The RS-25 team has conducted the latest hot-fire test of development engine 0528 at the Stennis Space Center in Mississippi on Wednesday. While the team continues to wait for delivery of flight versions of the new engine controller units (ECU), they took the opportunity to conduct an engine test firing to evaluate a design trade-off that was recently proposed by the Space Launch System (SLS) program.



Test plans and schedules:

The flight model ECUs are currently expected to start arriving at NASA facilities from Honeywell this week to begin testing to complete certification of the new engine control system, including several hot-fire tests throughout the year. And as Aerojet Rocketdyne restarts production of RS-25s, even more testing is planned over the next several years.

Personnel from NASA, engine prime contractor Aerojet Rocketdyne, and Stennis facilities contractor Syncom Space Services conducted the test firing, with ignition in the afternoon on February 22. Development engine 0528 was involved in the A-1 test stand.

“It’ll be an events-driven countdown, like we usually do for a test,” Steve Wofford, manager of the SLS Liquid Engines Office, said in an interview with NASASpaceflight.com ahead of the test.

The plan for the test called for a total duration of 380 seconds; during the test, the engine was throttled at 109 percent of its original power level for 205 seconds, at 100 percent for nine seconds, and at 80 percent for 118 seconds.

The different throttle settings are required to meet a series of objectives designed for the test. A low-pressure fuel duct was also “green run” as an acceptance test of that engine component, qualifying it to enter the flight hardware inventory.

The last RS-25 hot-fire test at Stennis was back in late August and at the time test firings at Stennis were planned to continue in the Fall of last year to begin green-running the new flight model ECUs that are being assembled by Honeywell at their Clearwater, Florida, facility.

At the time, the first flight model controllers were expected to be available in the early Fall, but the controllers are just now starting to be shipped.

“We are in the process of delivering our first flight box to qual [qualification] test (on February 17), so it’ll be coming up here to Huntsville for the hardware-in-the-loop lab testing and then it will go into formal qualification testing at Honeywell,” Mr. Wofford explained. That flight model ECU, FM1, should arrive in Huntsville this week.

The second controller, FM2, is now scheduled to ship to Stennis in time for the next hot-fire test on March 24.

Evaluating a recent “requirements change request”:

While waiting for the flight model ECUs, the Liquid Engines Office elected to go ahead with this hot-fire to evaluate a change request from the SLS Program.

“The primary reason we wanted to get into test earlier rather than later is because we had a fairly recent “requirements change request” from the program,” Mr. Wofford explained.

“The program came to us and said ‘hey can you handle it if we increase the LOX inlet pressure by about four percent?’ And we said ‘well, yeah we can handle it, but we’ll need to go demonstrate that in hot-fire.’

“The models predict that we can handle it and we’re pretty confident that the engine can handle it, but we have to go demonstrate it. So that’s the primary objective of this [test] is to demonstrate that we can handle this four percent increase in LOX inlet pressure.”

Testing of the RS-25, formerly known as the Space Shuttle Main Engine (SSME), began in early 2015 and a main objective was to qualify the existing design to operate in a significantly different flight environment. 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.

“We had demonstrated and were ready to go at a certain LOX inlet pressure for the SLS program,” Mr. Wofford added. “Now, mind you that LOX inlet pressure was way, way beyond what our Shuttle experience was and we had models that said we can handle it, but we had never demonstrated that in hot-fire and certainly not certified to it. So that’s what we did last year – we had the new inlet pressure and we certified and demonstrated that last Summer and the engine handled it like a champ.”

Design trade-offs:

The SLS vehicle and flight designs continue to be evaluated in detail analytically as the program continues through Design Development Test and Evaluation (DDT&E) prior to its first flight.

As the designs are being iteratively refined, analytical models are showing that maintaining the inlet pressure limits on the engines results in less structural margin on the forward solid rocket booster (SRB) to Core Stage attach struts than earlier predictions during a period late in the SRB burn.

“The SRBs attach to the [Core] Stage with these giant attach struts, so part of the design analysis cycle in any new vehicle is to do a stress analysis to make sure that…you maintain positive margins versus stress on those attach struts,” Mr. Wofford explained. “That analysis matured and said that…we’ve got a little bit lower margin than we thought — so how can we help ourselves? How can we reduce the stress on those attach struts?”

The time period of interest in the SLS flight profile for this case is near the end of the SRB burn. “At that point, the vehicle is accelerating to the point where it needs to throttle the engines in order to stay within that LOX inlet pressure requirement,” added Philip Benefield, Systems and Requirements Team Lead for the Liquid Engines Office.

“When the vehicle does that then the Core kind of ‘sags,’ if you will, while the boosters [thrust isn’t changing]…So that’s why the vehicle came to us and said ‘hey, can you guys handle about a four percent increase in that LOX inlet pressure requirement? And if you can, then we won’t have to send you throttle commands, we’ll just leave you at the higher power setting so that the Core can keep pushing as much as we can in order to minimize the stress on these attach struts.”

Incidentally, the engines were already planned to throttle down for a short period of time much closer to SRB separation for the opposite case.

“At that point [the boosters are] almost dead weight – they’re tailing off, their thrust is going down very quickly and so at that point the boosters become the sagging object if you will,” Mr. Benefield explained.

“That’s why the engines are throttled down [near SRB separation] so that the Core isn’t pushing as hard while those boosters are ‘sagging.’ That [throttle] bucket is still in effect – it’s still part of the planned mission profile.”

Mr. Benefield noted that this change request to remove the throttle down commands and fly at a slightly higher LOX inlet pressure covers a period of time about 30 seconds prior to the SRB separation “bucket.”

The past two years of testing the RS-25 under SLS operating conditions has provided additional confidence that the engines will be able to handle the incremental increase in the upper limit:

“Operating at that different set of inlet conditions so far outside of our [Shuttle] experience base is a really tremendous testament to RS-25’s operational flexibility,” Mr. Wofford commented.

“It is remarkably flexible in comparison to other rocket engines in what it can handle in terms of throttling and inlet conditions and performance and all that sort of stuff. You can operate it at all sorts of different points whereas most other engines operate at one or two set points…one of the big selling points of RS-25 is that operational flexibility.”

Hot-fire testing plans for the year:

Including this latest hot-fire test, the current baseline plan has four more test firings with engine 0528 that would run into the Summer.

Mr. Wofford is evaluating an option to do a fifth hot-fire on 0528 before it is removed from the A-1 stand to test other engines. Green runs of the new flight model controllers would be a main objective of those tests.

As noted above, flight model controller FM2 would be attached to the 0528 engine for the next test on March 24 and then for each subsequent hot-fire a new flight controller would be rotated onto the engine in the stand for its green run.

Controller FM3 would be green run in a test scheduled for April 27, then controller FM4 in a test scheduled for May 16.

“I have an option that I’m considering to do an additional test as conditions warrant on 0528 in the June time frame,” Mr. Wofford noted. “And then following that I will move into flight engine acceptance tests – the first one is engine 2063 in the July time-frame and then following that will be engine 2062 in the September time-frame.”

After each flight model controller is hot-fired in the stand, assuming it passes the test it will be taken to Aerojet Rocketdyne’s facility at Stennis and installed on one of the flight engines designated for the Exploration Mission-1 (EM-1) launch. Then the flight engine will be prepared to be shipped to the Michoud Assembly Facility (MAF) in New Orleans, Louisiana, for eventual installation on the first flight Core Stage.

“There will be some checkout activity associated with the engine after we get the controller installed, so we’ll need to check it out functionally to make sure it’s talking to that engine in the right way,” Mr. Wofford noted. “And then there will be some final leak check activities and then some normal handling stuff with those engines themselves.”

Mr. Wofford said that whether to hold the EM-1 flight engines at Stennis after the new controllers are installed for a while or ship them to MAF earlier is still being discussed.

“We’re talking about it in the program and we can do it either way, it’s no big deal,” he added.

Flight engines 2063 and 2062 are “new” – never acceptance tested; hot-firing them will help complete certification activities.

“The control system will finish its hot-fire certification portion with the first of those flight engine tests, which is engine 2063 in the July time-frame,” Mr. Wofford said. “We’ll still have some paperwork activities associated with saying that system is fully certified, but the hot-fire demonstration part of it will be complete this summer with 2063.”

Testing production restart hardware:

Following the acceptance tests of the two flight engines, current plans are to begin testing affordability changes associated with the restart of RS-25 production. In addition to improved affordability goals, the engines will be certified to run at 111 percent of their original, early Shuttle-era power level.

“The first round of that testing starts this year – this Fall – on engine 0528,” Mr. Wofford noted.

“And then next year in FY18, we’ll have a test series – right now it’s scheduled for fourteen tests – on engine 0525. And then starting in [FY]19 we’ll go back to [engine] 0528, and then there are several other test series down the road and then we’ll finally have our new affordability production restart engines certified in December of 2021.”

(Images: Via NASA and L2 – including photos from Philip Sloss and 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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