Generating 705,000 pounds of thrust at sea level, the Aerojet Rocketdyne RS-68A is the world’s most powerful hydrogen-fueled rocket engine.

The engine provides main propulsion on United Launch Alliance’s Delta IV rocket, which is used primarily by the U.S. Department of Defense. The Delta IV Heavy, featuring three vehicle cores in a side-by-side configuration, is capable of launching the Defense Department’s heaviest payloads.

The original variant of the engine, the RS-68, was developed with private company funds. The RS-68A represents an improvement over the RS-68 and provides 42,000 pounds of additional thrust. The upgraded RS-68A completed its first test firing in September 2008, was certified in April 2011 and made its inaugural flight in June 2012.

Key Features

  • World’s largest and highest thrust hydrogen-fueled engine
  • Commercially-developed
  • Produces more than 17 million horsepower

Program Milestones

  • November 2002: First flight – RS-68
  • September 2008: First test firing – RS-68A
  • November 2010: RS-68A certification testing complete
  • April 2011: RS-68A design certification complete
  • June 2012: First flight – RS-68A

Engine Specifications


Full Power

Minimum Power

Thrust (Vacuum): 800K lbf 412K lbf

Thrust (Sea Level):

705K lbf

318K lbf

Chamber Pressure:

1,580 psia

820 psia

Engine Mixture Ratio:



Isp, Vacuum:

411 sec

411 sec

Isp, Sea Level:

362 sec

317 sec


17.1 ft 17.1 ft


14,740 lbs 14,740 lbs


8 ft 8 ft
Expansion Ratio: 21.5 21.5