Pratt & Whitney
Continues Tests On NASA Rocketdyne RS-68 Engines
By Shane Nolan
September 22, 2011 – In an impressive display of power
and technology, Pratt & Whitney Rocketdyne successfully
completed a series of hot-fire tests on the certified
RS-68A engine, the world's most powerful hydrogen-fueled
The tests demonstrated the capability of the engine to
operate for 4,800 seconds of cumulative run time – four
times the design life of the engine and more than 10
times what’s needed to boost a United Launch Alliance
heavy-lift rocket into space. The tests took place at
John C. Stennis Space Center in Mississippi.
The Pratt & Whitney Rocketdyne RS-68 (Rocket System 68)
is a liquid-fuel rocket engine that burns liquid
hydrogen (LH2) with liquid oxygen (LOX). It is the
largest hydrogen-fueled engine in the world.
Development of the engine started in the 1990s with the
goal of producing a simpler, less-costly, heavy-lift
engine for the Delta IV launch system. The engine has
three versions: the original RS-68, the improved RS-68A,
and the RS-68B for NASA.
was developed at Rocketdyne Propulsion and Power, located in
Canoga Park, Los Angeles, California, to power the Delta IV
Evolved Expendable Launch Vehicle (EELV). The combustion chamber
burns liquid hydrogen and liquid oxygen at 1,486 lbf/in˛ (10.25
MPa) at 102% with a 1:6 engine mixture ratio.
maximum 102% thrust, the engine produces 758,000 pounds-force
(3,370 kN) in a vacuum and 663,000 pounds-force (2,950 kN) at
sea level. The engine's mass is 14,560 pounds (6,600 kg) at 96
inches (2.4 m). With this thrust, the engine has a
thrust-to-weight ratio of 51.2, and a specific impulse of 410 s
(4 kNs/kg) in a vacuum and 365 s (3.58 kNs/kg) at sea level. The
RS-68 is gimbaled hydraulically and is capable of throttling
between 58% and 101% thrust.
goal of the RS-68 program was to produce a simple engine that
would be cost-effective when jettisoned after a single launch.
To achieve this, the RS-68 has 80% fewer parts than the
multi-launch Space Shuttle main engine (SSME). Simplicity came
at the cost of lower thrust-efficiency versus the SSME: the
RS-68's thrust-to-weight ratio is significantly lower and the
RS-68's specific impulse is 10% lower.
benefit of the RS-68 is its reduced construction cost: To build
an RS-68 for the Boeing Delta IV program costs about $14
million, compared to $50 million for the SSME. While the SSME's
higher costs were designed to be spread across multiple
launches, the larger, less-costly, more powerful (50% more
thrust) RS-68 was a more cost-effective engine for an expendable
“We are proud to
celebrate this success with our United Launch Alliance customer on a
test series that went above and beyond in demonstrating the robustness
and reliability of the RS-68A engine,” said Dan Adamski, RS-68 program
manager, Pratt & Whitney Rocketdyne.
The RS-68A is a liquid-hydrogen/liquid-oxygen booster engine designed to provide increased thrust and improved fuel efficiency for the Delta IV family of launch vehicles. It evolved from the RS-68 engine, which was developed and certified for commercial use entirely on private company funds. Each RS-68A will provide 702,000 pounds of lift-off thrust, or 39,000 more pounds of thrust than the RS-68 engine, with increased combustion efficiency as well.
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