NASA, Aerojet Rocketdyne, and Syncom Space Services have conducted the first hot-fire test of a flight model RS-25 engine controller on Thursday afternoon. The eight-minute long test firing occurred in the A-1 test stand at the Stennis Space Center in Mississippi. RS-25 engines will power the Core Stage of NASA’s Space Launch System (SLS) launch vehicle, planned for the first launch in about two years.
Aerojet Rocketdyne is the prime contractor for the RS-25. Syncom Space Services is the prime contractor for Stennis facilities and operations.
The new Honeywell engine controller unit (ECU) is bolted and wired onto development engine 0528, which is being used for the current series of hot-fire tests in the A-1 stand at Stennis.
The test team at Stennis conducted ignition at 3 pm Central time (20:00 UTC). The primary objective of the test is to acceptance test (or “green run”) the new FM2 flight model ECU. Each RS-25 engine has a dedicated, redundant controller unit that controls its operation, monitors its health, and communicates with the SLS flight computers.
FM2 was delivered to Stennis in early this month and installed on development engine 0528 (E0528), following shortly behind the delivery of the FM1 unit last month to the Marshall Space Flight Center (MSFC) in Huntsville, Alabama.
The FM1 unit is dedicated for ground testing. After FM2 is green run it will be removed from E0528, taken to Aerojet Rocketdyne’s facility at Stennis, and installed on one of the flight engines designated for the first SLS launch. Eventually, the flight engines will be shipped to the Michoud Assembly Facility in New Orleans, Louisiana, for installation into the first flight Core Stage when that point in assembly is reached.
NASA spokesperson Kim Henry reported the planned duration of the test is 500 seconds, which is the expected operating time of the RS-25 engines for SLS launches. During the test, the engine was throttled at different thrust levels; at 109 percent of rated power level (RPL) for 280 seconds, at 100 percent for 35 seconds, at 90 percent for 27 seconds, and at 80 percent for 97 seconds.
The duration of the test appeared to be nominal from the live coverage on NASA TV.
Henry noted the test will also demonstrate that the engine can start satisfactorily with “weak” LOX inlet conditions and “strong” fuel inlet conditions.
The RS-25 was originally developed in the 1970s for the Space Shuttle Program when it was known as the Space Shuttle Main Engine (SSME).
Three SSMEs were used in the Shuttle system to help propel Shuttle orbiter vehicles into Earth orbit.
NASA decided on a design of the SLS that uses four RS-25 engines in the Core Stage; as with Shuttle, those are combined in SLS with two Solid Rocket Boosters that fire for the first two minutes of launch. The engines burn cryogenic liquid hydrogen (LH2) and liquid oxygen (LOX) that fed to them from separate Core Stage propellant tanks where they are stored.
RS-25 hot-fire testing began at Stennis in January 2015, to demonstrate and certify engine operation at the higher performance levels for SLS. The engines run in SLS at higher pressures and higher thrust than on Shuttle, and the hydrolox 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 provides some of the required data for certification.
Thursday’s hot-fire will be the fifth in a test series with E0528 that began in July of last year. So far, a total of twelve tests have been conducted using both development engines and one flight engine retained from the Shuttle Program.
All prior RS-25 hot-fire tests used an engineering model of the new controller that is functionally equivalent to the flight units but could be produced earlier. Certification of the new control system and controller also involves lab testing at MSFC, and at Aerojet Rocketdyne and Honeywell facilities around the United States.
Hot-fire tests are designed to meet several test objectives and one of the objectives in the last test was to evaluate a requirements change request from the SLS Program to run the engine at a slightly higher LOX inlet pressure than planned.
“The high LOX inlet pressure demonstration last test went very well,” Henry noted. “The actual value demonstrated slightly exceeded the new vehicle requirement, and the engine performed nominally.”
Running the engine at the slightly higher inlet pressure will maintain vehicle structural margins during parts of the boost phase of launch without needing to throttle the engines down. Future hot-fire tests will also continue to explore engine performance at the higher LOX inlet pressure.
Deliveries of the new flight model engine controllers for green run testing were originally expected last year. The current plan this year is to individually green run more of the flight ECUs installed in both development and flight engines. New flight controllers (FM3, FM4, and so on) will be rotated onto E0528 on the test stand with two or three more hot-fire tests planned for about once a month, followed by an acceptance test of two full flight engines in the Summer.
The engines designated for the first SLS launch will 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.
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