Following a quick end to a hot-fire test two months ago, NASA’s test team at the Stennis Space Center in Mississippi run RS-25 Development Engine 0525 in the A-1 test stand again on Wednesday. The last test on December 12 was manually cut off after only thirty seconds, coincident with a fire observed near the engine powerhead.

Wednesday’s test was the eighth in the “Retrofit 1b” series for prime contractor Aerojet Rocketdyne’s production restart program. Four RS-25s will power NASA’s Space Launch System (SLS) Core Stage and Aerojet Rocketdyne is using modern, updated manufacturing techniques aimed at reducing the production cost of new engines.



Hot-fire test first after quick December cut off

The previous test started at 3:24 pm Central Time on December 12 and ended just thirty seconds after ignition when the shutdown command was manually sent to the engine. A fire was observed around the engine powerhead in the live NASA Television coverage for several seconds prior to the manual shutdown.

NASA released a short update on December 20 that the test was manually terminated early due an anomaly in the data and that damage to the stand and the engine were limited. “The root cause has been traced to a non-flight test pressure transducer mounted near the engine,” Philip Benefield, Systems and Requirements Team Lead for NASA’s SLS Liquid Engines Office, said in an email.

“Visual indications of a small fire were seen a few seconds after engine start. The fire was from a gaseous hydrogen leak of [the] transducer mounted on the facility near the engine.”

Although the test didn’t last very long, Benefield explained: “Even with the short duration of the Dec. 12th test, test 1036, we achieved many of the planned objectives, including the primary objectives. Secondary objectives planned for but not achieved on test 1036 due to the short duration have been added to test 1037.”

“Test 1037 adds the new primary objective to Greenrun ECU FM4, as well as a few new secondary objectives not planned for test 1036,” he added. The ground test pressure sensor that triggered December’s abort is not needed or being used for Wednesday’s planned test.

Wednesday’s planned Test 1037

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 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 afternoon, Central time. The NASA TV schedule shows live coverage of the test is planned. The latest test fired up at 3:30pm Central.

“Primary objectives are the Greenrun of ECU (Engine Controller Unit) FM4, and continued evaluation of the HIP-bonded Main Combustion Chamber (MCC),” Benefield said. The flight model (FM) engine controllers are new to the RS-25 configuration that will fly on SLS, replacing the Shuttle era engine computers.

SLS will fly with four RS-25 liquid hydrogen (LH2), liquid oxygen (LOX) engines in the Core Stage and each one has a dedicated, redundant ECU that controls its operation, monitors its health, and communicates with the vehicle’s flight computers. Each new controller is acceptance tested or “green run” in a ground hot-fire test prior to installation on existing flight engines.

NASA has hardware for sixteen flight Space Shuttle Main Engines (SSME) left over from the Space Shuttle Program to fly in sets of four on the first four SLS launches. Testing, including two years of ground hot-fire tests, certified that the SSME flight hardware could meet SLS launch requirements largely as-is.

SLS will operate the engines at higher thrust, higher pressures, and lower temperatures than during Shuttle.

The “hot-isostatic press” (HIP)-bonded main combustion chamber (MCC) is a focus of the current Retrofit 1b series. The design takes advantage of modern manufacturing technology and is expected to reduce production time and unit cost for new flight engines that will be built for downstream SLS launches.

As part of ground test evaluation of the new MCC, Benefield noted the team will evaluate the performance of the new production restart components outside the center of the engine’s operating envelope. “We will demonstrate a low mixture ratio start,” he said. “The engine LOX inlet pressure will be set to the minimum value allowed, and the engine fuel inlet pressure will be set to the max value allowed.”

“The engine will operate primarily at 111% RPL, 378 seconds total,” he added. “The engine will operate as high as 113% RPL, for 133 seconds, and as low as 80% RPL, for a few seconds just before cut-off.”

Designed in the 1970s as the Space Shuttle Main Engine (SSME) for the Shuttle Program, the engine’s original rated power level (RPL) of 100 percent was approximately 375,000 pounds of thrust at sea-level and a vacuum thrust of approximately 470,000 pounds. SLS will run the engines at higher thrust levels than Shuttle through most of flight.

At the end of the Shuttle program, the engines were normally operated at 104.5 percent RPL; for SLS, the left-over Shuttle engines (also called adaptation engines) will run at 109 percent RPL and then 111 percent RPL for the future production restart units. The engines are throttleable, and in SLS as with Shuttle they will be throttled depending on in-flight conditions and performance. The duration of the test is over 500 seconds, but Benefield noted one of the test objectives could vary that by a few seconds. “This test will demonstrate the new ‘Non-emergency safe abort’ procedure, which is initiated by a manual input during a short time window. Cutoff from 80% RPL should occur sometime between 505 and 515 seconds after engine start.”

Benefield explained that the new procedure is for only for ground tests: “When initiated, the procedure automatically throttles the engine to min (minimum) power level prior to shutting down (vs simply shutting down while operating at a high power level.) The procedure reduces risk of hardware damage due to high power level shutdown.”

During ground tests, the pre-programmed throttle sequence generally has the engine at minimum power level when shutdown is command. Although at altitude or in near vacuum during Shuttle and early SLS flights the engines can be throttled down near 65 percent, the minimum power level for ground tests is 80 percent.

“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 a 2017 interview. “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.”

In addition to possibly being applicable during the last test, the abort procedure would have come into play in a test on August 14, when failure of an A-1 facility valve forced the test to be terminated early while the engine was running at 111 percent.

Second to last Retrofit 1b test

Wednesday’s test was the eighth of nine planned in the Retrofit 1b series focused on RS-25 production restart. In addition to the HIP-bonded MCC, a “3-D printed” pogo accumulator assembly and a new insulation system for the high-pressure fuel turbopump (HPFTP) are being tested in this series on E0525.

The 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, 2018.

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.

New engines will be needed beginning with the fifth vehicle, and Aerojet Rocketdyne began work several years ago to restart engine production in parallel with work to certify the existing SSMEs were adaptable to the SLS operating environment.

The current plan after today’s test is for one additional firing with E0525 at the end of February. Following that, Shuttle-era flight engine E2062 will be fired for the first time in an acceptance test in April in the A-1 stand.

The engine was an unflown, untested spare built at Kennedy Space Center near the end of the Shuttle Program, and is the last of the sixteen that hasn’t been fired. The other unflown unit, E2063 was acceptance tested in October, 2017, as a part of hot-fire testing in support of initial SLS certification. Both E2062 and E2063 are assigned to the second SLS launch and will serve as spares for the first launch campaign.

As part of the production restart program, components built to the new manufacturing specifications are being cumulatively retrofitted on the two existing development engines that NASA retained from the Shuttle Program. E0525 and E0528 will be cycled back and forth into the shop for hardware retrofitting and then to the A-1 stand for testing.

E0528 will eventually follow E2062 into the test stand. It is being retrofit with the current set of hardware with the new manufacturing improvements, along with additional components for the Retrofit 2 test series.