The latest RS-25 test at NASA’s Stennis Space Center in Mississippi – aimed at continuing the certification effort to adapt the Space Shuttle era cryogenic rocket engine for the first Space Launch System (SLS) launch in two years time – has been conducted, albeit resulting in an early shutdown. The test involved development Engine 0528, back in the test stand at Stennis for its first test firing in seven years.
E0528 was last fired from the nearby A-2 stand on July 1, 2009, in the second-to-last Space Shuttle Main Engine (SSME) ground test for the Shuttle program.
Now out of storage and ready to fire again, the Aerojet Rocketdyne engine is being used in a second series of tests of the RS-25 configuration that will fly on SLS.
NASA’s Steve Wofford, manager of the SLS Liquid Engines Office at Marshall Space Flight Center in Alabama, explained in an interview with NASASpaceflight.com how this hot-fire test fits into the overall certification program:
“This test series is a continuation of the broader objectives of the series from last year, and that is basically certifying the engine to the new start and run conditions on SLS versus what it was on Shuttle, [such as] different propellant inlet conditions.
“So this engine [Engine 0528] is a second sample towards that certification, whereas last summer’s test on Engine 0525 was the first sample.
“For this particular test, we decompose that broad objective into a whole bunch of test requirements that we call design verification requirements (DVR), so when we say that [the engine has] got different inlet conditions we decomposed exactly what the corners of those boxes are and we decomposed it into a bunch of different requirements in terms of start and run conditions, and shutdown the engine at certain power levels and certain percentages.
“We parsed all of those objectives out to various tests, so this test is getting some of those DVR objectives.”
The test was planned to last for 650 seconds, with the engine running for 277 seconds at the 109% power level and for 268 seconds at 80%.
“That’s all a part of [needing] so much time at [a] minimum power level and so much time at maximum power level and that’s how those shake out for this test,” Mr. Wofford noted ahead of the test.
“This hot-fire will also test nominal, “center of the box” inlet conditions for the engine at start for both the liquid hydrogen fuel and liquid oxygen oxidizer and will “green-run” a fuel flow sensor.
However, Thursday’s test appeared to cut off early, later confirmed by Stennis Space Center to NASASpaceflight.com.
“Today at 5:57 p.m. (CST) RS-25 developmental engine number 0528 initiated a test firing. There was a minor issue with the test stand that triggered an early shutdown of the test. The test systems in place responded properly by shutting down the test in an orderly fashion.”
In addition to objectives for the engine start and run conditions, another primary objective for the test is to continue testing the new engine control system and new engine controller. The first two hot-fire tests in this series are using the last planned engineering model of the controller.
Engine hot-fire tests are the most visible aspect of the certification effort, but a lot of work goes on away from the test stands.
“Part of it is lab work and part of it is software lab work and part of it is hot-fire verification,” Mr. Wofford explained. The certification work is transitioning from development testing of engineering hardware to qualification testing of flight hardware.
“Earlier this year, we completed what we call controller design verification testing [DVT] — think of it as a laboratory torture test that we put the controller through.
“That’s vibration in three axes, followed by functional testing, and then thermal cycling where we bake the controller under some pretty extremes of temperatures, followed by [more] functional testing — it goes through a whole regimen of this, over several months.
“What it is, is a durability test of the controller to make sure that it functions…after a long regimen of those extreme environments. It passed with flying colors, it did really, really well. At the completion of that, I was pretty comfortable and confident that the controller is durable [and] that we’ve got a durable design.
“Following that, when we get our first flight box, we’re going to put it through what we call qualification testing. Qual testing is kind of similar to DVT testing except it’s with the final flight design box. I’m pretty comfortable and confident that we’ll pass that because we passed DVT with a very similar design box.
“Qual [testing] is the laboratory end of certification for the controller. We’re also at the same time putting the software through its paces in the various simulation labs, a lot of which are hardware-in-the-loop type simulations here in Huntsville.”
After the first flight controller goes into the lab for qual testing, Mr. Wofford noted that subsequent flight controller builds will be installed on Engine 0528 at Stennis for hot-fire verification.
For the first SLS launch on Exploration Mission-1 (EM-1), NASA needs to have enough controllers for the flight set of four engines and for a full set of spare engines, but Mr. Wofford also noted, “really I’m not outfitting spare controllers, I’m outfitting spare engines.
“So my posture for the first mission, EM-1, is to have four [spare] engines in the barn ready to swap out.
“Now, really and truly I’ll have a lot more controllers completed, ten or twelve by then, but I only need four spares.”
Qualification testing is currently planned to begin in the Fall and continue into early 2017.
Another difference for the engines between Shuttle and SLS is that the engines will be closer to the plume from the SLS solid rocket boosters; an ablative insulation is being tested on the RS-25 engine nozzles to protect them from increased heat due to the closer location to the booster nozzles.
The insulation may also protect the engines from other hazards due to the closer location to the aft separation motors on the boosters and to the hydrogen exhaust ports for the Core Stage auxiliary power units.
“Early on in the SLS program we thought we were going to need a lot more ablative than we actually need now,” Mr. Wofford said. “The question right now is whether we need it at all. The analysis is kind of borderline as to whether we need or not, but due to those uncertainties…we are going to incorporate it. It’s possible that we may not need it, but we’re doing it for safety’s sake.”
Thursday’s test was the first RS-25 hot-fire test since March, when a flight engine, E2059 was fired.
“The test went really well, we got the high-pressure fuel pump green-run and we got the…DVR objectives just fine,” Wofford said of the results of the March test.
“We did have some noise on some of the accelerometers for our Advanced Health Management System (AHMS). We had what are called FIDs, which are Failure Identifications, which is basically a flag that the controller sets that says ‘hey, this measure was wrong.’
“We had some noisy data in some of those accelerometer signals from the Advanced Health Management System, and we were concerned that it was associated with the new controller.
“So we went into full investigation mode and it turns out that..the cause of the problem was a faulty connector, which has since been corrected. It was the connector between the cable and the accelerometer that was faulty and noisy and caused that issue. So that issue is put to bed, but that’s one of those things that happens in a test program.”
After Thursday’s test, Mr. Wofford said the plan is for another hot-fire in August followed by a short break until the Fall.
The three remaining tests in this hot-fire test series with E0528 will use different flight hardware controller boxes and are scheduled to be completed by the end of the calendar year. Two flight engine green-run tests will follow at Stennis in 2017.
Overall, Mr. Wofford is optimistic about how things are going with the test program for the RS-25s.
“I’d say I’m eighty percent done. Let me tell you why I feel that way,” he explained. “Last summer, we answered the question ‘does the engine handle the new SLS conditions?’ in terms of inlet temperatures and pressures. That question got pounded flat last summer with that test series — the answer is a resounding ‘yes’ — no problem. That’s a huge risk reduction there.
“The other thing on my mind was the control system — we answered a big question, again last summer with the test series, ‘does the new engine control system control the engine adequately?’ The answer is ‘yes, the controller works.’
“Then with DVT testing on the controller, we’ve proved that the controller is durable. So the design works, the design is durable, and in terms of having a viable solution I’m much, much less concerned than before we did all of this activity.
“We still have some forward work in terms of qual testing, and certifying the software, and finishing the hot-fire certification of the control system, but the technical risk is much, much less now than it was a year ago.”
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