J-2

The J-2 was the rocket engine that helped push Apollo astronauts toward the Moon. Built by Rocketdyne in the 1960s, it could do something rare for its time: switch off in space and then light again on command.

Quick facts

• Type: Cryogenic, gas-generator-cycle, liquid-propellant rocket engine. (“Cryogenic” means its fuel and oxidizer are kept as extremely cold liquids.)
• Propellants: Liquid hydrogen as fuel and liquid oxygen as the oxidizer (the substance that lets the fuel burn), at a mixture ratio of about 5.5 parts oxygen to 1 part hydrogen by weight.
• Thrust in vacuum: about 1,033 kN (232,250 pounds of force); about 486 kN at sea level. Thrust is the push the engine produces.
• Specific impulse: roughly 421 seconds in vacuum, a measure of fuel efficiency, and among the highest of any engine of its era.
• Burn time: about 500 seconds, and restartable once during a flight.
• Size and mass: about 3.4 m long, 2.1 m wide, and 1,788 kg (3,942 lb) dry.
• Maker and dates: Rocketdyne (designed with NASA’s Marshall Space Flight Center); development approved June 1960, first flight 26 February 1966, last flight 15 July 1975.

How it works

The J-2 uses a “gas-generator cycle.” A small amount of the hydrogen and oxygen is burned in a side chamber called a gas generator, and the hot gas spins two turbopumps. These pumps force the main propellants into the combustion chamber at high pressure. The hydrogen pump is a seven-stage axial pump turning around 28,000 rpm; the oxygen pump is a centrifugal type.

Before the hydrogen is burned, the cold liquid is routed through 540 tubes that form the bell-shaped nozzle and chamber walls, cooling them. This is called regenerative cooling, the same idea as running cold water through pipes to keep them from overheating. Two spark plugs in an Augmented Spark Igniter light the propellants and keep firing throughout the burn for reliable ignition. The turbine exhaust is dumped through the nozzle skirt, adding a little extra thrust, and a propellant-utilization valve trims the oxygen flow so both tanks empty together.

For restarting in space, a pressurized start tank stored gaseous hydrogen, refilled from the cooling jacket during the burn, to spin the pumps back up. That let the engine reignite in the vacuum of orbit.

Why it matters

The J-2 was the workhorse upper-stage engine of the Apollo program and was essential to landing humans on the Moon. Its efficient hydrogen and oxygen combustion gave the Saturn rockets the performance to push heavy Apollo spacecraft out of Earth orbit. Its restart ability made the mission possible: the single J-2 on the S-IVB stage first fired to reach a low parking orbit around Earth, then reignited later for trans-lunar injection, the burn that sent the crew toward the Moon. The J-2 also helped prove and mature large-scale liquid-hydrogen engine technology in the United States, influencing later cryogenic engines.

Where it was used and notable examples

• Saturn V S-II second stage: a cluster of five J-2 engines that propelled the rocket through the upper atmosphere toward orbit.
• Saturn V S-IVB stage: a single J-2 that fired twice, once to enter Earth parking orbit and again for trans-lunar injection on missions like Apollo 8 and Apollo 11.
• Saturn IB S-IVB stage: a single J-2 used on AS-201 (the engine’s first flight, 26 February 1966), Apollo 7, and the three crewed Skylab ferry flights.
• Apollo 6 (4 April 1968): an uncrewed test in which a resonant vibration cracked the fuel line of one engine’s igniter, leading two of the five J-2 engines on the S-II stage to shut down early. The problem appeared only in vacuum and was fixed before the first crewed Saturn V.

Engineering challenges

Liquid hydrogen gave the J-2 excellent efficiency but demanded great care. It is stored at about -423 F (-253 C), takes up a lot of room because it is so light, and causes hydrogen embrittlement, a weakening of metals. Engineers plated parts with copper or gold so hydrogen seeped in more slowly. Early copper injectors overheated and melted, tinting the exhaust green with vaporized copper, so Rocketdyne switched to a porous sintered-metal “Rigimesh” injector face borrowed from the RL10 engine. Later derivatives included the throttleable J-2S and the experimental aerospike-nozzle J-2T. Decades later NASA developed a modernized version, the J-2X (about 1,310 kN thrust), for the Constellation program and an early Space Launch System upper stage, though its development ended in 2014. Examples of the J-2 are preserved at the Smithsonian National Air and Space Museum.