Representatives of NASA, Boeing and Aerojet Rocketdyne gathered for a media briefing Tuesday afternoon (12 January) to discuss progress towards the critical eighth and final test in the year-long “Green Run” campaign to prepare the Boeing-built Core Stage of the Space Launch System (SLS) for its maiden voyage as early as November. The Hot Fire Test—targeted as a 485-second full-mission-duration “burn” of Aerojet Rocketdyne’s four RS-25 engines—is targeted for Saturday, 16 January.
After enduring a stop-start rhythm of success and frustration throughout 2020—tempered not only by the worldwide march of the COVID-19 coronavirus pandemic, but also by Mother Nature and technical troubles with the hardware—NASA finally announced Monday that it is targeting about 4 p.m. EST Saturday, 16 January for the long-awaited “Hot Fire Test”. This will see the Core Stage’s four shuttle-era RS-25 engines roar to life with a thrust in excess of 1.6 million pounds (750,000 kg). It was noted that although a full mission-length duration of 485 seconds is desirable, it is possible that requisite test data may be gathered from a slightly shorter burn-time.
Throughout the challenging year of 2020, the Core Stage resided within the confines of the B-2 Test Stand at NASA’s Stennis Space Center (SSC) in Bay St. Louis, Miss., as the eight functional and operational tests in support of the Green Run campaign got underway.
Standing 212 feet (64.6 meters) tall, the stage arrived at Stennis from NASA’s Michoud Assembly Facility (MAF) in New Orleans, La., aboard the Pegasus barge in the second week of last January—almost exactly one year ago—whereupon it was hoisted into the cavernous expanses of the B-2 facility.
Previously used for qualification test-firings of the RS-25 in its earlier incarnation as the Space Shuttle Main Engine (SSME), together with the Common Booster Core (CBC) for the United Launch Alliance (ULA) Delta IV and the S-IC first-stage of the mighty Saturn V, the stand has a long and chequered history which extends back almost five decades.
Within days of its arrival, the Core Stage was put through the first Green Run test to begin wringing out its systems under functional and operational conditions. The “Modal Test” utilized mechanized “shakers” to impart dynamic forces as a means of identifying bending modes to help validate vehicle models to operate the great rocket’s Guidance, Navigation and Control (GNC) systems.
However, the arrival of COVID-19 pushed Stennis into a “Level Four” posture on the scale of NASA’s pandemic response framework in March, with only essential personnel whose duties pertained to the safety and security of the center permitted to remain on site.
When crews returned to work in mid-May, the Core Stage systems were reactivated and checked out, both for the test stand the test control center. At the end of 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 “Fail-Safes Test” of the Core Stage’s safety systems concluded in early July, followed by the “Propulsion Test” in early August to check for leaks and evaluate command-and-control operations for the Main Propulsion System (MPS) elements which directly interface with the four RS-25 engines.
But in addition to COVID-19, Mother Nature also imparted her own pressures on the progress of the Green Run. August brought a pair of exceptionally powerful natural predators in the menacing forms of Hurricanes Marco and Laura, which devastated the Caribbean Sea and threatened the Gulf Coast.
Work was temporarily suspended for a few days in late August as both the B-2 stand and the Core Stage were secured until the storms passed. The fifth test concluded early in September and formed the final “functional” test, devoted to evaluating the Core Stage’s Thrust Vector Control (TVC) and hydraulics.
With the completion of the five functional tests, the final three tests were considered “operational” in nature, since all were geared towards an all-up countdown, fueling and test-firing of the four RS-25 engines for 485 seconds, mimicking a full mission duration as closely as practicable on the ground. However, the weather was not quite done with the Green Run, as even the countdown test was delayed a few days in response to the threat of Hurricane Sally.
It was finally completed in early October, with hopes that fueling the Core Stage with 733,000 gallons (3.3 million liters)—six barge-loads—of liquid oxygen and hydrogen for the Wet Dress Rehearsal (WDR) and Hot Fire Test might occur before year’s end.
By the end of October, efforts had again been temporarily suspended in response to the ravages of Hurricane Zeta, which caused some damage to buildings and roofs at Stennis but from which the B-2 Test Stand and the Core Stage emerged unscathed. During the pause of on-site work, engineers identified inconsistent performance from one of eight pre-valves responsible for supplying liquid hydrogen to the RS-25 engines.
The cause was traced to a faulty clutch and was repaired and satisfactorily tested in mid-November. This allowed Green Run personnel to press ahead with plans for the seventh test—the WDR—as soon as 7 December, with hopes of a Hot Fire Test during the week of the 21st.
A test readiness review in early December verified that all systems were in a state of readiness for the WDR. On the morning of the 5th, the cryogenic tanking process got underway. However, it became apparent that the liquid oxygen temperatures were warmer than anticipated and the NASA/Boeing team elected to pause the WDR to take a closer look at the test data before committing to pumping the full propellant load into the Core Stage’s tanks.
“After an integrated assessment of the Core Stage and the facility,” NASA noted on 10 December, “the team identified processes and equipment that could be modified to keep the liquid oxygen at the proper temperature during delivery to the stage.”
Finally, a week before Christmas, the Core Stage was powered up for a second attempt to execute the WDR. On 20 December, the tanks were loaded with liquid oxygen and hydrogen propellants, with initial indications pointing to a satisfactory performance of the hardware. Part of the test was to simulate the final phase of an SLS launch countdown, leading up to 33 seconds before the ignition of the RS-25 engines.
However, the test ended a few seconds shy of its planned countdown duration due to a timing issue associated with a valve closure. “Subsequent analysis of the data determined the valve’s predicted closure was off by a fraction of a second,” NASA noted on 5 January, “and the hardware, software and stage controller all performed properly to stop the test.”
Last week, with this timing issue corrected, the space agency announced its preparedness to proceed with the Hot Fire Test, no sooner than Sunday, 17 January. But following a test readiness review yesterday, it was revealed that the critical test would move to the left, with teams now targeting Saturday instead.
At today’s media teleconference, it was noted that the RS-25 engines—which have supported a grand total of 21 Space Shuttle missions, including the final flight of Atlantis, STS-135, and recorded 1.1 million seconds’ worth of prior “burn-time”—will ignite and quickly ramp up to 109 percent of rated performance.
That represents a marked increase over the 104.5-percent which typically characterized the RS-25 peak performance on Shuttle. True to their shuttle heritage, they will start up at 120-millisecond intervals, with ignition commanded at about T-6.6 seconds. NASA’s Tracy McMahan told AmericaSpace that a definitive “Go/No-Go” decision to press ahead with the Hot Fire Test is anticipated by 6 a.m. EST Saturday.
After ignition and stable burn at 109 percent, the four engines will be throttled back to about 95 percent of rated performance at approximately one minute into the Hot Fire Test, to mimic the SLS rocket’s passage through the period of maximum aerodynamic turbulence (known as “Max Q”).
Thirty seconds or so later, their thrust output will be returned to 109 percent, before being throttled down to 85 percent just prior to shutdown. Assuming that nothing untoward occurs during the test, the engines will be dried, the Core Stage inspected and where necessary—and particularly with regard to its thermal protection system—refurbished, ahead of shipment via the Pegasus barge to the Kennedy Space Center (KSC), no sooner than February.
Stacking of the twin five-segment Solid Rocket Boosters (SRBs) is already underway at KSC and is expected to be complete by the time the Core Stage arrives for integration.