Rocketdyne J-2

J-2

J-2 test firing
Country of origin	United States
First flight	26 February 1966 (AS-201)
Last flight	15 July 1975 (ASTP)
Designer	MSFC/Rocketdyne
Manufacturer	Rocketdyne
Application	Upper stage engine
Associated LV	Saturn IB (S-IVB) Saturn V (S-II and S-IVB)
Successor	HG-3 J-2X
Status	Retired
Liquid-fuel engine
Propellant	Liquid oxygen / Liquid hydrogen
Mixture ratio	5.5:1
Cycle	Gas generator
Configuration
Nozzle ratio	27.5:1
Performance
Thrust, vacuum	1,033.1 kN (232,250 lbf)
Thrust, sea-level	486.2 kN (109,302 lbf)
Thrust-to-weight ratio	73.18
Chamber pressure	5,260 kilopascals (763 psi)
Specific impulse, vacuum	421 seconds (4.13 km/s)
Specific impulse, sea-level	200 seconds (2.0 km/s)
Burn time	500 seconds
Dimensions
Length	3.4 metres (11.1 ft)
Diameter	2.1 metres (6.8 ft)
Dry weight	1,788.1 kilograms (3,942 lb)
References
References	[1][2][3]
Notes	Data is for SA-208/SA-504 version.

The J-2, commonly known as Rocketdyne J-2, was a liquid-fuel cryogenic rocket engine used on NASA's Saturn IB and Saturn V launch vehicles. Built in the United States by Rocketdyne, the J-2 burned cryogenic liquid hydrogen (LH₂) and liquid oxygen (LOX) propellants, with each engine producing 1,033.1 kN (232,250 lb_f) of thrust in vacuum. The engine's preliminary design dates back to recommendations of the 1959 Silverstein Committee. Rocketdyne won approval to develop the J-2 in June 1960 and the first flight, AS-201, occurred on 26 February 1966. The J-2 underwent several minor upgrades over its operational history to improve the engine's performance, with two major upgrade programs, the de Laval nozzle-type J-2S and aerospike-type J-2T, which were cancelled after the conclusion of the Apollo program.

The engine produced a specific impulse (I_sp) of 421 seconds (4.13 km/s) in a vacuum (or 200 seconds (2.0 km/s) at sea level) and had a mass of approximately 1,788 kilograms (3,942 lb). Five J-2 engines were used on the Saturn V's S-II second stage, and one J-2 was used on the S-IVB upper stage used on both the Saturn IB and Saturn V. Proposals also existed to use various numbers of J-2 engines in the upper stages of an even larger rocket, the planned Nova. The J-2 was America's largest production LH2-fuelled rocket engine before the RS-25. A modernized version of the engine, the J-2X, was considered for use on the Earth Departure Stage of NASA's Space Shuttle replacement, the Space Launch System.

Unlike most liquid-fueled rocket engines in service at the time, the J-2 was designed to be restarted once after shutdown when flown on the Saturn V S-IVB third stage. The first burn, lasting about two minutes, placed the Apollo spacecraft into a low Earth parking orbit. After the crew verified that the spacecraft was operating nominally, the J-2 was re-ignited for translunar injection, a 6.5 minute burn which accelerated the vehicle to a course for the Moon.