The J-2 is a restartable hydrogen and oxygen engine that powered the upper stages of the Saturn V.
rocketdyne-j-2 The J-2 is a liquid hydrogen and liquid oxygen engine that Rocketdyne developed for NASA under a contract dated 1 September 1960. Five J-2 engines powered the second stage of the Saturn V. A single J-2 powered the third stage, which restarted in Earth orbit to send Apollo toward the Moon. Rocketdyne delivered 152 production engines.
The chamber wall is made of stainless-steel tubes brazed together, with liquid hydrogen running down 180 tubes and back up 360 to cool them. The first injector was copper and melted in development. The production injector has a porous sintered-metal face, and part of the fuel flows through it. The propellants also lubricate the pump bearings.
Five J-2 engines powered the Saturn V second stage and one J-2 powered the third stage, which restarted in Earth orbit to send Apollo toward the Moon. The engine is a reference for early hydrogen-oxygen engine construction: a brazed stainless-steel tube chamber, a propellant-lubricated turbopump pair and a hydrogen-fed start tank.
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The chamber body is a set of longitudinal stainless-steel tubes, brazed together and held by external stiffening bands. Hydrogen runs down 180 tubes and back up 360, cooling the wall on the way. Exhaust from the turbines is dumped into the nozzle at the point where the 180 tubes split into 360.
The first injector had a flat copper face. It melted in development, and the vaporised copper turned the exhaust plumes green. The production face is Rigi-Mesh, a porous screen of sintered metal, and part of the fuel flows through it to keep it cool. An igniter with two spark plugs lights the chamber.
The fuel pump is a high-speed axial machine with an inducer and a seven-stage rotor and stator. Its design came from pumps used in nuclear rocket programmes. The oxygen pump is a single-stage centrifugal pump. Gas from the gas generator drives the fuel turbine and then the oxygen turbine, and the propellants lubricate the bearings.
A small combustion chamber with two spark plugs, the gas generator, makes the hot gas that spins the turbines. To start the engine, cold hydrogen from a start tank is discharged through both turbines. A heat exchanger in the turbine exhaust duct boils liquid oxygen to pressurise the oxygen tank.
The main fuel valve and the main oxygen valve are both butterfly valves, and the oxygen valve is spring loaded closed. Teflon-coated seals keep the hydrogen in at duct flanges and pipe joints. On the third stage, two hydraulic actuators swivel the engine.
The brazed chamber tubes, and the igniter fuel line with its braid-covered flexible bellows.
The face of the production injector. Part of the fuel flows through it to cool it.
The original flat-faced injector, which melted during development.
Seals at duct flanges and pipe joints that keep the hydrogen contained.
The fuel. It cools the chamber tubes and, with the liquid oxygen, lubricates the pump bearings.
Five J-2 engines sat at the back end of the Saturn V second stage, the S-II, and the four outer ones swivelled. One more powered the third stage, the S-IVB. The J-2 first flew on 26 February 1966, on a Saturn IB second stage, as part of AS-201. Rocketdyne built it, and Moog Industries made the third-stage gimbal actuators. Its fuel pump was derived from Rocketdyne's nuclear rocket programme pumps.
NASA
Five J-2 engines on the second stage; the four outer engines gimbal
One restartable J-2 on the third stage
Grumman / NASA
Five on the Saturn V S-II second stage; Saturn V second-stage engines; launcher engine, not carried by the vehicle
One restartable engine on the S-IVB; Saturn V third-stage engine; launcher engine, not carried by the vehicle
North American Aviation
Five on the Saturn V S-II second stage; Saturn V second-stage engines; launcher engine, not carried by the vehicle
One restartable engine on the S-IVB; Saturn V third stage and Saturn IB second stage engine; launcher engine, not carried by the vehicle
The axial fuel turbopump was derived from machines used in the nuclear rocket programs
Engine developer; development contract awarded 1 September 1960
Built the S-IVB gimbal actuators to MSFC specifications
Rocketdyne, Division of North American Aviation
Copper. Here: Original injector (flat-faced). There: Fuel orifice rings (14); Oxidizer orifice rings (15) and more.
liquid oxygen. Here: Oxidizer. There: Oxidizer.
Teflon-coated seals. Here: Hydrogen seals at duct flanges and pipe joints. There: Main oxidizer valve poppet seal; Main fuel valve poppet seal.
Aerojet Rocketdyne
stainless steel. Here: Chamber tubes; Igniter fuel line and more. There: RL10C-1 thrust chamber tubes; Fuel pump drive shaft and bearings and more.
Teflon-coated seals. Here: Hydrogen seals at duct flanges and pipe joints. There: Duct flange seals (Teflon-coated conical washers).
TRW
stainless steel. Here: Chamber tubes; Igniter fuel line and more. There: Pintle tip, original; Flow-control valve bellows and more.
Copper. Here: Original injector (flat-faced). There: Pintle tip, copper-slug version.
Published accounts name few of the metals. The grade of the stainless steel, the braze alloy and the alloy of the injector face are missing, as are the metals of the pump rotors and housings, the valve bodies, the gas generator and the hydrogen start tank. Rigi-Mesh is a trade name, not an alloy.
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 |
|---|---|
| Vacuum thrust | Up to 230,000 lbf |
| Cycle | Open-cycle gas generator |
| Specific impulse | 425 s |
| Chamber pressure | A little over 700 psia |
| Mixture ratio | 5.5:1 oxidizer to fuel |
| Alternate mixture ratio | 4.5:1 |
| Nozzle area ratio | 27.5:1 |
| Propellants | Liquid oxygen and liquid hydrogen |
| Restart | Restartable in space |
| Engines per vehicle (S-II) | 5 on the Saturn V second stage |
| Engines per vehicle (S-IVB) | 1 on the Saturn V third stage |
| Production | 152 production engines |
| Development contract | 1 September 1960 (Rocketdyne) |
| First flight | 26 February 1966, AS-201 (Saturn IB second stage) |
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 partsChamber tubes · Igniter fuel line · Igniter fuel-line bellows
Used in 1 partOriginal injector (flat-faced)
Used in 1 partFuel
Used in 1 partOxidizer
Used in 1 partInjector face (production)
Used in 1 partHydrogen seals at duct flanges and pipe joints
Stage use, cycle and layout are sourced from NASA SP-2009-4545 and EngineHistory.org. Stated materials: brazed stainless-steel chamber tubes, stainless-steel igniter fuel line and bellows, a sintered-metal injector face, the original copper injector and Teflon-coated hydrogen seals. Pumps, valves, gas generator and start tank are listed without a material.
| Part | Material | Finish and standard | Made by |
|---|---|---|---|
| Chamber tubes“The thrust chamber body was constructed of longitudinal stainless steel tubes that were brazed together” | stainless steel (grade not stated)stainless-steel | — | — |
| Tube circuit (180 down, 360 up)“LH2 fuel from the fuel manifold circulated downward through 180 tubes, and back upward through 360 tubes” | Not said | — | — |
| External stiffening bands“supported by external stiffening bands around the tubes” | Not said | — | — |
| Nozzle (27:1 expansion)“a divergent nozzle with an expansion ratio of 27:1” | Not said | — | Sources also give 27.5:1 for the engine |
| Turbine exhaust injection into the nozzle“we used the opening at the 2:1 split to dump the hot gas into the nozzle of the thrust chamber” | Not said | — | — |
| Part | Material | Finish and standard | Made by |
|---|---|---|---|
| Injector face (production)“The porous sintered metal screen material (Rigi-Mesh®) that formed the injector face” | porous sintered metal screen (Rigi-Mesh, alloy not stated)rigimesh-sintered-metal-screen | — | — |
| Original injector (flat-faced)“The original J-2 injector was a flat-faced copper affair” | coppercopper | — | Replaced after injectors melted in development |
| Augmented spark igniter chamber“The ASI chamber was an integral part of the injector.” | Not said | — | — |
| Augmented spark igniter spark plugs (two)“The thrust chamber was ignited by an augmented spark igniter in the middle with two spark plugs” | Not said | — | — |
| Igniter fuel line“The stainless-steel ASI fuel line was attached to the fuel inlet manifold” | stainless steel (grade not stated)stainless-steel | — | — |
| Igniter fuel-line bellows“Stainless-steel braid-covered flexible bellows in three places” | stainless steel (grade not stated)stainless-steel | — | Single-ply bellows later implicated in the Apollo 6 anomaly and removed (SP-2009-4545) |
| Part | Material | Finish and standard | Made by |
|---|---|---|---|
| Fuel turbopump (axial, self-lubricated)“self-lubricated high-speed axial-flow fuel turbopump” | Not said | — | — |
| Fuel pump rotor“consisted of an inducer and a seven-stage rotor/stator assembly” | Not said | — | — |
| Oxidizer turbopump (centrifugal)“The oxidizer turbopump was a self-lubricated, high-speed, single-stage centrifugal pump” | Not said | — | — |
| Series turbine drive“in a series turbine arrangement, through the fuel turbopump and then over into the oxidizer turbopump” | Not said | — | — |
| Propellant-lubricated bearings“The bearing lubrications were liquid oxygen and liquid hydrogen, so there was no separate lubrication system.” | Not said | — | — |
| Part | Material | Finish and standard | Made by |
|---|---|---|---|
| Gas generator“The gas generator consisted of a combustion chamber containing two spark plugs” | Not said | — | — |
| Heat exchanger (LOX tank pressurisation)“LOX boiled by the turbine exhaust duct heat exchanger pressurized the LOX tank.” | Not said | — | — |
| Hydrogen start tank“it was this start tank that would discharge cold hydrogen through the two turbines to get the engine started” | Not said | — | — |
| Part | Material | Finish and standard | Made by |
|---|---|---|---|
| Main fuel valve“The main fuel valve (MFV) was a butterfly-type valve” | Not said | — | — |
| Main oxidizer valve“The MOV was a butterfly-type valve, spring loaded in the closed position” | Not said | — | — |
| Hydrogen seals at duct flanges and pipe joints“They also developed special Teflon®-coated seals to keep hydrogen contained” | Teflon-coated sealsteflon-ptfe | — | — |
| Part | Material | Finish and standard | Made by |
|---|---|---|---|
| Gimbal actuators (two)“Two linear hydraulic actuator assemblies gimbaled the J-2 engine.” | Not said | — | Douglas (DAC) contracted Moog Industries to build the actuators to MSFC specifications |
| Part | Material | Finish and standard | Made by |
|---|---|---|---|
| Oxidizer“Propellants were liquid oxygen and liquid hydrogen.” | liquid oxygen (LOX)liquid-oxygen | — | — |
| Fuel“Propellants were liquid oxygen and liquid hydrogen.” | liquid hydrogen (LH2)liquid-hydrogen | — | — |
No alloy grades, braze alloy or standards stated. Turbomachinery and valve materials not published in the sources read.
Maker credit: Rocketdyne (for NASA). Collected from public pages and documents, checked 4 Nov 2026.
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