RS-25 hot fires with 3D printed Pogo – Facility issue cuts test short

by Philip Sloss

Following closely on the heels of certifying one set of RS-25 engine upgrades and updates for the first Space Launch System (SLS) flights, NASA and RS-25 prime contractor Aerojet Rocketdyne have begun the next phase of testing on Wednesday afternoon.  This involved the hot firing of development engine 0528 (E0528) at the Stennis Space Center at 14:45 local time, although the test ended early.

RS-25 Test:

The RS-25 team is gearing up to begin manufacturing new engines for future SLS launches and are introducing modern manufacturing and production methods in hopes of reducing costs and improving performance.

Development engine 0528 (E0528) is back in the A-1 Test Stand at the Stennis Space Center, sporting a new “3D printed” pogo accumulator assembly.  The beachball-sized engine part is the first “production restart” hardware to be hot-fired.

“All future RS-25 tests plan to incorporate production restart hardware,” Philip Benefield, Systems and Requirements Team Lead for the SLS Liquid Engines Office, told in an email.

3D printed pogo accumulator:

With hot-fire testing to support certification of the “adaptation” version of the RS-25 complete in October, NASA and Aerojet Rocketdyne are beginning tests of hardware for the follow-on version.

“This is the first RS-25 hot-fire test with production restart hardware,” Benefield said.

NASA will run out of existing RS-25 engines after four SLS launches and awarded a contract to Aerojet Rocketdyne to restart production of new engines.  In addition to restarting supply chains, manufacturing, and production, a primary goal for the future “production restart” engines is a reduction in the cost to produce new units.

The first hardware to be tested is a new pogo accumulator assembly manufactured using additive manufacturing (AM) processes (also known as “3D printing”) that are hoped will help with the cost reduction goals.

The pogo assembly, a development test unit, was manufactured at Aerojet Rocketdyne’s facility in Canoga Park, outside of Los Angeles, using an AM process called selective laser melting (SLM).

“This is one of the larger parts to be built with the AM/SLM process and we are using one of the largest machines available to achieve this part manufacturing,” Jeff Haynes, Aerojet Rocketdyne Additive Manufacturing Lead, noted in an email.

“Previously, we did extensive machining, which included welding structures from forgings and sheet metal [The redesigned manufacturing process] reduced the fabrication cycle time by 50 percent — printing the part is just a fraction of the total fabrication cycle time.”

According to information the redesign reduced the number of parts needed to assemble the accumulator from twenty-eight to six, eliminated 123 welds and one bolted joint.  The extensive changes would have been impractical or impossible using conventional manufacturing means, and have also reduced the unit cost by a third.

Although the manufacturing process was modernized, the assembly is built with a similar metal alloy to the conventionally manufactured one.  This results in a slightly heavier part, though Haynes added that the difference is within the SLS program’s weight allocations for the engines and the new part has slightly increased strength.

The unit was installed on E0528 in Aerojet Rocketdyne’s facility in Building 9101 at Stennis in November prior to the engine being transported back out to the A-1 test stand for reinstallation there.  The old unit, a ground test unit, becomes a development spare.

Although using new manufacturing methods, the pogo assembly retains the same form, fit, and function, which is to help to absorb and dampen the magnitude of oscillations that can start within the engine during mainstage operation.  The accumulator is connected in the engine’s oxidizer system between the low and high pressure liquid oxygen turbopumps.

Benefield noted that a primary objective for the hot-fire test on Wednesday is to “demonstrate [the part] at nominal operating conditions.”

Wednesday’s test:

The test team of personnel from NASA, Aerojet Rocketdyne, and Stennis facilities contractor Syncom Space Services (S3) conducted another countdown on Wednesday morning and afternoon, leading up to the eighth and final hot-fire of the year.  It was the nineteenth RS-25 hot-fire test at Stennis for SLS since January 2015.

The test team used the more typical event-driven approach — rather than targeting a specific time of ignition, the test will start when all of the prerequisite steps prior to ignition are complete and the hardware and the people are ready.  Ignition typically occurs in the mid-afternoon Central time. The firing began at 2:45 PM local time.

Notably, the test ended before the completion of its full duration firing, although this was no fault of the engine and all objectives were completed.

“The test was cut 70 seconds early due to a facility issue, but we achieved all test objectives,” noted Stennis PAO to “There are no issues with the engine.”

Another primary objective for the hot-fire was to “acceptance test” or “green run” another engine controller unit (ECU), FM3.  The unit was originally tested in May, but a retest became necessary.

“FM3 went back [to ECU prime contractor Honeywell] for an inspection [after its green run in May]; when they re-assembled the unit and took it through its ATP (Acceptance Test Procedure) it had an issue that they had to troubleshoot and ended up changing a board, so they had to put it back in the green run mix,” Benefield noted in an earlier interview.

After the test, FM3 should be integrated with one of the existing RS-25 flight engines in storage at Stennis.  The first four ECUs to pass their tests are already installed on the engines designated for the first SLS launch, Exploration Mission-1 (EM-1).

Wednesday’s test was the planned duration of 470 seconds.  During the firing, the engine was throttled at thrust levels from 80 percent to 111 percent of rated power level (RPL).

The engine was throttled at 109 percent RPL for 253 seconds, at 80 percent RPL for 85 seconds, and at 111 percent RPL for 50 seconds, which is the first significant run-time at that throttle setting for the RS-25 in the SLS program.  The production restart engines will be certified to fly at 111 percent, with future plans to ground test up to 113 percent RPL.

Engine retrofits:

The “heritage” Space Shuttle Main Engine (SSME) hardware and its Shuttle-era, reusable design was adapted for use in the SLS Core Stage.  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.

Although largely the same engine design and hardware, the RS-25 adaptation engines integrated a new control system for use on SLS, which includes new engine controller hardware and software.  Hardware for sixteen SSME flight units remained after the Shuttle program ended in 2011, and those will fly in the Core Stage on the first four SLS launches.

Aerojet Rocketdyne is working with NASA to make further changes to the RS-25 for SLS flights beyond the first four.  Although the changes are primarily aimed at cost reduction, the production restart design will also incorporate operational requirements changes such as increasing the nominal throttle setting from 109 to 111 percent RPL, which will benefit SLS performance when the engines begin flying.

The two development engines remaining from the Shuttle program, E0525 and E5028, will continue to be used for most of the hot-fire testing.  Both engines are being retrofitted with new production restart hardware in phases.

The SLM pogo accumulator was installed on E0528 as a part of Retrofit 1a, which was planned to last for a series of four hot-fire tests.  The next test is currently planned for mid-January.

Meanwhile, E0525 is getting the next major replacement component for Retrofit 1b.  “A production restart Main Combustion Chamber (MCC) will be incorporated on the next test series on E0525.  This MCC was built using a more simplified manufacturing technique requiring much less time and cost than its predecessor,” Benefield said.  That test series will begin towards the middle of 2018.

The development engines will retain their designations, but by the end of the certification program for the production restart design they should largely be different engines.

They will alternate time in the A-1 test stand qualifying the new hardware; while E0525 is testing another set of new hardware in the Retrofit 1b series, E0528 will be retrofit with that and more and go back in the test stand for the Retrofit 2 test series, followed by E0525 with Retrofit 3.

Honeywell is continuing to build and assemble engine controller units and acceptance testing of those continues.  “Each of the upcoming 4 tests on E0528 will greenrun a flight ECU to be used on an adaptation flight engine.  Flight ECUs will also be greenrun on the subsequent E0525 test series,” Benefield noted.

As they make it through testing those flight ECUs will be installed on the existing engines to fly on the second, third and fourth planned SLS launches.

Also in parallel with the development testing, the hardware for new production restart flight engines will be also be assembled to begin flying on the fifth SLS Core Stage.

(Images: Via NASA, Aerojet Rocketdyne and Philip Sloss/L2. To join L2, click here:

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