RS-25 Second Life: SLS Core Stage engine set for summer testing

by Chris Bergin

The RS-25 – the engine that enjoyed a hugely successful career with the Space Shuttle fleet – is set to begin its new life with the Space Launch System (SLS) this summer. Testing at the Stennis Space Center will initially focus on a new Main Engine Controller (MEC) and the ability to accommodate the full range of propellant conditions in its new configuration with the Heavy Lift Launch Vehicle (HLV).


The RS-25D, better known as the Space Shuttle Main Engine (SSME), provided safe flight for the Shuttle’s eight and a half minute ride to orbit since debuting with Columbia during her launch in 1981.

The engines proved their worth with only one major malfunction during its flight history, namely STS-51F (ME-1), resulting in a safe Abort To Orbit (ATO).

For Shuttle, the RS-25D was capable of achieving 400,000 lbs of thrust with an ISP of 453 seconds in a vacuum, or 363 seconds at sea level. The engines consist of over 50,000 parts and could be reused up to 20 times during their role with Shuttle.

As NASA moved towards the new era, the RS-25 was initially provided with a role with the Ares I Upper Stage, as part of the Constellation Program (CxP). However, the engine’s future was then placed into doubt, as Ares managers opted to move to the J-2X for the Upper Stage role.

SSME Production Shutdown Plan, via L2NASA’s Transition Control Board (TCB) – a body that was tasked with redirection of agency assets to the CxP – directed the shutdown of engine production capabilities in 2007, claiming only four more engines would be required throughout the remainder of the Space Shuttle Program (SSP).

Only a year later, the situation became more convoluted, as  a NASA Authorization Act in 2008 placed a temporary hold on the complete shutdown of RS-25 fabrication assets – mainly from the standpoint of spare hardware availability – but also in relation to continued evaluations into short-term and long-term Shuttle extension possibilities.

With the RS-25D’s reusability as its trump card, numerous Shuttle extension efforts pointed out only one additional “full engine” would have been required, had a two year extension – past its original 2010 end date – occurred during the evaluations.

In the end, only the Contingency Logistic Flights (CLFs) of STS-133 and STS-134 – along with the addition of STS-135 – were added to the manifest, despite at least two high level attempts to stop the Shuttle from entering retirement.

The RS-25 still had one out remaining during the CxP, as managers debated a potential switch of the RS-68 on the core stage of the Ares V.

Ares V Engine Evaluations, via L2The study – available on L2 – noted that exhaust plumes of six RS-68 engines, combined with the two SRBs, interact to reduce the efficiency of the engines, and cause extreme heating on the base of the core stage.

The study claimed the RS-25s may have been better suited than RS-68s in mitigating the plume impingement and base heating issues, adding the regenerative nozzle of the RS-25 held an advantage over the ablative nozzle on the RS-68s, by providing more resilience.

Ares V died with the end of the CxP, but the RS-25s yet again fought against retirement, as lawmakers realigned NASA future vehicle aspirations – via the law of the 2010 Authorization Act – to select a vehicle that used both Shuttle and former Constellation Program hardware.

SLS Option in the RAC study, via L2Evaluated via the RAC (Requirements Analysis Cycle) effort – tasked with finding the best baseline for the new Space Launch System (SLS) rocket – the RS-25s were confirmed as the preferred option for all derivatives of the evolvable HLV.

In preparation for this new milestone for the famous engines, Stennis engineers have been building and installing a new 7,755-pound thrust frame adapter for the A-1 Test Stand, in order to enable testing of the RS-25s ahead of their role on SLS’ core.

Z27The first engine that will grace the test stand for the opening tests of the RS-25 for SLS will be Engine 0525.

This engine never flew in space, as it was one of two development engines used for component testing on Stand A-2 to support shuttle flights – 0528 was the second development engine.

Final testing – using these units was conducted in 2009 – with engine 0525 earmarked for any hot-fire tests that may have been required to support the final STS missions during 2010.

In response to questions from, Aerojet Rocketdyne noted 0525 will be used to test the first couple of new engine controllers, which have progressed from the Critical Design Review (CDR) phase through to integrated testing.

The RS-25 controller provides complete and continuous monitoring and control of engine operation. In addition, it performs maintenance and start preparation checks, and collects data for historical and maintenance purposes.

The controller is an electronic package that contains five major sections; power supply section, input electronics section, output electronics sections, computer interface section, and digital computer unit.

Pressure, temperature, pump speed, flowrate, and position sensors supply the input signals. Output signals operate spark igniters, solenoid valves, and hydraulic actuators.

The controller is dual redundant, which provide normal, fail-operate, and fail-safe operational mode capability.

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Aerojet Rocketdyne note the first controllers are developmental units. However, they are nearly identical to those that will follow for flight use.  With these developmental units, the company noted they have the flexibility to fine tune the design prior to building the first flight units.

A319A total of 15 RS-25Ds left the Kennedy Space Center (KSC) to be reunited with spare engines and parts that have since congregated at Stennis.

The shipment from Florida included nine of the last SSMEs to fly with the Space Shuttle, all of which performed admirably.

Per L2 documentation, Discovery flew with Main Engine 1 (ME-1) – serial number 2044, ME-2 – 2048 and ME-3 – 2058 during her final mission, STS-133.

For Endeavour’s swansong, ME-1 – 2059, ME-2 – 2061, and ME-3 – 2057 helped begin the flight phase of the successful STS-134 mission, while Atlantis closed out the Space Shuttle Program, flying with engines ME-1 – 2047, ME-2 – 2060 and ME-3 – 2045 during STS-135.

Z4Notably, the first flight controller is scheduled to be completed in time to support the retrofitting of RS-25 engine 2047 in early summer of 2015.

ME-2047, as noted above, was one of the noisy trio that lofted Atlantis into orbit during STS-135.

“Our focus this year is to demonstrate two very important adaptations of the RS-25 engines through a series of live engine tests,” said Walt Janowski, Program Manager, SLS Programs to this week.  “The first is to confirm that the new, modern engine controller we developed for this engine will properly monitor and manage the engine operation.

Z3“The second is to show that we can safely accommodate the full range of propellant conditions that we expect when we use this engine in the new vehicle architecture.”

This second test objective relates to  greater height/mass of propellant in the tanks, which is compounded by higher accelerations. This leads to higher pressure in the propellant feed lines – especially the LOX – than on Shuttle.

This is something the engines need to cope with during start-up and operation, and that range of inlet pressures is one of the major elements of their test program.

Aerojet Rocketdyne expect to be in a position to create 16 flight engines from their RS-25D stock. They are contracted for the first four SLS launches, prior to an expected – if not yet contracted – switch to the RS-25E, a variant that is designed to be expendable.

“Over the next couple of years, we’re taking the existing engines that served the space shuttle program and getting them ready to safely and reliably support the first four flights of the SLS vehicle,” added Mr. Janowski.

“These engines have undergone at least three major upgrades during the shuttle era and this new controller, that was derived from our highly successful J-2X experience, will serve this system well into the future. “

Z7The company also added they continue to make significant progress toward SLS affordability by consolidating its manufacturing footprint by 60 percent, demonstrating advanced low-cost additive manufacturing technologies, and securing a sustainable supply chain using common key suppliers.

“The 2017 launch for SLS is a go do for us,” Mr. Janowski added. “We’re doing our part by leveraging the benefit of an extremely reliable and capable asset.

“We understand this engine very well and are confident it will do its part to expand human exploration to new destinations.”

(Images: Via L2 content. Other images via NASA)

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