The RS-25 is the Space Shuttle Main Engine, now used on the core stage of NASA's Space Launch System.
aerojet-rs-25 The RS-25 burns liquid hydrogen with liquid oxygen. It began as the Space Shuttle Main Engine, three to an orbiter, and flew on 135 shuttle missions from 12 April 1981. Four now power the core stage of NASA's Space Launch System, which first flew on 16 November 2022.
In its staged-combustion design, two preburners make hot gas that drives the turbopumps, and that exhaust then burns in the main chamber. Hydrogen flowing through channels and tubes cools the chamber and nozzle. Inconel 718, a nickel superalloy, makes up roughly 51 percent of the engine's weight.
NASA’s Space Launch System core stage uses RS-25 engines, the same design that powered the Space Shuttle for 30 years. The engine shows how a copper-alloy liner and a nickel superalloy jacket survive hydrogen-oxygen combustion, so it is a common reference for reusable high-pressure engines.
The inner wall of the main chamber is NARloy-Z, a copper alloy with silver and zirconium that Rocketdyne developed, cut with 430 coolant channels. An electroformed nickel layer covers the outside, and a jacket of Inconel 718 holds in a pressure of 3,000 psi.
The nozzle wall is lined with 1,080 stainless steel tubes, each an eighth of an inch across, that carry hydrogen to cool it. Four layers of metallic batting under Nichrome foil insulate the outside. The exhaust leaves the nozzle at 17,000 feet per second.
The two preburners have Inconel 718 bodies and burn most of the hydrogen with part of the oxygen to drive the turbopumps. The high-pressure fuel pump, the engine's most complex part, spins at about 36,000 rpm. New oxygen-pump ball bearings are silicon nitride, a ceramic 30 percent harder and 40 percent lighter than steel.
The shuttle-era controller, built by Honeywell with two redundant computers, ran start-up, throttling and shutdown, updating every 20 milliseconds. Five hydraulic valves controlled the propellant flow. Two actuators swivelled each engine, up to 10.5 degrees in pitch and 8.5 in yaw.
SLS engines have new controllers and software, and nozzle insulation to protect them from booster exhaust. A 3D-printed pogo accumulator, a beachball-sized shock absorber that regulates the oxygen flow, removed more than 100 welds. New-production engines, from Artemis V, are rated for 111 percent thrust against the shuttle engines' limit of 109.
The inner wall of the main combustion chamber, a copper-silver-zirconium alloy cut with 430 coolant channels.
The chamber support jacket and preburner bodies. It is in about 1,500 components.
The outer layer of the chamber liner, applied by electroforming.
The 1,080 nozzle cooling tubes and the oxygen heat exchanger coil. NASA does not name the grades.
Ball bearings in the high-pressure oxygen pump of the Block I engine.
Foil and screen over the batting on the outside of the nozzle.
Three RS-25s sat at the back of each Space Shuttle orbiter, and four power the SLS core stage. The first four SLS missions use 14 flight-proven shuttle engines and two assembled from shuttle-era parts. Pratt & Whitney Rocketdyne made the nozzle, chamber and powerhead. Honeywell built the controller and HR Textron the valve actuators. L3Harris has restarted production, aiming for a 30 percent lower cost.
Three engines per orbiter: centre, left, right
Engines are mounted on the aft end of the orbiter
First flown on STS-1, 12 April 1981
NASA
Core stage: four engines, in every SLS configuration
Formerly the Space Shuttle Main Engine
NASA / Boeing
Main engines of the orbiter that lifted Destiny; launcher engine, not carried by the vehicle
Identity link: RS-25 is the former Space Shuttle Main Engine; launcher engine, not carried by the vehicle
NASA
Four RS-25 engines on the SLS core stage lift Orion; launcher engine, not carried by the capsule
RS-25 is the SSME, reused on SLS: 14 flight-proven engines plus two assembled from shuttle parts for the first four missions
Nozzle, chamber, powerhead, low-pressure pumps, valves and ducts
Restarted engine production (30 percent cost-reduction goal)
Main engine controller, Clearwater, Florida
Engine valve actuators, Valencia, California
Inconel alloy 718 named as a key alloy
Developed the nickel-based super alloy of the original turbine blades
Aerojet Rocketdyne
stainless steel. Here: Nozzle coolant tubes (1,080 tubes, brazed to jacket); Oxidizer heat exchanger coil. There: Chamber liner.
structural nickel. Here: Chamber liner exterior. There: Chamber outer shell.
NASA's detailed report covers the shuttle-era engine, so the SLS version's changes are described only in outline. The grades of the turbine blade superalloy, nozzle tubes and heat exchanger tube are not named, and neither are the materials of most rotors, injectors, valves and ducts.
What is written above is built from these numbers and quotes. Each row links to the page it was taken from. Get all of it as JSON, or ask our MCP server.
| What | Value |
|---|---|
| Propellants | Liquid oxygen and liquid hydrogen |
| Cycle | Staged combustion |
| Sea-level thrust (rated power level) | About 380,000 lb |
| Throttle range | 67 to 109 percent of rated power |
| Mass | About 7,775 lb |
| Size | About 14 ft long, 7.5 ft diameter at nozzle exit |
| Chamber pressure | 3,000 psi (main combustion chamber) |
| Nozzle area ratio | 77.5:1 |
| Thrust, sea level, full power level | about 420,000 lbf at 109 percent |
| Thrust levels, vacuum | about 470,000 / 490,000 / 512,000 lbf at 100 / 104.5 / 109 percent |
| First SLS flight with four RS-25 engines | 16 Nov 2022 (Artemis I) from Launch Complex 39B |
| First Space Shuttle flight with three RS-25 engines | STS-1, Columbia, 12 April 1981 |
| SLS operating thrust | 109 percent, about 512,300 lbf (2,279 kN) in vacuum |
| Part count | about 50,000 parts, 7,000 tracked for replacement |
| Flight experience in the shuttle program | over 57 hours of flight time and 246.7 hours of ground tests |
| Design life | 55 starts and 27,000 s of run time |
| Shuttle missions flown | 135 space shuttle missions |
| Burn | about eight and a half minutes from ignition to main engine cutoff |
| Propellant burned | over 1.6 million lb (about 528,000 gal) per flight |
| Nozzle size | 10.3 in diameter at the throat, 90.7 in at the exit |
| Nozzle propellant flow | 1,035 lb/s at 100 percent power |
| Nozzle exit velocity | 17,000 ft/s |
| Fleet leader | Engine 2019 flew nineteen missions |
| New-production affordability | design changes and advanced manufacturing to eliminate more than 700 welds and 700 parts |
| New-production engines | Artemis V onward, rated for 111 percent thrust |
| Engines being built | 24 engines |
Each material named in the sources: where it is used here, which other products use it, and its key numbers when its material page exists.
Used in 3 partsPreburner body and inlet manifold · Chamber support jacket · Inconel 718 across about 1,500 components
Used in 2 partsNozzle coolant tubes (1,080 tubes, brazed to jacket) · Oxidizer heat exchanger coil
Used in 1 partHot gas manifold outer wall liner
Used in 1 partLow-pressure fuel turbopump housing insulation
Used in 1 partChamber liner (inner wall, 430 coolant channels)
Used in 1 partNozzle external thermal shield
Used in 1 partHigh-pressure fuel turbopump turbine blades (original design)
Used in 1 partHigh-pressure oxidizer turbopump bearings (Block I)
Used in 1 partChamber liner exterior
Chamber, preburner, manifold, nozzle and heat-exchanger materials are named in the NASA HAER report for the Shuttle-era Block II engine. Turbopump rotors and injectors are mostly unnamed. Components, suppliers and links were added from public sources. A material is left blank where no source states it.
| Part | Material | Finish and standard | Made by |
|---|---|---|---|
| Hot gas manifold outer wall liner“The structural outer walls consisted of an alloy 903 sheet metal liner” | alloy 903alloy-903 | — | Two halves joined by electron-beam welding |
| Part | Material | Finish and standard | Made by |
|---|---|---|---|
| Preburner body and inlet manifold“The structural body and inlet manifold of each preburner were machined from Inconel alloy 718” | Inconel alloy 718inconel-718 | — | — |
| Part | Material | Finish and standard | Made by |
|---|---|---|---|
| Chamber liner (inner wall, 430 coolant channels)“Rocketdyne developed NARloy-Z, a high conductivity copper-based alloy that contained silver and zirconium.” | NARloy-Z (copper-silver-zirconium)narloy-z | — | — |
| Chamber liner exterior“The exterior of the liner was made from structural nickel which was applied by an electroforming process.” | structural nickelelectroformed-nickel | — | — |
| Chamber support jacket“The support jacket of the main combustion chamber was made from Inconel alloy 718.” | Inconel alloy 718inconel-718 | — | — |
| Part | Material | Finish and standard | Made by |
|---|---|---|---|
| Nozzle coolant tubes (1,080 tubes, brazed to jacket)“The inside wall of the nozzle was lined with 1,080, 1/8” stainless steel cooling tubes that carried hydrogen.” | stainless steel (grade not stated)stainless-steel | — | — |
| Nozzle external thermal shield“four layers of metallic batting covered with a metallic foil (Nichrome)” | Nichrome foil and screen over metallic battingnichrome | — | — |
| Part | Material | Finish and standard | Made by |
|---|---|---|---|
| Oxidizer heat exchanger coil“continuous piece of coiled stainless steel alloy tubing” | stainless steel alloy (grade not stated)stainless-steel | — | — |
| Part | Material | Finish and standard | Made by |
|---|---|---|---|
| High-pressure fuel turbopump turbine blades (original design)“The blades were cast from a nickel-based super alloy developed by Martin Metals.” | nickel-based super alloy (grade not stated)nickel-base-superalloy | — | Later Block I/II blades used a different material |
| High-pressure oxidizer turbopump bearings (Block I)“new ball bearings made of silicon nitride, a ceramic material 30 percent harder and 40 percent lighter than steel” | silicon nitridesilicon-nitride | — | — |
| Low-pressure fuel turbopump housing insulation“Foam insulation encased in a Kevlar jacket covered the pump housing.” | Kevlar jacket over foamkevlar | — | — |
| Low-pressure oxidizer turbopump (LPOTP)Boosts LO2 pressure so the high-pressure pump can run fast without cavitating“The Low-Pressure Oxidizer Turbopump (Figure No. C-19) contained an axial-flow inducer driven by a six-stage hydraulic turbine.” | Not said | — | Supplier: Pratt & Whitney Rocketdyne (Canoga Park) |
| Low-pressure fuel turbopump (LPFTP)Boosts LH2 pressure; turbine runs on gaseous hydrogen“The Low-Pressure Fuel Turbopump (Figure No. C-20) contained an axial-flow inducer driven by a two-stage, axial-flow turbine powered with gaseous hydrogen.” | Not said | — | Supplier: Pratt & Whitney Rocketdyne (Canoga Park) |
| High-pressure oxidizer turbopump (HPOTP), mainstage pumpDouble-entry centrifugal impeller between two inducers, three-stage hot-gas turbine“The mainstage pump was a double entry centrifugal impeller flanked by two inducers.” | Not said | — | Supplier: Pratt & Whitney Rocketdyne (West Palm Beach, Block II) |
| HPOTP preburner pumpSingle centrifugal impeller that raises LO2 to 7,420 psia for the preburners“The final path entered the preburner impeller to raise the LO2’s pressure from 4,300 psia to 7,420 psia for use in both preburners.” | Not said | — | — |
| High-pressure fuel turbopump (HPFTP)Three-stage centrifugal pump on a two-stage hot-gas turbine; the most complex part of the engine“The three-stage centrifugal pump was driven by a twostage, hot gas turbine.” | Not said | — | Supplier: Pratt & Whitney Rocketdyne (West Palm Beach, Block II) |
| Low-pressure to high-pressure duct bellowsFlexible bellows that take up load between the pumps“contained flexible bellows that enabled them to flex when loads were applied” | Not said | — | — |
| LPOTP speed transducerTriple-redundant magnetic speed sensor on the turbine end“A triple-redundant, magnetic-type, speed transducer was located on the turbine end.” | Not said | — | — |
| Part | Material | Finish and standard | Made by |
|---|---|---|---|
| Inconel 718 across about 1,500 components“The latter, a nickel-based superalloy, was used in approximately 1,500 engine components and comprised roughly 51 percent of the SSME, by weight.” | Inconel alloy 718inconel-718 | — | — |
| Part | Material | Finish and standard | Made by |
|---|---|---|---|
| Main injector (600 coaxial elements)Injects LO2 through hollow posts into the main chamber; sits in the centre of the hot gas manifold“It included 600 coaxial elements which injected LO2 through their center posts.” | Not said | — | — |
| Main injector flow shieldsBolted to the outer row of elements against hot gas erosion“Flow shields, bolted to the outer row of elements, helped to protect them from damage and erosion from the high-velocity gas.” | Not said | — | — |
| Fuel preburner injector (264 coaxial elements)First-stage combustion of hydrogen-rich gas to drive the fuel turbopump“The injector was made up of 264 coaxial elements, arranged in a concentric row pattern.” | Not said | — | — |
| Oxidizer preburner injector (120 coaxial elements)First-stage combustion to drive the oxidizer turbopump“The injector was comprised of 120 coaxial elements, arranged in a concentric row pattern.” | Not said | — | — |
| Preburner combustion baffles (x3 per preburner)Elements that support and cool baffles to stabilize combustion“supported and cooled three baffles that helped to stabilize combustion” | Not said | — | — |
| Dual-redundant spark igniters (main chamber and both preburners)Light the engine and switch off after about three seconds“The dual-redundant igniter was used during the engine start sequence to initiate combustion.” | Not said | — | — |
| Two-duct hot gas manifold (redesign, first flown July 1995)Replaced three small fuel ducts with two enlarged ducts“The redesigned two-duct hot gas manifold, first flown in July 1995, replaced the three small fuel ducts with two enlarged ducts.” | Not said | — | — |
| Part | Material | Finish and standard | Made by |
|---|---|---|---|
| Nozzle hatbands (x9) and structural jacketWelded around the jacket for hoop strength“Nine hatbands were welded around the jacket for hoop strength” | Not said | — | — |
| Steerhorn hydrogen feed line (1.625 in)Carries hydrogen to the nozzle coolant circuit“a hydrogen feed line (“steerhorn”) measuring 1.625” in diameter also was attached to the nozzle exterior” | Not said | — | — |
| Nozzle coolant manifolds, fuel transfer ducts and drain linesTop and bottom manifolds with three transfer ducts and six drains“Coolant manifolds were welded to the top and bottom of the nozzle, along with three fuel transfer ducts and six drain lines.” | Not said | — | — |
| Engine heat shield support ringAttach point at the nozzle throat for the engine heat shield“A support ring welded to the throat of the nozzle was the attach point for the engine heat shield.” | Not said | — | — |
| Part | Material | Finish and standard | Made by |
|---|---|---|---|
| Five hydraulically actuated propellant valvesPreburner oxidizer valves set thrust and mixture ratio; main valves feed the chamber“the oxidizer preburner oxidizer valve, the fuel preburner oxidizer valve, the main oxidizer valve, the main fuel valve, and the chamber coolant valve” | Not said | — | Supplier: HR Textron (valve actuators) |
| Valve actuatorsHydraulic actuators driven by controller signals“Major SSME subcontractors were HR Textron (also known as Woodward HR Textron and Hydraulic Research, Inc.) in Valencia, California, for engine valve actuators” | Not said | — | Supplier: HR Textron (Hydraulic Research), Valencia, California |
| Main engine controllerOn-board digital computer that runs start, throttle, mixture ratio and shutdown“The pressurized, thermally conditioned controller, manufactured by Honeywell” | Not said | — | Supplier: Honeywell, Clearwater, Florida |
| Bleed valves (one LH2, one LO2)Circulate propellant for thermal conditioning and dump LH2 after cutoff“Two bleed valves were contained in each SSME, including one LH2 bleed valve and one LO2 bleed valve.” | Not said | — | — |
| Helium systemCloses the five main valves if hydraulics fail and purges the HPOTP intermediate seals“Helium was used to pneumatically close the five main hydraulically-actuated valves in the propellant lines should a hydraulic failure occur.” | Not said | — | — |
| Pneumatic control assemblySolenoid valves for seal purge, pogo postcharge and pneumatic shutdown“Each SSME had one pneumatic control assembly.” | Not said | — | — |
| Thrust vector control actuators (x2: pitch and yaw)Gimbal each engine; pitch up to 10.5 degrees, yaw up to 8.5 degrees“Two main engine TVC actuators were connected to the powerhead of each SSME.” | Not said | — | — |
| Part | Material | Finish and standard | Made by |
|---|---|---|---|
| Pogo accumulator assembly (3D printed)Beachball-sized shock absorber that regulates LOX movement to stop pogo vibration“The 3-D printed part tested, called the pogo accumulator, is a beachball-sized piece of hardware” | Not said | — | — |
| New engine controller and nozzle insulation (SLS upgrades)New controllers and software, nozzle insulation for booster exhaust“Upgrades to the engines include development of new engine controllers and software and the addition of nozzle insulation” | Not said | — | — |
Grades for turbine blade superalloy, nozzle tubes and heat exchanger tube not given. No data on the SLS-era RS-25 changes. Injector, valve and bearing materials (other than Block I silicon nitride) not covered. More detail: Block II shuttle-era hardware (NASA HAER TX-116) is the only component-level source; alloys of the turbopump rotors, injectors, valves, bearings and ducts remain unnamed except those already in the entry. SLS-era differences (new controller, nozzle insulation, additively manufactured pogo accumulator and other new-production parts) are described only at program level on NASA pages. The Space Shuttle is not a catalog product, so the orbiter relations use a plain-name target. Honeywell and HR Textron supplier statements are Shuttle-era; current SLS controller and actuator makers are not named in the NASA pages read. More detail: Block II shuttle-era hardware (NASA HAER TX-116) is the only component-level source; alloys of the turbopump rotors, injectors, valves, bearings and ducts remain unnamed except those already in the entry. SLS-era differences (new controller, nozzle insulation, additively manufactured pogo accumulator and other new-production parts) are described only at program level on NASA pages. The Space Shuttle is not a catalog product, so the orbiter relations use a plain-name target. Honeywell and HR Textron supplier statements are Shuttle-era; current SLS controller and actuator makers are not named in the NASA pages read.
Maker credit: Aerojet Rocketdyne (originally Rocketdyne). Collected from public pages and documents, checked 4 Nov 2026.
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