The RS-25 engine test team at NASA’s Stennis Space Center in Mississippi ran the next hot-fire test in the A-1 test stand on Tuesday afternoon. The third test-firing in a series of nine continues the evaluation of engine components built by prime contractor Aerojet Rocketdyne using modern, updated manufacturing techniques aimed at reducing the cost to build new “production restart” engines for delivery in the early 2020s.

Development Engine 0525 (E5025) explored data points outside the center of the engine’s operating envelope during Tuesday’s eight-plus minute long test. Another engine computer was installed on E0525 for an acceptance test that will add the part to the inventory of Core Stage Engine hardware for the first launches of NASA’s Space Launch System (SLS).



Third test in series

Personnel from NASA, Aerojet Rocketdyne, and Stennis facilities contractor Syncom Space Services (S3) conduct RS-25 tests, taking the engine through an event-driven countdown that typically begins early on test morning. The test started when all of the prerequisite steps prior to ignition were complete and the hardware and the people were ready. Ignition was timed at 3:30 pm Central time.

Philip Benefield, Systems and Requirements Team Lead for the SLS Liquid Engines Office, said one of the primary objectives of Tuesday’s hot-fire is an acceptance test or “Green Run” of engine controller unit (ECU) FM9. “Also, we continue to demonstrate performance of the HIP-bonded MCC and the SLM POGO,” he added.

SLS will fly with four RS-25 engines in the Core Stage and each one has a dedicated, redundant ECU that controls its operation, monitors its health, and communicates with the SLS flight computers located on Block 1 vehicles in the Core Stage forward skirt.

The outline of the test is similar to the last two tests using E0525 in mid-August and early this month, with a planned duration of 500 seconds – which appeared to hit the mark during the actual firing. During the firing, E0525 was throttled at 111 percent of the RS-25’s rated power level (RPL) for about 310 seconds, and at 80 percent RPL for approximately 120 seconds.

The RS-25 engines flew throughout the Space Shuttle Program as “Space Shuttle Main Engines (SSME)” whose original RPL was 375,000 pounds of thrust at sea level, 470,000 pounds thrust at vacuum. They are “hydrolox” engines that use liquid hydrogen (LH2) for fuel and liquid oxygen (LOX) for oxidizer.

Tuesday’s test evaluated the performance of the new production restart components outside the center of the engine’s operating envelope. “We plan to demonstrate ‘high MR (mixture ratio) start’ conditions: min fuel pressure, max LOX pressure, and min LOX temp at start,” Benefield noted. “Also, this test will demonstrate both min and max run box pressures required for flight for both the fuel and LOX systems.”

Development engine 0525 was one of the last two ground test units in service at the end of the Shuttle program. E0525, E0528, and hardware for sixteen flight SSMEs were part of the flight and test engine hardware passed to the SLS Program, where they became RS-25s.

Prior to today’s hot-fire, E0525 has 87 ground test starts at Stennis in its service for Shuttle and SLS, accumulating 43,133 seconds of run time on its powerhead. Similarly, E0528 has 109 ground test starts, accumulating 58,842 seconds of run time on its powerhead at Stennis.

Both engines will continue to be retrofitted with new hardware over the next few years and rotated onto the A-1 test stand to help certify production restart changes. They will also continue to acceptance test new or refurbished components for the engine program’s hardware inventory.

Retrofit 1b overview

This was the third of nine tests planned in the Retrofit 1b series, which is testing the first new “hot-isostatic press” (HIP) bonded main combustion chamber (MCC), a “3-D printed” pogo accumulator assembly, and a new insulation system for the high-pressure fuel turbopump (HPFTP).

The HIP-bonded MCC at the heart of the engine is also the center of attention for this test series. Manufacturing of the MCC at Aerojet Rocketdyne’s Canoga Park facility in Canoga Park, California, now employs the same process the company uses for its RS-68 engines.

“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,” Dan Adamski, Aerojet Rocketdyne’s RS-25 Program Director, explained.

“[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 what that does, 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.”

New pogo units are now being built using “selective laser melting” (SLM), an additive manufacturing (also known as “3-D printing”) technique. This test series continues use of the unit first tested in the Retrofit 1a test series that concluded in February.

The new insulation system for the HPFTP also borrows production techniques from the RS-68 program, injecting the insulating material into a mold that fits around the pump. The old system was assembled from several different, individually built pieces and then fitted over the pump in a more labor-intensive process.

The production restart components maintain the form, fit, and function of the heritage SSME designs with the goal of maximizing affordability reductions in production cost and time.

Beginning in December, 2017, the focus of RS-25 ground testing shifted to development and certification of the “production restart” design. Testing from 2015 through most of 2017 helped to certify the already-built “adaptation” engines and a new engine control system to the SLS flight environment.

The first four SLS launches will use engines built during the Shuttle Program and fly them during most of ascent at a throttle setting of 109 percent RPL. Production restart engines will be flown at 111 percent RPL.

In addition to testing the performance of hardware built with the new manufacturing methods, NASA and Aerojet Rocketdyne are acceptance testing new engine controller units (ECU) that will fly on all the SLS launches. The ECU design was certified during the previous adaptation test series and new units produced by Honeywell are being delivered to Stennis.

ECUs for the first two sets of flight engines and beyond have already been tested and new units will continue to fill out the inventory for the first four launch sets in hot-fire tests on the A-1 stand.

Shutdown sequence

The RS-25 engine can be throttled anywhere within (and slightly outside) its normal operating range, and in SLS as with Shuttle they will be throttled depending on in-flight conditions and performance. The production restart engines will be certified to throttle between 80 percent and 111 percent RPL during flight.

80 percent RPL is also the lowest the engines can be throttled on the ground today, although at altitude or in near vacuum during Shuttle and early SLS flights the engines can be throttled down near 65 percent RPL. “For a ground test without a diffuser, which is what we have on the A-1 test stand, you can’t throttle that low because the nozzle flow separates and it would damage the nozzle,” Benefield explained in an interview last year.

“So we can only throttle down to 80 percent on the ground…if we needed to throttle lower than 80 percent, we’d have to go back to a stand like A-2 that has a diffuser.”

Although the engine ran at 111 percent RPL during most of the test, it was also throttled back a few times, sometimes in an incremental stair-step or step-down fashion and sometimes in a linear fashion from one setting to the next. “The ‘stair-step’ throttle down near the end of the test simulates flight,” Benefield explained.

“Near the end of flight the vehicle flight computers must slowly throttle the engines back in a ‘stair-step’ fashion in order for the vehicle to remain within its max acceleration limit. Other times in flight the vehicle requires a ‘constant ramp down’ from one power level to another; hence, this capability is demonstrated during ground test as well.”

Benefield said the next test in the series, number four, is planned for October 11th.