RS-25 engine installation into the Artemis II SLS Core Stage begins

by Philip Sloss

Aerojet Rocketdyne, an L3Harris Technologies Company, is helping install four of its RS-25 liquid-propellant rocket engines in NASA’s second Space Launch System (SLS) core stage at the agency’s Michoud Assembly Facility (MAF) in New Orleans. The beginning of installation of the upgraded former Space Shuttle Main Engines (SSME) in Core Stage-2 at MAF on Sept. 11 comes several months later than previous plans.

The engines were originally uncrated and prepped for installation early this year, around the time that the core stage engine section was mated to the rest of the stage, but core stage readiness was delayed, the engines went back in their storage containers sometime around March and stayed there until the end of August, when pre-install preparations were redone. After the engines are all installed in the stage and all the other stage hardware is fully assembled, final integrated testing can be completed ahead of delivery to its Kennedy Space Center (KSC) launch site, which NASA says will occur in November or late fall.

First RS-25 installed on Sept. 11

The first engine installed in the core stage for the Artemis II crewed, lunar-flyby flight test was engine 2059, which was bolted into the number two position on Sept. 11. The stage is currently rotated so that the number two position is top-left, with number one top-right, number three bottom-left and number four bottom-right.

“We prefer to put the top ones in first and then take the spacer out of the installer to lower it, then put [engine] 3 and 4 in,” Bill Muddle, Lead RS-25 Field Integration Engineer for Aerojet Rocketdyne, said in an Aug. 29 interview with NSF. The baseline plan was to install the engines in position order, 1-2-3-4, but as it turned out engine 2 went in first, same as with the first core stage in October 2019.

The procedures for installing the engines, getting them “soft-mated” and then eventually “hard-mated” is similar to last time, four years ago, but the engine install team reviewed the process and made some improvements.

Credit: NASA/Michael Democker.

(Photo Caption: The RS-25 engine carrier holding engine 2059 is lowered onto the engine installer at the Michoud Assembly Facility on Sept. 8. The engine was installed in the stage’s number two position on Sept. 11.)

After Artemis I we sat down with the team and we actually went through an after-action item review and we came up with approximately 60 items,” Muddle noted. “I’ll call them ‘do betters,’ and we categorized them as ‘must have, nice to have’; we categorized them that way, we’ve been working through that laundry list of things, and we’ve enhanced the procedure.”

“We’ve actually upgraded our training, we created an interface simulator on our handler, that blue handler that you see the engine sits on and we enhanced that. We created a more real-life simulator so that we could practice, because that’s a very tight tolerance between the engine and the vehicle and we wanted to practice that, so the guys had a better representation of what it was actually like to align those two items up.”

“We’ve practiced that twice with both our first shift team and our second shift team and [the] move directors feel very comfortable with the team and the knowledge that the team has gained during those training operations,” he added.

The clearances between different parts of the RS-25 engine and the core stage during the install process are small; although each engine is oriented differently, Muddle said the clearances are pretty much the same. “With all four engines, however they are clocked, I think some of the difficulties is getting to the area that they have to inspect,” he said.

“Because of the clocking, a low-pressure LOX (liquid oxygen) duct is out in the fairing area and to get over the top of the engine to look down into the gap that the gimbal operator has to look at to line those two up. [That] is a little difficult, but it’s just the orientation and just how that person has to get there to be able to get access, but once he’s there the access is exactly the same for all four engines.”

Engine install preparations originally started early in the year, install delayed until September

Two of the four engines, E2047 and E2059, which are expected to be the first two installed, are veterans of several Space Shuttle missions. Engine 2047 flew on 15 different Shuttle launches beginning with STS-91 on Orbiter Discovery in June 1998 and ending with the final Shuttle flight, STS-135, that launched in July 2011. Engine 2059 flew on five Shuttle launches beginning with STS-117 on Orbiter Atlantis and ending with STS-134 on Orbiter Endeavour.

Credit: NASA/Michael Democker.

(Photo Caption: Transportation containers carrying the four RS-25 engines for the Artemis II core stage are seen at MAF on Sept. 22, 2022, following their delivery from the Stennis Space Center. At the time, the engines were expected to be installed by the end of 2022.  To the left is a Pathfinder ground support engine that is used to practice engine handling and installation procedures.)

The other two were new SSME part sets that were spares for at the end of the Shuttle program. Engine 2062 was assembled at KSC prior to the last Shuttle launch, but assembly of engine 2063 did not occur until it was property of the SLS program a few years later, following shipment to Stennis Space Center in Mississippi.

The two new engines were acceptance tested at Stennis in October 2017 and April 2019 as a part of work to adapt the SSME design to the SLS operating conditions and to upgrade the engine control system to new engine controller hardware and software.  Engine 2059 was also test-fired at Stennis in March 2016 for SLS, serving as a calibration unit for a newly installed thrust measurement system at that time for the Fred Haise (A-1) Test Stand.

All four engines were trucked on the relatively short drive from Aerojet Rocketdyne’s Stennis facility to Michoud a year ago in September 2022; at the time the engines were expected to be installed beginning as early as the Thanksgiving 2022 time period ahead of a March 2023 delivery of the completed stage.

Subsequent delays in production of the core stage pushed the final engine preparations for stage installation into early 2023. Standalone integration of the engine section/boattail assembly for the stage was completed in early February and the element was rotated from vertical to horizontal in mid-February.

Credit: NASA/Jack Pfaller.

(Photo Caption: Engine 2062 is seen at KSC in April 2010 at the beginning of its assembly in the SSME engine shop co-located with what was then Orbiter Processing Facility Bay 3.)

Around that time, Muddle and his team began final engine preps for install. “We left the engines in the shipping containers for a period of time until early this year when we thought we were getting close to engine installation, so we made the decision to pull them out of the shipping container and actually do the pre-installation inspections and the [installation] preps,” he said. “Once we knew that engine installation was slipping, we put them back into semi-storage I’ll call it, where we desiccated the interfaces and put the shipping container back over [the engines].”

“[For example,] we are getting into hurricane season and so if something happens at MAF we want to at least try to protect it from falling debris off a roof or water intrusion into the facility, we at least want to protect it from that type of damage. We’ve been keeping them in semi-storage since probably March of this year.”

The engine section was finally bolted to the rest of the stage in mid-March, but by that time completion of the stage and delivery to KSC had been delayed into the second half of the year. NASA officials attributed some of the delays in production to repairing one or more sections of the liquid oxygen feedlines, also known as “downcomers,” that run along the exterior of the stage connecting the LOX propellant tank at the top of the stage to the engines at the bottom.

Readiness of the stage for engine installation was delayed to June, then August, and then an issue with one of the liquid oxygen prevalves was reported that pushed the start of installations until September. Delivery of the stage was also delayed until November or late fall of this year, but it isn’t needed for stacking in Florida until next year, so the schedule still has some positive margin in case there are any subsequent issues.

At the time of the interview on August 29, Muddle noted that Aerojet Rocketdyne had just taken the engines back out of their containers. “Just yesterday we pulled them out of the shipping containers and have [redone] some of the pre-installation inspections that we [invalidated] when we put them in that state of storage,” he said. “We finish those up today.”

The Artemis II core stage is seen at MAF on March 17 following the bolting of the engine section to the rest of the rocket. Credit: NASA/Michael Democker.

At the end of August, the rest of the Aerojet Rocketdyne team was waiting to travel to New Orleans to support engine installs. “We’re in communications with [core stage prime contractor] Boeing, they’re keeping us up to speed, we’re probably talking on a daily basis now because we’re getting relatively close,” Muddle said at the time.

“They’ve got a bunch of constraints that they have to work off [before they are ready for] engine installation and as they work through that we’ve been talking about a tentative date because I do have a bunch of guys from KSC that need to travel down there to support engine installation.”

Now that the stage reached the milestone of being ready to accept the engines and installation has begun, getting all four engines soft-mated first is the next major milestone. “That’s always our goal, to put one in right after the other, it allows the team to be focused on their task,” Muddle noted.

“We’ve been stressing that we prefer a focused team and [installing all] the engines in a relatively timely manner to keep the team focused on the task at hand because putting an engine in is pretty critical and there’s some close tolerances.”

Post-installation steps

After the engines are fully secured to the stage, or “hard-mated,” the Aerojet Rocketdyne team will support leak checks to verify the integrity of the interfaces between the stage and the engines.

“The first thing is we’ll do all the interface leak checks, to make sure all that is [properly sealed],” Muddle said. “That’s very critical because if you have one of the low-pressure fuel or LOX pumps that starts leaking, that’s a lot of work to back that pump out and change that seal out and we give ourselves three attempts at correcting the leak and then that’s a potential engine removal.”

“We’re only talking about a [small] gap that we can pull the pump back to get the seal out of there, so you have limited view into what the cause of the potential leak was.”

Engine 2059 is seen installed in Core Stage-2 on Sept. 11. Credit: NASA/Eric Bordelon.

The post-installation work that the Aerojet Rocketdyne team will be supporting is a little different than with Core Stage-1 because there’s no Green Run design verification campaign planned at Stennis this time around; after completion, the stage will be delivered from MAF to KSC for launch. “Once those leak checks are all good, then they’ll start powering up the vehicle and we’ll go through all our engine checks,” Muddle explained.

“There’s three categories that we always check, we’ll do all our electrical checks, and then we’ll do our pneumatic checkouts, and then we’ll go through our hydraulic testing and then after that we’ve got a whole bunch of other tests that they need to do. [From a pretty high-level], then we’ll start doing our closeout inspections for shipment and then configuring the engines for transport to KSC. So we’ve got a bunch of work left.”

“Now remember on Artemis I, we did the interface leak checks and then we did the electrical checkouts and then we punted the pneumatics and the hydraulic stuff to Stennis [for Green Run], so this will be actually the first time that we’re doing the pneumatic checks and hydraulic checks at MAF.”

Core Stage-2 will be the last to go through final assembly at Michoud; final production of all future units will be completed in High Bay 2 of the Vehicle Assembly Building at KSC, so those could also be the last time those checks are done at Michoud. These are also the last engine installations planned at MAF and also in a horizontal orientation.

Beginning with the third SLS core stage, final assembly will also be done in a vertical orientation, including installation of the engines into the engine section/boattail. “We have an engine service area or ESA in the VAB, it’s just south of High Bay 1 which is opposite of High Bay 2 where Boeing is going to go do their work, so we’ve working with [the SLS Liquid Engines Office] to transform that into a pre-shipment processing area,” Muddle said.

The area is the previous location of the engine shop for SSMEs during the early era of Space Shuttle flights in the 1980s and 1990s. “I’ve come full circle,” Muddle said. “I started in that engine shop and will probably finish my career in that engine shop.”

(Lead image: The first RS-25, engine 2059, is installed into the Artemis II core stage at MAF on Sept. 11. Credit: NASA/Eric Bordelon.)

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