Hot-fire tests of new RS-25 engine components will resume in 2021 to certify a new manufacturing process during production. Over one hour of engine run-time is planned for the Retrofit 2 series to test an expanded set of new engine parts that serve to achieve a primary objective of the production restart program: improving affordability by applying modern manufacturing technologies like additive manufacturing, also called “3-D printing,” to building new engines.
Aerojet Rocketdyne plans to complete two other development engine builds in 2021 that will go into the test stand for subsequent test series, including an all-new build that will demonstrate the first new engine powerhead and nozzle assemblies built for RS-25 engines in nearly two decades.
Retrofit 2 test series set to start
Retrofit RS-25 development engine 0528 (E0528) has been installed in the refurbished A-1 Test Stand at the Stennis Space Center in southern Mississippi, where it will soon begin another series of hot-fire tests working towards the goal of certifying the restart of engine production for the Space Launch System (SLS) Program.
“Retrofit 2 is what we call a series of tests that’s going to be using one of our development engines, which is unit 0528, one of our two workhorse development engines,” Johnny Heflin, manager of the Liquid Engines Office for NASA’s SLS Program, said in a November 17 interview.
Heflin noted the facility would then undergo checkouts after engine installation, with a goal to be ready to enter the test campaign as soon as the SLS Core Stage completes its Green Run operations at the neighboring B-2 test stand. SLS inherited 16 flight units of the former Space Shuttle Main Engines (SSME) from the Space Shuttle Program to power expendable Core Stages for the first four launches.
NASA originally contracted with Aerojet Rocketdyne to certify those existing “adaptation” engines from Shuttle days to the higher-performance SLS operating conditions; in late 2015, a separate contract was awarded to restart production of new engines and certify the new build process. A primary objective of the program was improving affordability by applying modern manufacturing technologies like additive manufacturing, also called “3-D printing,” to building new engines.
(Photo Caption: A graphic showing the basics of the evolution of the multi-flight, reusable requirements for the SSME to the higher-performance, reduced service life RS-25 production restart requirements. New engine components will be certified for only a small number of firings that culminate in a single flight on the expendable SLS vehicle.)
The form, fit, and function of the existing SSMEs are being carried forward, along with the design’s reliability, but SLS is an expendable vehicle and more engines will need to be built to support launches compared to the reusable Shuttle orbiter vehicles. The new engines will also be certified to fly at a higher performance level of 111% of the original Shuttle rated power level (RPL); the late 1970s original RPL of the engine was 375,000 pounds of thrust at sea-level, 470,000 pounds thrust at vacuum.
The Retrofit 2 campaign will hot-fire test new engine components as they come off of restarted production lines. E0528 is configured for the series with a larger set of new production restart components — including an additively-manufactured pogo accumulator and a hot-isostatic press (HIP) bonded Main Combustion Chamber — that have been retrofitted to the Shuttle-era powerhead and new Retrofit 1 units
“We’re going to run eight tests for a total of about 4,150 seconds in this series, and what we’re going to do is… we have the second unit, the second HIP-bonded Main Combustion Chamber that’s come off the production line. So that unit will be new; it’ll be the second sample,” Heflin noted. “When we do this early development testing, we like to get as many samples as we can.”
“There’s variability from unit to unit, so we try to get a lot of samples where we can. So this will be the second Main Combustion Chamber of the new design to be tested. That’s another really large objective for us. We’re also going to put that additively manufactured pogo back on and continue to gather data on its performance,” he added.
“That’s a huge additive advancement for us. That pogo accumulator used to be, I think, 38 different parts. Now it is three pieces and one weld, one circumferential weld around the outside. The outer body of the pogo used to require a lot of welds.”
Along with continuing to test the new pogo and Main Combustion Chamber components, the engine is also retrofitted with new engine turbopumps. “We reassembled a high-pressure fuel pump with some new parts in it,” Heflin noted. “It’s not a new pump per se, but it does have new manufactured turbine blades for example in that high-pressure fuel pump.”
Another component with new 3-D printed parts is one of the main engine valves: the oxidizer preburner oxidizer valve (OPOV). “We’re also for the first time going to be testing an additively-manufactured ball-shaft on one of our valves on the engine, on the OPOV,” Heflin explained. “That’s the first time we will have gotten any hot-fire data on those SLM (selective laser melting) ball-shafts, so that’s another big objective for us. The housing is legacy design, but the ball-shaft is new.”
Heflin noted that Retrofit 2 has 37 test objectives total. “Some are performance-related, some are hardware-related,” he said. “We’re going to get some more time at a 113% power level.”
“That was a big deal for us in Retrofit 1. [It was] one of the few times that the engines have been run at a 113%, so we’re going to continue to gain test time at that higher power level.” New flight engines will be run at 111% RPL on future SLS launches, but the ground test program is running development engines at 113% to demonstrate operating margin.
(Photo Caption: Several of the major RS-25 components getting updated manufacturing processes to improve the affordability of the overall engine units. The cost reduction objectives are shown in red.)
The single-engine tests will resume in the A-1 Test Stand after over a year of inactivity. The last Retrofit 1 test was in late February 2019, followed by an acceptance test of the last unflown Shuttle-era engine, number 2062, in early April 2019.
Since then, with exceptions for the initial COVID-19 shutdown and tropical weather standdowns, NASA Stennis performed maintenance work on the A-1 stand’s infrastructure. In addition to upgrades to facility control systems, the test stand flame deflector and run-tanks were repaired.
A previously-installed facility thrust vector control (TVC) system will be used during a couple of Retrofit 2 hot-fire tests to prepare for testing new RS-25 engine hardware in following certification test series.
“We’re going to activate the facility TVC system on this test series, so we’ll gimbal the engines, not to capture an engine requirement but to wring out and demonstrate that the new TVC system on the facility operates as expected,” Heflin said.
“The engine has been gimbaled for many, many, many years in Shuttle, but in Retrofit 3 we’re bringing in new flex ducts that are designed to move the way the old flex ducts did on the SSME. The new flex ducts will be much lower cost.”
“The way we’re building them, we’re using flex hoses as opposed to old, very complex and very difficult to build articulating joints on the engines,” he added. “So for this test series we’re going to gimbal the engine while we’re hot-fire testing just to demonstrate on the facility side the TVC actuators react accordingly.”
The current plan for Retrofit 2 is to gimbal the engine during the second and third tests in the series. The engine will be moved for a total of about two minutes of the overall run-time across those two tests.
Production restart certification testing to begin in 2021
The other Shuttle-era ground test development engine, number 0525 (E0525), will receive a more thorough retrofit of production restart parts. “All the hardware is on schedule to get delivered to support that, and we’re planning on starting assembly in March  with testing on the Retrofit 3 engine in the summertime,” Heflin said. “Right now, it’s around the June timeframe.”
The Retrofit 3 tests are the second of the two hot-fire series that will be used to demonstrate the higher engine performance requirements using a larger set of new hardware. Aside from its powerhead, E0525 will be retrofitted with newly-produced engine components for the test series.
(Photo Caption: An assembled RS-25 nozzle jacket is seen at Aerojet Rocketdyne’s facility in Canoga Park, California. Manufacturing of the jacket was changed to reduce the number of pieces from a few dozen down to four.)
The test series is divided into two parts, Retrofit 3a and 3b. “3a is six tests and 3b is 12,” Heflin noted. “So 18 total.” Retrofit 3 and the following test series that will be run on the certification engine will help certify the production restart engine program.
“The certification engine is a little bit of a misnomer,” Heflin explained. “The cert requirements are based on the components, and there’s a whole set of criteria on how you define that. But the certification is also taking advantage of the Retrofit 3 series.”
“Some of the components require hot-fire tests on two different engines. So it’s a development engine, but we’ll certify some of the components. That hot-fire test [series] will be a part of our certification campaign. So Retrofit 3, as well as the cert engine, all that test time makes up our certification program.”
After the six tests in Retrofit 3a, E0525 will be removed from the stand and brought back to Aerojet Rocketdyne’s final assembly facility on the NASA Stennis federal city grounds. The series was divided into two parts because the first engine nozzle assemblies are trailing production of other components.
When E0525 returns to the shop, it will be retrofitted with one of the new engine nozzles. The first nozzle assembly, unit 6001, is scheduled to be delivered to Stennis in 2021.
“We’re making really good progress on the nozzle,” Heflin said. “It’s through all of what we call the ‘braze book’ or the braze operations. When it comes out of the furnace it still has a lot of work to do that is part of what we call the braze book.”
“That has been completed, and that nozzle is now over in the post-braze machining and assembly process. We’ve still got some work to do but closing the braze book is really significant because the affordability changes that were made on that nozzle all center on the jacket, the nozzle jacket that the tubes are brazed to.”
(Photo Caption: RS-25 nozzle unit 6001 is positioned in the open braze furnace at Aerojet Rocketdyne’s Canoga Park facility early in 2020. Over 1,000 cooling tubes are laid out on the inside of the nozzle jacket prior to entering the furnace, which fuses the tubes to the jacket wall. Hydrogen propellant flows through the tubes while the engine is firing to cool the nozzle from the 3,315°C combustion temperature.)
“Once you get through that braze operation, you’re past the point where all the pieces were changed by that design change,” he added. “[Now] we’re into the more familiar operations doing the post-braze machining and assembly. So we’re past the point where I think we’re going to be learning a lot due to first time through activities on the nozzle and we’re sort of past the new development part of manufacturing of the nozzle and into the more traditional operations.”
Which nozzle unit goes on which engine is still to be determined; the second unit was beginning the brazing process in November. “The second, the follow-up nozzle, 6002, is about to braze as we speak,” Heflin said in the mid-November interview. “It’s in the braze furnace right now, and we’re expecting to start those braze operations by the end of the month, so making really good progress on that nozzle as well.”
First new engine expected to be completed in 2021 for ground testing
When E0525 returns to the Aerojet Rocketdyne final assembly facility, the components for an all-new ground test development engine could already be in assembly there. “We’re calling it the certification engine, it got dubbed that because it’s a clean sheet engine, all new parts,” Heflin said.
The certification engine, which Heflin said will be engine number 10001, includes the first new powerhead; along with the nozzle, the powerhead is the other component that takes the longest to build. “The internal parts, the main injector, the preburners, the heat exchanger, all the internal operations are completed at this point,” Heflin said.
Those parts are assembled around the hot-gas manifold, which is the central structure of the powerhead. “They’re all assembled, and it is into machining and assembly operations where we’re going in and we’re drilling the bolt pours around the flanges and getting ready to install all the ducting that goes around the outside of that powerhead,” he added.
(Photo Caption: Former development SSME 0528, now a retrofitted RS-25, is lifted into the A-1 Test Stand in mid-November. The engine is outfitted with newly-manufactured RS-25 components including a new Main Combustion Chamber, pogo accumulator assembly, and other components now built using additive-manufacturing technologies.)
“If you’ve ever seen a picture of it, there’s a lot of external parts that have to be assembled onto that powerhead before it’s completed. It’s really coming along well; it looks like a powerhead.”
Assembly of the certification engine at Stennis is currently scheduled to begin in the late Summer or early Fall of 2021; depending on the progress of the Retrofit 3a test series and when the first nozzle, unit 6001, is delivered to Stennis, the certification engine could also get the first new nozzle.
“The Retrofit 3 series, 3a/3b, we’re going to test the cert engine in between those,” Heflin said. “So we’ll run 3a and then we’ll pull that development engine (E0525) and we’ll put the certification engine in the stand and start testing itt. And that’s going to happen in early calendar year .”
“What that does for us is it allows us to get test time earlier on the powerhead and some of the other components while the nozzle is being changed out [on E0525].”
After the six Retrofit 3a hot-fire tests, the all-new certification engine would go in the A-1 test stand for a set of 12 hot-fire tests.
Following behind the certification engine and the certification test program will be all-new flight engines. “They’re on track to deliver the first one in FY 23 (Fiscal Year 2023),” Heflin said. “It’s scheduled to test in the Spring or early Summer of  and the other engines will follow on about three-month centers after that.”
The engines won’t come off a production line; they will be assembled from components built around the country. “All the major components, that’s all four pumps on the engine, the powerhead, the nozzle, the Main Combustion Chamber, the engine controller, all those parts basically come off of their [own] production lines and then they all get shipped down to Stennis,” Heflin explained.
With the exception of the powerhead at the heart of the engine, components like pumps and nozzles and controllers are interchangeable, and they are the focus of the restarting production lines at two of Aerojet Rocketdyne’s facilities in Canoga Park, California, and West Palm Beach, Florida. Honeywell produces the RS-25 engine controllers at their Clearwater, Florida, facility.
(Photo Caption: From the October issue of NASA SSC”s Lagniappe, the newly refurbished A-1 Test Stand at Stennis in the early Fall of 2020. The yellow flame bucket and the large, white run tanks were among the systems that were reserviced over the last year and a half.)
“So I’ve got Main Combustion Chambers in production, and they’re lined up in the queue. I’ve got nozzles in the queue and I’ve got powerheads in the queue. And some of those are more advanced in [restarting] production than others. So right now we’ve got a lot of nozzles in work, but they’re not as far in the production cycle as say an Main Combustion Chamber might be because they’re a little bit farther.”
“It’s been challenging,” he added. “Restarting production on such a complex piece of machinery is always difficult, and we have dealt with those issues and what we call ‘first-time through’ activities and we’re getting pretty much past most of that. And the manufacturing is really stabilizing and coming along really well.”
“All six of the [flight] engines that were on contract previously are in one state of manufacturing or another. And the following 18, the contract that was awarded this year, those engines… we’re already getting a lot of material in the door. So those will follow right behind those six as soon as they’re ready to be queued up in the manufacturing operation.”
Lead image credit: NASA/SSC.