Four Aerojet Rocketdyne RS-25 engines are fully installed in a NASA Space Launch System (SLS) Core Stage for the first time. The Space Shuttle Main Engines (SSME) donated to SLS from the forty-year long program that ended in 2011 were hard-mated in Core Stage-1 last week after being physically attached between mid-October and early November.



The lower stages of SLS were designed in general around Shuttle rockets and the Core Stage was designed in particular around the RS-25. Although largely the same as when they flew in Shuttle, NASA certified the design and new engine controller avionics to fly in the more demanding SLS operating environment and they will be flying to space as a quartet on SLS, compared to flying in a trio on Shuttle.

After getting new controllers, the engines were delivered by Aerojet to NASA two years ago and transported to the Michoud Assembly Facility (MAF) in June as the first Core Stage neared readiness to receive them. Post-install, the first working Core Stage is now going through final assembly checkouts ahead of transport to the Stennis Space Center in Mississippi for the Stage Green Run test campaign, where the RS-25 engines will help verify the performance of the new stage.

First SLS engine installs complete

The remaining connections between the four SSMEs and the Core Stage were completed last week in the final assembly area at MAF, and final integrated checkout of the stage is underway prior to shipment to Stennis. “This thing is massive, it’s going to be an amazing sight when it leaves MAF and goes to Stennis and then when it leaves Stennis and it goes to KSC (Kennedy Space Center),” Andrew Rostron, RS-25 engine installation field engineer with Aerojet Rocketdyne, said in a recent interview.

“It’s incredibly exciting to see this rocket progress and now you’ve got all four engines on there and it looks amazing. It’s been an exciting run. I’m looking forward to Green Run and then the first launch.”

All four engines were soft-mated first; the final engine was attached to Core Stage-1 in the number four position late on November 6. “We started with engine two top-left, then we did engine one,” Rostron noted.

“We did what’s called ‘soft install,’ we transferred the weight of the engine from our GSE (ground support equipment) to the vehicle and we did all four engines in that manner as Boeing was working through the vehicle constraints to get the vehicle ready. So when I say a ‘soft install,’ we bring the engine behind the vehicle, we get it onto [the structure], [and] put on a number of bolts and hardware.”

(Photo Caption: The first engine installed in the stage, Number 2056, is lined up with the number two position on October 19. The engine is held in a yellow carrier that was the main part carried over from the installer used in the Shuttle program to do horizontal installations. It now sits on a structure specialized for Core Stage engine installs.)

“We use stiff arms to secure the weight of the engine, so that kind of temporarily holds the engines and properly keeps them in a safe configuration as a soft install.”

Engine 2056 was the first engine installed in the number two position on October 19, top-left in the current stage orientation. Engine 2045 was attached on October 29 in the top-right, number one position, followed by engine 2058 in the bottom-left/number three position, and engine 2060 in the bottom-right/number four position.

“We actually got engine three and four installed in a matter of three days, so we knocked those two out and now we’re in the process of securing the engines,” Rostron noted on November 11. “That’s part of hard install.”

“It’s all a flow process that’s based on the access to the vehicle. We use a lot of modeling to figure out a proper way to remove our ground support equipment and properly secure the engines.”

“We use a lot of the folks that are my leads with that [experience] on Space Shuttle and their knowledge and we use a lot of model-based [analysis] to figure out the best way to get our folks inside the vehicle and get access and then we work through the flow process of properly securing the engine,” he added.

During launch the four engines are fed propellant through their inlets from the large liquid hydrogen and liquid oxygen tanks that make the stage as long as it is, but each one is also connected to the flight computers and overall network of avionics boxes and instrumentation in the stage. They also receive electrical power, a constant supply of gaseous helium while they are running, and hydraulics that allows them to control valve positions for ignition, mainstage, and shutdown, along with throttling.

The engines are bolted into the stage’s structure and each has its own pitch and yaw thrust vector control (TVC) actuators; the actuators allow the flight computers to gimbal the engines to help steer the launch vehicle during flight.

(Photo Caption: A pair of Boeing’s “indoor” Manufacturing, Assembly, and Operations (MAO) Self-Propelled Modular Transporters (SPMT) is prepared to begin installation of Engine 2045 into the number one position in the Core Stage on October 28.)

As in the Shuttle Orbiter boattail, the engines are clocked in different orientations for a better layout of all the Main Propulsion System (MPS) equipment inside the stage to service the engines. In contrast to Shuttle, gaseous hydrogen tapped off the engines while they are running not only feeds the stage’s repressurization system but also keeps the turbines spinning in each of the four Core Auxiliary Power Units (CAPU) that power the four hydraulic systems in the stage.

Most of the empty volume in the boattail is now taken up by the engine powerheads with just enough space for work platforms. The tight access inside drives the approach to finishing the installations.

“We are limited on our platforms, so what we try to do is we secure two engines at one time. So we focused on the lower ones first, engines three and four, then we moved up to engines one and two and that’s what we’ve been working on.”

The “heritage” SSME hardware and its reusable Block II design was adapted for use in the SLS Core Stage largely unchanged. The resulting “adaptation” version of the engine, now going by its RS-25 designation, will fly a final launch in sets of four on the expendable launch vehicle.

NASA integrated a new control system with the Shuttle-era adaptation engines for use on SLS, which includes new engine controller hardware and software. Two and a half years of hot-fire testing of two Shuttle ground-test engines by NASA and prime contractor Aerojet Rocketdyne certified the Shuttle design and the new control system in the SLS operating environment.

(Photo Caption: Engine 2056 along with the engine carrier is lifted into place on top of the engine installer in Building 114 on October 8.)

The first four launches with the adaptation engines from the leftover Shuttle inventory will normally fly at 109 percent of the original SSME “rated power level” (RPL) of 375,000 pounds of thrust at sea level, 470,000 pounds thrust at vacuum; at the end of the Shuttle Program the Block II engines nominally ran at 104.5 percent RPL. Aerojet Rocketdyne is restarting production of the engine and new builds will be flown at 111 percent RPL.

For SLS, in addition to operating at a higher power level, the liquid hydrogen and liquid oxygen propellant is also fed to the engines by the Core Stage at higher pressures and colder temperatures at their inlets.

Engines delivered in 2017

The four adaptation engines assigned to Core Stage-1 were trucked from Aerojet Rocketdyne’s facility at the Stennis Space Center to Michoud in late June, after being outfitted with new engine controllers and prepped for installation a few years ago. They were figuratively delivered to NASA in place at Stennis in October, 2017.

While waiting for first-time Core Stage assembly to reach a similar readiness for engine installs, Aerojet worked with Boeing to identify any get-ahead tasks that could done at Stennis.

(Photo Caption: The four engines assigned to Core Stage-1 and the Artemis 1 launch after arrival and unpacking at MAF in late June. From left to right: Engine 2045, Engine 2056, Engine 2058, and Engine 2060.)

“Back in 2017 we had all four Artemis 1 engines shipped in place and working with Boeing we were looking at how we can condense the schedule,” Rostron said. “We saw that we didn’t need to wait until post-Green Run to install that ablative material, it would take some time away, post [hot-fire] refurbishment, from the Green Run schedule.”

The material was applied to the two engine nozzles in the vicinity of the aft separation motors of the SLS Boosters. The nozzles for the engines and boosters are essentially next to each other in the SLS configuration; in Shuttle, the side-mounted orbiter rode higher above the boosters.

Most of the engine nozzle area is actively cooled during mainstage operation, with elements like the hatbands and drain lines fitted with passive thermal protection during Shuttle for ascent and re-entry heating. The additional thermal protection remains for SLS, with a limited amount of ablative material added over the top.

“Engines one and four have ablative material inline with the SRBs (Solid Rocket Booster) jettison motors, we have a lot of thermal heat sink in that area,” Rostron noted back in late June. “To protect the nozzle, we’ve added ablative material in certain areas of engine four and engine one as well.

“It’s because of the jettison motors and the analysis shows the thermal sink is going to be higher in that area.” Two engines face the aft SRB separation motors, with engine one closest to the left booster sep motors and engine four closest to the right booster sep motors.

(Photo Caption: A diagram of the SLS launch vehicle looking up from the bottom, showing the locations of engines one and four adjacent to the left and right aft SRB separation motors, respectively. For additional margin/protection when the boosters separate and the motors fire, ablative thermal protection material was added in selected locations on the nozzles that aren’t actively cooled during ascent, such as the structural hatbands.)

“Boeing asked us to install their modal sensors earlier than to do it at MAF, so while we had the engines uncanned we installed those modal sensors and ground test instrumentation and the ablative and then we re-canned the engines and they were ready for when the vehicle was ready.”

The ground test instrumentation fitted on the engines is not a working part of the engine, it is for the modal test that will be performed at the outset of the Stage Green Run test campaign.

Final engine install preps at MAF

Each engine was staged on a mobile GSE structure for installation into the Core Stage. “With the crane height you can’t do those operations in the engine processing area, so we transport the engine,” Rostron explained.

“We have a tug puller that brings the engine down to Building 114. We transfer the weight of the engine from our blue handler, which is our ground support equipment that the engine can rotate and transport on, to the engine carrier and then the carrier takes the weight of the engine.”

(Photo Caption: The engine installer with Engine 2056 loaded is parked in the engine processing area on October 8. Boeing was still preparing the Core Stage and ground support equipment installed inside the boattail for the installs, so the engine and installer were brought back to the processing area to wait. In the lower-left, engines 2058 and 2060 sit waiting for their turns to be installed that would come in early November.)

“We do our final prep work for engine installation and we work with Boeing,” he added. “They lift it up onto the engine installer and then we transport it over to final assembly, Area 47/48, and then we get up there [on to work platforms] while they are bringing it in and we do a final alignment.”

“We want to make sure that the engine installer is square to the vehicle so that when we come in, we come in perfectly straight as we’re transitioning through the boattail fairing of the Core Stage.”

Five technicians were positioned on the work platforms that Boeing had installed inside the boattail volume. “You’re forty-five feet in the air for engines one and two getting all that equipment up there, getting all that prep work you need, getting properly situated, your fall protection,” Rostron explained.

As Aerojet Rocketdyne explained in a news feature, while technicians had 360-degree access around the three engine positions in the Shuttle orbiter boattail they have only about half that access in the Core Stage engine section with equipment four engines packed in its aft compartment.

“As we’re moving in we have to manipulate the engine around to get our low-pressure fuel duct inside and get it back lined up and get it optimally put in all five axes to make sure that it’s perfectly centered,” he added.

(Photo Caption: A composite of NASA illustrations showing the arrangement of the low-pressure fuel (LH2, red-highlighted in the left drawing) and oxidizer (LOX, green-highlighted) ducts and the different clockings of the engines in Shuttle (left) and the SLS Core Stage(right). The low-pressure fuel duct, in particular, requires some maneuvering during install to clear the openings.)

Last two engines installed in three days

After engine 2045 was installed in the number one engine position at the end of October, the installer was reconfigured for the bottom two engines. A spacer section was removed so that the last two engines loaded into the installer would line up with the engine position in the stage.

This was the first time engines had been installed in a Core Stage engine compartment, but Rostron said that getting the last two engines loaded in three days wasn’t completely the result of a classic learning curve.

(Photo Caption: Looking up from the floor at the four soft-installed engines in the Core Stage-1 on November 7, the day after the last one was put in. Propellant, hydraulic, pneumatic, structural and electrical connections from the Core Stage to the engines were fully mated at the end of last week.)

“Yes and no, because each engine is angled differently inside the Core Stage so it depends on the platforms inside the vehicle,” he explained. “You know when you’re forty-five feet in the air for engines one and two, it changes when you’re only fifteen feet in the air.”

“Obviously when you’re over flight hardware all of our tools and hardware is tethered, so no matter whether you’re at forty feet or fifteen feet you still have those [precautions] but in terms of I’ll say hauling up all of your equipment and tooling you have less platforms to go up, you have better access if you missed something. [There’s] a little bit more time-saving.”



“We did a lot of after-action reviews to see what we could learn, what we could adjust, and we made changes to our sequence and our planning to optimize the best case for each engine after that,” Rostron added. “So as we went through we started getting some lessons learned, what we can utilize better, both teams that we had talked to each other very well to indicate what worked and what didn’t work.”

“As we got to [engines] three and four, we utilized those lessons and we were able to install them with far less issues.”

Now that the engines are securely installed in the stage, they are ready for testing at Stennis next year. “As far as configuration-wise minus removing some covers and closures that are just protective covers and closures for transport, the engines are ready they don’t need anything else added prior to the Core Stage Green Run,” Rostron said.

The engines will be leak checked during the stage integrated testing going on for the next month at MAF.

Lead Image Credit: NASA/Jude Guidry.