RS-25 fires up again in another controller test for SLS

by Philip Sloss

A little more than two weeks after the last test, the RS-25 test team acceptance tested the next completed engine controller unit (ECU) that will eventually fly on NASA’s Space Launch System (SLS) launch vehicle.  Another flight-duration hot-fire of Development Engine 0528 (E0528) occurred Wednesday afternoon local time in the A-1 test stand at the Stennis Space Center (SSC) in Mississippi.

RS-25 Hot Fire:

Once again NASA, RS-25 prime contractor Aerojet Rocketdyne, and Stennis facilities contractor Syncom Space Services (S3) performed the test firing with the latest modern ECU built and delivered by Honeywell to Stennis.

Following the most recent hot-fire test on July 25, the latest controller was rotated onto the test engine to begin final preparations for the test.

Each RS-25 engine has a dedicated, self-redundant controller unit that controls its operation, monitors its health, and communicates with the launch vehicle flight computers.

Honeywell is building these new, modern units at their Clearwater, Florida, facility and as each of the flight model controllers are completed they are transported to Stennis, installed on an engine in the test stand, and hot-fired in an acceptance test (or “green run”).

Philip Benefield, Systems and Requirements Team Lead for the SLS Liquid Engines Office, said in an email that Wednesday’s test will green run the FM6 controller.

This is the fifth unit to be completed and the fourth to be hot-fired.

The FM2, FM3, and FM5 ECUs were green run on E0528 during the three most recent hot-fire tests on E0528 in March, May, and July, respectively.

As was the case for the tests of the other flight units, Wednesday’s test was planned to be a 500-second flight-duration firing, which is the approximate operating time of the RS-25 engines for SLS launches.  The first flight model unit (FM1) is being used for lab testing only.

As with the last test in July, the test team at Stennis was again using the more typical event-driven approach.  When all of the prerequisite steps prior to ignition are complete and the hardware and the people are ready, the test will start.  “Assuming normal operations, [the] test will occur around 3 p.m.,” Benefield wrote.  A 3 pm local or Central time start would be 2000 UTC. The ignition was timed at two minutes past the opening of the window.

Benefield said during the test that the engine will be throttled at thrust levels from 80 percent of rated power level (RPL) “up to and including” 109 percent.  “[The] majority of the test will be spent at 109% RPL, 280 seconds,” he wrote.  “35 seconds will be spent at 100% RPL, and 97 seconds will be spent at 80% RPL.”

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 latest 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 an ECU green run, the test will also continue verification of SLS program requirements for the engines.  During the test series, engines have been tested to demonstrate that they can start satisfactorily in a range of temperature and pressure conditions at its inlet.  In the case of Wednesday’s test, Benefield wrote that “this will be a test at nominal propellant start conditions.”

Wednesday’s hot-fire was the eighth in a test series with E0528 that began in July of last year.  So far, a total of fifteen 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.

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 in Building 9101 at Stennis for integration with the flight engines that are stored there.

Until recently, the FM4 controller was scheduled to be tested as one of the units assigned to fly on the first SLS launch on Exploration Mission-1 (EM-1).

Originally, it was to be tested after the FM3 test in May and was expected to be the controller tested in Wednesday’s hot-fire.  With both FM5 and FM6 now completed before that controller, it was unclear what FM4’s current status is.

Benefield noted that the controller tested in the last hot-fire test, FM5, performed well.  “FM5 has been moved to SSC Building 9101 where it will be installed on EM-1 engine 2058,” he added.

A recent schedule has the four first-flight engines being checked out after the ECUs are installed and “delivered” at the end of the (Northern Hemisphere) Summer between the end of August and the end of September.

Plans were for the engines to remain at Stennis until some time next Spring, when they would make the relatively short trip to the Michoud Assembly Facility (MAF) in New Orleans where first Core Stage (CS-1) is under construction.

After the major CS-1 elements have been joined together and started final assembly at MAF, the engines will be installed horizontally in the engine section.  Currently, the engines are assigned the following positions in the Core Stage: E2045 in position one, E2056 in position two, E2058 in position three, and E2060 in position four.

The next RS-25 hot-fire test planned at Stennis will green run flight engine 2063 (E2063) with another flight ECU.  E2063 is one of two flight engines that haven’t been acceptance tested yet and it is expected to take E0528’s place in the A-1 test stand in late September for a test planned for October.

(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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