Last Saturday’s premature shutdown of all four RS-25 engines on the first Space Launch System (SLS) was triggered by intentionally conservative test parameters, according to a NASA update on Tuesday evening. After more than a year in the historic B-2 Test Stand at the Stennis Space Center (SSC) in Bay St. Louis, Miss., the giant rocket’s 212-foot-tall (64.6-meter) Core Stage—which carries more than 733,000 gallons (3.3 million liters) of liquid oxygen and hydrogen propellants and provides about 20 percent of the thrust at liftoff—roared to life at 5:27 p.m. EST Saturday, for a hoped-for, eight-minute-long full-flight-duration “burn”.
However, a little over a minute into the Hot Fire Test, one engine exceeded its highly conservative pre-test limits and the Core Stage was commanded to shut down after 67.2 seconds. Teams are working to determine if a second firing may be needed.
“This is the biggest test NASA has run in 50 years,” said SLS Program Manager John Honeycutt of the Marshall Space Flight Center (MSFC) in Huntsville, Ala. He underlined the intentional conservatism of the test parameters on Saturday, since “this is not just a test article, it’s the flight article”.
In spite of disappointment at not running to the full duration, Mr. Honeycutt was philosophical. He explained that engineers gained a “tremendous amount of data on the engines and the stage” in what was, after all, the very first simultaneous test-firing of four RS-25 engines, all of which reached their requisite 109-percent rated power level and generated 1.6 million pounds (750,000 kg) of thrust.
As previously detailed by AmericaSpace, the Hot Fire Test was the eighth and final step in the “Green Run”, a year-long campaign to wring out the Core Stage’s myriad systems ahead of the rocket’s maiden voyage and the uncrewed Artemis-1 mission around the Moon, possibly as soon as November 2021.
Five “functional” tests to validate the rocket’s Guidance, Navigation and Control (GNC) systems, evaluate its avionics and safety systems and check out its Main Propulsion System (MPS), Thrust Vector Control (TVC) and hydraulics were completed between January and September 2020. Satisfactory completion of these steps allowed Stennis teams to press into three “operational” tests, beginning last fall, which saw the Core Stage put through a mock countdown, fueled with its full load of propellants in a so-called “Wet Dress Rehearsal” (WDR) and all four RS-25 engines hot-fired.
Original plans last Saturday called for the four engines—all of which are refurbished Space Shuttle Main Engines (SSMEs), with over 1.1 million seconds’ worth of “burn-time” and a total of 25 shuttle missions to their credit—to be fired for up to 485 seconds, approximating as closely as possible the conditions that they will encounter during the raging, eight-minute climb to orbit on a real mission.
To mimic the passage through a period of maximum aerodynamic turbulence (“Max Q”), about a minute after liftoff, the RS-25s were to be throttled back from their maximum 109-percent thrust level to 95 percent for about 30 seconds, then returned to full power. It was also expected that the engines would be “gimbaled” under TVC control to demonstrate their steering capabilities.
The Hot Fire Test got underway at 5:27 p.m. EST Saturday, with the sparkler-like blaze of the hydrogen burn-off igniters that previously offered such a visually impressive prelude to Main Engine Start on 135 shuttle missions between April 1981 and July 2011. These igniters ensured that unburned quantities of gaseous hydrogen beneath the RS-25s was cleared ahead of the ignition sequence. With a characteristic low-pitched rumble, which gradually built up into a thunderous crescendo, the engines roared to life, a quartet of perfect “Mach-diamonds” forming as exhaust gases exceeded supersonic velocities.
“Engine Start!” came the call as all four engines came alive and billowing clouds of steam poured from the flame bucket of the B-2 Test Stand. The first minute of stable thrust proceeded without incident.
At 60 seconds, the pre-planned gimbaling test of the engines under TVC control got underway. “We began to go through an aggressive gimbal profile of all four engines,” noted Mr. Honeycutt in his remarks last night. Responsibility for gimbaling each engine fell to the TVC actuators, each powered by a Core Stage Auxiliary Power Unit (CAPU).
At approximately 61 seconds, CAPU-2—serving the Core Stage’s No. 2 engine—detected low hydraulic fluid levels and after a series of verification checks over the next two or three milliseconds to validate this reading, it shut itself down. The other three CAPUs momentarily increased their hydraulic pressures to 105 percent to compensate for this evolving situation. CAPU-2 then commanded the Core Stage flight computer to shut down the other engines. This was executed safely over the next few seconds and the Hot Fire Test ended after 67.2 seconds, which represented less than 15 percent of a full-flight-duration burn.
Designated “Engine No. 2056”, the Core Stage’s No. 2 engine has a long and chequered past as a Space Shuttle Main Engine (SSME). It first saw service aboard shuttle Atlantis in July 2001, when it helped push STS-104 to orbit to deliver the Quest airlock to the International Space Station (ISS). It flew next on Columbia’s last fully successful voyage, STS-109 in March 2002 to service the Hubble Space Telescope (HST).
Those two missions both saw it fly as an interim “Block IIA” engine, boasting a raft of safety enhancements, including the Large Throat Main Combustion Chamber (LTMCC), improved low-pressure turbopumps and certification to operate at 104.5 percent Rated Power Level.
It was then fully upgraded as a “Block II” SSME with the addition of the Alternate High Pressure Fuel Turbopump (AHPFT) and went on to pull double duty to power Discovery towards low-Earth orbit on her two Return to Flight (RTF) missions, STS-114 in July 2005 and STS-121 in July 2006. Following the retirement of the shuttle fleet, Engine 2056 was assigned to the first SLS mission.
In summing up the behavior of the Core Stage systems during the Hot Fire Test, NASA noted that—had this been a “real” flight—the CAPU margins would have been higher and CAPU-2 would have continued to function nominally. “The specific logic that stopped the test is unique to the ground test, where the Core Stage is mounted in the B-2 Test Stand at Stennis,” NASA explained. “If this scenario occurred during a flight, the rocket would have continued to fly using the remaining CAPUs to power the Thrust Vector Control systems for the engines.”
“In flight, we would have continued to fly after encountering the condition that we saw on the test stand,” said Mr. Honeycutt. “But for the Hot Fire Test…the control systems were set up to “safe” the vehicle for this condition.”
Another observed incident, at just 1.5 seconds after Engine Start, was a sensor reading for a Major Component Failure (MCF) which resulted from the failure of one of four pressure sensors—and a corresponding loss of one “leg” of redundancy—in the Core Stage’s No. 4 engine. The other three pressure sensors behaved without incident and RS-25 Program Director Jeff Zotti of engine manufacturer Aerojet Rocketdyne stressed that if such a situation occurred on a “real” launch, the mission would have continued normally.
The No. 4 engine, incidentally, has three previous launches to its credit, including the final shuttle mission, Atlantis’ STS-135 in July 2011. “The team plans to investigate and resolve the Engine 4 instrumentation issue before the next use of the Core Stage,” NASA noted.
Following additional reports of a “flash” near the engines, visual inspections have revealed some “exterior scorching” in the vicinity of their protective thermal blankets. Due to the proximity of the blankets to the exhausts from the RS-25s and the CAPUs, this finding was not unanticipated. “Sensor data indicate temperatures in the Core Stage engine section were normal,” NASA explained. “Both observations are an early indication the blankets did their job and protected the rocket from the extreme heat generated by the engines and CAPU exhaust.”
As to the next steps, Mr. Honeycutt said that the “data is going to drive us and inform our decision as to whether we either proceed to launch or we perform an additional Hot Fire Test”. Original plans were for the Core Stage to be delivered via the Pegasus barge to the Kennedy Space Center (KSC) in Florida, possibly as soon as late February, for integration with the twin five-segment Solid Rocket Boosters (SRBs) and associated hardware in readiness for Artemis-1 in the fall.
But Mr. Honeycutt added that, in either eventuality, there is “no date yet on that decision-point”. Drying of the four RS-25 engines and post-test recycling activities are underway as part of the critical path towards either delivering the Core Stage to KSC or proceeding with another Hot Fire Test.