NASA ready to power up the RS-25 engines for SLS

by Philip Sloss

NASA is finally ready to begin the test fire and development program for the engines that will push the Space Launch System (SLS) uphill in 2018 and beyond. The RS-25 – formally known as the Space Shuttle Main Engine (SSME) – will enjoy a test program that will also evaluate higher inlet pressures and a state of the art controller, as the veteran engines gain a new lease of life.


The RS-25 engine enjoyed an esteemed history with the Space Shuttle Program (SSP), a career lasting over three decades, during which the engine received numerous upgrades on both performance and safety.

Now re-purposed for use on the Space Launch System (SLS), the RS-25s are back at the Stennis Space Center in preparation for their role with Heavy Lift Launch Vehicle’s first flight in 2018.

Development testing of the RS-25 engine is currently targeted to begin in mid-January – delayed from 2014 due to a contamination issue – on the A-1 test stand at the test site in Mississippi.

While this first RS-25 – Engine 0525 – that will be fired up on the test stand is a development engine, the following engines will be the units that will be put through their paces ahead of their launch with the EM-1 SLS.

“We’ve got three things that we’re really interested in making sure that we shake out on these engines, because you’re actually talking about engines that have flown in space before. These are engines that have flown on the Shuttle before – they’re qualified engines,” noted Todd May, Program Manager for SLS, to

Z7For the debut flight – known as Exploration Mission-1 (EM-1) – NASA’s Liquid Engine Office selected the first four engines that will loft the monster rocket uphill.

The four engines – ME-2045, ME-2056, ME-2058, and ME-2060 – are all established Shuttle veterans with numerous successful missions under their belts.

Mr. May explained that while the engines flew on the Space Shuttle in the past, the operating environment for SLS is different. As such, the program will be looking at ways to increase the engine’s performance.

“There are some parameters that are changing on SLS. If you remember the way the engines were configured on the Shuttle, there was a pretty arduous path from the External Tank to the engines.

“Now we’re very efficient in getting the fuel (liquid hydrogen) and LOX (liquid oxygen) to the engines, we actually get it there cooler than we would like to – we’re actually below the original start box for the engines. So we want to explore the low end of the start box to see if we can expand that a little lower.”

For the interim, Mr. May noted they will incorporate measures to keep the engine’s operating conditions closer to what were used on Shuttle.

“Right now we’re actually doing something that is counter-intuitive. We’ve had to install heaters to heat up our fuel and LOX to get it back into the start box because we deliver it too efficiently.

“It’s a counter-intuitive thing, it’s not something we would like to have done, but it becomes a constraint to the systems engineering process when you’re using something you’ve already had.”

2015-01-02 19_47_21-ssme schematic - Google SearchAnother key change for the RS-25 from its Shuttle era, to its role on the SLS, relates to inlet and head pressures.

“We would like to start testing at higher inlet pressures because you have a higher head pressure – the core (stage) is taller (than the External Tank) and when you fly payloads you want to be able fly with higher Gs which (adds) even more head pressure, so we’re going to be exploring a higher head pressure,” added Mr. May.

NASA is currently looking at a pressure of 260 pounds per square inch (psi) at the engine’s LOX inlet for both the 70 metric-ton and 130 metric ton versions of SLS.

In comparison, the Space Shuttle Main Engine was certified to a maximum pressure of about 220 psi at the engine LOX inlet, but during Shuttle launches max pressure was typically around 175 psi. For SLS, NASA is looking at a maximum acceleration of about 3.3 Gs, whereas with Shuttle it was 3.0 Gs.

2015-01-02 19_21_02-controller_on_engine_0525_in_bldg_9101_at_ssc_cropped.jpg (709×795)Another objective of the testing relates to the new engine control unit or controller (MEC) that will be a key part of the RS-25’s new role.

“We’ve got a new, state-of-the-art controller (which) we actually took from the J-2X controller, and have used about 60 percent of that design,” Mr. May added.

“We needed a brand new, state-of-the-art controller that cost about one-fifth of what the controller for the Shuttle cost. So we’re really going to be shaking out that new controller.

“We’ve got a development controller that’s going into this particular test series that we’re running now, so we’ll be running it through its paces.”

2015-01-02 19_43_51-SLS MAF - Google SearchWhile testing of SLS propulsion elements continue throughout 2015, production of the hardware for the first flight Core Stage is continuing at Michoud Assembly Facility (MAF) in Louisiana, along with hardware for structural test article (STA) elements that will be sent to Marshall Space Flight Center in Alabama for testing.

“The five structural test articles are the engine section, the forward section (forward skirt), the intertank, the hydrogen tank and the LOX tank; we’ll have a test article for each one of those things,” Mr. May confirmed.

2015-01-02 19_44_33-SLS Structural Test Article Stands at MSFC - Construction Photos and Updates“The test articles will come off the machines first, and then they go up to Marshall – we’re actually building a large strongback for the hydrogen tank and then another for the LOX tank, and then we have an existing structure for the other pieces.”

Testing of both the STAs – and the first flight unit – will run in parallel, up to a point.

“We’ll actually move forward with building the structures for the flight units, but we will stop short of firing on the B-2 stand until we’ve completed all the appropriate sequencing of the structural tests that we think we need to do before that,” added Mr. May, referring to the “Green Run” testing of the first flight unit Core Stage at Stennis.

2015-01-02 19_49_16-index.php (946×1314)“To put that into perspective, I worked on the New Horizons mission and we were stacked on the pad when they were doing a structural test on their core stage because it was the first Atlas 551; they had added a belly band on the inside and they were still doing their structural tests.

“The key is that our folks won’t let us do it until we think we’ve done enough to know that we’re going to be safe running the tests, but there is some parallelism – we’re not going to wait, run all the STAs and then go back and try to manufacture.

“With today’s tools we feel fairly confident that we’ve got a design that’s going to work.”

The flow for the production and assembly of the first flight unit is proceeding at MAF, utilizing an array of new machinery that is dedicated to fabricating the giant structures of the SLS.

2014-09-12 16_28_15-NASA Public“There are six major weld tools in the factory,” explained Mr. May. “One of them welds up the rings, which are the things that connect the cylinder sections to the domes. One of them does the gore panels for the domes.

“(Also) one of them puts the top circle on the dome, the Vertical Weld Center actually takes cylinder sections and makes a full cylinder and then the VAC (Vertical Assembly Center) is where those are all stacked up and welded together.”

“So the VAC will build an entire tank, it does both the hydrogen tanks and LOX tanks. Then on the bottom (hydrogen) tank you put the engine section and on the top (oxygen) tank you put the intertank and the forward section (forward skirt).”

2014-11-04 02_00_35-L2 Level_ Space Launch System Update Notes And Discussion - Sept, 2014 onwardsThe rocket is so tall that the sections then have to be translated to the horizontal, allowing for the application of the foam Thermal Protection System (TPS).

Notably, as expected, there was no mention of painting the core, given the current NASA materials showing an all-white rocket are not accurate. (Image left: L2 render of what SLS will actually look like).

“So you lay those sections on their side, and then you put the TPS on and you outfit the avionics and in the engine section you outfit it with the engines,” Mr. May continued.

“Then it’s too tall to stack, so you have to take the two halves and the final thing you do is put the two halves together horizontally. And then from that point on, you put it on the transporter and it goes off (to) the (Self Propelled Module Transporter (SPMT)) and barge and goes to Stennis, stands up on the B-2 stand, full testing, back down on the barge to KSC.”

The Green Run testing on the B-2 test stand under renovation at Stennis will see the four RS-25 engines on the first Core Stage fired for a full mission duration of approximately 500 seconds.

“I think we’re talking about doing two (firings) at this point, but the idea is to do a full mission sequence,” Mr. May added.

“We’ll obviously modify it in certain ways to make sure we are safe. We’re using a software version that’s very much like the flight software, but it has protections built-in, so it has safe shutdown and things like that.

“The idea is to put it through its paces, make it like a flight test.”

(Images: Via NASA and L2 content from L2’s SLS specific L2 section. SLS Render by L2 artist Nathan Koga)

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