Aerojet Rocketdyne refurbishing RS-25 engines for Artemis 1 launch and production restart testing
With the completion of another single-engine RS-25 development test, Aerojet Rocketdyne is now refurbishing five… The post Aerojet Rocketdyne refurbishing RS-25 engines for Artemis 1 launch and production restart testing appeared first on NASASpaceFlight.com.
With the completion of another single-engine RS-25 development test, Aerojet Rocketdyne is now refurbishing five engines at the Stennis Space Center in Mississippi for their next use. After the second test firing of the Retrofit 2 series in the A-1 Test Stand on April 6, development engine 0528 is going through a streamlined refurbishment prior to its next firing as a part of the production restart Retrofit 2 test series.
In parallel, the four RS-25 engines installed on the first Space Launch System (SLS) Core Stage in the nearby B Test Stand are being taken through a longer refurbishment checklist for flight engine hardware. The next planned ignition for those Core Stage engines is for the first SLS launch on the Artemis 1 mission to send an Orion spacecraft to lunar orbit.
Production restart testing resumes
Hot-fire testing of the two current RS-25 ground test engines resumed on April 6 with the second test in the Retrofit 2 series. The test team of engineers from NASA, Aerojet Rocketdyne, and Stennis facility prime contractor Syncom Space Services fired engine 0528 (E0528) for 500 seconds.
Aerojet Rocketdyne (AR) said the primary objectives of the test were to demonstrate engine thrust vector control (TVC) and operating characteristics of the new oxidizer preburner oxidizer valve (OPOV) retrofitted on E0528.
The RS-25 engines support gimbaling, but the muscle itself is provided externally. The single-engine, hydrogen-oxygen A-1 test stand was modified a few years ago to support gimbaling the RS-25 and other possible future customers. The ground-side TVC system has been thoroughly tested, but the April 6 test was its first operation during an engine test-firing.
AR noted that the demonstration included 118 cumulative seconds of engine gimbaling by the new test stand system.
(Photo Caption: Aerojet Rocketdyne is now processing the five RS-25 engines shown in this composite image. Engine 0528 on the left is a development engine that is ground-testing newly-produced engine components retrofitted to its powerhead. Engines 2045, 2056, 2058, and 2060 are seen firing during one of the SLS Core Stage Green Run Hot-Fire tests; all five engines were originally Space Shuttle Main Engines used by the program for flight operations and engineering support.)
The Retrofit 2 test series is the second of four in support of certifying the restart and modernization of RS-25 engine production. Sixteen Block II Space Shuttle Main Engines (SSME) were retained from the Space Shuttle Program and will support the first four SLS launches.
The SSMEs were adapted largely as-is to the higher SLS operating requirements, which were recently demonstrated in the second Core Stage Green Run Hot-Fire test in the B-2 position of the B Test Stand at Stennis on March 18. SLS is an expendable vehicle and new engine builds will be needed by the program, first as spares and then for flight beginning with the fifth SLS launch.
Manufacturing improvements will allow the production restart engines to operate at 111 percent of the original SSME rated power level; during Shuttle, the engines operated as high as 104.5 percent, and the RS-25 adaptation engines will fly their last missions at 109 percent, just as they ran in the Green Run Hot-Fire tests.
In the April 6 single-engine test in the A-1 stand, E0528 started and throttled up to 100 percent power level before throttling up to 111 percent a little over six and a half seconds after ignition, simulating what the new flight engines will do during SLS launches immediately after liftoff.
Nominal “start box” conditions at the liquid oxygen and liquid hydrogen inlets were used for the test; for some development testing, the test stand can be configured to supply pressure and temperature conditions at the engine inlets at the edges or corners of the engine’s standard pressure-temperature ranges at start.Artemis 1 Updates
Aerojet Rocketdyne also said that E0528 was throttled at 111 percent power level for 384 seconds during the test, with another 23 seconds at 90 percent and 40 seconds at 80 percent power level where the engine was shut down. The restarted production lines of new RS-25 engine components are introducing modern manufacturing techniques, like additive manufacturing (AM).
The new OPOV installed on E0528 for Retrofit 2 is the first unit with parts built using 3-D printing, a form of AM. The new valve retains the same form, fit, and function of the engine design with a 3-D printed ball shaft. Goals of the production restart program are to reduce the cost of new engine production while maintaining the engine’s high reliability.
The two Shuttle-era development SSMEs, E0528 and E0525, are being retrofitted with production restart components in phases. Both engines participated in the Retrofit 1 test series, which began testing a 3-D printed pogo accumulator and hot-isostatic press (HIP) bonded Main Combustion Chamber.
With the April 6 test completed, the development engine joins its four former SSME teammates in refurbishment for their next firings. E0528 is being refurbished for its next ground test firing later in April; after two ground tests of the Core Stage, the next ignition of the four RS-25 flight engines will be for the first SLS launch on Artemis 1.
RS-25 refurbishment integral to Core Stage Green Run shipping schedule
The RS-25 engine design is largely the same between the development engines being retrofitted with production restart components and the engines that will power the Core Stage on Artemis 1; however, the usage environments are different and the requirements account for that.
“Requirements drive everything, so requirements on the single-engine tests are different than requirements on the vehicle,” Bill Muddle, Lead RS-25 Field Integration Engineer for Aerojet Rocketdyne, said. “They’re tweaked because [single-engine] testing can do a lot of things differently from valve configurations and access, things like that. They can do things differently than the vehicle.”
(Photo Caption: The four RS-25 engines in Core Stage-1 fire in the B-2 position of the B Test Stand at Stennis Space Center on March 18. The veteran engines served as test support equipment for the Green Run, which was a test of the new SLS Core Stage. The second Hot-Fire ran a full 500 seconds in duration, accomplishing all planned test objectives.)
“Because the vehicle is designed to go fly we have limitations,” he added. “Based on those limitations I have to diverge my requirements so that I can fit them into the Core Stage processing. Access is a big deal but if you go look at a borescope inspection of a high-pressure fuel pump blade it is exactly the same whether it’s on a single-engine test or whether it’s on the Core Stage.”
The way the engines are used is also different. Ground testing of development engines often go beyond flight specifications, not just in terms of temperatures and pressures within a single firing, but also in terms of the number of firings conducted.
“Development [tests] run in different regimes,” Muddle said. “Our ICD, our interface control document, tells us what [operating] box we can live in [with the flight engines].”
“We can’t go outside those [operating] boxes like development testing can go do.” Minimizing wear and tear on the flight engines and operating within the specified limits during flight use provides extra margins of safety that are mandatory for human-rated spaceflight.
In contrast, 24-hour turnaround operations were prototyped on an AR-22 engine, which was assembled from earlier generation SSME hardware that Aerojet Rocketdyne had maintained in storage. New Honeywell engine controller units that were developed as a part of adapting the Block II SSMEs for use on SLS as the RS-25 were coupled with pre-Block II hardware on an AR-22 engine that was test-fired ten times in ten days in the summer of 2018.
After firing, the RS-25 engines are purged to begin the process of getting them ready to fire again. The first purge is a part of the engine shutdown sequence; for vehicle firings, the gaseous helium comes from the supply stored in five large composite overwrapped pressure vessels (COPV) in the engine section of the Core Stage instead of from the test stand.
“Part of that shutdown sequence is that there’s a bunch of purges that come on the engines to help push all the residual propellant out,” Muddle explained.
“If there’s any LOX or hydrogen or hot gas or the combination of the two are left in the engine, we try to push that out so that it doesn’t sit there and boil off and then [possibly] ‘pop,’ which could actually cause damage to the engine so we try to push all that propellant out of the engine as part of the shutdown purge.”
(Photo Caption: An Aerojet Rocketdyne AR-22 engine fires in the A-1 Test Stand in July, 2018, as a part of a rapid-reuse demonstration of the SSME technology for the Defense Advanced Research Projects Agency’s since-cancelled (DARPA) Experimental Space Plane (XSP). The test team at Stennis Space Center fired the engine, assembled from retired pre-Block II SSME hardware, ten times in ten days. The average time between tests was about 18.5 hours.)
After residuals are evacuated from the interior of the engine powerheads during shutdown, the next purge uses heated, ground-supply gaseous nitrogen (GN2) to start the process of eliminating any moisture left inside. “Once the [shutdown] purge is complete, we bring on a purge from the ground through the vehicle to the engines and basically that becomes more of an inerting purge and also what we call a warmup purge,” Muddle explained.
“We start warming the engine back up again to get it back to ambient [temperature] and to help any residual hydrogen that’s left in the LH2 feedlines to start boiling off. That runs for approximately twelve hours and we call that a warming purge.”
After the Core Stage is inerted and it is safe for workers to return to the test stand, engine drying purges begin. “One of the first things to go do within 48 hours is to start drying the engines, getting that moisture out of the critical areas of the engine,” Muddle said. “It’s in two phases.”
Muddle said the first phase dries critical areas of the engines turbopumps and powerhead. “That’s where you can use the conditioned air or the GN2 and either one of them is heated to help dry the moisture out of those critical areas,” he noted. “That’s the first drying.”
“And that’s one of the four critical things that we have to do before we say that the engines are acceptable [for travel] before you pull the Core Stage out of the test stand,” he added. Muddle said the second phase of drying goes farther into the interior of the engine powerhead and the engine nozzle.
(Photo Caption: Closeup render of the SLS engine section during flight.)
A second critical task is to inspect and document the immediate condition of the engine hardware after firing. “We want to see if there’s any hot-fire related damage to the engine that we didn’t see on sensors or something like that,” he said. “We want to look at the exterior because once you start processing now you get into the potential for collateral damage when they’re bringing platforms in and doing work around the engine. So we want to see the condition of the engine before we start working around that engine.”
Following both hot-fire tests on the Core Stage as a part of the Green Run design verification campaign, both the engines and parts of the Core Stage were refurbished for their next firing. Tasks within the overall refurbishment work on the engines and the stage are divided and organized so they can be performed in parallel with each other as much as possible to maximize productivity.
In order to get hands-on access for other inspections inside the engine compartment, a multi-level, internal platform kit is installed inside the engine section to allow for up-close access to the hardware. “And also taking the heat-shield blankets off around each of the four [engines],” Muddle added.
The thermal blankets protect the engine powerheads from the heat generated when they are firing and the eight-minute long, static ground test exacerbated the heat flux on the bottom of the stage; for inspections and servicing inside and outside, they are removed.
“The way we work is we work from the bottom and we work from the interior,” Muddle said. “We have enough room in the engine opening to stand through that engine opening hole and do a lot of the inspections and a lot of the work around the engine through the engine opening.”
“We look at what is the best access to do the job and the technicians decide whether they want to go into the door to the engine section or they’ll pop their head up through the engine opening to do that work. It all depends on where the access is that they need to go work.”
Access to an engine in the A-1 Test Stand is greater than inside the four engine cluster installed in the Core Stage. The engine powerhead is not covered by any flight or ground test equipment in the single-engine test stand, where the whole test stand is substituting for a rocket stage as a engine services provider.
(Photo Caption: An infographic published by Aerojet Rocketdyne providing an overview of the steps involved in refurbishing an RS-25 engine from one firing to the next. Some of these steps are performed in parallel to complete the overall process faster and more efficiently.)
Aerojet Rocketdyne has iteratively refined the Green Run post-firing refurbishment timeline. Starting from their decades-long Shuttle post-flight knowledge bases, they have shortened the timeline and minimized the amount of work that will need to be deferred or “traveled” from Stennis to the Kennedy Space Center (KSC) launch site for Artemis 1.
“We’ve got tons of history on this Block II SSME version that we’re running today,” Muddle noted. “So we can [say] ‘we have no history of seeing this kind of damage in this area’ so we have high confidence that we don’t need to do this inspection.’ [Or] ‘We have high confidence that this leak check never failed.'”
An initial estimate of a couple of months worth of refurbishment work on the engines was refined to about six weeks last Summer, with half of the work done in the B-2 stand at Stennis and the remainder in the Vehicle Assembly Building at KSC after the Core Stage is stacked with its Artemis 1 Solid Rocket Boosters. By the time the plan was implemented after the January 16 Hot-Fire test, almost all the engine refurbishment work was expected to be completed at Stennis.
“Most of the travelled work has been minimized or eliminated as the refurbishment processes have matured,” Jeff Zotti, Aerojet Rocketdyne’s RS-25 program director, said in late January. “The post-test engine checkouts and inspections are very similar whether we re-test or ship to KSC, so the schedule is basically the same.”
In the case of repeating the Hot-Fire test at Stennis to complete all the Green Run objectives, the engines were not only fully refurbished but also fully prepared and checked out for another test-firing. They were ready to support a February 25 test date before issues with a Core Stage prevalve pushed the test to March 18.
A third critical area of refurbishment work after the engines are fired is inspecting the 1080 coolant tubes in each engine’s nozzle for leaks. The tubes circulate hydrogen to cool the inside of the nozzle from the hot, exiting, combustion gas.
“You want to know what you have to go repair because there’s areas on that nozzle that I’ll say could take a lot of time to get to actually repair the leakage,” Muddle explained. “So you want to know whether the leakage that you saw and the areas that you saw can be repaired or it can be accepted ‘use as-is.'”
(Photo Caption: The Artemis 1 Solid Rocket Boosters are stacked on the Mobile Launcher in the Vehicle Assembly Building at Kennedy Space Center.)
Another early area of refurbishment work is to do an internal inspection of the bottom of the engine nozzle. The aft manifold distributes the hydrogen from the engine powerhead through the nozzle coolant tubes. “We have to go in and borescope around the bottom of the nozzle to look for some potential contamination,” Muddle said.
Those four areas were previously prioritized as critical to removing the Core Stage from the stand to place it back on the Pegasus barge for transport to KSC. With the updated timelines, the rest of the RS-25 refurbishment work is also expected to be done at Stennis.
Any nozzle tube damage repairs would be performed in the stand. “We’ll get those repaired on all four of the [engines],” Muddle said. “Then what we start doing is we start getting into removing the ground test instrumentation. There’s some old modal testing instrumentation, all that stuff starts getting pulled off the engine. So all that non-flight instrumentation gets removed from the engine.”
Internal inspections of powerhead components would also be performed. “We start opening ports up on the engine so that we can start doing borescope inspections on the engine,” Muddle explained.
“We’ve also got to pull a filter off in the [oxygen] system that’s good for one hot-fire. Once we do those inspections, we’re pretty much at the end so now we’re going to start buttoning the engine up.”
Muddle also explained that opening up those areas to apply the drying purges and for access to make the inspections means they have to be retested when they are closed back up and put into a flight/firing configuration. “We’re going to get into doing the leak checks for all those disturbed joints,” he said.
“[We do leak checks on] all the joints that we disturbed as a part of drying [and] all the joints that we disturbed to do the internal inspections and removing the filter. So we get into those leak checks next.”
One set of leak checks will be deferred until the Core Stage is at KSC; before the stage is shipped from Stennis, blanking plates are installed in the Core Stage Main Propulsion System (MPS) for transportation. “We have to put those blanking plates back in to support the configuration of the vehicle so that we can have the [MPS] prevalves open,” Muddle explained.
(Photo Caption: Core Stage-1 arrives at Stennis in January, 2020, on NASA’s Pegasus barge. The stage will again be placed on the barge while lying in a transportation fixture and some stage systems will be configured specifically for transportation. Sometime after the stage arrives at Kennedy Space Center (KSC), one of the final engine refurbishment tasks will be a few changes to fully configure the engines for flight.)
“So we have to put the engines back in the transportation configuration, which is basically we’ve got to put [in] a transportation throat plug.” Leak checks deferred until after the Core Stage arrives in Florida for launch preparations and the blanking plates are removed, then those engine-MPS interfaces can be leak checked with the engines in their flight configuration while stacked in the VAB.
“The final thing that we do is a final inspection of the engines to make sure that we didn’t cause any collateral damage when we were processing it,” Muddle added. “Make one final look at the engine and basically after that we turn that engine over to TOSC (test and operations support contractor Jacobs) and then they start their nominal flow processing to get ready for Wet Dress Rehearsal and flight.”
Once Aerojet Rocketdyne completes their work, refurbishment is complete and EGS and Jacobs will take over ownership of final pre-flight processing. The launch team will perform the engine flight readiness test (FRT) that checks out the electrical, computer, hydraulic, and pneumatic systems in all four engines, along with their built-in redundancy, and runs the valves through a countdown and ascent timeline.
As with the sequence of events during the Green Run campaign at Stennis, the engines will go through a full countdown sequence during the WDR and be ready to start when the countdown part of the dress rehearsal is concluded. They will then go through a repeat of the countdown sequence again before finally firing again for the Artemis 1 launch.
(Lead image credit: NASA/SSC.)
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