View source for RS-25

{{short description|Space Shuttle and SLS main engine}}
{{redirect|SSME|other uses|SSME (disambiguation)}}
{{good article}}
{{Use mdy dates|date=May 2012}}

{{infobox rocket engine
| image                      = Shuttle Main Engine Test Firing.jpg
| image_size                 = 250
| image_alt                  = A rocket engine firing. A blue flame is projecting from a bell-shaped nozzle with several pipes wrapped around it. The top of the nozzle is attached to a complex collection of plumbing, with the whole assembly covered in steam and hanging from a ceiling-mounted attachment point. Various pieces of transient hardware are visible in the background.
| caption                    = RS-25 test firing. The bright area at the bottom of the picture is a [[shock diamond]].
| name                       = RS-25
| country_of_origin          = United States
| designer                   = [[Rocketdyne]]
| manufacturer               = {{ubli
  | Rocketdyne (1981–2005)
  | [[Pratt & Whitney Rocketdyne]] (2005–2013)
  | [[Aerojet Rocketdyne]] (2013–present)
   }}
| predecessor                = [[HG-3 (rocket engine)|HG-3]]
| first_date                 = {{Start date|1981|04|12}} ([[STS-1]])
| associated                 = {{Unbulleted list 
  | [[Space Shuttle]]
  | [[Space Launch System]]
   }}
| type                       = liquid
| fuel                       = {{chem2|LH2|link=liquid hydrogen}}
| oxidiser                   = [[LOX]]
| thrust(SL)                 = {{cvt|418000|lbf|kN|sigfig=3|order=flip}}<ref name="aerojet"/>
| thrust(Vac)                = {{cvt|512300|lbf|kN|sigfig=4|lk=on|order=flip}}<ref name="aerojet"/>
| thrust_to_weight           = 73.1<ref name="EASSME"/>
| specific_impulse_vacuum    = {{cvt|452.3|isp}}<ref name="aerojet">{{cite web |title=RS-25 Engine |url=http://www.rocket.com/rs-25-engine  |archive-url=https://web.archive.org/web/20140703043723/https://www.rocket.com/rs-25-engine |archive-date=July 3, 2014 |publisher=[[Aerojet Rocketdyne]] }}</ref>
| specific_impulse_sea_level = {{cvt|366|isp}}<ref name="aerojet" />
| chamber_pressure           = {{cvt|2994|psi|MPa}}<ref name="aerojet" />
| cycle                      = [[Staged combustion cycle|Fuel-rich dual-shaft staged combustion]]
| nozzle_ratio               = 78:1<ref name="aerojet" />
| length                     = {{cvt|168|in|m|order=flip}}
| diameter                   = {{cvt|96|in|m|order=flip}}
| dry_weight                 = {{cvt|7,004|lb|kg|order=flip}}<ref name="EASSME"/>
| status                     = In use
| references                 = <ref name="P&WFS">{{cite web|url=http://www.pw.utc.com/products/pwr/assets/pwr_SSME.pdf |title=Space Shuttle Main Engine |publisher=[[Pratt & Whitney Rocketdyne]] |access-date=November 23, 2011 |year=2005  |archive-url=https://web.archive.org/web/20120208191620/http://www.pw.utc.com/products/pwr/assets/pwr_SSME.pdf |archive-date=February 8, 2012 }}</ref><ref name="EASSME">{{cite web|url=http://www.astronautix.com/s/ssme.html|archive-url=https://web.archive.org/web/20161228143022/http://astronautix.com/s/ssme.html|archive-date=December 28, 2016|title=SSME|first=Mark|last=Wade|publisher=Encyclopedia Astronautica|access-date=December 28, 2017}}</ref>
| notes                      = Data is for RS-25D at 109% of rated power level.
| mixture_ratio              = 6.03:1
| throttle_range             = 67–109%
| mass_flow                  = {{cvt|1134.26|lb/s|kg/s|order=flip}}
}}

The '''RS-25''', also known as the '''Space Shuttle Main Engine''' ('''SSME'''),<ref name="aerojet" /> is a [[liquid-fuel rocket|liquid-fuel]] [[cryogenic rocket engine]] that was used on [[NASA]]'s [[Space Shuttle]] and is used on the [[Space Launch System]].

Designed and manufactured in the United States by [[Rocketdyne]] (later [[Pratt & Whitney Rocketdyne]] and [[Aerojet Rocketdyne]]), the RS-25 burns [[Cryogenic fuel|cryogenic]] (very low temperature) [[liquid hydrogen]] and [[liquid oxygen]] propellants, with each engine producing {{convert|1859|kN|abbr=on|sigfig=3}} [[thrust]] at liftoff. Although RS-25 heritage traces back to the 1960s, its concerted development began in the 1970s with the first flight, [[STS-1]], on April 12, 1981. The RS-25 has undergone upgrades over its operational history to improve the engine's thrust, reliability, safety, and maintenance load.

The engine produces a [[specific impulse]] (''I''<sub>sp</sub>) of 452 seconds (4.43&nbsp;kN-sec/kg) in vacuum, or 366 seconds (3.59&nbsp;kN-sec/kg) at sea level, has a mass of approximately {{convert|3.5|t|lb|abbr=off}}, and is capable of throttling between 67% and 109% of its [[#Engine throttle/output|rated power level]] in one-percent increments. Components of the RS-25 operate at temperatures ranging from {{convert|-253|to|3300|°C|°F|-2|abbr=on}}.<ref name="aerojet"/>

The Space Shuttle used a cluster of three RS-25 engines mounted at the stern of the [[Space Shuttle orbiter|orbiter]], with fuel drawn from the [[Space Shuttle External Tank|external tank]]. The engines were used for propulsion throughout the spacecraft ascent, with total thrust increased by two [[Space Shuttle Solid Rocket Booster|solid rocket boosters]] and the orbiter's two [[AJ10]] [[Space Shuttle Orbital Maneuvering System|orbital maneuvering system]] engines. Following each flight, the RS-25 engines were removed from the orbiter, inspected, refurbished, and then reused on another mission.

Four RS-25 engines are installed on each Space Launch System, housed in the engine section at the base of the [[Space Launch System core stage|core stage]], and expended after use. The first four Space Launch System flights use modernized and refurbished engines built for the Space Shuttle program. Subsequent flights will make use of a simplified RS-25E engine called the Production Restart, which is under testing and development.

== Components ==
{{multiple image
 | direction = vertical
 | width = 300
 | footer = RS-25 propellant flow
 | image1 = Ssme_schematic_(updated).svg
 | alt1 = A diagram showing the components of an RS-25 engine. See adjacent text for details.
 | caption1 = RS-25 schematic
 | image2 = Simplified diagram of RS-25 rocket engine(SSME) , Jan 2021 .png
 | alt2 = A diagram showing the components of an RS-25 engine. See adjacent text for details.
 | caption2 = Simplified diagram of RS-25
 | image3 = RS-25 Fuel Flow.png
 | alt3 = A flowchart showing the flow of liquid hydrogen fuel through an RS-25 engine. See adjacent text for details.
 | caption3 = Fuel flow
 | image4 = RS-25 Oxidiser Flow.png
 | alt4 = A flowchart showing the flow of liquid oxygen oxidizer through an RS-25 engine. See adjacent text for details.
 | caption4 = Oxidizer flow
 }}
<!--[[File:Orbiter main propulsion system.svg|thumb|Space Shuttle Main Propulsion System (MPS)|alt=A diagram showing the major plumbing elements in a Space Shuttle orbiter's aft structure. The orbiter's three engines are visible, each supplied by a fuel line (indicated in red) and an oxidizer line (indicated in blue). In each system, the three lines split off from a mainline which disappears into the base of the structure towards where the external tank would be, as indicated by a smaller diagram in the bottom-right of the diagram. Finally, a small fill and drain line branches off each mainline, with the two lines exiting the aft structure on opposite sides.]]-->
The RS-25 engine consists of pumps, valves, and other components working in concert to produce [[thrust]]. [[Rocket propellant|Fuel]] ([[liquid hydrogen]]) and [[oxidizing agent|oxidizer]] ([[liquid oxygen]]) from the Space Shuttle's [[Space Shuttle external tank|external tank]] entered the [[Space Shuttle Orbiter|orbiter]] at the [[Space Shuttle external tank#Hardware|umbilical disconnect valves]] and from there flowed through the orbiter's main propulsion system (MPS) feed lines; whereas in the [[Space Launch System]] (SLS), fuel and oxidizer from the rocket's core stage flow directly into the MPS lines. Once in the MPS lines, the fuel and oxidizer each branch out into separate paths to each engine (three on the Space Shuttle, four on the SLS). In each branch, pre-valves then allow the propellants to enter the engine.<ref name="2.16">{{cite tech report |author1=United Space Alliance |author1-link=United Space Alliance |title=Shuttle Crew Operations Manual |url=https://www.nasa.gov/centers/johnson/pdf/390651main_shuttle_crew_operations_manual.pdf#%5B%7B%22num%22%3A1989%2C%22gen%22%3A0%7D%2C%7B%22name%22%3A%22XYZ%22%7D%2C129%2C716%2Cnull%5D |publisher=[[NASA]] |access-date=May 23, 2023 |archive-url=https://web.archive.org/web/20230411024639/https://www.nasa.gov/centers/johnson/pdf/390651main_shuttle_crew_operations_manual.pdf |archive-date=April 11, 2023 |pages=577-618 |date=December 15, 2008 |url-status=live |id=USA007587 |section=2.16 Main Propulsion System (MPS) }}</ref><ref name="NSFSLS"/>

Once in the engine, the propellants flow through low-pressure fuel and oxidizer [[turbopump]]s (LPFTP and LPOTP), and from there into high-pressure turbopumps (HPFTP and HPOTP). From these HPTPs the propellants take different routes through the engine. The oxidizer is split into four separate paths: to the oxidizer [[heat exchanger]], which then splits into the oxidizer tank pressurization and [[Pogo oscillation|pogo]] suppression systems; to the low-pressure oxidizer turbopump (LPOTP); to the high-pressure oxidizer pre-burner, from which it is split into the HPFTP turbine and HPOTP before being reunited in the hot gas manifold and sent on to the main combustion chamber (MCC); or directly into the main combustion chamber (MCC) injectors.

Meanwhile, fuel flows through the main fuel valve into [[Regenerative cooling (rocket)|regenerative cooling]] systems for the [[rocket engine nozzle|nozzle]] and MCC, or through the chamber coolant valve. The fuel passing through the MCC cooling system then passes back through the LPFTP turbine before being routed either to the fuel tank pressurization system or to the hot gas manifold cooling system (from where it passes into the MCC). Fuel in the nozzle cooling and chamber coolant valve systems is then sent via pre-burners into the HPFTP turbine and HPOTP before being reunited again in the hot gas manifold, from where it passes into the MCC injectors. Once in the injectors, the propellants are mixed and injected into the main combustion chamber where they are ignited. The ejection of the burning propellant mixture through the throat and bell of the engine's nozzle creates the thrust.<ref name="2.16"/>

===Turbopumps===

====Oxidizer system====
The low-pressure oxidizer turbopump (LPOTP) is an [[axial flow pump|axial-flow pump]] which operates at approximately 5,150 [[Revolutions per minute|rpm]] driven by a six-stage [[turbine]] powered by high-pressure liquid oxygen from the high-pressure oxidizer turbopump (HPOTP). It boosts the liquid oxygen's pressure from {{convert|0.7|to|2.9|MPa|psi|abbr=on}}, with the flow from the LPOTP then being supplied to the HPOTP. During engine operation, the pressure boost permits the high-pressure oxidizer pump to operate at high speeds without [[cavitation|cavitating]]. The LPOTP, which measures approximately {{convert|450|by|450|mm|in|abbr=on}}, is connected to the vehicle propellant ducting and supported in a fixed position by being mounted on the launch vehicle's structure.<ref name="2.16"/>

Then, mounted before the HPOTP, is the [[pogo oscillation]] suppression system accumulator.<ref name="Orientation"/> For use, it is pre-and post-charged with {{chem|He}} and charged with gaseous {{chem|O|2}} from the heat exchanger, and, not having any membrane, it operates by continuously recirculating the charge gas. A number of baffles of various types are present inside the accumulator to control sloshing and turbulence, which is useful of itself and also to prevent the escape of gas into the low-pressure oxidizer duct to be ingested in the HPOTP.

The HPOTP consists of two single-stage [[centrifugal pump]]s (the main pump and a pre-burner pump) mounted on a common shaft and driven by a two-stage, hot-gas turbine. The main pump boosts the liquid oxygen's pressure from {{convert|2.9|to|30|MPa|psi|abbr=on}} while operating at approximately 28,120&nbsp;rpm, giving a power output of {{convert|23260|hp|MW|abbr=on|lk=on}}. The HPOTP discharge flow splits into several paths, one of which drives the LPOTP turbine. Another path is to, and through, the main oxidizer valve and enters the main combustion chamber. Another small flow path is tapped off and sent to the oxidizer [[heat exchanger]]. The liquid oxygen flows through an anti-flood valve that prevents it from entering the heat exchanger until sufficient heat is present for the heat exchanger to utilize the heat contained in the gases discharged from the HPOTP turbine, converting the liquid oxygen to gas. The gas is sent to a manifold and then routed to pressurize the liquid oxygen tank. Another path enters the HPOTP second-stage pre-burner pump to boost the liquid oxygen's pressure from 30 to 51 MPa (4,300 [[Pounds per square inch|psia]] to 7,400 psia). It passes through the oxidizer pre-burner oxidizer valve into the oxidizer pre-burner and through the fuel pre-burner oxidizer valve into the fuel pre-burner. The HPOTP measures approximately {{convert|600|by|900|mm|in|abbr=on}}. It is attached by flanges to the hot-gas manifold.<ref name="2.16"/>

The HPOTP turbine and HPOTP pumps are mounted on a common shaft. Mixing of the fuel-rich hot gases in the turbine section and the liquid oxygen in the main pump can create a hazard and, to prevent this, the two sections are separated by a cavity that is continuously purged by the engine's helium supply during engine operation. Two seals minimize leakage into the cavity; one seal is located between the turbine section and the cavity, while the other is between the pump section and cavity. Loss of helium pressure in this cavity results in automatic engine shutdown.<ref name="2.16"/>

====Fuel system====
The low-pressure fuel turbopump (LPFTP) is an axial-flow pump driven by a two-stage turbine powered by gaseous hydrogen. It boosts the pressure of the liquid hydrogen from 30 to 276 psia (0.2 to 1.9 MPa) and supplies it to the high-pressure fuel turbopump (HPFTP). During engine operation, the pressure boost provided by the LPFTP permits the HPFTP to operate at high speeds without cavitating. The LPFTP operates at around 16,185 [[Revolutions per minute|rpm]], and is approximately {{convert|450|by|600|mm|in|abbr=on}} in size. It is connected to the vehicle propellant ducting and is supported in a fixed position by being mounted to the launch vehicle's structure.<ref name="2.16"/>

The HPFTP is a three-stage centrifugal pump driven by a two-stage hot-gas turbine. It boosts the pressure of the liquid hydrogen from 1.9 to 45 MPa (276 to 6,515 psia), and operates at approximately 35,360 rpm with a power of {{convert|71140|hp|MW|abbr=on|lk=on}}. The discharge flow from the turbopump is routed to, and through, the main valve and is then split into three flow paths. One path is through the jacket of the main combustion chamber, where the hydrogen is used to cool the chamber walls. It is then routed from the main combustion chamber to the LPFTP, where it is used to drive the LPFTP turbine. A small portion of the flow from the LPFTP is then directed to a common manifold from all three engines to form a single path to the liquid hydrogen tank to maintain pressurization. The remaining hydrogen passes between the inner and outer walls of the hot-gas manifold to cool it and is then discharged into the main combustion chamber. A second hydrogen flow path from the main fuel valve is through the engine nozzle (to cool the nozzle). It then joins the third flow path from the chamber coolant valve. This combined flow is then directed to the fuel and oxidizer pre-burners. The HPFTP is approximately {{convert|550|by|1100|mm|in|abbr=on}} in size and is attached to the hot-gas manifold by flanges.<ref name="2.16"/>

===Powerhead===
[[File:IMG 9440 edited-1.JPG (7392553592).jpg|thumb|alt=The SSME is a compact tangle of pipework attached to a much larger rocket nozzle.|The large silver pipe across the top carries fuel from the low-pressure fuel turbopump (not visible) to the high-pressure fuel turbopump (HPFTP, silver drum at lower left). The top of the HPFTP is bolted to part of the hot gas manifold (black, with brown diagonal pipe), and above that is the fuel pre-burner (also black, with brown pipe entering at right).<ref name="Orientation"/><!--Views 6 and 7-->]]

====Preburners====
<!--[[File:SSME powerhead.jpg|thumb|The RS-25's powerhead and high-pressure pumps.|alt=A diagram showing the RS-25's powerhead and high-pressure pumps. See adjacent text for details.]]-->
The oxidizer and fuel pre-burners are [[welded]] to the hot-gas manifold. The fuel and oxidizer enter the pre-burners and are mixed so that efficient combustion can occur. The augmented [[Electric arc|spark]] igniter is a small combination chamber located in the center of the injector of each pre-burner. Two dual-redundant spark igniters are activated by the engine controller and are used during the engine start sequence to initiate combustion in each pre-burner. They are turned off after approximately three seconds because the combustion process is then self-sustaining. The pre-burners produce the fuel-rich hot gases that pass through the turbines to generate the power needed to operate the high-pressure turbopumps. The oxidizer pre-burner's outflow drives a turbine that is connected to the HPOTP and to the oxidizer pre-burner pump. The fuel pre-burner's outflow drives a turbine that is connected to the HPFTP.<ref name="2.16"/>

The speed of the HPOTP and HPFTP turbines depends on the position of the corresponding oxidizer and fuel pre-burner oxidizer valves. These valves are positioned by the engine controller, which uses them to throttle the flow of liquid oxygen to the pre-burners and, thus, control engine thrust. The oxidizer and fuel pre-burner oxidizer valves increase or decrease the liquid oxygen flow, thus increasing or decreasing pre-burner chamber pressure, HPOTP and HPFTP turbine speed, and liquid oxygen and gaseous hydrogen flow into the main combustion chamber, which increases or decreases engine thrust. The oxidizer and fuel pre-burner valves operate together to throttle the engine and maintain a constant 6.03:1 propellant mixture ratio.<ref name="P&WFS"/>

The main oxidizer and main fuel valves control the flow of liquid oxygen and liquid hydrogen into the engine and are controlled by each engine controller. When an engine is operating, the main valves are fully open.<ref name="2.16"/>

====Main combustion chamber====
The engine's main combustion chamber (MCC) receives fuel-rich hot gas from a hot-gas manifold cooling circuit. The gaseous hydrogen and liquid oxygen enter the chamber at the injector, which mixes the propellants. The mixture is ignited by the "Augmented Spark Igniter", an H{{sub|2}}/O{{sub|2}} flame at the center of the injector head.<ref name="ASI_igniter">{{Cite web |url=https://blogs.nasa.gov/J2X/2014/01/24/inside-the-leo-doghouse-light-my-fire/ |title=Inside the LEO Doghouse: Light My Fire! |last=Greene |first=Bill |date=January 24, 2014 |publisher=[[NASA]] |access-date=March 15, 2019 |url-status=live |archive-url=https://web.archive.org/web/20221211233847/https://blogs.nasa.gov/J2X/2014/01/24/inside-the-leo-doghouse-light-my-fire/ |archive-date=December 11, 2022 }}</ref> The main injector and dome assembly are welded to the hot-gas manifold, and the MCC is also bolted to the hot-gas manifold.<ref name="2.16"/> The MCC comprises a structural shell made of [[Inconel|Inconel 718]] which is lined with a [[copper]]-[[silver]]-[[zirconium]] [[alloy]] called NARloy-Z, developed specifically for the RS-25 in the 1970s. Around 390 channels are machined into the liner wall to carry liquid hydrogen through the liner to provide MCC cooling, as the temperature in the combustion chamber reaches {{convert|3300|°C|°F|abbr=on}} during flight – higher than the [[boiling point]] of [[iron]].<ref name="NARloy-Z">{{Cite web |url=http://www.copper.org/publications/newsletters/cutopics/Ct73/shuttle_engine.html |title=NASA Relies on Copper for Shuttle Engine |year=1992 |work=Discover Copper Online |publisher=Copper Development Association |access-date=January 19, 2012  |archive-url=https://web.archive.org/web/20130920154705/http://www.copper.org/publications/newsletters/cutopics/Ct73/shuttle_engine.html |archive-date=September 20, 2013 }}</ref><ref name="Enhancements"/>

An alternative for the construction of RS-25 engines to be used in SLS missions is the use of advanced structural ceramics, such as [[thermal barrier coatings]] (TBCs) and [[Ceramic matrix composite|ceramic-matrix composites]] (CMCs).<ref>{{Cite journal|last=Padture|first=Nitin P.|date=August 2016|title=Advanced structural ceramics in aerospace propulsion|journal=Nature Materials|volume=15|issue=8|pages=804–809|doi=10.1038/nmat4687|pmid=27443899|bibcode=2016NatMa..15..804P|issn=1476-4660}}</ref> These materials possess significantly lower thermal conductivities than metallic alloys, thus allowing more efficient combustion and reducing the cooling requirements. TBCs are thin ceramic oxide layers deposited on metallic components, acting as a thermal barrier between hot gaseous combustion products and the metallic shell. A TBC applied to the Inconel 718 shell during production could extend engine life and reduce cooling costs. Further, CMCs have been studied as a replacement for Ni-based superalloys and are composed of high-strength fibers (BN, C) continuously dispersed in a SiC matrix. An MCC composed of a CMC, though less studied and farther from fruition than the application of a TBC, could offer unprecedented levels of engine efficiency.

===Nozzle===
[[File:SSME1.jpg|thumb|upright|The nozzles of {{OV|102}}'s three RS-25s following the landing of [[STS-93]]. The bright spot in engine 3's nozzle (bottom right) is from damage that occurred during liftoff.|alt=Three bell-shaped rocket engine nozzles projecting from the aft structure of a Space Shuttle orbiter. The cluster is arranged triangularly, with one engine at the top and two below. Two smaller nozzles are visible to the left and right of the top engine, and the orbiter's tail fin projects upwards toward the top of the image. In the background is the night sky and items of purging equipment.]]

The engine's [[rocket engine nozzle|nozzle]] is {{convert|121|in|m|abbr=on}} long with a diameter of {{convert|10.3|in|m}} at its throat and {{convert|90.7|in|m}} at its exit.<ref name="P&WNozzle">{{cite web |author=O'Leary |first1=R.A. |last2=Beck |first2=J. E. |year=1992 |title=Nozzle Design |url=http://www.engineeringatboeing.com/articles/nozzledesign.htm |archive-url=https://web.archive.org/web/20080316084228/http://www.engineeringatboeing.com/articles/nozzledesign.htm |archive-date=March 16, 2008 |work=Threshold |publisher=[[Pratt & Whitney Rocketdyne]]}}</ref> The nozzle is a bell-shaped extension bolted to the main combustion chamber, referred to as a [[de Laval nozzle]]. The RS-25 nozzle has an unusually large [[Rocket engine expansion|expansion ratio]] (about 69:1) for the chamber pressure.<ref name="SSMEFTY">{{cite book |author=Biggs |first=Robert E. |title=History of Liquid Rocket Engine Development in the United States 1955–1980 |date=May 1992 |publisher=American Astronautical Society |isbn=978-0-87703-350-9 |editor=Stephen E. Doyle |series=AAS History Series |pages=69–122 |chapter=Space Shuttle Main Engine: The First Ten Years |access-date=December 12, 2011 |chapter-url=http://www.enginehistory.org/ssme.shtml |archive-url=https://web.archive.org/web/20111225163215/http://www.enginehistory.org/ssme.shtml |archive-date=December 25, 2011}}</ref> At sea level, a nozzle of this ratio would normally undergo flow separation of the jet from the nozzle, which would cause control difficulties and could even mechanically damage the vehicle. However, to aid the engine's operation Rocketdyne engineers varied the angle of the nozzle walls from the theoretical optimum for thrust, reducing it near the exit. This raises the pressure just around the rim to an absolute pressure between {{convert|4.6|and|5.7|psi|abbr=on}}, and prevents flow separation. The inner part of the flow is at much lower pressure, around {{convert|2|psi|abbr=on}} or less.<ref>{{cite web|url=http://www.k-makris.gr/RocketTechnology/Nozzle_Design/nozzle_design.htm|title=Nozzle Design|access-date=November 23, 2011|date=March 16, 2009|archive-url=https://web.archive.org/web/20111002122814/http://www.k-makris.gr/RocketTechnology/Nozzle_Design/nozzle_design.htm|archive-date=October 2, 2011}}</ref> The inner surface of each nozzle is cooled by liquid hydrogen flowing through [[brazing|brazed]] stainless steel tube wall coolant passages. On the Space Shuttle, a support ring welded to the forward end of the nozzle is the engine attach point to the orbiter-supplied heat shield. Thermal protection is necessary because of the exposure portions of the nozzles experience during the launch, ascent, on-orbit and entry phases of a mission. The insulation consists of four layers of metallic batting covered with a metallic foil and screening.<ref name="2.16"/>

===Controller===
[[File:99723290 SSME Blk II Controller.jpg|upright|thumb|A Block II RS-25D main engine controller|alt=A black, rectangular box, with cooling fins mounted to its outer surface. Various tubes and wires project from the side of the box facing the camera, with the other side mounted to a complex of silvery plumbing. The box is nestled in amongst other wires and pieces of hardware, and some warning stickers are attached to the casing.]]

Each engine is equipped with a main engine controller (MEC), an integrated computer which controls all of the engine's functions (through the use of valves) and monitors its performance. Built by [[Honeywell Aerospace]], each MEC originally comprised two [[Redundancy (engineering)|redundant]] [[Honeywell]] [[HDC-601]] computers,<ref name="SSMEC1">{{cite web|url=https://history.nasa.gov/computers/Ch4-7.html|work=Computers in Spaceflight: The NASA Experience|publisher=NASA|title=Computers in the Space Shuttle Avionics System|access-date=November 23, 2011|date=July 15, 2005 |archive-url=https://web.archive.org/web/20220924220822/https://history.nasa.gov/computers/Ch4-7.html |archive-date=September 24, 2022 |url-status=live }}</ref> later upgraded to a system composed of two doubly redundant [[Motorola 68000]] (M68000) processors (for a total of four M68000s per controller).<ref name="SSMEC2">{{cite web|url=http://www.hq.nasa.gov/office/pao/History/computers/Ch4-8.html|title=The future of the shuttle's computers|access-date=November 23, 2011|date=July 15, 2005|publisher=NASA |archive-url=https://web.archive.org/web/20221004030412/http://www.hq.nasa.gov/office/pao/History/computers/Ch4-8.html |archive-date=October 4, 2022  }}</ref> Having the controller installed on the engine itself greatly simplifies the wiring between the engine and the launch vehicle, because all the sensors and actuators are connected directly to only the controller, each MEC then being connected to the orbiter's [[Space Shuttle#Flight systems|general purpose computers]] (GPCs) or the SLS's avionics suite via its own engine interface unit (EIU).<ref name="SSMEC3">{{cite web|url=http://spaceflight.nasa.gov/shuttle/reference/shutref/orbiter/prop/controllers.html|archive-url=https://web.archive.org/web/20010124060000/http://spaceflight.nasa.gov/shuttle/reference/shutref/orbiter/prop/controllers.html|archive-date=January 24, 2001|title=Space Shuttle Main Engine Controllers|publisher=NASA|access-date=December 8, 2011|date=April 4, 2004}}</ref> Using a dedicated system also simplifies the software and thus improves its reliability.

Two independent dual-CPU computers, A and B, form the controller; giving redundancy to the system. The failure of controller system A automatically leads to a switch-over to controller system B without impeding operational capabilities; the subsequent failure of controller system B would provide a graceful shutdown of the engine. Within each system (A and B), the two M68000s operate in [[Lockstep (computing)|lock-step]], thereby enabling each system to detect failures by comparing the signal levels on the buses of the two M68000 processors within that system. If differences are encountered between the two buses, then an interrupt is generated and control turned over to the other system. Because of subtle differences between M68000s from Motorola and the second source manufacturer [[TRW Inc.|TRW]], each system uses M68000s from the same manufacturer (for instance system A would have two Motorola CPUs while system B would have two CPUs manufactured by TRW). Memory for block I controllers was of the [[Plated wire memory|plated-wire]] type, which functions in a manner similar to magnetic [[Magnetic-core memory|core memory]] and retains data even after power is turned off.<ref name="SSMEC">{{cite web |author=Mattox |first1=Russell M. |last2=White |first2=J. B. |date=November 1981 |title=Space Shuttle Main Engine Controller |url=https://ntrs.nasa.gov/api/citations/19820003911/downloads/19820003911.pdf |url-status=live |archive-url=https://web.archive.org/web/20210625073012/https://ntrs.nasa.gov/api/citations/19820003911/downloads/19820003911.pdf |archive-date=June 25, 2021 |access-date=December 15, 2011 |publisher=NASA}}</ref> Block II controllers used conventional [[CMOS]] static [[Random-access memory|RAM]].<ref name="SSMEC2"/>

The controllers were designed to be tough enough to survive the forces of launch and proved to be extremely resilient to damage. During the investigation of the [[Space Shuttle Challenger disaster|''Challenger'' accident]] the two MECs (from engines 2020 and 2021), recovered from the seafloor, were delivered to Honeywell Aerospace for examination and analysis. One controller was broken open on one side, and both were severely corroded and damaged by marine life. Both units were disassembled and the memory units flushed with [[Purified water#Deionization|deionized water]]. After they were dried and [[Bake-out|vacuum baked]], data from these units was retrieved for forensic examination.<ref>{{cite web|url=https://history.nasa.gov/rogersrep/v1ch4.htm|title=The Cause of the Accident|work=Report of the Presidential Commission on the Space Shuttle Challenger Accident|access-date=December 8, 2011|date=June 6, 1986|publisher=[[NASA]] |archive-url=https://web.archive.org/web/20230510115414/https://history.nasa.gov/rogersrep/v1ch4.htm |archive-date=May 10, 2023 |url-status=live }}</ref>

====Main valves====
To control the engine's output, the MEC operates five hydraulically actuated propellant valves on each engine; the oxidizer pre-burner oxidizer, fuel pre-burner oxidizer, main oxidizer, main fuel, and chamber coolant valves. In an emergency, the valves can be fully closed by using the engine's helium supply system as a backup actuation system.<ref name="2.16"/>

In the Space Shuttle, the main oxidizer and fuel bleed valves were used after shutdown to dump any residual propellant, with residual liquid oxygen venting through the engine and residual liquid hydrogen venting through the liquid hydrogen fill and drain valves. After the dump was completed, the valves closed and remained closed for the remainder of the mission.<ref name="2.16"/>

A [[coolant]] control valve is mounted on the combustion chamber coolant bypass duct of each engine. The engine controller regulates the amount of gaseous hydrogen allowed to bypass the nozzle coolant loop, thus controlling its temperature. The chamber coolant valve is 100% open before the engine start. During engine operation, it is 100% open for throttle settings of 100 to 109%. For throttle settings between 65 and 100%, its position ranged from 66.4 to 100%.<ref name="2.16"/>

===Gimbal===
{{external media|width=|video1=[https://www.youtube.com/watch?v=7Wtg_3Y4lFc RS-25 gimbal test]}}
[[File:Shuttle Main Engine Firing in Gimbal Test - GPN-2000-000552.jpg|thumb|RS-25 gimbal test]]

Each engine is installed with a [[gimbal]] [[bearing (mechanical)|bearing]], a universal [[Ball joint|ball and socket joint]] which is bolted to the launch vehicle by its upper [[flange]] and to the engine by its lower flange. It represents the thrust interface between the engine and the launch vehicle, supporting {{convert|7,480|lb|kg|abbr=on}} of engine weight and withstanding over {{convert|500,000|lbf|N|abbr=on}} of thrust. As well as providing a means to attach the engine to the launch vehicle, the gimbal bearing allows the engine to be pivoted (or "gimballed") around two axes of freedom with a range of ±10.5°.<ref name="MPS">{{cite web |author=Dumoulin |first=Jim |date=August 31, 2000 |title=Main Propulsion System |url=http://science.ksc.nasa.gov/shuttle/technology/sts-newsref/sts-mps.html |archive-url=https://web.archive.org/web/20211123164917/https://science.ksc.nasa.gov/shuttle/technology/sts-newsref/sts-mps.html |archive-date=November 23, 2021 |access-date=January 16, 2012 |publisher=NASA}}</ref> This motion allows the engine's thrust vector to be altered, thus steering the vehicle into the correct orientation. The comparatively large gimbal range is necessary to correct for the pitch momentum that occurs due to the constantly shifting center of mass as the vehicle burns fuel in flight and after booster separation. The bearing assembly is approximately {{convert|290|by|360|mm|in|abbr=on}}, has a mass of {{convert|105|lb|kg|abbr=on}}, and is made of [[titanium]] alloy.<ref name="Orientation">{{cite web|access-date=December 12, 2011|title=Space Shuttle Main Engine Orientation|publisher=Boeing/Rocketdyne|date=June 1998|url=http://www.lpre.de/p_and_w/SSME/SSME_PRESENTATION.pdf |archive-url=https://web.archive.org/web/20220831201441/http://www.lpre.de/p_and_w/SSME/SSME_PRESENTATION.pdf |archive-date=August 31, 2022 |url-status=live }}</ref>

The low-pressure oxygen and low-pressure fuel turbopumps were mounted 180° apart on the orbiter's aft fuselage thrust structure. The lines from the low-pressure turbopumps to the high-pressure turbopumps contain flexible bellows that enable the low-pressure turbopumps to remain stationary while the rest of the engine is gimbaled for thrust vector control, and also to prevent damage to the pumps when loads were applied to them. The liquid-hydrogen line from the LPFTP to the HPFTP is insulated to prevent the formation of liquid air.<ref name="2.16"/>

===Helium system===
In addition to fuel and oxidizer systems, the launch vehicle's main propulsion system is also equipped with a helium system consisting of ten storage tanks in addition to various regulators, check valves, distribution lines, and control valves. The system is used in-flight to purge the engine and provides pressure for actuating engine valves within the propellant management system and during emergency shutdowns. During entry, on the Space Shuttle, any remaining helium was used to purge the engines during reentry and for repressurization.<ref name="2.16"/>

==History==

===Development===
[[File:SSME test A-1.ogv|thumb|RS-25 testing at [[John C. Stennis Space Center|Stennis Space Center]] in early 2015|alt=A video showing RS-25 testing. The video opens with a night view of a large scaffold structure (the test stand), lit with internal lights. The view then switches to show the nozzle of a rocket engine, mounted within the structure, lighting and beginning to fire. The view then cuts back to the view of the scaffold, from which large amounts of steam are now billowing out, towards the right of the frame. Wide and close-up views of this plume follow, before the view switches back to the engine nozzle, which shuts down.]]

The history of the RS-25 traces back to the 1960s when [[NASA]]'s [[Marshall Space Flight Center]] and [[Rocketdyne]] were conducting a series of studies on high-pressure engines, developed from the successful [[J-2 (rocket engine)|J-2 engine]] used on the [[S-II]] and [[S-IVB]] upper stages of the [[Saturn V]] rocket during the [[Apollo program]]. The studies were conducted under a program to upgrade the Saturn V engines, which produced a design for a {{cvt|350,000|lbf|kN}} upper-stage engine known as the [[HG-3 (rocket engine)|HG-3]].<ref name="HG-3">{{cite web |author=Wade |first=Mark |title=HG-3 |url=http://www.astronautix.com/engines/hg3.htm |archive-url=https://web.archive.org/web/20111115163306/http://www.astronautix.com/engines/hg3.htm |archive-date=November 15, 2011 |access-date=December 13, 2011 |publisher=Encyclopedia Astronautica}}</ref> As funding levels for Apollo wound down the HG-3 was cancelled as well as the upgraded [[F-1 (rocket engine)|F-1 engines]] already being tested.<ref>{{cite web|url=https://archive.org/details/nasa_techdoc_19740077747|title=F-1A Task Assignment Program|last=NON|date=January 15, 1970|via=Internet Archive}}</ref> It was the design for the HG-3 that would form the basis for the RS-25.<ref name="RS-25MSFC">{{cite web|url=http://history.msfc.nasa.gov/saturn_apollo/propulsion_center.html|archive-url=https://web.archive.org/web/20051115064042/http://history.msfc.nasa.gov/saturn_apollo/propulsion_center.html|archive-date=November 15, 2005|publisher=NASA|access-date=December 13, 2011|title=MSFC Propulsion Center of Excellence is Built on Solid Foundation|year=1995}}</ref>

Meanwhile, in 1967, the [[US Air Force]] funded a study into advanced rocket propulsion systems for use during [[Project Isinglass]], with Rocketdyne asked to investigate [[aerospike engine|aerospike]] engines and [[Pratt & Whitney]] (P&W) to research more efficient conventional [[de Laval nozzle]]-type engines. At the conclusion of the study, P&W put forward a proposal for a 250,000&nbsp;lb<sub>f</sub> engine called the [[Pratt & Whitney XLR-129|XLR-129]], which used a two-position [[expanding nozzle]] to provide increased efficiency over a wide range of altitudes.<ref name="Haynes">{{cite book |author=Baker |first=David |title=NASA Space Shuttle |date=April 2011 |publisher=Haynes Publishing |isbn=978-1-84425-866-6 |series=Owners' Workshop Manuals}}</ref><ref name="spacereview-20100412">{{Cite web |url=https://www.thespacereview.com/article/1602/1 |title=A bat outta Hell: the ISINGLASS Mach 22 follow-on to OXCART |last=Day |first=Dwayne |date=April 12, 2010 |publisher=The Space Review |access-date=January 8, 2012 |url-status=live |archive-url=https://web.archive.org/web/20220926190816/https://www.thespacereview.com/article/1602/1 |archive-date=September 26, 2022 }}</ref>

In January 1969 NASA awarded contracts to General Dynamics, Lockheed, McDonnell Douglas, and North American Rockwell to initiate the early development of the Space Shuttle.<ref name="SSME30"/> As part of these 'Phase A' studies, the involved companies selected an upgraded version of the XLR-129, developing {{cvt|415,000|lbf|kN}}, as the baseline engine for their designs.<ref name="Haynes"/> This design can be found on many of the planned Shuttle versions right up to the final decision. However, since NASA was interested in pushing the [[state of the art]] in every way they decided to select a much more advanced design in order to "force an advancement of rocket engine technology".<ref name="SSMEFTY"/><ref name="Haynes"/> They called for a new design based on a high-pressure combustion chamber running around {{cvt|3000|psi}}, which increases the performance of the engine.

Development began in 1970, when NASA released a [[request for proposal]] for 'Phase B' main engine concept studies, requiring development of a throttleable, [[Staged combustion cycle (rocket)|staged combustion]], de Laval-type engine.<ref name="SSMEFTY"/><ref name="Haynes"/> The request was based on the then-current design of the Space Shuttle which featured two reusable stages, the orbiter and a crewed fly-back booster, and required one engine which would be able to power both vehicles via two different nozzles (12 booster engines with {{cvt|550,000|lbf|kN}} sea level thrust each and 3 orbiter engines with {{cvt|632,000|lbf|kN}} vacuum thrust each).<ref name="SSMEFTY"/> Rocketdyne, P&W and [[Aerojet|Aerojet General]] were selected to receive funding although, given P&W's already-advanced development (demonstrating a working {{cvt|350,000|lbf|kN}} concept engine during the year) and Aerojet General's prior experience in developing the {{cvt|1,500,000|lbf|kN}} [[M-1 (rocket engine)|M-1 engine]], Rocketdyne was forced to put a large amount of private money into the design process to allow the company to catch up to its competitors.<ref name="Haynes"/>

By the time the contract was awarded, budgetary pressures meant that the shuttle's design had changed to its final orbiter, external tank, and two boosters configuration, and so the engine was only required to power the orbiter during ascent.<ref name="SSMEFTY"/> During the year-long 'Phase B' study period, Rocketdyne was able to make use of their experience developing the HG-3 engine to design their SSME proposal, producing a prototype by January 1971. The engine made use of a new Rocketdyne-developed [[copper]]-[[zirconium]] alloy (called NARloy-Z) and was tested on February 12, 1971, producing a chamber pressure of {{cvt|3172|psi}}. The three participating companies submitted their engine development bids in April 1971, with Rocketdyne being awarded the contract on July 13, 1971—although work did not begin on engine development until March 31, 1972, due to a legal challenge from P&W.<ref name="SSMEFTY"/><ref name="Haynes"/>

Following the awarding of the contract, a preliminary design review was carried out in September 1972, followed by a critical design review in September 1976 after which the engine's design was set and construction of the first set of flight-capable engines began. A final review of all the Space Shuttle's components, including the engines, was conducted in 1979. The design reviews operated in parallel with several test milestones, initial tests consisting of individual engine components which identified shortcomings with various areas of the design, including the HPFTP, HPOTP, valves, nozzle, and fuel pre-burners. The individual engine component tests were followed by the first test of a complete engine (0002) on March 16, 1977, after its final assembly line was established in the main Rocketdyne factory in [[Canoga Park, Los Angeles]].<ref name="Canoga Park">{{cite web | title=Rocketdyne plant that built world's most powerful rocket engines being razed | website=Canoga Park Neighborhood Council | date=August 8, 2016 | url=https://canogaparknc.org/2016/08/rocketdyne-plant-built-worlds-powerful-rocket-engines-razed/ | access-date=July 3, 2024}}</ref> NASA specified that, prior to the Shuttle's first flight, the engines must have undergone at least 65,000 seconds of testing, a milestone that was reached on March 23, 1980, with the engine having undergone 110,253 seconds of testing by the time of [[STS-1]] both on test stands at [[John C. Stennis Space Center|Stennis Space Center]] and installed on the [[MPTA-098|Main Propulsion Test Article]] (MPTA). The first set of engines (2005, 2006 and 2007) was delivered to [[Kennedy Space Center]] in 1979 and installed on {{OV|102|full=no}}, before being removed in 1980 for further testing and reinstalled on the orbiter. The engines, which were of the first manned orbital flight (FMOF) configuration and certified for operation at 100% rated power level (RPL), were operated in a twenty-second flight readiness firing on February 20, 1981, and, after inspection, declared ready for flight.<ref name="SSMEFTY"/>

===Space Shuttle program===
{{See also|List of Space Shuttle missions}}
[[File:020408 STS110 Atlantis launch.jpg|thumb|{{OV|104}}'s three RS-25D main engines at liftoff during [[STS-110]]|alt=Three bell-shaped rocket engine nozzles projecting from the aft structure of a Space Shuttle orbiter. The cluster is arranged triangularly, with one engine at the top and two below, with two smaller nozzles visible to the left and right of the top engine. The three larger engines are firing, with white-hot flames visible projecting from each nozzle. The Space Shuttle's left solid rocket booster (a white, cylindrical rocket) is visible in the background, with the two large, grey tail service masts visible to the left and right of the orbiter's aft structure.]]
[[File:SSME startup & shutdown.ogv|thumb|SSME startup and shutdown sequences|alt=The video starts with a view of the NASA and Space Shuttle program logos followed by titles. The first view is of the base of a Space Shuttle stack as seen from a camera mounted above on the external tank. White vapor can be seen curling around the base of the shuttle. The view cuts to a wide shot of the launchpad, before moving back to the original view, at which point large jets of water can be seen pouring into the flame trench beneath the shuttle. The video then moves to a view from underneath the stack, showing the three RS-25 engines mounted in the orbiter in slow motion. Orange sparks begin to project from service structures on either side of the engines, then the video changes to a quartered view, with shots of both sides of the engine cluster and a clock. The engines then ignite, in the sequence right, left, top. At first, the engines burn with a yellow flame, but the flames then progress through white and blue, eventually forming perfect cones with each discs visible lower in the plume. A side view of this in real-time is then shown, followed by a return to the original view where the real-time process repeats. The movement of the top of the stack as this occurs is shown next, followed by the retraction of the umbilicals mounted on either side of the orbiter's aft structure and to the external tank, the ignition of the shuttle's solid rocket boosters, and the liftoff of the stack. Several views of the first stage ascent, including SRB separation, are then shown. Next, the view changes to a view from another external tank camera pointing aft, showing the engines' plume, in the form of a large white ring, glowing around the aft of the shuttle. This gradually fades as the engines throttle back and shut down, before the orbiter separates from the external tank, accompanied by bright thruster firings. Finally, the video fades and is replaced by another logo.]]

Each Space Shuttle had three RS-25 engines, installed in the aft structure of the [[Space Shuttle orbiter]] in the [[Orbiter Processing Facility]] prior to the orbiter being transferred to the [[Vehicle Assembly Building]]. If necessary the engines could be changed on the pad. The engines, drawing propellant from the Space Shuttle external tank (ET) via the orbiter's main propulsion system (MPS), were ignited at T−6.6 seconds prior to liftoff (with each ignition staggered by 120&nbsp;[[millisecond|ms]]<ref name="Hale">{{cite web |author=Hale |first1=Wayne |author-link1=Wayne Hale |last2=& various |date=January 17, 2012 |title=An SSME-related request |url=http://forum.nasaspaceflight.com/index.php?topic=27783 |access-date=January 17, 2012 |publisher=NASASpaceflight.com}}</ref>), which allowed their performance to be checked prior to ignition of the [[Space Shuttle Solid Rocket Booster]]s (SRBs), which committed the shuttle to the launch.<ref name="Countdown101">{{Cite web |url=https://www.nasa.gov/mission_pages/shuttle/launch/countdown101.html |title=Countdown 101 |last=Ryba |first=Jeanne |date=September 17, 2009 |publisher=NASA |access-date=January 8, 2012 |url-status=live |archive-url=https://web.archive.org/web/20230308190149/https://www.nasa.gov/mission_pages/shuttle/launch/countdown101.html |archive-date=March 8, 2023 }}</ref> At launch, the engines would be operating at 100% RPL, throttling up to 104.5% immediately following liftoff. The engines would maintain this power level until around T+40 seconds, where they would be throttled back to around 70% to reduce aerodynamic loads on the shuttle stack as it passed through the region of maximum dynamic pressure, or [[max q|max. ''q'']].<ref group="note">The level of throttle was initially set to 65%, but, following review of early flight performance, this was increased to a minimum of 67% to reduce fatigue on the MPS. The throttle lever was dynamically calculated based on initial launch performance, generally being reduced to a level around 70%.</ref><ref name="Haynes"/><ref name="Hale"/> The engines would then be throttled back up until around T+8 minutes, at which point they would be gradually throttled back down to 67% to prevent the stack exceeding 3&nbsp;[[g-force|g]] of acceleration as it became progressively lighter due to propellant consumption. The engines were then shut down, a procedure known as main engine cutoff (MECO), at around T+8.5 minutes.<ref name="Haynes"/>

After each flight the engines would be removed from the orbiter and transferred to the Space Shuttle Main Engine Processing Facility (SSMEPF), where they would be inspected and refurbished in preparation for reuse on a subsequent flight.<ref name="Sidemount">{{Cite web |url=https://www.nasa.gov/pdf/361842main_15%20-%20Augustine%20Sidemount%20Final.pdf |title=Shuttle-Derived Heavy Lift Launch Vehicle |last=Shannon |first=John |date=June 17, 2009 |publisher=[[NASA]] |url-status=live |archive-url=https://web.archive.org/web/20230404220952/https://www.nasa.gov/pdf/361842main_15%20-%20Augustine%20Sidemount%20Final.pdf |archive-date=April 4, 2023 }}</ref> A total of 46 reusable RS-25 engines, each costing around US$40 million, were flown during the Space Shuttle program, with each new or overhauled engine entering the flight inventory requiring [[Flight-qualify|flight qualification]] on one of the test stands at [[John C. Stennis Space Center|Stennis Space Center]] prior to flight.<ref name="Hale"/><ref name="P&WChart">{{cite web|url=http://collectspace.com/review/sts133_ssmechart-lg.jpg|title=SSME Flight Experience|publisher=Pratt & Whitney Rocketdyne|date=November 2010|format=JPEG}}</ref><ref name="Cost">{{Cite news |url=https://www.nasaspaceflight.com/2007/12/constellation-transition-phased-retirement-plan-for-the-ssme-set/ |title=Constellation transition – phased retirement plan for the SSME set |last=Bergin |first=Chris |date=December 3, 2007 |work=NASASpaceflight.com |access-date=January 23, 2012 |url-status=live |archive-url=https://web.archive.org/web/20230205181229/https://www.nasaspaceflight.com/2007/12/constellation-transition-phased-retirement-plan-for-the-ssme-set/ |archive-date=February 5, 2023 }}</ref>

====Upgrades====
[[File:SSME Flight History.png|thumb|Flight history of the Space Shuttle Main Engines|alt=A chart showing the flight history of each RS-25 used during the Space Shuttle program, sorted by engine version.]]

Over the course of the Space Shuttle program, the RS-25 went through a series of upgrades, including combustion chamber changes, improved welds and turbopump changes in an effort to improve the engine's performance and reliability and so reduce the amount of maintenance required after use. As a result, several versions of the RS-25 were used during the program:<ref name="Enhancements">{{Cite web |url=https://www.nasa.gov/centers/marshall/news/background/facts/ssme.html |title=Space Shuttle Main Engine Enhancements |last=Roy |first=Steve |date=August 2000 |publisher=[[NASA]] |access-date=December 7, 2011 |url-status=live |archive-url=https://web.archive.org/web/20230419051749/https://www.nasa.gov/centers/marshall/news/background/facts/ssme.html |archive-date=April 19, 2023 |id=FS-2000-07-159-MSFC }}</ref><ref name="Haynes"/><ref name="SSME30">{{cite web |author=Jue |first=Fred H. |title=Space Shuttle Main Engine: 30 Years of Innovation |url=http://history.msfc.nasa.gov/shuttle/documents/space_shuttle_main_engine_30_years_innovation.pdf |archive-url=https://web.archive.org/web/20100528085851/http://history.msfc.nasa.gov/shuttle/documents/space_shuttle_main_engine_30_years_innovation.pdf |archive-date=May 28, 2010 |access-date=November 27, 2011 |publisher=Boeing}}</ref><ref name="Hale"/><ref name="SSMEAT">{{Cite tech report |url=https://ntrs.nasa.gov/api/citations/19930012456/downloads/19930012456.pdf |title=Report of the SSME Assessment Team |date=January 1993 |publisher=[[NASA]] |access-date=November 27, 2011 |url-status=live |archive-url=https://web.archive.org/web/20230525075734/https://ntrs.nasa.gov/api/citations/19930012456/downloads/19930012456.pdf |archive-date=May 25, 2023 }}</ref><ref name="AIAA1">{{cite web |author=Jue |first1=Fred |last2=Kuck |first2=Fritz |date=July 2002 |title=Space Shuttle Main Engine (SSME) Options for the Future Shuttle |url=http://www.pwrengineering.com/dataresources/SpaceShuttleMainEngine(SSME)OptionsForTheFutureShuttle.doc |archive-url=https://web.archive.org/web/20071009152328/http://www.pwrengineering.com/dataresources/SpaceShuttleMainEngine%28SSME%29OptionsForTheFutureShuttle.doc |archive-date=October 9, 2007 |access-date=November 27, 2011 |publisher=American Institute of Aeronautics and Astronautics |format=DOC}}</ref><ref name="Reference Engines">{{Cite web |url=http://www.alternatewars.com/BBOW/Space/Reference_Spacecraft_Engines.htm |title=Reference Spacecraft Engines |last=Crierie |first=Ryan |date=November 13, 2011 |access-date=January 8, 2012 |archive-url=https://web.archive.org/web/20200105185400/http://www.alternatewars.com/BBOW/Space/Reference_Spacecraft_Engines.htm |archive-date=January 5, 2020 |url-status=live }}</ref><ref name="Roar">{{cite web|url=https://science.nasa.gov/headlines/y2002/06nov_ssme.htm |archive-url=https://web.archive.org/web/20021108104951/https://science.nasa.gov/headlines/y2002/06nov_ssme.htm  |archive-date=November 8, 2002 |title=The Roar of Innovation |publisher=NASA |date=November 6, 2002 |access-date=December 7, 2011 }}</ref><ref name="msfc6976.2">{{Cite web |url=https://www.nasa.gov/centers/marshall/ppt/134663main_6976_coalition_MSFC_ESAS_Rollout-092205.ppt |title=MSFC and Exploration: Our Path Forward |date=September 2005 |publisher=[[NASA]] |url-status=live |archive-url=https://web.archive.org/web/20220331235411/https://www.nasa.gov/centers/marshall/ppt/134663main_6976_coalition_MSFC_ESAS_Rollout-092205.ppt |archive-date=March 31, 2022 }}</ref>
*FMOF (first manned orbital flight): Certified for 100% rated power level (RPL). Used for the orbital flight test missions [[STS-1]] – [[STS-5]] (engines 2005, 2006 and 2007).
*Phase I: Used for missions [[STS-6]] – [[STS-51-L]], the Phase I engine offered increased service life and was certified for 104% RPL. Replaced by Phase II after the [[Challenger Disaster]].
*Phase II (RS-25A): First flown on [[STS-26]], the Phase II engine offered a number of safety upgrades and was certified for 104% RPL & 109% full power level (FPL) in the event of a contingency.
*Block I (RS-25B): First flown on [[STS-70]], the Block I engines offered improved turbopumps featuring ceramic bearings, half as many rotating parts, and a new casting process reducing the number of welds. Block I improvements also included a new, two-duct powerhead (rather than the original design, which featured three ducts connected to the HPFTP and two to the HPOTP), which helped improve hot gas flow, and an improved engine heat exchanger.
*Block IA (RS-25B): First flown on [[STS-73]], the Block IA engine offered main injector improvements.
*Block IIA (RS-25C): First flown on [[STS-89]], the Block IIA engine was an interim model used whilst certain components of the Block II engine completed development. Changes included a new large throat main combustion chamber (which had originally been recommended by Rocketdyne in 1980), improved low-pressure turbopumps, and certification for 104.5% RPL to compensate for a {{convert|2|isp}} reduction in [[specific impulse]] (original plans called for the engine to be certified to 106% for heavy [[International Space Station]] payloads, but this was not required and would have reduced engine service life). A slightly modified version first flew on [[STS-96]].
*Block II (RS-25D): First flown on [[STS-104]], the Block II upgrade included all of the Block IIA improvements plus a new high-pressure fuel turbopump. This model was ground-tested to 111% FPL in the event of a [[Space Shuttle abort modes#Ascent abort modes|contingency abort]], and certified for 109% FPL for use during an [[Space Shuttle abort modes#Intact abort modes|intact abort]].
*RS-25E: It will be used on the [[Space Launch System]] for future [[Artemis program]] missions beginning with [[Artemis V]], as the RS-25D stock is intentionally being used up. Unlike previous versions, this engine is designed to be expendable.<ref name="NSFSLS">{{Cite news |url=https://www.nasaspaceflight.com/2011/09/sls-finally-announced-nasa-forward-path/ |title=SLS finally announced by NASA – Forward path taking shape |last=Bergin |first=Chris |date=September 14, 2011 |work=NASASpaceflight.com |access-date=December 14, 2011 |url-status=live |archive-url=https://web.archive.org/web/20230322214504/https://www.nasaspaceflight.com/2011/09/sls-finally-announced-nasa-forward-path/ |archive-date=March 22, 2023 }}</ref> The powerhead is almost completely redesigned ({{as of|2023|September|lc=y}} the specific design changes from the -25D have not been announced), and intended to incorporate various cost-saving measures and innovations in manufacturing. The first testing engine, E10001, passed all its qualifications and tests at NASA's Stennis Space Center, and demonstrated both a 113% FPL and a 30% increase in thrust.<ref name="June 22 Stennis tweet"/>

====Engine throttle/output====
The most obvious effects of the upgrades the RS-25 received through the Space Shuttle program were the improvements in engine throttle. Whilst the FMOF engine had a maximum output of 100% RPL, Block II engines could throttle as high as 109% or 111% in an emergency, with usual flight performance being 104.5%. Existing engines used on the Space Launch System are throttled to 109% power during normal flight, while new RS-25 engines produced for the Space Launch System are to be run at 111% throttle,<ref name="FS-2020-10-42-MSFC">{{Cite web |id=FS-2020-10-42-MSFC |url=https://www.nasa.gov/sites/default/files/atoms/files/sls_rs25_engine_fs_508.pdf |title=Space Launch System RS-25 Core Stage Engine |date=January 29, 2020 |website=NASA Facts |publisher=[[Marshall Space Flight Center]] |access-date=May 24, 2023 |url-status=live |archive-url=https://web.archive.org/web/20230211050940/https://www.nasa.gov/sites/default/files/atoms/files/sls_rs25_engine_fs_508.pdf |archive-date=February 11, 2023 }}</ref> with 113% power being tested.<ref name="nsf-20180221">{{Cite news |url=https://www.nasaspaceflight.com/2018/02/rs-25-hot-fire-sls-engine-113-percent/ |title=RS-25 hot fire pushes SLS engine to record 113 percent thrust level |last=Sloss |first=Philip |date=February 21, 2018 |work=NASASpaceFlight.com |access-date=May 24, 2023 |url-status=live |archive-url=https://web.archive.org/web/20230404150603/https://www.nasaspaceflight.com/2018/02/rs-25-hot-fire-sls-engine-113-percent/ |archive-date=April 4, 2023 }}</ref><ref name="nasa-20230524">{{Cite web |url=https://www.nasa.gov/centers/stennis/news/image-feature/2023/NASA-Continues-Key-Test-Series-with-Moon-Rocket-Engine-Hot-Fire |title=NASA Continues Key Test Series with Moon Rocket Engine Hot Fire |last=Dean |first=LaToya |date=23 May 2023 |publisher=[[NASA]] |access-date=24 May 2023 |url-status=live |archive-url=https://web.archive.org/web/20230524115152/https://www.nasa.gov/centers/stennis/news/image-feature/2023/NASA-Continues-Key-Test-Series-with-Moon-Rocket-Engine-Hot-Fire/ |archive-date=24 May 2023 }}</ref> These increases in throttle level made a significant difference to the thrust produced by the engine:<ref name="Orientation"/><ref name="Hale"/>

{| class="wikitable"
|-
! rowspan=2 |
! rowspan=2 | Of {{abbr|RPL|Rated power level}} <br/>(%)
! colspan=2 | Thrust
|-
! Sea level
! Vacuum
|-
| Minimum power level (MPL)
| {{fsp}}67
| || {{convert|316100|lb-f|kN|abbr=on|order=flip}}
|-
| Rated power level (RPL)
| {{n/a|100}}
| {{convert|1670|kN|abbr=on}} || {{convert|2090|kN|abbr=on}}
|-
| Nominal power level (NPL)
| 104.5
| {{convert|1750|kN|abbr=on}} || {{convert|2170|kN|abbr=on}}
|-
| Full power level (FPL)
| 109
| {{convert|1860|kN|abbr=on}} || {{convert|2280|kN|abbr=on}}
|-
| SLS Production Restart
| 111
||| {{convert|521000|lb-f|kN|abbr=on|order=flip}}
|-
| Production Restart Abort
| 113
| {{convert|1887|kN|abbr=on}}|| {{convert|2362|kN|abbr=on}}
|}

Specifying power levels over 100% may seem nonsensical, but there was a logic behind it. The 100% level does not mean the maximum physical power level attainable, rather it was a specification decided on during engine development—the expected rated power level. When later studies indicated the engine could operate safely at levels above 100%, these higher levels became standard. Maintaining the original relationship of power level to physical thrust helped reduce confusion, as it created an unvarying fixed relationship so that test data (or operational data from past or future missions) can be easily compared. If the power level was increased, and that new value was said to be 100%, then all previous data and documentation would either require changing or cross-checking against what physical thrust corresponded to 100% power level on that date.<ref name="SSMEFTY"/> Engine power level affects engine reliability, with studies indicating the probability of an engine failure increasing rapidly with power levels over 104.5%, which was why power levels above 104.5% were retained for contingency use only.<ref name="SSMEAT"/>

====Incidents====
[[File:Space Shuttle abort panel.jpg|thumb|alt=refer to caption|This Shuttle control panel is set to select the abort to orbit (ATO) option, as used in the STS-51-F mission. After orbit was achieved, the mission continued normally and the orbiter returned to Earth with the crew.]]
[[File:Columbia's Main Engine Powerheads - GPN-2003-00076.jpg|thumb|alt=refer to caption|Recovered power-head of one of ''Columbia''{{'}}s main engines. ''Columbia'' was lost on re-entry, from a heat shield failure.]]

During the course of the Space Shuttle program, a total of 46 RS-25 engines were used (with one extra RS-25D being built but never used). During the 135 missions, for a total of 405 individual engine-missions,<ref name="P&WChart"/> Pratt & Whitney Rocketdyne reports a 99.95% reliability rate, with the only in-flight SSME failure occurring during {{OV|99}}'s [[STS-51-F]] mission.<ref name="P&WFS"/> The engines, however, did suffer from a number of pad failures (redundant set launch sequencer aborts, or RSLSs) and other issues during the course of the program:
*[[STS-41-D]] {{OV|103|full=no}} – No.&nbsp;3 engine caused an RSLS shutdown at T−4 seconds due to loss of redundant control on main engine valve, stack rolled back and engine replaced.<ref>{{cite book |author=Mullane |first=Mike |author-link=Mike Mullane |url=https://archive.org/details/ridingrocketsout00mull_0 |title=Riding Rockets: The Outrageous Tales of a Space Shuttle Astronaut |date=February 3, 2007 |publisher=[[Charles Scribner's Sons|Scribner]] |isbn=978-0-7432-7682-5 |url-access=registration}}</ref>
*[[STS-51-F]] {{OV|99|full=no}} – No.&nbsp;2 engine caused an RSLS shutdown at T−3 seconds due to a coolant valve malfunction.<ref name="ksc-51-f">{{Cite web |url=http://science.ksc.nasa.gov/shuttle/missions/51-f/mission-51-f.html |title=51-F |last=Dumoulin |first=Jim |date=June 29, 2001 |publisher=[[NASA]] |access-date=January 16, 2012  |archive-url=https://web.archive.org/web/20211217060254/https://science.ksc.nasa.gov/shuttle/missions/51-f/mission-51-f.html |archive-date=December 17, 2021 }}</ref><ref name="Challenger"/>
*[[STS-51-F]] {{OV|99|full=no}} – No.&nbsp;1 engine (2023) shutdown at T+5:43 due to faulty temperature sensors, leading to an [[Abort to Orbit|abort to orbit]] (although the mission objectives and length were not compromised by the ATO).<ref name="Hale"/><ref name="Challenger">{{cite book |author=Evans |first=Ben |title=Space Shuttle Challenger: Ten Journeys into the Unknown |publisher=Springer-Praxis |year=2007 |isbn=978-0-387-46355-1 |location=Warwickshire, United Kingdom}}</ref>
*[[STS-55]] {{OV|102|full=no}} – No.&nbsp;3 engine caused an RSLS shutdown at T−3 seconds due to a leak in its liquid-oxygen preburner check valve.<ref name="ksc-sts-55">{{Cite web |url=http://science.ksc.nasa.gov/shuttle/missions/sts-55/mission-sts-55.html |title=STS-55 |last=Dumoulin |first=Jim |date=June 29, 2001 |publisher=[[NASA]] |access-date=January 16, 2012  |archive-url=https://web.archive.org/web/20220120161816/https://science.ksc.nasa.gov/shuttle/missions/sts-55/mission-sts-55.html |archive-date=January 20, 2022 }}</ref>
*[[STS-51]] {{OV|103|full=no}} – No.&nbsp;2 engine caused an RSLS shut down at T−3 seconds due to a faulty hydrogen fuel sensor.<ref name="ksc-sts-51">{{Cite web |url=http://science.ksc.nasa.gov/shuttle/missions/sts-51/mission-sts-51.html |title=STS-51 |last=Dumoulin |first=Jim |date=June 29, 2001 |publisher=[[NASA]] |access-date=January 16, 2012  |archive-url=https://web.archive.org/web/20220402154413/http://science.ksc.nasa.gov/shuttle/missions/sts-51/mission-sts-51.html |archive-date=April 2, 2022 }}</ref>
*[[STS-68]] {{OV|105|full=no}} – No.&nbsp;3 engine (2032) caused an RSLS shutdown at T−1.9 seconds when a temperature sensor in its HPOTP exceeded its [[redline]].<ref name="ksc-sts-68">{{Cite web |url=http://science.ksc.nasa.gov/shuttle/missions/sts-68/mission-sts-68.html |title=STS-68 |last=Dumoulin |first=Jim |date=June 29, 2001 |publisher=[[NASA]] |access-date=January 16, 2012  |archive-url=https://web.archive.org/web/20220103165246/https://science.ksc.nasa.gov/shuttle/missions/sts-68/mission-sts-68.html |archive-date=January 3, 2022 }}</ref>
*[[STS-93]] {{OV|102|full=no}} – An Orbiter Project AC1 Phase A electrical wiring short occurred at T+5 seconds causing an under voltage which disqualified SSME{{nbsp}}1A and SSME{{nbsp}}3B controllers but required no engine shut down. In addition, a 0.1-inch diameter, 1-inch long gold-plated pin, used to plug an oxidizer post orifice (an inappropriate SSME corrective action eliminated from the fleet by redesign) came loose inside an engine's main injector and impacted the engine nozzle inner surface, rupturing three hydrogen cooling lines. The resulting three breaches caused a leak resulting in a premature engine shutdown, when four external tank LO{{sub|2}} sensors flashed dry resulting in low-level cutoff of the main engines and a slightly early main engine cut-off with a {{convert|16|ft/s|abbr=on}} underspeed, and an 8 nautical mile lower altitude.<ref>{{cite book |author=Evans |first=Ben |title=Space Shuttle Columbia: Her Missions and Crews |date=August 30, 2005 |publisher=Springer Praxis |isbn=978-0-387-21517-4}}</ref>

=== Constellation ===
[[File:Pratt Whitney Rocketdyne space shuttle main engines.jpg|thumb|left|The six RS-25Ds used during [[STS-134]] and [[STS-135]] in storage at [[Kennedy Space Center]]|alt=Six rocket engines, consisting of a large bell-shaped nozzle with working parts mounted to the top, stored in a large warehouse with white walls decorated with flags. Each engine has several pieces of red protective equipment attached to it and is mounted on a yellow wheeled pallet-like structure.]]

During the period preceding final [[Space Shuttle retirement]], various plans for the remaining engines were proposed, ranging from them all being kept by NASA, to them all being given away (or sold for US$400,000–800,000 each) to various institutions such as museums and universities.<ref name="shuttle_sale">{{cite news |url=https://abcnews.go.com/Technology/wireStory?id=9574776|title=Recession Special: NASA Cuts Space Shuttle Price|last=Dunn|first=Marcia|date=January 15, 2010|work=[[ABC News (United States)|ABC News]]|archive-url=https://web.archive.org/web/20100118122624/https://abcnews.go.com/Technology/wireStory?id=9574776|archive-date=January 18, 2010}}</ref> This policy followed changes to the planned configurations of the [[Constellation program]]'s [[Ares V]] cargo-launch vehicle and [[Ares I]] crew-launch vehicle rockets, which had been planned to use the RS-25 in their first and second stages respectively.<ref name="NSFAres"/> While these configurations had initially seemed worthwhile, as they would use then-current technology following the shuttle's retirement in 2010, the plan had several drawbacks:<ref name="NSFAres">{{cite web |author=Harris |first1=David |last2=Bergin |first2=Chris |date=December 26, 2008 |title=Return to SSME – Ares V undergoes evaluation into potential switch |url=http://www.nasaspaceflight.com/2008/12/ssme-ares-v-undergoes-evaluation-potential-switch/ |access-date=December 15, 2011 |publisher=NASASpaceflight.com}}</ref>
*The engines would not be reusable, as they would be permanently attached to the discarded stages.
*Each engine would have to undergo a test firing prior to installation and launch, with refurbishment required following the test.
*It would be expensive, time-consuming, and weight-intensive to convert the ground-started RS-25D to an air-started version for the Ares I second stage.

Following several design changes to the Ares I and Ares V rockets, the RS-25 was to be replaced with a single [[J-2 (rocket engine)|J-2X]] engine for the Ares I second stage and six modified [[RS-68 (rocket engine)|RS-68]] engines (which was based on both the SSME and Apollo-era J-2 engine) on the Ares V core stage; this meant that the RS-25 would be retired along with the Shuttle fleet.<ref name="NSFAres"/> In 2010, however, NASA was directed to halt the Constellation program, and with it development of the Ares I and Ares V, instead of focusing on building a new heavy-lift launcher.<ref name="BBCHLV">{{Cite news |url=https://www.bbc.co.uk/news/science-environment-11518049 |title=Obama signs Nasa up to new future |last=Amos |first=Jonathan |date=October 11, 2010 |work=BBC News  |archive-url=https://web.archive.org/web/20221114013133/https://www.bbc.co.uk/news/science-environment-11518049 |archive-date=November 14, 2022 }}</ref>

=== XS-1 ===
{{See also|XS-1 (spacecraft)}}
On May 24, 2017, [[DARPA]] announced that they had selected [[The Boeing Company]] to complete design work on the XS-1 program. The technology demonstrator was planned to use an [[Aerojet Rocketdyne]] AR-22 engine. The AR-22 was a version of the RS-25, with parts sourced from Aerojet Rocketdyne and NASA inventories from early versions of the engine.<ref name="darpa-20170524">{{Cite press release |url=https://www.darpa.mil/news-events/2017-05-24 |title=DARPA Picks Design for Next-Generation Spaceplane |date=24 May 2017 |publisher=[[DARPA]] |access-date=February 13, 2018 |url-status=live |archive-url=https://web.archive.org/web/20221221203036/https://www.darpa.mil/news-events/2017-05-24 |archive-date=December 21, 2022 }}</ref><ref>{{Cite press release|url=http://www.rocket.com/article/aerojet-rocketdyne-selected-main-propulsion-provider-boeing-and-darpa-experimental|title=Aerojet Rocketdyne Selected As Main Propulsion Provider for Boeing and DARPA Experimental Spaceplane |publisher=[[Aerojet Rocketdyne]] |date=May 24, 2017 |location=Los Angeles, California |access-date=2018-02-13|archive-date=May 30, 2017|archive-url=https://web.archive.org/web/20170530103911/http://www.rocket.com/article/aerojet-rocketdyne-selected-main-propulsion-provider-boeing-and-darpa-experimental}}</ref>  In July 2018 Aerojet Rocketdyne successfully completed ten 100-second firings of the AR-22 in ten days.<ref name="insider-20180712">{{Cite news |url=https://www.spaceflightinsider.com/organizations/aerojet-rocketdyne/ar-22-engine-fired-10-times-in-as-many-days/ |title=AR-22 engine fired 10 times in as many days |last=Rhian |first=Jason |date=July 12, 2018 |work=SpaceFlight Insider |access-date=January 20, 2021 |url-status=live |archive-url=https://web.archive.org/web/20230202020110/https://www.spaceflightinsider.com/organizations/aerojet-rocketdyne/ar-22-engine-fired-10-times-in-as-many-days/ |archive-date=February 2, 2023 }}</ref>

On January 22, 2020, Boeing announced its departure from the XS-1 program, leaving no role for the AR-22.<ref>{{Cite web|date=2020-01-22|title=Boeing drops out of DARPA Experimental Spaceplane program|url=https://spacenews.com/boeing-drops-out-of-darpa-experimental-spaceplane-program/|access-date=2021-01-20|website=SpaceNews|language=en-US}}</ref>

== Present use==
=== Space Launch System ===
{{See also|Space Launch System}}
[[File:Artemis 1 RS-25.jpg|thumb|Aft view of the bottom of the [[Space Launch System]]'s core stage with four RS-25 engines attached, at the [[Michoud Assembly Facility]] in Building 103, on 7 November 2019.]]

On 14 September 2011, following the [[Space Shuttle retirement|retirement of the Space Shuttle]], NASA announced that it would be developing a new launch vehicle, known as the [[Space Launch System]] (SLS), to replace the shuttle fleet.<ref name="SLSPR">{{cite web|publisher=NASA |access-date=December 14, 2011 |title=NASA Announces Design For New Deep Space Exploration System |url=http://www.nasa.gov/home/hqnews/2011/sep/HQ_11-301_SLS_Decision.html  |archive-url=https://web.archive.org/web/20110921160158/http://www.nasa.gov/home/hqnews/2011/sep/HQ_11-301_SLS_Decision.html |archive-date=September 21, 2011 }}</ref> The design for the SLS features the RS-25 as part of its [[Space Launch System core stage|core stage]], with different versions of the rocket being equipped with between three and five engines.<ref name="NSFTrades">{{Cite news |url=https://www.nasaspaceflight.com/2011/10/sls-trades-opening-four-rs-25s-core-stage/ |title=SLS trades lean towards opening with four RS-25s on the core stage |last=Bergin |first=Chris |date=October 4, 2011 |work=NASASpaceflight.com |access-date=December 14, 2011 |url-status=live |archive-url=https://web.archive.org/web/20230327093406/https://www.nasaspaceflight.com/2011/10/sls-trades-opening-four-rs-25s-core-stage/ |archive-date=March 27, 2023 }}</ref><ref name="25DTransition">{{Cite news |url=https://www.nasaspaceflight.com/2012/01/ssme-family-sls-core-stage-role-shuttle-success/ |title=SSME family prepare for SLS core stage role following Shuttle success |last=Bergin |first=Chris |date=January 13, 2012 |work=NASASpaceflight.com |access-date=January 16, 2012 |url-status=live |archive-url=https://web.archive.org/web/20221128144838/https://www.nasaspaceflight.com/2012/01/ssme-family-sls-core-stage-role-shuttle-success/ |archive-date=November 28, 2022 }}</ref> The initial flights of the new launch vehicle are making use of previously flown Block II RS-25D engines, with NASA keeping such engines in a "purged safe" environment at Stennis Space Center, "along with all of the ground systems required to maintain them."<ref name=aw20110329>{{cite news|last=Carreau |first=Mark |title=NASA Will Retain Block II SSMEs |url=http://www.aviationweek.com/aw/generic/story_channel.jsp?channel=space&id=news/asd/2011/03/29/08.xml&headline=NASA%20Will%20Retain%20Block%20II%20SSMEs |archive-url=https://web.archive.org/web/20110420033217/http://www.aviationweek.com/aw/generic/story_channel.jsp?channel=space&id=news%2Fasd%2F2011%2F03%2F29%2F08.xml&headline=NASA%20Will%20Retain%20Block%20II%20SSMEs  |archive-date=April 20, 2011 |access-date=March 30, 2011 |newspaper=Aviation Week |date=March 29, 2011 }}</ref><ref name="SLS MPS">{{Cite news |url=https://www.nasaspaceflight.com/2012/01/engineers-removing-orbiter-mps-components-donation-sls/ |title=Engineers begin removing orbiter MPS components for donation to SLS |last=Bergin |first=Chris |date=January 22, 2012 |work=NASASpaceflight.com |access-date=January 23, 2012 |url-status=live |archive-url=https://web.archive.org/web/20230201200426/https://www.nasaspaceflight.com/2012/01/engineers-removing-orbiter-mps-components-donation-sls/ |archive-date=February 1, 2023 }}</ref> For Artemis I, the RS-25D units with serial numbers E2045, E2056, E2058, and E2060 from all three orbiters were used.<ref>{{cite web | last=Dvorsky | first=George | title=Artemis 1's RS-25 Engines Have Gone to Space Many Times Before | website=Gizmodo | date=September 2, 2022 | url=https://gizmodo.com/nasa-artemis-1-sls-rs-25-engines-space-shuttle-1849490202 | access-date=June 15, 2023}}</ref> They were installed on the core stage by November 6, 2019.<ref name="Artemis 1 attachments 2019">{{cite web |date=November 9, 2019 |title=All Four Engines Are Attached to the SLS Core Stage for Artemis I Mission |last=Harbaugh|first=Jennifer|url=https://www.nasa.gov/exploration/systems/sls/multimedia/four-engines-attached-to-sls-core-stage-for-artemis-I-mission.html |url-status=live |archive-url=https://web.archive.org/web/20191112214328/https://www.nasa.gov/exploration/systems/sls/multimedia/four-engines-attached-to-sls-core-stage-for-artemis-I-mission.html |archive-date=November 12, 2019 |access-date=September 25, 2023|publisher=NASA}} {{PD-notice}}</ref> For Artemis II, the units with serial numbers E2047, E2059, E2062, and E2063 were slated to be used.<ref name="Mohon O'Brien 2022">{{cite web | last1=Mohon | first1=Lee|first2=Kevin|last2=O'Brien | title=Space Launch System Engines: Launching Artemis Astronauts to the Moon | website=NASA | date=October 27, 2022 | url=http://www.nasa.gov/exploration/systems/sls/space-launch-system-engines-launching-artemis-astronauts-to-the-moon.html | access-date=June 26, 2023}}</ref> They were installed on the core stage by September 25, 2023.<ref name="Mohon September 2023">{{cite web |last=Mohon|first=Lee | title=All Engines Added to NASA's Artemis II Moon Rocket Core Stage – Artemis | website=NASA Blogs | date=September 25, 2023 | url=https://blogs.nasa.gov/artemis/2023/09/25/all-engines-added-to-nasas-artemis-ii-moon-rocket-core-stage/ | access-date=September 25, 2023}}</ref> In spring 2025, engine E2063 was replaced with E2061 after a leak was discovered in its oxygen valve hydraulics.<ref name="Clark 2025">{{cite web | last=Clark | first=Stephen | title=NASA just swapped a 10-year-old Artemis II engine with one nearly twice its age | website=Ars Technica | date=April 30, 2025 | url=https://arstechnica.com/space/2025/04/nasa-just-swapped-a-10-year-old-artemis-ii-engine-with-one-nearly-twice-its-age/ | access-date=May 6, 2025}}</ref>

In addition to the RS-25Ds, the SLS program makes use of the Main Propulsion Systems (MPS, the "plumbing" feeding the engines) from the three remaining shuttle orbiters for testing purposes (having been removed as part of the orbiters' decommissioning), with the first two launches ([[Artemis I]] and [[Artemis II]]) originally predicted to make use of the MPS hardware from Space Shuttles {{OV|104|full=no}} and {{OV|105|full=no}} in their core stages.<ref name="25DTransition"/><ref name="SLS MPS"/><ref name="NSFMPS">{{Cite news |url=https://www.nasaspaceflight.com/2011/09/prcb-recommend-atlantis-endeavour-become-sls-donors/ |title=PRCB managers recommend Atlantis and Endeavour become SLS donors |last=Bergin |first=Chris |date=September 20, 2011 |work=NASASpaceflight.com |access-date=December 14, 2011 |url-status=live |archive-url=https://web.archive.org/web/20230127054638/https://www.nasaspaceflight.com/2011/09/prcb-recommend-atlantis-endeavour-become-sls-donors/ |archive-date=January 27, 2023 }}</ref> The SLS's propellants are supplied to the engines from the rocket's core stage, which consists of a modified Space Shuttle external tank with the MPS plumbing and engines at its aft, and an [[Space Launch System#Upper stages|interstage]] structure at the top.<ref name="NSFSLS"/>

For the first two Artemis missions, the engines were installed on the SLS core stage in Building 103 of the [[Michoud Assembly Facility]];<ref>{{cite web | title=SLS Rockets for Artemis 3 and 4 Being Assembled | website=Futuramic | date=August 2, 2022 | url=https://futuramic.com/sls-rockets-for-artemis-3-and-4-being-assembled-at-the-nasas-michoud-assembly-facility/ | access-date=June 24, 2023}}</ref> they will be installed in the [[Space Station Processing Facility]] at Kennedy beginning with [[Artemis III]].<ref name="NASA-20221219">{{cite web | last=Cawley | first=James | title=Artemis III Core Stage Engine Section Arrives at Kennedy | website=NASA | date=December 19, 2022 | url=http://www.nasa.gov/image-feature/artemis-iii-core-stage-engine-section-arrives-at-kennedy | access-date=June 24, 2023}}</ref><ref name="SFN-20221207">{{cite web | last=Foust | first=Jeff | title=NASA and Boeing change SLS core stage assembly process | website=SpaceNews | date=December 7, 2022 | url=https://spacenews.com/nasa-and-boeing-change-sls-core-stage-assembly-process/ | access-date=June 24, 2023}}</ref>

Once the remaining RS-25Ds are exhausted, they are to be replaced with a cheaper, expendable version designated the RS-25E.<ref name="NSFSLS"/>  In 2023, Aerojet Rocketdyne reported reductions in manufacturing time and labour requirements during manufacturing of new-production RS-25 engines, such as a 15% reduction in fabrication time for the powerhead and a 22-month reduction in the time needed to produce a main combustion chamber.<ref>{{Cite web |title=To The Moon And Back, Aerojet Wants To Power Future Cislunar Economy {{!}} Aviation Week Network |url=https://aviationweek.com/defense-space/space/moon-back-aerojet-wants-power-future-cislunar-economy |access-date=2023-05-24 |website=aviationweek.com}}</ref>

On 1 May 2020, NASA awarded a contract extension to manufacture 18 additional RS-25 engines, with associated services, for $1.79 billion, bringing the total SLS contract value to almost $3.5 billion.<ref name="nasa-20200501">{{Cite press release |url=https://www.nasa.gov/press-release/nasa-commits-to-future-artemis-missions-with-more-sls-rocket-engines |title=NASA Commits to Future Artemis Missions with More SLS Rocket Engines |last=Potter |first=Sean |date=May 1, 2020 |publisher=[[NASA]] |access-date=May 4, 2020 |url-status=live |archive-url=https://web.archive.org/web/20230129080441/https://www.nasa.gov/press-release/nasa-commits-to-future-artemis-missions-with-more-sls-rocket-engines/ |archive-date=January 29, 2023 |id=20-050 }}</ref>

On 29 August 2022, Artemis I was delayed by a problem with engineering sensors on RS-25D #3 (serial number E2058) erroneously reporting that it hadn't chilled down to its ideal operating temperature.<ref name="nblogs-20220830">{{Cite web |url=https://blogs.nasa.gov/artemis/2022/08/30/nasa-targets-sept-3-for-next-artemis-i-moon-mission-launch-attempt/ |title=NASA Targets Sept. 3 for Next Artemis I Moon Mission Launch Attempt – Artemis |last=Kraft |first=Rachel |date=August 30, 2022 |website=blogs.nasa.gov |publisher=[[NASA]] |access-date=August 31, 2022 |url-status=live |archive-url=https://web.archive.org/web/20230329144153/https://blogs.nasa.gov/artemis/2022/08/30/nasa-targets-sept-3-for-next-artemis-i-moon-mission-launch-attempt/ |archive-date=March 29, 2023 }}</ref>

On 16 November 2022, Artemis I launched from Kennedy Space Center [[Kennedy Space Center Launch Complex 39B|Launch Complex 39B]], the first time the RS-25 engine had flown since the Space Shuttle's final flight, [[STS-135]], on 21 July 2011.<ref name="nasa-22-117">{{Cite press release |last1=Potter |first1=Sean |last2=Hambleton |first2=Kathryn |last3=Fairley |first3=Tiffany |last4=Cheshier |first4=Leah |id=22-117 |date=2022-11-16 |title=Liftoff! NASA's Artemis I Mega Rocket Launches Orion to Moon |url=https://www.nasa.gov/press-release/liftoff-nasa-s-artemis-i-mega-rocket-launches-orion-to-moon |access-date=2022-12-23 |publisher=[[NASA]] |archive-url=https://web.archive.org/web/20230509181249/https://www.nasa.gov/press-release/liftoff-nasa-s-artemis-i-mega-rocket-launches-orion-to-moon/ |archive-date=2023-05-09 |url-status=live }}</ref>

==== Engine tests ====
In 2015, a test campaign was conducted to determine RS-25 engine performance with a new engine controller unit, under lower liquid-oxygen temperatures, with greater inlet pressure due to the taller SLS core-stage liquid-oxygen tank and higher vehicle acceleration; and with more nozzle heating due to the four-engine configuration and its position in-plane with the SLS booster exhaust nozzles. New ablative heat-shield insulation was to be tested as well.<ref name="nasa-20150611" />{{Better source needed|reason=Details of the eventual conduct and number of these tests requires sources from after their conclusion|date=October 2017}} Tests occurred on January 9 (500 seconds),<ref name="nasa-15-007">{{cite press release |title=RS-25 Engine Testing Blazes Forward for NASA's Space Launch System |url=https://www.nasa.gov/press/2015/january/rs-25-engine-testing-blazes-forward-for-nasas-space-launch-system |publisher=[[NASA]] |access-date=23 May 2023 |archive-url=https://web.archive.org/web/20221207233417/http://www.nasa.gov/press/2015/january/rs-25-engine-testing-blazes-forward-for-nasas-space-launch-system/ |archive-date=7 December 2022 |date=9 January 2015 |url-status=live |id=15-007 }}</ref> May 28 (450 seconds),<ref name="nasa-20150529">{{cite web |last1=Dean |first1=LaToya |title=Steamy Summer Begins for SLS with RS-25 Test |url=https://www.nasa.gov/image-feature/steamy-summer-begins-for-sls-with-rs-25-test |publisher=[[NASA]] |access-date=23 May 2023 |archive-url=https://web.archive.org/web/20221206025324/http://www.nasa.gov/image-feature/steamy-summer-begins-for-sls-with-rs-25-test |archive-date=6 December 2022 |date=29 May 2015 |url-status=live}}</ref> June 11 (500 seconds),<ref name="nasa-20150611">{{cite web |last1=Mohon |first1=Lee |last2=Henry |first2=Kim |title=We have Ignition: NASA Space Launch System RS-25 Engine Fires Up for Third Test in Series |url=https://www.nasa.gov/exploration/systems/sls/we-have-ignition-nasa-space-launch-system-rs-25-engine-fires-up-for-third-test-in-series.html |publisher=[[NASA]] |access-date=23 May 2023 |archive-url=https://web.archive.org/web/20221211222406/https://www.nasa.gov/exploration/systems/sls/we-have-ignition-nasa-space-launch-system-rs-25-engine-fires-up-for-third-test-in-series.html |archive-date=11 December 2022 |location=[[Marshall Space Flight Center]] |date=11 June 2015 |url-status=live}}</ref> June 25 (650 seconds),<ref name="nasa-20150626">{{cite web |last1=Dean |first1=LaToya |title=Longest SLS Engine Test Yet Heats Up Summer Sky |url=https://www.nasa.gov/image-feature/longest-sls-engine-test-yet-heats-up-summer-sky |publisher=[[NASA]] |access-date=23 May 2023 |archive-url=https://web.archive.org/web/20221211222347/https://www.nasa.gov/image-feature/longest-sls-engine-test-yet-heats-up-summer-sky/ |archive-date=11 December 2022 |date=26 June 2015 |url-status=live}}</ref> July 17 (535 seconds),<ref name="nasa-20150717">{{cite web |last1=Harbaugh |first1=Jennifer |title=Pedal to the Metal – RS-25 Engine Revs Up Again |url=https://www.nasa.gov/exploration/systems/sls/multimedia/pedal-to-the-metal-rs-25-engine-revs-up-again.html |publisher=[[NASA]] |access-date=23 May 2023 |archive-url=https://web.archive.org/web/20221211222330/https://www.nasa.gov/exploration/systems/sls/multimedia/pedal-to-the-metal-rs-25-engine-revs-up-again.html |archive-date=11 December 2022 |date=17 July 2015 |url-status=live}}</ref> August 13 (535 seconds)<ref name="nasa-20150814">{{cite web |last1=Harbaugh |first1=Jennifer |title=Countdown to Deep Space Continues with Latest RS-25 Test |url=https://www.nasa.gov/exploration/systems/sls/multimedia/countdown-to-deep-space-continues-with-latest-rs-25-test.html |publisher=[[NASA]] |access-date=May 23, 2023 |archive-url=https://web.archive.org/web/20221001052103/https://www.nasa.gov/exploration/systems/sls/multimedia/countdown-to-deep-space-continues-with-latest-rs-25-test.html |archive-date=October 1, 2022 |date=August 14, 2015 |url-status=live}}</ref> and August 27 (535 seconds).<ref name="nasa-15-178">{{cite press release |last1=Hambleton |first1=Kathryn |last2=Buckingham |first2=Valerie |last3=Northon |first3=Karen |title=NASA Concludes Series of Engine Tests for Next-Gen Rocket |url=https://www.nasa.gov/press-release/nasa-concludes-series-of-engine-tests-for-next-gen-rocket |publisher=[[NASA]] |access-date=May 23, 2023 |archive-url=https://web.archive.org/web/20221223093455/https://www.nasa.gov/press-release/nasa-concludes-series-of-engine-tests-for-next-gen-rocket |archive-date=December 23, 2022 |date=August 27, 2015 |url-status=live |id=15-178 }}</ref>

Following these tests, four more engines were scheduled to enter a new test cycle.<ref name="nasa-20150717" />{{Better source needed|reason=As above, the preceding citation should be expanded upon by the use of additional pages providing greater detail on the series of tests; additionally its format is rough and could benefit from more thorough information in template parameters|date=October 2017}} A new series of tests designed to evaluate performance in SLS-use cases was initiated in 2017.<ref name="aiaa-jacobs-2018">{{cite conference |last1=Vetcha |first1=Naveen |last2=Strickland |first2=Matthew B. |last3=Philippart |first3=Kenneth D. |last4=Giel |first4=Thomas V. Jr. |title=1 Overview of RS-25 Adaptation Hot-Fire Test Series for SLS, Status and Lessons Learned |url=https://ntrs.nasa.gov/api/citations/20180006338/downloads/20180006338.pdf |publisher=[[Jacobs Solutions|Jacobs Space Exploration Group]]/ESSCA/[[Marshall Space Flight Center|/NASA Marshall Space Flight Center]] |access-date=May 23, 2023 |archive-url=https://web.archive.org/web/20220526124715/https://ntrs.nasa.gov/api/citations/20180006338/downloads/20180006338.pdf |archive-date=May 26, 2022 |date=July 9, 2018 |url-status=live |location=Cincinnati, Ohio |conference=AIAA 2018 Joint Propulsion Conference |id=20180006338 }}</ref>

On February 28, 2019, NASA conducted a 510-second test burn of a developmental RS-25 at 113 percent of its originally designed thrust for more than 430 seconds, about four times longer than any prior test at this thrust level.<ref name="nasa-20190228">{{cite web |last1=Dean |first1=LaToya |title=RS-25 Engine Fired At Highest Power Level, Four Times Longer Than Prior Tests |url=https://www.nasa.gov/image-feature/rs-25-engine-fired-at-highest-power-level-four-times-longer-than-prior-tests |publisher=[[NASA]] |access-date=May 23, 2023 |archive-url=https://web.archive.org/web/20221211220241/https://www.nasa.gov/image-feature/rs-25-engine-fired-at-highest-power-level-four-times-longer-than-prior-tests/ |archive-date=December 11, 2022 |date=February 28, 2019 |url-status=live}}</ref>

On January 16, 2021, the RS-25 engines were fired again, during a hot-fire test as part of the Artemis program. The test was originally scheduled as an 8-minute test but was terminated at the 67th second due to intentionally conservative test parameters being breached in the hydraulic system of Engine 2's (serial number E2056) Core Stage Auxiliary Power Unit (CAPU) during the thrust vector control (TVC) system test. Engine 2's CAPU was shut down automatically, although if this issue had occurred during flight, it would not have caused an abort, as the remaining CAPUs are capable of powering the TVC systems of all four engines.<ref name="nblogs-20210119">{{cite web |last1=Harbaugh |first1=Jennifer |title=Green Run Update: Data and Inspections Indicate Core Stage in Good Condition |url=https://blogs.nasa.gov/artemis/2021/01/19/green-run-update-data-and-inspections-indicate-core-stage-in-good-condition/ |website=NASA Blogs |publisher=[[NASA]] |access-date=January 20, 2021 |archive-url=https://web.archive.org/web/20230419095204/https://blogs.nasa.gov/artemis/2021/01/19/green-run-update-data-and-inspections-indicate-core-stage-in-good-condition/ |archive-date=April 19, 2023 |date=January 19, 2021 |url-status=live}}</ref> The engine also suffered a different "Major Component Failure", in the engine control system, that was caused by instrumentation failure. This {{em|would}} have triggered an abort of the launch countdown during an actual launch attempt.<ref name="wapo-20210119">{{cite news |last1=Davenport |first1=Christian |title=Before shortened NASA SLS rocket engine test, officials predicted only a 50 percent chance of complete success |url=https://www.washingtonpost.com/technology/2021/01/19/nasa-sls-hotfire-test-artemis/ |access-date=20 January 2021 |archive-url=https://web.archive.org/web/20230329101900/https://www.washingtonpost.com/technology/2021/01/19/nasa-sls-hotfire-test-artemis/ |archive-date=29 March 2023 |date=19 January 2021 |url-status=live |newspaper=Washington Post }}</ref>

On March 18, 2021, the four RS-25 core-stage engines were once again fired as part of the second SLS core stage hot-fire test, which lasted the full duration of 500 seconds,<ref name="nblogs-20210318">{{cite web |last1=Harbaugh |first1=Jennifer |title=Green Run Update: Full Duration Hot Fire Successfully Completed on Mar. 18 |url=https://blogs.nasa.gov/artemis/2021/03/18/green-run-update-full-duration-hot-fire-successfully-completed-on-mar-18/ |website=NASA Blogs |publisher=[[NASA]] |access-date=May 23, 2023 |archive-url=https://web.archive.org/web/20230404055935/https://blogs.nasa.gov/artemis/2021/03/18/green-run-update-full-duration-hot-fire-successfully-completed-on-mar-18/ |archive-date=April 4, 2023 |date=March 18, 2021 |url-status=live}}</ref> successfully certifying the Artemis I core stage for flight.

On December 14, 2022, a single development RS-25E, serial number E10001, attempted a 500-second hot-fire test. The test aborted at T+209.5 due to test systems subsequently interpreting signals from a group of improperly configured accelerometers during the hot fire as exceeding acceptable vibration limits.<ref name="nasa-s23-001">{{cite press release |last1=Thompson |first1=C. Lacy |last2=Dean |first2=LaToya |title=NASA Conducts RS-25 Engine Hot Fire at Stennis for Future Artemis Missions |url=https://www.nasa.gov/centers/stennis/news/releases/2023/NASA-Conducts-RS-25-Engine-Hot-Fire-at-Stennis-for-Future-Artemis-Missions |publisher=[[NASA]] |access-date=May 23, 2023 |archive-url=https://web.archive.org/web/20230118160101/https://www.nasa.gov/centers/stennis/news/releases/2023/NASA-Conducts-RS-25-Engine-Hot-Fire-at-Stennis-for-Future-Artemis-Missions/ |archive-date=January 18, 2023 |date=January 12, 2023 |url-status=live |id=S23-001 }}</ref> Tests of the engine continued in 2023; on February 8, 2023, it was fired for 500 seconds at 111% power, fitted with a new-production nozzle.<ref name="nasa-s23-015">{{Cite press release |url=https://www.nasa.gov/centers/stennis/news/releases/2023/NASA-Conducts-First-2023-Test-of-Redesigned-Moon-Rocket-Engine |title=NASA Conducts First 2023 Test of Redesigned Moon Rocket Engine |last1=Dean |first1=LaToya |last2=Thompson |first2=C. Lacy |date=February 8, 2023 |publisher=[[NASA]] |access-date=May 24, 2023 |url-status=live |archive-url=https://web.archive.org/web/20230320230826/https://www.nasa.gov/centers/stennis/news/releases/2023/NASA-Conducts-First-2023-Test-of-Redesigned-Moon-Rocket-Engine/ |archive-date=March 20, 2023 |id=S23-015 }}</ref> Subsequent tests included a 600-second test at 111% power on February 22,<ref name="NASA-20230223">{{cite web | last=Valentine | first=Andre | title=NASA TV VIDEOFILE RUNDOWN – for Thursday February 23, 2023 – Evening | website=NASA | date=February 23, 2023 | url=http://www.nasa.gov/feature/nasa-tv-videofile-rundown-for-thursday-february-23-2023-evening | access-date=June 20, 2023 | archive-date=June 20, 2023 | archive-url=https://web.archive.org/web/20230620133644/https://www.nasa.gov/feature/nasa-tv-videofile-rundown-for-thursday-february-23-2023-evening/ | url-status=dead }}</ref> a 520-second test at 113% power on March 8,<ref name="nasa-s23-021">{{Cite press release |url=https://www.nasa.gov/centers/stennis/news/release/2023/NASA-Continues-Test-Series-for-Redesigned-Artemis-Moon-Rocket-Engines |title=NASA Continues Test Series for Redesigned Artemis Moon Rocket Engines |last=Dean |first=LaToya |date=March 8, 2023 |publisher=[[NASA]] |access-date=May 24, 2023 |url-status=live |archive-url=https://web.archive.org/web/20230524114306/https://www.nasa.gov/centers/stennis/news/release/2023/NASA-Continues-Test-Series-for-Redesigned-Artemis-Moon-Rocket-Engines/ |archive-date=May 24, 2023 |id=S23-021 }}</ref> a 600-second test at 113% power on March 21,<ref name="nasa-20230321">{{cite web | last=Dean | first=LaToya | title=NASA Conducts Long Duration Hot Fire of RS-25 Certification Engine | website=NASA | date=March 21, 2023 | url=http://www.nasa.gov/centers/stennis/news/image-feature/2023/NASA-Conducts-Long-Duration-Hot-Fire-of-RS-25-Certification-Test | access-date=June 20, 2023}}</ref> a 500-second, 113% power level test on April 5,<ref name="NASA-20230407">{{cite web | last=Daines | first=Gary | title=This Week @NASA, April 7, 2023 | website=NASA | date=April 7, 2023 | url=http://www.nasa.gov/mediacast/this-week-nasa-april-7-2023 | access-date=June 20, 2023 | archive-date=June 20, 2023 | archive-url=https://web.archive.org/web/20230620223937/https://www.nasa.gov/mediacast/this-week-nasa-april-7-2023/ | url-status=dead }}</ref> a 720-second fire that tested the engine's thrust vectoring gimbal system on April 26,<ref name="NASA-20230426">{{cite web | last=Dean | first=LaToya | title=NASA Tests Critical In-Flight Capability During RS-25 Engine Hot Fire | website=NASA | date=April 26, 2023 | url=http://www.nasa.gov/centers/stennis/news/releases/2023/NASA-Tests-Critical-In-Flight-Capability-During-RS-25-Engine-Hot-Fire | access-date=June 20, 2023}}</ref> a 630-second test on May 10,<ref>{{Cite tweet|user=NASAStennis|number=1656047416649367565|title=Operators are aiming for a test duration of more than 10 minutes (630 seconds), which is longer than the 500 seconds the engines must fire to help launch the @NASA_SLS (Space Launch System) to orbit and helps provide a margin of operational safety.}}</ref> and five more 500-second, 113% power level tests without gimbaling on May 23,<ref name="nasa-20230524" /> June 1,<ref name="NASA-20230601">{{cite web | last=Dean | first=LaToya | title=NASA Enters Stretch in Critical Moon Rocket Engine Test Series | website=NASA | date=June 1, 2023 | url=http://www.nasa.gov/centers/stennis/news/image-feature/2023/NASA-Enters-Stretch-in-Critical-Moon-Rocket-Engine-Test-Series | access-date=June 20, 2023}}</ref> June 8,<ref>{{Cite tweet|user=NASA_SLS|number=1667939936794804233|title=On June 8, @NASA conducted the 10th certification test of an RS-25 engine at @NASAStennis, continuing a critical hot fire series to facilitate the production of new engines for future SLS (Space Launch System) flights. Watch the engine ignite!}}</ref> June 15,<ref name="nasa-20230615">{{cite web | last=Dean | first=LaToya | title=NASA Nears Completion of Key RS-25 Certification Test Series | website=NASA | date=June 15, 2023 | url=http://www.nasa.gov/centers/stennis/news/image-feature/2023/NASA-Nears-Completion-of-Key-RS-25-Certification-Test-Series | access-date=June 20, 2023}}</ref> and June 22.<ref>{{Cite tweet|user=NASAStennis|number=1671591381079400456|title=RS-25 engine testing on the Fred Haise Test Stand at NASA's Stennis Space Center}}</ref><ref name="June 22 Stennis tweet">{{Cite tweet|user=NASAStennis|number=1671885257765949448|title=Today's RS-25 engine test on the Fred Haise Test Stand is targeted between 1:30-3:30 p.m. CDT on Facebook Live and YouTube! We will go live 15 minutes early, so tune in to learn more about RS-25 engine testing for future Artemis missions.}}</ref>

The RS-25E developmental unit E0525, with significant inclusion of new components including a redesigned nozzle, hydraulic actuators, flex ducts and turbopumps, was hot fire tested to 111% power levels for 550 seconds in the first in a series of certification tests beginning October 17, 2023.<ref>{{Cite tweet|user=NASAStennis|number=1671888203522138112|title=Today's test is the 12th (and final) test in the current series using a certification engine with dozens of improvements to make production more efficient and affordable while maintaining high performance and reliability. Another certification engine will be tested this fall.}}</ref><ref name="Stennis Press Release October 2023">{{cite web | title=Start Your Engines: NASA to Begin Critical Testing for Future Artemis Missions | website=NASA | date=October 13, 2023 | url=https://www.nasa.gov/news-release/start-your-engines-nasa-to-begin-critical-testing-for-future-artemis-missions/ | access-date=October 24, 2023}}</ref><ref name="Confirmation October 2023">{{cite web | title=NASA Conducts 1st Hot Fire of New RS-25 Certification Test Series | website=NASA | date=October 18, 2023 | url=https://www.nasa.gov/centers-and-facilities/stennis/nasa-conducts-1st-hot-fire-of-new-rs-25-certification-test-series/ | access-date=October 24, 2023}}</ref> It was tested to 113% power levels for 500 seconds on November 15,<ref>{{cite tweet|number=1724807535541325870|user=NASAStennis|title=Today's test of RS-25 Engine 0525 on the Fred Haise Test Stand has a planned duration of 500 sec. with a max. 113% power level.|date=November 15, 2023}}</ref><ref>{{cite tweet|number=1724915231527203172|user=NASAStennis|title=Today, NASA conducted the 2nd hot fire in a final 12-test certification series paving the way for production of new RS-25 engines to help power the @NASA_SLS rocket on future Artemis missions to the Moon and beyond|date=November 15, 2023}}</ref> and to 113% for 650 seconds with gimbaling on November 29, 2023,<ref name="November 29 2023">{{cite web | title=NASA Tests In-Flight Capability of Artemis Moon Rocket Engine | website=NASA | date=November 29, 2023 | url=https://www.nasa.gov/centers-and-facilities/stennis/nasa-tests-in-flight-capability-of-artemis-moon-rocket-engine/ | access-date=December 2, 2023}}</ref> to 113% for 500 seconds on January 17, 2024,<ref name="SciNews 2024-01-17">{{cite web | title=SLS RS-25 Engine Test, 17 January 2024 | website=YouTube |publisher=SciNews| url=https://www.youtube.com/watch?=wAjeWYUj11g | access-date=January 30, 2024}}</ref><ref name="NASA 2024-01-18">{{cite web | title=NASA Continues Artemis Moon Rocket Engine Tests with 1st Hot Fire of 2024 | website=NASA | date=January 18, 2024 | url=https://www.nasa.gov/news-release/nasa-continues-artemis-moon-rocket-engine-tests-with-1st-hot-fire-of-2024/ | access-date=January 30, 2024}}</ref><ref name="Howell 2024-01-20">{{cite web | last=Howell | first=Elizabeth | title=Fire! NASA kicks off Artemis moon program's 2024 with big engine test (video) | website=Space.com | date=January 20, 2024 | url=https://www.space.com/nasa-artemis-moon-program-rs-25-engine-test-january-2024 | access-date=January 30, 2024}}</ref> January 23,<ref name="SciNews 2024-01-23">{{cite web | title=SLS RS-25 Engine Test, 23 January 2024 | website=YouTube | date=January 23, 2024 | url=https://www.youtube.com/watch?v=jRLsxPeq6dY|publisher=SciNews | access-date=January 30, 2024}}</ref><ref name="Howell 2024-01-24">{{cite web | last=Howell | first=Elizabeth | title=NASA fires up powerful Artemis moon rocket engine in key test (video) | website=Space.com | date=January 24, 2024 | url=https://www.space.com/moon-engine-rocket-test-rs-25-artemis-mission-january-2024 | access-date=January 30, 2024}}</ref> and January 29,<ref name="https://www.wlox.com 2024">{{cite web | title=Halfway there: NASA completes 6 of 12 RS-25 engine tests | website=www.wlox.com | date=January 29, 2024 | url=https://www.wlox.com/2024/01/29/halfway-there-nasa-completes-6-12-rs-25-engine-tests/ | access-date=January 30, 2024}}</ref><ref name="NASA 2024-01-29">{{cite web | title=NASA Marks Halfway Point for Artemis Moon Rocket Engine Certification Series | website=NASA | date=January 29, 2024 | url=https://www.nasa.gov/centers-and-facilities/stennis/nasa-marks-halfway-point-for-artemis-moon-rocket-engine-certification-series/ | access-date=January 30, 2024}}</ref> to 113% for 550 seconds on February 23,<ref name="SciNews 2024-02-23">{{cite web | title=SLS RS-25 Engine Test, 23 February 2024 | website=YouTube | date=February 23, 2024 |publisher=SciNews| url=https://www.youtube.com/watch?v=Wk4k-8A99i0 | access-date=February 26, 2024}}</ref><ref name="NASA 2024-02-22">{{cite web | title=NASA to Continue Testing for New Artemis Moon Rocket Engines | website=NASA | date=February 22, 2024 | url=https://www.nasa.gov/centers-and-facilities/stennis/nasa-to-continue-testing-for-new-artemis-moon-rocket-engines/ | access-date=February 26, 2024}}</ref> to 111% for 615 seconds on February 29,<ref name="Leap Day test 2024">{{cite web | title=SLS RS-25 Engine Test, 29 February 2024 (615 seconds test) | website=YouTube |publisher=SciNews| date=February 29, 2024 | url=https://www.youtube.com/watch?v=SGE-uv0O3Ec | access-date=March 9, 2024}}</ref> and to 113% for 600 seconds on March 6<ref name="SciNews March 6 2024">{{cite web | title=SLS RS-25 Engine Test, 6 March 2024 (600 seconds test) | website=YouTube | date=March 6, 2024 | url=https://www.youtube.com/watch?v=PuEnE5_aQmI | access-date=March 9, 2024}}</ref><ref name="Howell 2024 March 15">{{cite web | last=Howell | first=Elizabeth | title=Artemis 2 moon astronauts celebrate engine test for future lunar missions (video) | website=Space.com | date=March 15, 2024 | url=https://www.space.com/artemis-2-moon-astronauts-rs-25-engine-test-nasa | access-date=March 22, 2024}}</ref><ref name="Noble 2024">{{cite web | last=Noble | first=Noah | title=Stennis Space Center updates RS-25 engine program, new missile production facility | website=www.wlox.com | date=March 20, 2024 | url=https://www.wlox.com/2024/03/20/stennis-space-center-updates-rs-25-engine-program-new-missile-production-facility/ | access-date=March 22, 2024}}</ref> and 500 seconds on March 22<ref name="SciNews 2024-03-22">{{cite web | title=SLS RS-25 Engine Test, 22 March 2024 | website=YouTube | date=March 22, 2024 | url=https://www.youtube.com/watch?v=Mf5X_xTT84w | access-date=March 28, 2024}}</ref> and 27,<ref name="SciNews 2024-03-27">{{cite web | title=SLS RS-25 Engine Test, 27 March 2024 | website=YouTube | date=March 27, 2024 | url=https://www.youtube.com/watch?v=-Q3Yrts_cDU | access-date=March 28, 2024}}</ref> and April 3.<ref name="NASA 2024-04-03">{{cite web | title=NASA Achieves Milestone for Engines to Power Future Artemis Missions | website=NASA | date=April 4, 2024 | url=https://www.nasa.gov/centers-and-facilities/stennis/nasa-achieves-milestone-for-engines-to-power-future-artemis-missions/ | access-date=April 8, 2024}}</ref>

On February 20, 2025, engine no. E20001 was installed at the test stand, the first full production RS-25E to undergo testing.<ref name="E20001">{{cite web | title=NASA Stennis Teams Install New Production RS-25 Engine for Upcoming Hot Fire | website=NASA | date=March 6, 2025 | url=https://www.nasa.gov/image-article/stennis-teams-install-new-production-rs-25-engine/ | access-date=May 6, 2025}}</ref> It was tested to 111% power levels for 500 seconds on June 20.<ref name="June 20 2025">{{cite web | title=SLS RS-25 Engine Test, 20 June 2025 (the first new production engine) | website=YouTube | date=July 5, 2025 | url=https://www.youtube.com/watch?v=dJRQNrepFG4 | publisher=SciNews|access-date=July 14, 2025}}</ref> On November 12, the second production RS-25E, serial number E20002, was tested to the same levels and time limit.<ref name="Engine Test 2025-11-12">{{cite web | title=SLS RS-25 Engine Test, 12 November 2025 | website=YouTube|publisher=SciNews | date=November 20, 2025 | url=https://www.youtube.com/watch?app=desktop&v=Lscxp5RJovY | access-date=December 2, 2025}}</ref>

== See also ==
* [[Shuttle-C]]
* [[RD-0120]]

== Notes ==
{{external media|width=|video1=[https://www.youtube.com/watch?v=1sxOrqJFPCo STS-49 Flight Readiness Firing]|video2=[https://www.youtube.com/watch?v=alFid8YchG8 Time-lapse video of STS-135 SSME installation]|video3=[https://www.youtube.com/watch?v=y4jxnA7mtJE RS-25 test for SLS on 28 May 2015]|video4=[https://www.youtube.com/watch?v=6n6quVTC1h4 RS-25 engine controller system test on 27 July 2017]|video5=[https://www.youtube.com/watch?v=yyY_C-udQCc RS-25 test for SLS on 17 October 2023] }}
{{reflist|group=note}}
{{clear}}

== References ==
{{Include-NASA}}
{{reflist|30em}}

== External links ==
{{Commons category|RS-25 (rocket engine)}}
*[https://archive.today/20151109200753/http://nasatech.net/ntSSMEPF_PAGE.html Spherical panoramas of RS-25D in SSME Processing Facility prior to shipping to Stennis Space Center]
*[http://libarchstor.uah.edu:8081/repositories/2/resources/114 Lawrence J. Thomson Collection, The University of Alabama in Huntsville Archives and Special Collections] Files of Lawrence J. Thomson, chief engineer for the SSME from 1971 to 1986
*{{HAER |survey=TX-116-I |id=tx1115 |title=Space Transportation System, Space Shuttle Main Engine, Lyndon B. Johnson Space Center, 2101 NASA Parkway, Houston, Harris County, TX |photos=20 |dwgs=2 |cap=8}}

{{Space Shuttle}}
{{Rocket engines}}

{{DEFAULTSORT:Space Shuttle Main Engine}}
[[Category:Rocketdyne engines|RS-25]]
[[Category:Space Launch System]]
[[Category:Articles containing video clips]]
[[Category:Historic American Engineering Record in Texas]]
[[Category:Rocket engines of the United States]]
[[Category:Space Shuttle program|Main Engine]]
[[Category:Rocket engines using hydrogen propellant]]
[[Category:Rocket engines using the staged combustion cycle]]