work on the core stage in May after introducing new guidelines for physical distancing and other measures to guard against COVID-19.
Ground teams switched on the core stage’s avionics in June and performed a thorough checkout of the rocket, then proceeded into safing checks, which verified controllers can send commands to shut down the rocket’s engines and other major systems in the event of a problem.
Then engineers commenced testing of the core stage’s main propulsion system, searching for any signs of leaks and verifying all connections between the engines and the rocket’s tankage. That test, known as Test 4 in the Green Run, also included engine igniter checks and tests of the engine control valves.
With that test passed successfully over the summer, teams moved on to a checkout of the core stage’s hydraulic system, which drives thrust vector control actuators to swivel the four RS-25 engines and steer the rocket in flight.
Teams activated the rocket’s auxiliary power units, which drive the hydraulic loops for engine steering system. The engines then gimbaled individually to ensure they could be swiveled within an 8-degree cone, followed by gimbal test profiles to simulate how the engines will move together in flight.
The hydraulic thrust vector control test concluded Sept. 13, wrapping up a series of functional checks on the rocket as engineers moved into final reviews and rehearsals for the wet dress rehearsal and hotfire.
The next milestone, known as Test 6, was completed Oct. 5 after control teams at Stennis ran through a simulated 48-hour launch countdown to validate activation, fuel loading, and pressurization sequences.
Engineers and technicians from NASA, Boeing, and engine-builder Aerojet Rocketdyne participated in the countdown simulation, which provided a “refresher course” for test controllers to ensure they are ready to load cryogenic propellants into the rocket for the first time, according to Mark Nappi, Boeing’s Green Run test director.
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