RS-68

RS-68

An RS-68 engine undergoing hot-fire testing at NASA's Stennis Space Center during its developmental phase.
Country of origin	United States
First flight	20 November 2002 (2002-11-20)
Last flight	9 April 2024 (2024-04-09)
Designer	Rocketdyne
Manufacturer	Rocketdyne (2002–2005) Pratt & Whitney Rocketdyne (2005–2013) Aerojet Rocketdyne (2013–2024)
Application	First stage engine
Associated LV	Delta IV · Delta IV Heavy
Status	Retired
Liquid-fuel engine
Propellant	LOX / LH2
Cycle	Gas-generator
Configuration
Nozzle ratio	21.5:1
Performance
Thrust, sea-level	RS-68: 2,950 kN (660,000 lbf) RS-68A: 3,137 kN (705,000 lbf)[1]
Thrust-to-weight ratio	RS-68: 45.3:1 RS-68A: 47.4:1
Chamber pressure	1,488 psi (10.26 MPa)
Specific impulse, vacuum	RS-68: 410 s (4.0 km/s) RS-68A: 411.9 s (4.039 km/s)[2]
Dimensions
Length	5.20 m (17.1 ft)
Diameter	2.43 m (8 ft 0 in)
Dry mass	RS-68: 6,600 kg (14,560 lb) RS-68A: 6,740 kg (14,870 lb)[1]

The RS-68 (Rocket System-68) was a liquid-fuel rocket engine that used liquid hydrogen (LH₂) and liquid oxygen (LOX) as propellants in a gas-generator cycle. It was the largest hydrogen-fueled rocket engine ever flown.^[3]

Designed and manufactured in the United States by Rocketdyne (later Pratt & Whitney Rocketdyne and Aerojet Rocketdyne). Development started in the 1990s with the goal of producing a simpler, less costly, heavy-lift engine for the Delta IV launch system. Two versions of the engine have been produced: the original RS-68 and the improved RS-68A. A third version, the RS-68B, was planned for the National Aeronautics and Space Administration's (NASA) Ares V rocket before the cancellation of the rocket and the Constellation Program in 2010.