NASA and Aerojet Rocketdyne have conducted an opening test of the RS-25 rocket engine that will help power the Space Launch System (SLS) during launch. Engine 2059, a flight engine that helped launch five Space Shuttle missions, was test-fired on Thursday afternoon in the A-1 test stand at Stennis Space Center in Mississippi to continue qualifying a new engine controller design and to help calibrate the A-1 facility itself for future testing.
Typically, hot-fire tests such as this are planned to start when the engine and the test team are ready, rather than planned for a specific time.
As such there wasn’t an ignition T-0 available until close to when the engine was ready to fire up. Ignition was marked at approximately 15:41 pm. Central time.
“The test will be in the early to mid-afternoon,” said Steve Wofford, SLS Liquid Engines Office at Marshall Space Flight Center in Alabama, in an interview with NASASpaceflight.com.
“We don’t set a (specific) time for the test as it’s more dictated by when propellant conditions are in the right state to be able to test the engines, so you do prep work all throughout the day and then when conditions are correct, you proceed with the test.”
The engine ran for 500 seconds during the test, which is a SLS flight duration. Ahead of the test, Mr. Wofford said that 323 of those seconds would be at the SLS power level of 109 percent.
One of the objectives of Thursday’s test is to help calibrate a post-Shuttle change made to the A-1 test stand.
The A-1 facility was one of the stands at Stennis used to test RS-25 engines, formerly known as Space Shuttle Main Engines (SSME), during the Space Shuttle era. The last SSME test on the A-1 stand was in 2006, after which the stand was converted for use testing the J-2X engine for the Constellation program.
Following those tests (and cancellation of the Constellation program), the stand was reconfigured to once again support RS-25 testing that began early last year, this time as a part of SLS development work.
“We installed a new thrust measurement system on test stand A-1 following the end of the Shuttle program,” Mr. Wofford explained, “so we need to run a calibration test on a flight engine.
“We’ve got previous calibration data for this flight engine with the old thrust measurement system; we want to run another sample with this flight engine with the new thrust measurement system to see how those compare so that we can isolate that variable with that new change.”
While the design of the engine for the Space Shuttle has largely been retained, the operating environment between Shuttle and SLS does have significant differences.
SLS will be flying with four RS-25 engines versus Shuttle’s three and with more propellant, and last year’s test firings verified that the engine handles the higher inlet pressures and lower inlet temperatures during SLS launches while running at a higher thrust level than Shuttle.
“The inlet conditions are in a region that was previously outside of our test history, but we had model data that said that the engines could handle that,” Mr. Wofford explains. “So we proved that out in the hot-fire test series last year and it doesn’t bother the engine.
“The engine gets in steady-state and you can run it at that power level at those inlet conditions and it doesn’t bother the engine – the throttling is for vehicle driven needs.”
In addition to the calibration of the facility thrust measurement system for A-1, other objectives for this test include another test sample of the SLS inlet conditions and testing of a rebuilt high-pressure fuel turbopump.
The engine used in this test, E2059, flew on five Space Shuttle missions; it first flew three times on Shuttle Orbiter Atlantis (STS-117, STS-122, and STS-125) and then on Orbiter Endeavour’s last two flights (STS-130 and STS-134). It is currently assigned to fly installed in the second Core Stage on Exploration Mission-2 (EM-2), which is planned to be the first crewed SLS/Orion flight.
The primary hardware change from the SSME to the RS-25 is the new engine controller; the SSME controller hardware was judged to be both obsolete – parts were no longer available – and incompatible with the SLS power and data architectures.
The new design was evolved from the controller that was in development for use with the J-2X engine; development to certify the controller for the RS-25 on SLS is still on-going, with engineering models being used to qualify the hardware and software for flight units that will follow.
Hot-fire testing of RS-25 engines resumed last year, with a series of seven tests conducted using development engine E0525 on the A-1 stand between January and August, 2015.
Among the objectives for last year’s series, the tests explored the different engine inlet conditions between Shuttle and SLS, provided development run time for the new engine controller, and tested a variety of thrust profiles; Mr. Wofford called the completed test series both a success and a relief.
“The results of the tests last year were extremely successful – I was really, really pleased with that test series,” he said.
“We threw some really tough thrust profiles at the engine – kind of unprecedented in their complexity – and all of our tests went full duration and all of our tests came off on time. And really and truly I think we got about 80 percent of the way there in terms of hardware confidence on being ready to fly with the test series last summer.
“Now we (still) need to…get additional (test) samples and certification of our new ablative insulation and our development flight instrumentation, but in terms of hardware confidence, I breathed a lot easier following that test series last summer.”
Thursday’s test used an Engineering Model 5 controller; originally planned for late last year, it was decided to wait to do the hot-fire test until the EM5 controller was ready.
“EM5 was a little late coming off the production line due to some parts availability issues on getting it completed and we slipped this test until EM5 was ready,” Mr. Wofford noted. “So it’s ready to go – it’s installed on the engine and that’s what we’re going to test.”
Mr. Wofford explained why they waited for the newer controller, noting it revolved around their certification planning and they had the time available to acquire it for use during testing.
“Last summer was all about development time for the controller system on a ground test engine. Part of our certification plan for the new control system was to also get hot-fire time on a flight engine and then hot-fire time on a different development engine.
“So it’s now time to get that hot-fire time for the new control system on a flight engine and this was a convenient window to accomplish that. This test wasn’t critical path – we could do it at several different windows – but it was a convenient window to do it in.”
Test firings at Stennis are one part of overall development of the new engine controller, with Mr. Wofford noting there is a broader perspective in the development process.
“In terms of the overall controller development, engine hot-fire time and demonstration is just a single facet of the way we develop and certify that system,” he explained.
“So in addition to hot-fire testing we do lots and lots of lab testing in the various ‘hardware-in-the-loop’ labs, we do something that we call design verification testing or DVT testing – and we’re doing that right now. That’s going on in the labs right now on one of our controller development units. Following that we do controller hardware qual (qualification) testing.
“The DVT testing has been going on since October and DVT is where we really put the hardware through its paces in the lab with repeated thermal, vibration, and functional tests – it’s really kind of a torture test – and the testing for DVT is far more severe than anything the hardware would see in flight.
“The intention there is to shake out any problems ahead of controller qual tests. So think of DVT as risk-reduction testing leading into qual testing.”
Following Thursday’s test, another series of development hot-fire tests will be conducted using the other development engine, E0528. A series of five tests are planned to start in the July time-frame.
Mr. Wofford said that the EM5 controller is the last planned development version, with flight boxes being the next units to be produced.
One of the objectives of the next test series is to “green run” three of the four controllers that will fly with the first Core Stage on the EM-1 flight currently planned for late 2018.
The first two tests of the next series will use EM5 units installed on E0528; the last three hot-fire tests will use different flight units installed on the engine. The fourth flight unit controller will get its green run during the full Core Stage test on the B-2 stand at Stennis, currently planned for late 2017.
After the E0528 test series, one “green run” test each of newly assembled flight engines (E2063 and E2062) is planned in the late-Fall / early Winter time-frame.
One of those tests was earlier scheduled in between Thursday’s test and the E0528 test series. However, Mr. Wofford said that they are not in the critical path of development and both were moved to the other side of the E0528 tests.
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