Spaceflight Insider

Space Launch System (SLS) engine testing update

RS-25 engine № 0528 tested at NASA's Stennis Space Center (SSC) on August 18, 2018, in preparation for the SLS.

RS-25 engine № 0528 tested at NASA’s Stennis Space Center (SSC) on August 18, 2016, in preparation for the SLS. Photo Credit: Curt Godwin / SpaceFlight Insider

BAY ST. LOUIS, Miss. — This past week, on August 18, SpaceFlight Insider was on hand at NASA’s Stennis Space Center (SSC), in south Mississippi, for the latest RS-25 engine test in preparation for its use in the agency’s new super-heavy-lift vehicle – the Space Launch System (SLS).

RS-25 engine № 0528 tested at NASA's Stennis Space Center (SSC) on August 18, 2018, in preparation for the SLS.

Guests watch as RS-25 № 0528 unleashes a torrent of steam at Stennis’ A-1 test stand. Photo Credit: Scott Johnson / SpaceFlight Insider

Rain in the area caused some concern as to whether the test would proceed as scheduled. However, Rick Gilbrech, SSC Director, explained that “a shower this morning had me a little edgy, but, as long as we don’t have any lightning, we’re ok.”

The test, one in a series on engine № 0528, an unflown Shuttle-era developmental engine, took place as scheduled at 5:10 p.m., local time, in SSC’s A-1 test stand. The firing lasted for the planned 420 seconds, and all reports indicate that the test was successful.

Prior to the test, SpaceFlight Insider had the opportunity to speak with Steve Wofford, manager of the SLS Liquid Engines Office at NASA’s Marshall Space Flight Center (MSFC), concerning the timing of the test.

“This is a target time test. It’s usually an events based thing,” explained Wofford. “You proceed to test whenever inlet conditions are right. You condition the engine, you condition the facility, all day, and when it’s ready, you test.”

Wofford also explained that this test would involve no gimbaling of the engine and that controllers were aiming test conditions at the “edge of the start-box today. Not corners, but [the] edge of the start box – so it’s high pressure, nominal temperature.”

Prior to the test, NASA had announced that controllers would throttle the engine between 80 percent and 111 percent. SpaceFlight Insider asked Wofford if on-site observers would be able to distinguish between the thrust levels.

“Most definitely,” Wofford stated. “We’re going to be doing a lot of throttling today, and you can tell the difference in the pitch, and the loudness, between high power levels and lower power levels. And […] if the wind blows the sound to you, it’ll sound a lot louder. But, yeah, high power levels are most definitely louder.”

Wofford also explained that one aspect of the firing was continued testing of the new engine controller, or “brain”: “The new RS-25 controller that we’re making was derived from the J-2X controller. So, the J-2X controller was kind of a stepping stone to this RS-25 controller. The RS-25 engine has a much, much more sophisticated control system than the J-2X because it’s a more complicated engine cycle. So, we had to build that extra functionality into the RS-25 controller.”

SLS Engine Operating Environment

Steve Wofford, SLS Engine Manager, discusses the August 18, 2016, test. Image Credit: NASA TV

Steve Wofford, SLS Engine Manager, discusses the August 18, 2016, test. Image Credit: NASA TV

Later, Wofford addressed the differences between Shuttle and SLS RS-25 engine operating environments.

“It’s a lot different, particularly in terms of thermals and acoustics,” explained Wofford. “We’ve got models that predict that environment. But, the big differences are you’ve got four engines in a cluster now, instead of three. And, on Shuttle, the three engines were kind of tucked up out of harm’s way. Now, they’re right at the exit plane, and co-planer, with the solids. So, it’s very, very different.”

Wofford further explained that “we’ve got models that predict that we can handle that environment, but that’s a big part of why we’re going to run this stage test [– the SLS core stage “Green Run” –] next year. So, we’re going to instrument the heck out of the engines to gather data, all sorts of data, in terms of temperature, pressure, flow, and acoustics, in that stage test environment.”

“There’s some extra insulation on the nozzles to protect them from that different thermal environment,” Wofford continued. “The vast majority of the nozzle is cool; it’s got hydrogen running through it. Some of it is not cool – the structural path and the drain lines are not cool – and they’ve got extra insulation.”

Future RS-25 Testing

On upcoming RS-25 testing, Wofford responded: “We’re going to run a lot more tests on 0528. We’re going to run, […] counting today, six more tests in this series, on this engine, and then two tests on flight engines for SLS. And then, following that, we’ll be certified for flight with SLS, so we can kind of stand down the test program to certify for flight. Then, following that, in the fall of next year, we’re going to be moving into a development test series for our new engines – for our affordability changes.”

And, when asked about testing of the “flight” versions of the new RS-25 engine controller, Wofford responded: “That’ll be in the fall. That’ll actually be the second flight model controller – the first one will be the qualification unit – that’ll be [brought] out of the laboratories […] controller FM-1 will never fly. It’s a lab unit. Controller FM-2 will fly on EM-1 [and will be tested] in the fall.”

Future RS-25 Production

NASA has 16 (14 of which are previously flown) Shuttle-era RS-25 “flight” engines in storage at SSC. These engines will be installed on the SLS core stages, four at a time, and flown on missions EM-1 through EM-4.

SLS RS-25 flight engine mission assignments.

SLS RS-25 flight engine mission assignments. Photo Credit: Aerojet Rocketdyne

Missions subsequent to EM-4 will require new engines. As a result, NASA signed, in 2015, a $1.16 billion contract with Aerojet Rocketdyne for the production of six (four “flight” and two “certification”) additional RS-25s.

“We’ve started work on the future engines,” said Wofford. “Number one, we’re re-starting production lines that have been idle for as long as twenty years, so that’s no small feat in itself. Number two is to continue to get more time and explore how the engine handles the new SLS propellant engine conditions [at] colder and higher pressure.”

RS-25 engine № 0528 tested at NASA's Stennis Space Center (SSC) on August 18, 2018, in preparation for the SLS.

RS-25 engine № 0528 test at SSC on Aug. 18, 2016. Photo Credit: Curt Godwin / SpaceFlight Insider

“The other thing is [there are] some obsolescence changes that we’ve had to make for these, and [there are] some parts that we couldn’t make again if we wanted to, particularly some of the electronics.”

Wofford continued, “But, the really exciting category is the affordability changes. We’re taking advantage of forty years worth of manufacturing lessons learned, advances in technology, like 3-D printing […]. We need to ‘move’ the world’s most complex rocket engine to make it much, much more affordable for this expendable application.”

When asked about the benefits of new 3-D printing, or additive manufacturing (AM), technology, Wofford stated that “it can help a lot […] we have upwards of forty parts that we’re going to be making via additive manufacturing. Some of the really, really difficult, expensive, time-consuming parts to make, we’re going to be making by additive – a lot cheaper, a lot faster.”

“Some of the ducts that we’re going to be using on the engine are going to be made by AM,” explained Wofford. “For instance, you can make a duct by AM that has a lot of complex geometry, and a lot of complex bends in it, whereas before, that would have to be made in multiple pieces, with welds in them, now you can make one monolithic piece. As an engineer, a weld is a built-in brittle discontinuity that I don’t like – one piece is better.”

However, and despite advances in technology, SLS is a Shuttle-heritage design, conceived to take advantage of existing architecture and hardware.

“We have to kind of tread a thin line. It has to have the same form, fit, and function, and fit in [the] same hole that it did before,” explained Wofford. “You’re somewhat constrained in that regard because you’re retrofitting new parts into in an existing engine. But, given that, we’re still doing lots of redesigns that do meet that form, fit, and function requirement.”

Potential RS-25 Cost-Savings

Wofford was also asked about potential cost-savings associated with new RS-25 production, as compared with Shuttle-era production: “33 percent is our target and we’re on track to make that […]. We would be there, at the 33 percent, on the third or fourth engine […]. It’s high pay-off. When you integrate that 33 percent, over time with four engines per vehicle, […] it’s [a] really good return on our investment.”

Video Courtesy of NASA Stennis



Scott earned both a Bachelor's Degree in public administration, and a law degree, from Samford University in Birmingham, Alabama. He currently practices law in the Birmingham suburb of Homewood. Scott first remembers visiting Marshall Space Flight Center in 1978 to get an up-close look at the first orbiter, Enterprise, which had been transported to Huntsville for dynamic testing. More recently, in 2006, he participated in an effort at the United States Space and Rocket Center (USSRC) to restore the long-neglected Skylab 1-G Trainer. This led to a volunteer position, with the USSRC curator, where he worked for several years maintaining exhibits and archival material, including flown space hardware. Scott attended the STS - 110, 116 and 135 shuttle launches, along with Ares I-X, Atlas V MSL and Delta IV NROL-15 launches. More recently, he covered the Atlas V SBIRS GEO-2 and MAVEN launches, along with the Antares ORB-1, SpaceX CRS-3, and Orion EFT-1 launches.

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