RS-25 program gearing up for production restart, next test series

by Philip Sloss

Restart of RS-25 rocket engine production is ramping up at prime contractor Aerojet Rocketdyne’s Canoga Park facility in the Los Angeles area. Components for the next test series that will help certify design changes and modernization of manufacturing techniques are already assembled on a development engine at the Stennis Space Center in Mississippi.

Workhorse ground test engine 0525 (E0525) now sports a new main combustion chamber (MCC) built using methods taken from other engines that are intended to make building new RS-25s more affordable. The first test in the series will be next month.

Current work is focused on re-establishing the supply chain for parts and manufacturing of components using modern methods, culminating in assembly of a completely new engine in 2021 that will be used for certification testing. Canoga Park is where Aerojet Rocketdyne (AR) does most of the engineering for the engines, along with component production and testing.

“Retrofit 1b” starts in August

Aerojet Rocketdyne was awarded a contract by NASA in late 2015 to restart production of the RS-25, formerly known as the Space Shuttle Main Engine (SSME). The Space Launch System (SLS) Program has sixteen engines left over from the Space Shuttle era, which will be expended on the first four launches.

The existing engines are referred to as “adaptation” engines and testing to certify they are ready to fly on SLS was completed in the Fall of 2017. “Right now we are pretty much done with what we’ve called the adaptation series of testing, which is the sixteen heritage SSMEs and those will operate [at] our 109 percent power level,” Dan Adamski, RS-25 Program Director for Aerojet Rocketdyne said in an interview.

RS-25 “adaptation” engines at AR Stennis, July 2. Flight engine 2063, left, is assigned to the second SLS Core Stage. Development engine 0528, right, is partially disassembled. It will next be used in the Retrofit 2 ground test series. Credit: Philip Sloss for NSF/L2

“So right now we’ve pretty much gone through that test series [and] everything now is on the restart certification…which is 111 percent power level standard.”

Beginning in December, 2017, the focus of testing shifted from certifying the adaptation engines to development and certification of the “production restart” design. The new engines will be flown at 111 percent of the original SSME “rated power level” (RPL) of 375,000 pounds of thrust at sea level, 470,000 pounds thrust at vacuum.

E0525 will begin the second production restart test series, called “Retrofit 1b.” The center piece of the new series is the new MCC that sits at the heart of the engine.

“I’d say the exciting thing obviously is the HIP-bonded MCC,” Adamski said. “Last test series we were all excited about the first additive-manufactured pogo assembly and we actually demonstrated 113 percent power level last time, which is the highest power level we’ve ever run on the engine, so that was kind of exciting.”

Development engine 0528 runs at 113 percent RPL during test at Stennis on February 21. Credit: NASA

The new MCCs use a method called “hot-isostatic pressing” (HIP) bonding to form the jacket around the liner. “That’s a great thing for the program,” he said.

“Right now it’s all about driving the affordability — maintain the safety margins, maintain the reliability of the engine [and] the performance of the engine, but drive down the cost. And the HIP-bonded MCC, that’s the single biggest affordability change going into the engine.”

“If I look at a single component, that’s it,” he added. “I think the number is on the order of like 50 percent reduction in cost, compared to the heritage Space Shuttle Main Engine MCC.”

“So that’s a huge thing for us — and it’s not only a cost driver, but then you also look at driving down the cycle time, how long it takes. I think that’s about a 50 percent reduction also.”

“The heritage SSME MCC worked great, no problems,” Adamski explained. “You’ve got the liner itself and you’ve got a structural jacket on the outside of that and the heritage SSME used a structural nickel plating process as that outside shell.  “[It] was good, it worked, but it’s a lot of plating, a lot of time, a lot of expense associated with that.”

First production restart main combustion chamber (MCC) at AR Stennis, April 2018. After the jacket is bonded to the liner, the outside of the jacket is machined down to reduce weight. Credit: Aerojet Rocketdyne

For the new HIP-bonded MCCs, Adamski explained: “You’ve got the liner and you’ve got the jacket separately, you put that into the furnace and that hot, high-pressure allows you to make that bond between the liner and jacket and that’s what we’re utilizing now on the MCC.”

“[It] is exactly the same process that’s used on the RS-68 engine and exactly the same process that we used on the J-2X engine and ultimately we were able to reduce the cost and the cycle time on the MCC by over 50 percent of what it was for heritage SSME.”  Like the RS-25, those two engines use liquid hydrogen as fuel and liquid oxygen as oxidizer.

The first new MCC, number 8001, was completed in April and shipped to Stennis where engine final assembly and hot-fire testing occurs.

The additive-manufactured pogo accumulator assembly was the first major production restart component completed. It was test fired four times between December and February on Engine 0528 as a part of the Retrofit 1a test series. Aerojet Rocketdyne is using modern manufacturing techniques and the pogo accumulator was fabricated using an additive manufacturing / 3-D printing method called selective laser melting (SLM).

The first unit will also be used in this second test series and has already moved over to E0525. Major parts of the second pogo assembly are already 3-D printed at Canoga Park.

Aerojet Rocketdyne technician points to the 3-D printed pogo accumulator as installed on E0528 in November, 2017. After four hot-fire tests late last year and early this year, this same unit is now installed on E0525 for the upcoming test series. Credit: Aerojet Rocketdyne

As with lead components like the SLM pogo assembly, the MCC production line is up and running. The second MCC unit, number 8002, has already completed the jacket-to-liner HIP braze, with machining of the jacket upcoming. Once delivered to Stennis, that unit will join the second pogo unit for installation on E0528 for the next test series, which is Retrofit 2.

The upcoming Retrofit 1b test series is currently planned to begin with the first hot-fire in the A-1 test stand at Stennis, targeted for around August 14th. The outline of the first test is for a nominal flight duration of 500 seconds with a standard throttle profile, as would be seen in an engine “green run” or acceptance test.

There are nine total tests planned in this series.  “We’ll be green-running or acceptance testing a new flight controller every test, so we’ll put a new one on, take it off,” Adamski noted.

“The other big thing obviously [is] we will be characterizing the MCC. It’s a different MCC; we’ve done all the flow tests on it, things like that. It will the first test series with the new MCC, [so] we’ll be going through the start box and various things — different inlet conditions, different mixture ratios, and different power levels.”

Honeywell is continuing to assemble new engine controller units (ECUs) at its Clearwater, Florida, facility and ship them to Stennis. ECUs for the first two flight sets have already been acceptance tested; the tests in this series will help complete the hardware for the sixteen Shuttle-era adaptation engines.

“We’ve got three controllers waiting at Stennis to be green-run and we’ll be delivering the rest of them over the next few weeks,” he added.

The engine is in the final stages of assembly and checkout at the Stennis AR facility. In addition to the new MCC and pogo accumulator, the insulation for the high-pressure fuel turbopump is another affordability improvement being demonstrated. “[The] heritage SSME system was effective, it was good, but it was multiple pieces that would get clamshelled around the engine,” Adamski explained.

Development engine 0525 in final assembly at AR Stennis, July 2. Credit: Philip Sloss for NSF/L2

“We’re going to be using a different type of insulation system which is used on RS-68, on their fuel pump. We’re pretty much going to clamshell a mold around it and then you inject the foam inside that mold and so you use that as the insulation system.”

“It’s very effective and much cheaper and much easier to work with than what we were using before, so we’re going to be demonstrating that on the high-pressure fuel pump,” he added.

The A-1 test stand underwent a period of maintenance and upgrades after the Retrofit 1a series finished in late February. E0525 is slated to be installed in the A-1 stand in mid to late July.

Retrofit 2

Following the Retrofit 1b series, the current plan is to perform the green run on Engine 2062, which is one of the adaptation engines in the second SLS flight set. Those four engines will serve as spares for the first flight set and E2062 is the last of the sixteen Shuttle-era engines that needs to be acceptance tested on the ground.

Although the hardware was built before Shuttle flights ended in 2011, the engine is one of two that hasn’t flown. With delays in SLS development, the acceptance test has moved out on the calendar behind higher priority testing.

During the maintenance period earlier this year, the A-1 test stand was outfitted with a new thrust vector control (TVC) system that will see use during the next development test series, called Retrofit 2. E0528 is pointed at that test series, which will include new units of all the components being demonstrated in the first two retrofit series along with new flex hoses.

Testing of the new TVC system in the A-1 test stand in May using an RS-25 mass-simulator. The system will be used by the RS-25 program beginning with the Retrofit 2 test series. Credit: NASA Stennis Space Center

“So it will be the second MCC, it will also be the second pogo,” Adamski said.  “[The] big thing on that series is we’re planning to get the flex-hoses on the ducts. That’s one of the big changes that we’re making to the engine is we’re doing away with the fairly complicated flex joints that we’ve had.”

“We’ve got a little bit lower gimbaling requirement for SLS so that allowed us to go to flex-hoses, which is a tremendous affordability improvement for the engine. So that’ll be a big thing going into Retrofit 2 on 0528.”

Retrofit 2 is planned to include twelve hot-fire tests, and the flex-hoses and the ground TVC system will be exercised as gimbaling is introduced into some of the test firings.  The final retrofit test series, Retrofit 3, is planned with E0525 and will include the first production restart nozzle assembly.

New engines

The development phase of the production restart program will culminate in production and assembly of the first complete new engine. AR is calling this first new engine a “certification engine.”

It won’t fly, but will be a fleet leader going through a ground test series that will certify the new integrated design, performance parameters, and production methods.  The first of six new flight engines should follow close behind and long lead items for those engines are already being fabricated in Canoga Park.

“The development and certification series that we’re in the midst of now and that will conclude with a design certification review (DCR) in the December 2021 time-frame, that will now baseline and certify the engine,” Adamski said. “That’s the certification for those six engines.”

“So right now the main focus of the program is to implement affordability design changes, things like that since the last time that we’ve built these engines,” he added. “In some cases, with some of these components, that’s 10-15 years ago, plus.”

“And so it’s implementing all that affordability — design changes, manufacturing changes, affordability improvements — that’s what the focus of the program is now, so when you get to that certification engine, that’s the culmination of all that work and now gives you the certified configuration, which is that six-engine production configuration.”

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