The four RS-25 engines from Aerojet Rocketdyne successfully propelled the Core Stage, Interim Cryogenic Propulsion System, and Orion/Service Module stack into its initial Earth orbit during the system’s debut launch on Nov. 16, 2022.
The engines fired for a full duration of nearly eight and a half minutes, inserting the SLS stack into a 30 x 1,800 km initial Earth orbit trajectory before their destructive plunge with the Core Stage back into the atmosphere near the Hawaiian islands.
The reentry ended the careers for RS-25 engines 2045, 2060, 2056, and 2058, all four of which were responsible for launching 118 individual people into low Earth orbit during their use with the Shuttle fleet.
During their final use with Artemis I, the engines saw the performance they returned during the Shuttle era: “perfect.”
“As our chief engineer likes to say, the performance was perfect. And that’s not too far from the facts,” said Doug Bradley, RS-25 Deputy Program Director, Aerojet Rocketdyne, in an interview with NASASpaceflight.
Aerojet Rocketdyne completed an initial quick-look data review of the four RS-25 engines on Artemis I within an hour of the launch via telemetry sent back to the ground during flight.
A more in-depth review followed a week later, with a joint Aerojet Rocketdyne/NASA review coming on Monday, Nov. 28.
The joint review was a moment for both teams to come together and review their independent analyses of the four RS-25 engines’ performances.
“We like to do them independently to start with to see if we’re coming up with the same answers,” noted Bradley.
Those reviews all point to excellent and within family performance of all four engines.
To this, Bradley said, “Before the flight, we’ll predict where they should run within a couple of sigmas statistical accuracy. And boy they were just going right between those lines, all of the different parameters. So it was very satisfying and a relief when you see the engines working the way they should.”
Part of the pre-flight performance predictions for the engines stemmed not only from their Green Run test firings at the Stennis Space Center in 2021 but also from their individual flight histories with the Shuttle Orbiters, with engine 2045 having flown 12 times while the least flown engine, 2060, had flown three times.
But these particular post-flight data reviews for Artemis I are different than the ones conducted for the RS-25 series of engines, which were formally known as the Space Shuttle Main Engines (SSME), during the Shuttle Program.
Previously, the engines were recovered after each flight and disassembled and inspected to confirm data readings telemetered to the ground during the flight.
(Photo acronyms: HPOTP: High-Pressure Oxidizer Turbopump; LPFTP: Low-Pressure Fuel Turbopump; LPOTP: Low-Pressure Oxidizer Turbopump; HPFTP: High-Pressure Fuel Turbopump; MCC: Main Combustion Chamber. Credit: Aerojet Rocketdyne)
So how does Aerojet Rocketdyne ensure the same degree of reliability and safety now that the engines are expended?
“Most rocket engines don’t come back,” said Bradley. “And we have to deal in that world for some of our engines. But [RS-25s] engines are so well known we can really derive a lot since in the Shuttle Program we flew them for 30 years.”
“We would look at data, you [brought] the engines back and [saw] what the hardware [looked] like. And you [did] that over and over and over again.”
“And so now it’s not so important to have the hardware back again because we can look at the data and we know how that hardware is performing and what it looks like. What we have learned is really benefiting us now for this program.”
Part of this, too, stemmed from the design and use of a new engine controller, replacing the old 90s and early 2000s technology that closed out the Shuttle era.
The new controllers check and monitor the health of the engines 50 times per second and are the main tool Aerojet Rocketdyne relies on to report back on the health of the engines during flight.
“We want to hold on to reliability for the safety of the astronauts and also for the success of the mission,” stated Bradley. “So we didn’t change our philosophy about reliability and how we analyze the engine. We did get a really good picture of how the engines [ran]. We’ve got over 100 measurements, and we’ve got about 60 measurements that we really focus in on.”
Those measurements include information on pressures and temperatures, flows and speeds, and vibrations throughout the engine.
“It’s a complete survey of the engine,” said Bradley. “So we’ve really got a lot of instruments covering this engine to make sure we know exactly how it’s running. So we’re comfortable, even though we don’t get the engines back, we’re very comfortable with its operation.”
The RS-25s were not the only propulsion element Aerojet Rocketdyne contributed to the SLS, with the RL10B-2 engine on the Interim Cryogenic Propulsion Stage performing near flawlessly with its various burn executions.
Additionally, the Launch Abort System (LAS) jettison motors are also provided by Aerojet Rocketdyne. These are the motors that pull the LAS away from Orion during a nominal launch.
The actual abort motors which would pull Orion and its crew away from the SLS in the event of an emergency during launch are provided by Northrop Grumman.
For the LAS jettison motors on Artemis I, Bradley noted that they “ran beautifully as well.”
As post-flight data reviews continue on the first set of SLS RS-25 engines, preparations to install the four RS-25s onto the Core Stage that will power the Artemis II mission and carry the first Artemis program crew to the Moon continues at the Michoud Assembly Facility in Louisiana.
Current timelines from Boeing, the prime contractor for the Core Stage of SLS, indicate engine installation later this month.
Beyond Artemis II, Aerojet Rocketdyne continues to prepare the final eight Shuttle-era RS-25 engines for their use on the Artemis III and IV missions before the new-build RS-25s debut on Artemis V.
This all takes place while Aerojet Rocketdyne continues to develop an even-more-powerful variant of the engine which will operate at a nominal thrust setting of 111% compared to the current SLS RS-25s that operate at 109% rated performance.
The rated performance level is calculated against the original 100% rated power level of the first RS-25 series of engines which flew with the first handful of Shuttle fights in the early 1980s.
(Lead image: The four Core Stage RS-25 engines ignite on LC-39B on Nov. 16, 2022, ahead of Artemis I’s launch. Credit: Nathan Barker/NSF)