RS-25 conducts another hot fire as SLS flight engines line up for EM-1

by Philip Sloss

The RS-25 engine team has conducted the latest engine controller unit (ECU) hot-fire acceptance test on Tuesday in the A-1 test stand at the Stennis Space Center in Mississippi. NASA, RS-25 prime contractor Aerojet Rocketdyne, and Stennis facilities contractor Syncom Space Services (S3) performed a flight-duration test firing of Development Engine 0528 (E0528) with the third of four ECUs that are planned to fly on the first launch of NASA’s Space Launch System (SLS) launch vehicle.

RS-25 Test:

The latest Honeywell controller was installed on the test engine earlier this month as NASA looks to complete work to deliver the flight engines for the Exploration Mission-1 (EM-1) launch.

Philip Benefield, Systems and Requirements Team Lead for the SLS Liquid Engines Office, said in an email head of the firing that the primary objective for this test is to acceptance test (or “green run”) the FM5 flight model ECU.  Each engine has a dedicated, self-redundant controller unit that controls its operation, monitors its health, and communicates with the launch vehicle flight computers.

Tuesday’s test was the third in a string of engine controller green run tests that began earlier this year.  The FM2 and FM3 ECUs were green run on E0528 during the last two hot-fire tests, similar flight-duration firings in March and May, respectively.  Benefield noted that both units performed well and passed their tests.  The first flight model unit (FM1) is being used for lab testing only.

Originally scheduled to be the fourth in line, FM5 jumped in front of FM4 in the line for testing when it completed final assembly and checkouts at Honeywell first.  The final controller needed for the first launch, FM4 will get its Green Run in the next RS-25 hot-fire test, currently planned for early to mid-August.

In contrast to the last two hot-fire tests that targeted a time of ignition, the test team at Stennis is using the more typical event-driven approach for this test.  When all of the prerequisite steps prior to ignition are complete and the hardware and the people are ready, the test will start.  Going into the test, the window for ignition was in the mid-afternoon Central time.

The planned duration of the test was 500 seconds, which is the approximate operating time of the RS-25 engines for SLS launches.  During the test, the engine were throttled at thrust levels between 80 percent of rated power level (RPL) and 111 percent.  273 seconds of run time during the test was at the 109 percent RPL, which is the nominal power level for launches.  The engine was throttled at 80 percent RPL for 97 seconds, and seven seconds of run time was spent at 111 percent.

Originally developed in the 1970s for the Space Shuttle Program when it was known as the Space Shuttle Main Engine (SSME), RS-25 hot-fire testing began at Stennis in January 2015, to demonstrate and certify engine operation at the higher performance levels for SLS.  SLS is both physically longer and at times will accelerate faster during launch than Shuttle did, requiring different starting and running inlet conditions for the engines through powered flight.

Four engines will fly in the SLS Core Stage, burning cryogenic liquid hydrogen (LH2) and liquid oxygen (LOX) fed to them from the stage’s propellant tanks.  The engines will run at higher pressures and higher thrust than on Shuttle, and the propellant is also fed to them at colder temperatures.  A new engine control system, including a new engine controller, is also being certified to fly with the SLS vehicle; this green run test of the flight model ECU also continues to accumulate the required data for certification.

Each test is designed to meet several test objectives, and although it is primarily a green run of the ECU, the test will also continue verification of SLS program requirements for the engines.

One of the other objectives of the test is to continue to demonstrate that the engine can start satisfactorily in a range of temperature and pressure conditions at its inlet.  Similar to the prior test, Benefield said the engine will be started with “poor combustion conditions” at the engine inlet — higher than normal LOX temperatures and lower than normal LOX pressures.

Tuesday’s hot-fire was the seventh in a test series with E0528 that began in July of last year. Fourteen tests have been conducted using both development engines (E0525 and E0528) and one flight engine retained from the Shuttle Program.

Certification of the new control system and controller also involves lab testing at Marshall Space Flight Center in Huntsville, Alabama, and at Aerojet Rocketdyne and Honeywell facilities in other parts of the United States.  Qualification testing with the FM1 ECU is currently forecast to be completed in mid-September.

Production of flight engine controllers continues at Honeywell, with the next unit to be delivered (FM4) completing the set planned for use in the first SLS launch.  As new units are completed, most of them will be rotated onto E0528 on the test stand for individual green run tests before being integrated into the existing engine inventory at Aerojet Rocketdyne.

In addition to the ECU green run hot-fire tests, two flight engines that were assembled and added to the inventory are still planned to be green run.  E2062 and E2063 have never been hot-fired as an integrated unit and so will get their green run tests as more flight ECUs become available.

One of the certification requirements is to demonstrate run time on a flight engine and the upcoming full engine green run tests will help meet that.  E2062 and E2063 are currently assigned to the second SLS Core Stage, but along with other flight engines will also serve as program spares in the meantime.

After the new controllers are green run, they are being removed from the development engine on the test stand and taken back to Aerojet Rocketdyne’s facility at Stennis for integration with the flight engines that are stored there.

The current schedule shows the four engines in the first flight set being “delivered” at the end of the (Northern Hemisphere) Summer between the end of August and the end of September.

The assignments of controller to flight engine have changed; FM2 is still assigned to E2045, but FM5 is now assigned to E2058, FM4 to E2056, and FM3 to E2060.

A recent forecast (L2 SLS Update Section) shows E2045 being delivered around August 22, E2058 around August 31, and E2056 and E2060 around September 26.

Although the delivery milestone of the flight engines will essentially signify they are ready for installation, they will wait at Stennis for the time being because the first Core Stage (CS-1) is not ready for them yet.

CS-1 is under construction at the Michoud Assembly Facility (MAF) in New Orleans; recent estimates show the flight RS-25s being transported to MAF sometime next Spring.  After the Core Stage elements have been joined together and started final assembly, the engines will be installed horizontally in the engine section.

The engines will eventually get an additional pre-launch test firing together in a green run of the completed Core Stage; that test will occur on the nearby B-2 test stand at Stennis after the stage is barged there from MAF.

(Images: Via NASA and L2 – including SLS renders from L2 artist Nathan Koga – The full gallery of Nathan’s (SpaceX Dragon to MCT, SLS, Commercial Crew and more) L2 images can be *found here*)

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