The RS-68 (Rocket System-68) was a liquid-fuel rocket engine that used

liquid hydrogen Liquid hydrogen () is the liquid state of the element hydrogen. Hydrogen is found naturally in the molecule, molecular H2 form. To exist as a liquid, H2 must be cooled below its critical point (thermodynamics), critical point of 33 Kelvins, ...

(LH₂) and

liquid oxygen Liquid oxygen, sometimes abbreviated as LOX or LOXygen, is a clear cyan liquid form of dioxygen . It was used as the oxidizer in the first liquid-fueled rocket invented in 1926 by Robert H. Goddard, an application which is ongoing. Physical ...

(LOX) as propellants in a

gas-generator cycle The gas-generator cycle, also referred to as the GG cycle or colloquially as an open cycle, is one of the most commonly used power cycles in bipropellant liquid rocket engines. Propellant is burned in a gas generator (analogous to, but distinct ...

. It was the largest hydrogen-fueled rocket engine ever flown. Designed and manufactured in the United States by

Rocketdyne Rocketdyne is an American rocket engine design and production company headquartered in Canoga Park, California, Canoga Park, in the western San Fernando Valley of suburban Los Angeles, California, Los Angeles, in southern California. Rocketdyne ...

(later

Pratt & Whitney Rocketdyne Pratt & Whitney Rocketdyne (PWR) was an American company that designed and produced rocket engines that use liquid rocket propellants, liquid propellants. It was a division of Pratt & Whitney, a fully owned subsidiary of United Technologies Corpo ...

and

Aerojet Rocketdyne Aerojet Rocketdyne is a subsidiary of American Arms industry, defense company L3Harris that manufactures rocket, Hypersonic flight, hypersonic, and electric propulsive systems for space, defense, civil and commercial applications. Aerojet traces ...

). 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 The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the United States's civil space program, aeronautics research and space research. Established in 1958, it su ...

's (NASA) Ares V rocket before the cancellation of the rocket and the

Constellation Program The Constellation program (abbreviated CxP) was a crewed spaceflight program developed by NASA, the space agency of the United States, from 2005 to 2009. The major goals of the program were "completion of the International Space Station" and a " ...

in 2010.

Design and development

One of the main goals of the RS-68 program was to produce a simple engine that would be cost-effective when used for a single launch. To achieve this, the RS-68 has 80% fewer parts than the multi-launch

RS-25 The RS-25, also known as the Space Shuttle Main Engine (SSME), is a liquid-fuel cryogenic rocket engine that was used on NASA's Space Shuttle and is used on the Space Launch System. Designed and manufactured in the United States by Rocketd ...

Space Shuttle Main Engine (SSME)."AIAA 2002-4324, Propulsion for the 21st Century—RS-68"
. AIAA, 8–10 July 2002. The adverse consequences of this simplicity were a significantly lower thrust-to-weight ratio and a 10% lower specific impulse compared to the SSME. The benefit of this simplicity is the RS-68's reduced construction cost. The RS-68 was developed at

Propulsion and Power, located in

Canoga Park, Los Angeles Canoga Park is a neighborhood in the San Fernando Valley region of the City of Los Angeles, California. Before the Mexican–American War, the district was part of a Ranchos of California, rancho, and after the American victory it was converted ...

, California, where the SSME was manufactured. It was designed to power the Delta IV Evolved Expendable Launch Vehicle (EELV). The initial development engines were assembled at the nearby

Santa Susana Field Laboratory The Santa Susana Field Laboratory (SSFL), formerly known as Rocketdyne, is a complex of industrial research and development facilities located on a portion of Southern California in an unincorporated area of Ventura County in the Simi Hills betw ...

where the

Saturn V The Saturn V is a retired American super heavy-lift launch vehicle developed by NASA under the Apollo program for human exploration of the Moon. The rocket was human-rated, had multistage rocket, three stages, and was powered by liquid-propel ...

's Rocketdyne F-1 engines were developed and tested for the Apollo missions to the Moon. The initial testing of the RS-68 occurred at the

Air Force Research Laboratory The Air Force Research Laboratory (AFRL) is a scientific research and development detachment of the United States Air Force Air Force Materiel Command, Materiel Command dedicated to leading the discovery, development, and integration of direct- ...

(AFRL),

Edwards Air Force Base Edwards Air Force Base (AFB) is a United States Air Force installation in California. Most of the base sits in Kern County, California, Kern County, but its eastern end is in San Bernardino County, California, San Bernardino County and a souther ...

, California, and later at NASA's

Stennis Space Center The John C. Stennis Space Center (SSC) is a NASA rocket testing facility in Hancock County, Mississippi, United States, on the banks of the Pearl River (Mississippi–Louisiana), Pearl River at the Mississippi–Louisiana border. , it is NASA ...

. The RS-68 was certified in December 2001 for use on Delta IV rockets. An RS-68 was part of each Delta IV Common Booster Core. The largest of the launch vehicles, the Delta IV Heavy, used three CBCs mounted together. The engine produced in a vacuum and at sea level. The engine's mass was . With this thrust, the engine had a thrust-to-weight ratio of 51.2 and a

specific impulse Specific impulse (usually abbreviated ) is a measure of how efficiently a reaction mass engine, such as a rocket engine, rocket using propellant or a jet engine using fuel, generates thrust. In general, this is a ratio of the ''Impulse (physics), ...

of in a vacuum and at sea level. The RS-68 was gimbaled hydraulically and was capable of throttling between 58% and 102% thrust. The RS-68A is an updated version of the RS-68, with increased specific impulse and thrust (to over at sea level). The first launch on 29 June 2012, from the

Cape Canaveral Air Force Station Cape Canaveral Space Force Station (CCSFS) is an installation of the United States Space Force's Space Launch Delta 45, located on Cape Canaveral in Brevard County, Florida. Headquartered at the nearby Patrick Space Force Base, the sta ...

used three RS-68A engines mounted in a Delta IV Heavy rocket. The RS-68 was retired as of the last Delta IV Heavy flight in April 2024.

Proposed uses

In 2006, NASA announced an intention to use five RS-68 engines instead of SSMEs on the planned Ares V. NASA chose the RS-68 because of its lower cost, about $20 million per engine including the cost of NASA's upgrades. The upgrades included a different ablative nozzle to accommodate a longer burn, a shorter start sequence, hardware changes to limit free hydrogen at ignition, and a reduction in the amount of helium used during countdown and flight. Thrust and specific impulse increases would occur under a separate upgrade program for the Delta IV rocket. Later, the Ares V was changed to use six RS-68 engines, designated the RS-68B. Ares V was dropped as part of the cancellation of the

Constellation program The Constellation program (abbreviated CxP) was a crewed spaceflight program developed by NASA, the space agency of the United States, from 2005 to 2009. The major goals of the program were "completion of the International Space Station" and a " ...

in 2010. NASA's current successor heavy-lift vehicle, the

Space Launch System The Space Launch System (SLS) is an American Super heavy-lift launch vehicle, super heavy-lift Expendable launch system, expendable launch vehicle used by NASA. As the primary launch vehicle of the Artemis program, Artemis Moon landing progra ...

, uses four

engines instead.

Human-rating

In 2008, it was reported that the RS-68 needed over 200 changes to receive a human-rating certification. NASA has stated that those changes include health monitoring, removal of the fuel-rich environment at liftoff, and improving the robustness of its subsystems.

Variants

* RS-68 is the original version. It produces thrust at sea level. * RS-68A is an improved version. It produces thrust at sea level and thrust in a vacuum. Its specific impulse in a vacuum is . Certification was completed in April 2011. * RS-68B was a proposed upgrade to be used in the Ares V launch vehicle for NASA's Constellation program. The Ares V was to use six RS-68B engines on a diameter core stage, along with two 5.5-segment solid rocket boosters. It was later determined that the ablative nozzle of the RS-68 was poorly suited to this multi-engine environment, causing reduced engine efficiency and extreme heating at the base of the vehicle.

References

External links